energies

Article AC “Back to Back” Switching Device in Industrial † Application

Roberto Faranda 1 , Hossein Hafezi 2,* , Kishore Akkala 1 and Massimo Lazzaroni 3

1 Department of Energy, Politecnico di Milano, 20156 Milan, Italy; [email protected] (R.F.); [email protected] (K.A.) 2 School of Technology and Innovations, Electrical Engineering, University of Vaasa, 65200 Vaasa, Finland 3 Dipartimento di Fisica, Università degli Studi di Milano, 20133 Milan, Italy; [email protected] * Correspondence: hossein.hafezi@uwasa.fi This paper is an extended version of our paper published in 2018 IEEE International Conference on † Environment and Electrical Engineering and 2018 IEEE Industrial and Commercial Power Systems Europe (EEEIC/I&CPS Europe), Palermo, Italy, 12–15 June 2018; doi:10.1109/EEEIC.2018.8494482.



Received: 13 May 2020; Accepted: 29 June 2020; Published: 9 July 2020


Abstract: In industrial applications, among several varieties of semiconductor devices available, a silicon-controlled rectifier (SCR) is often used in managing and protecting various systems with different applications. Hence, it is of the utmost importance to design a control system which can operate over a range of electrical loads without any modifications in its hardware and/or software. This paper analyzes and investigates in detail the power circuit effects on conduction delay and SCR functioning. Moreover, two different commonly used driving systems for SCR application have been introduced, discussed, and evaluated. Concerning driving systems, here, three aspects have paramount importance and are consequently taken into consideration, namely the driver system losses, the conduction delay, and in particular, some power quality indices. The conduction delay is a parameter of great importance, as being able to control and reduce it to the minimum allowed by the application can bring significant practical advantages (both in terms of application and economic terms, as better summarized in the article). Theoretical analysis has been performed, followed and verified by simulation studies and, for some cases, laboratory experimental test results are presented which provide credibility to the study.

Keywords: back to back; electromagnetic compatibility (EMC); inductance; industrial automation; measurement; power converters; power quality; silicon-controlled rectifier (SCR)

1. Introduction In industrial applications, the silicon-controlled rectifier (SCR) (power electronics-based switch and breaker) is a widely-used semi-controlled device due to its controllability and flexibility over diodes (uncontrolled switches) and its reliability and affordable prices with respect to fully-controlled power electronic switches [1–4]. Moreover, in diverse industrial fields, among several varieties of semiconductor devices available, SCRs are a preferred choice in managing and protecting various systems with different applications, especially in the field of industrial control systems [5–7] where it is mandatory to consider the peak value of current and its duration. As examples, in magnetic applications (where the magnets are sensitive to the peak current) and electric protection devices, it is of importance to take into account the peak current value, its shape and its time duration, else it might result in the damage of the system [8] and/or the protected load. SCR due to its characteristics (controllability, reliability, and affordable price) is the first choice in several other industrial applications as well [4]. Controllability refers to the ability to move the system

Energies 2020, 13, 3539; doi:10.3390/en13143539 www.mdpi.com/journal/energies Energies 2020, 13, 3539 2 of 20 or the device (as it is in this situation) in the proper way as desired and in its entire configuration space using only certain admissible manipulations (as it will be clear in the following). Reliability, instead, is the ability of an item to perform the required function under given conditions for a given time interval. Finally, components should be evaluated also taking into account the cost versus the given functions. Flexible AC transmission systems (FACTS) is a wide area where SCRs are often used, particularly in Static VAR compensators (SVC) which are utilized in different sectors in electric power system (distribution and transmission lines/substations). Authors in [9] comprehensively reviewed the applications of SVC in different positions in electric railway systems. The evolution of SVC led to the development of static synchronous compensator (STATCOM), where in many topologies SCRs are still being used due to its economic benefits [10]. SCRs have also found their applications in high voltage DC (HVDC) systems, where they act as a converter interface between AC and DC systems and provides smooth operation through phase-control. Other interesting applications, which are under development include a hybrid DC vacuum circuit breaker for medium voltage and is reported in [11]. A self-supplied SCR for medium voltage application for a solid-state starter is proposed in [12]. In addition, Phase-controlled converters in high power AC machine drives is another vast field of application for SCRs and is a topic for many research articles. The recent works on SCR deal with the mitigation effects of common-mode current, as investigated in [13], and harmonic reduction of three-phase controlled converters, as studied in [14]. Furthermore, the wide range of applications of SCRs is not only limited to high voltage or high power. The diode triggered SCR for high temperature, low voltage electrostatic discharge application has been recently introduced and studied [15–17]. Given the wide range of applications of SCRs, under normal operation, a precise control to turn on the SCR (SCR conduction) and with emphasis to close the circuit at the right instant is important. In modern industrial applications, where it is mandatory to obtain high reliability with a mixture of optimal control techniques and diagnostics (particularly techniques related to detect failures), this target can be achieved [18,19]. The study on SCR as a static switch presented in this paper is mainly focused on applications related to power quality, demand response, current management for magnetic load, and in smart grid system applications [20]. Considering that the only way to properly use a SCR is to utilize an electronic control circuit, it is of utmost importance to design an universal SCR driver system which, without any modifications in its hardware and/or software, can operate over a wide variety of applications [21,22], and in turn, it can achieve optimal functionality and high level of system reliability [23]. In order to achieve this goal, it is important to understand the device behaviour and investigate different factor’s (e.g., supply voltage or load characteristics, and driver control circuit characteristics) effects on SCRs ignition. Although the behaviour of the SCR itself is familiar for the scholars and engineers in the field, the effects of different elements on SCR operation have not been studied or reported in any publication. These effects on SCR ignition reported in this paper, are very essential to design an effective control system which can operate over wide range of applications minimizing the SCRs side effects. This paper fill-in the gaps existing in current literature and investigates in detail the issues in general format and formulate the minimum pulse duration for ignition of an SCR as function of electrical parameters and different driver topologies, in order to lead to a system with better controllability, higher power quality, and to minimize design efforts to allow the industry to reuse a solution in several applications with minor or no changes. The paper is an extended version of previous work [24] and is organized in the following way. In Section2, a general review of SCR devices and its application is given. In Section3, the problem formulation is explained considering both general and practical aspects. In Section4, we provide Energies 2020, 13, 3539 3 of 20

EnergiesevidenceEnergies 20202020 concerning,, 1313,, xx FORFOR PEERPEER the driverREVIEWREVIEW circuits used, the simulation, and the obtained experimental results33 ofof are 2020 discussed. Finally, the conclusions are summarized in Section5. 2.2. AnAn OverviewOverview onon SCRSCR andand ItsIts ApplicationApplication 2. An Overview on SCR and Its Application SCRSCR isis aa semiconductorsemiconductor devicedevice widelywidely usedused asas aa powerpower switch.switch. ItsIts usageusage overover thethe yearsyears increasedincreased SCR is a semiconductor device widely used as a power switch. Its usage over the years increased duedue toto itsits characteristicscharacteristics ofof fastfast switchingswitching current,current, duringduring switchswitch onon andand sometimessometimes duringduring switchswitch off,off, induein comparisoncomparison to its characteristics toto thethe moremore of traditional fasttraditional switching andand current, historicallyhistorically during usedused switch electromechanicalelectromechanical on and sometimes breaker.breaker. during switch off, in comparisonTheThe switchingswitching to the onon more ofof thethe traditional devicedevice cancan and bebe historically controlledcontrolled used withwith electromechanical aa gategate signalsignal byby meansmeans breaker. ofof well-definedwell-defined biasingbiasingThe conditions.conditions. switching ThroughThrough on of the this,this, device thethe can electricalelectrical be controlled parametersparameters with ofof a thethe gate loadload signal voltage,voltage, by means current,current, of well-defined andand averageaverage absorbedbiasingabsorbed conditions. powerpower byby load Throughload cancan bebe this, controlled.controlled. the electrical parameters of the load voltage, current, and average absorbedTheThe SCR powerSCR devicedevice by load isis cantypicallytypically be controlled. manufacturedmanufactured byby fourfour layerslayers ofof PP andand NN typetype semiconductorsemiconductor materialsmaterialsThe SCR placedplaced device oneone is afterafter typically anotheranother manufactured inin alternatingalternating by fashion.fashion. four layers TheThe of operationoperation P and N type ofof thethe semiconductor SCRSCR cancan bebe explainedexplained materials byplacedby aa pairpair one ofof after tightlytightly another coupledcoupled in alternating bipolarbipolar junctionjunction fashion. Thetrantran operationsistors,sistors, arrangedarranged of the SCR inin such cansuch be aa explainedwayway toto causecause by a pairaa self-self- of latchingtightlylatching coupled actionaction asas bipolar depicteddepicted junction inin FigureFigure transistors, 1.1. Here,Here, arranged twotwo transistorstransistors in such aareare way shown,shown, to cause namelynamely a self-latching PNPPNP andand actionanan NPNNPN as devices.depicteddevices. in Figure1. Here, two transistors are shown, namely PNP and an NPN devices.

FigureFigure 1. 1. SCRSCR equivalentequivalent circuit.circuit. TheThe operationoperation ofof thethe SCRSCR cancan bebe simplysimply explainedexplained by by a a pairpair ofof tightlytightly coupledcoupled bipolar bipolar junction junction transistors, transistors, arrangedarranged asas inin figurefigurefigure inin suchsuch aa way way to to cause cause a a self-latchingself-latching action. action.

When the V is positive with respect to the cathode, the SCR operates in its forward WhenWhen thethe VVGATEGATE isis positivepositive withwith respectrespect toto thethe cathode,cathode, thethe SCRSCR operatesoperates inin itsits forwardforward characteristics.characteristics. FigureFigureFigure2 2graphically2 graphicallygraphically reports reportsreports SCR’s SCR’sSCR’s forward forwardforward characteristics. characteristics.characteristics. In Figure InIn FigureFigure2, three 2,2, I-V threethree curves I-VI-V are plotted in order to show gate current (designated as I ) effects on SCR’s forward voltage (reported curvescurves areare plottedplotted inin orderorder toto showshow gategate currentcurrent (designated(designatedG asas IIGG)) effectseffects onon SCR’sSCR’s forwardforward voltagevoltage (reportedon(reported abscissa onon axis) abscissaabscissa and theaxis)axis) forward andand thethe current forwardforward (reported currentcurrent on (reported(reported the ordinate onon thethe axis). ordinateordinate In particular axis).axis). InIn itparticularparticular represents itit three curves covering I = 0, relatively low gate current (I ) and slightly higher gate current (I ). representsrepresents threethree curvescurvesG coveringcovering IIGG == 0,0, relativelyrelatively lowlow gategateG1 currentcurrent ((IIGG11)) andand slightlyslightly higherhigher gategateG2 It can be noticed that I-V cures follow a similar shape unless the breakover point occurs sooner with currentcurrent ((IIGG22).). ItIt cancan bebe noticednoticed thatthat I-VI-V curescures followfollow aa similarsimilar shapeshape unlessunless thethe breakoverbreakover pointpoint occursoccurs higher I . soonersooner withwithG higherhigher IIGG..

Figure 2. Different kind of SCR Static I-V Characteristics (qualitative representation, not in scale). Figure 2. DifferentDifferent kind kind of of SCR Static I-V Characteristics (qualitative (qualitative representation, representation, not not in in scale). scale). Legend:Legend: VVB0B0 == ForwardForward BreakoverBreakover Voltage,Voltage, VVBRBR == ReverseReverse BreakoverBreakover Voltage,Voltage, IIGG == GateGate Current,Current, IILL == Legend: VB0 = Forward Breakover Voltage, VBR = Reverse Breakover Voltage, IG = Gate Current, LatchingLatching Current,Current, IIHH == HoldingHolding Current.Current. IL = Latching Current, IH = Holding Current.

InIn FigureFigure 3,3, thethe qualitativequalitative variationvariation ofof ForwardForward BreakdownBreakdown VoltageVoltage ((VVBB00)) vs.vs. GateGate CurrentCurrent ((IIGG)) isis shown.shown. ItIt putsputs inin evidenceevidence thatthat thethe correctcorrect wayway toto turnturn onon aa SCR,SCR, isis toto applyapply aa fastfast currentcurrent pulsepulse withwith suitablesuitable amplitudeamplitude toto thethe gategate terminalterminal whenwhen aa positivepositive voltagevoltage isis providedprovided betweenbetween anodeanode andand cathodecathode inin orderorder toto reducereduce thethe breakoverbreakover voltagevoltage ofof thethe device.device.

Energies 2020, 13, 3539 4 of 20

In Figure3, the qualitative variation of Forward Breakdown Voltage ( VB0) vs. Gate Current (IG) is shown. It puts in evidence that the correct way to turn on a SCR, is to apply a fast current pulse Energies 2020, 13, x FOR PEER REVIEW 4 of 20 with suitable amplitude to the gate terminal when a positive voltage is provided between anode and Energiescathode 2020 in, 13 order, x FOR to PEER reduce REVIEW the breakover voltage of the device. 4 of 20

Figure 3. Qualitative variation of Breakdown Voltage (VB0) vs. Gate Current (IG): the correct way to

turn on a SCR device is to apply a fast current pulse with suitable amplitude on the gate terminal Figure 3. Qualitative variation of Breakdown Voltage (VB0) vs. Gate Current (IG): the correct way to whenFigure a 3.positiveQualitative voltage variation is applied of Breakdownbetween anode Voltage and cathode (VB0) vs. to Gate reduce Current the Breakover (IG): the correctVoltage way of the to turndevice.turn on aa SCRSCR devicedevice is is to to apply apply a fasta fast current current pulse pulse with with suitable suitable amplitude amplitude on the on gate the terminalgate terminal when whena positive a positive voltage voltage is applied is applied between between anode anode and cathode and cathode to reduce to reduce the Breakover the Breakover Voltage Voltage of the device. of the device.Therefore, the gate current is usually made high enough to ensure that the SCR is switched to the ONTherefore, state at the the proper gate current time and is usually it is usually made necessary high enough for just toensure an instant that because the SCR a isconstant switched gate to currentthe ONTherefore, is state not atrequired the gate proper to current trigger time andis the usually it SCR is usually andmade it necessaryhighwould enough only for cause justto ensure an more instant that power becausethe lossesSCR ais constantthat switched needs gate to thebecurrent dissipated ON state is not at within required the proper the to device. trigger time andTherefore, the it SCR is usually and once it the wouldnecessary device only hasfor cause beenjust morean switched-on instant power because losses by the thata gateconstant needs terminal, togate be currentitdissipated remains is notlatched within required thein the device. to ON trigger state Therefore, the and SCR it does once and not theit would need device aonly hascontinuous cause been switched-onmore supply power of gate bylosses the current gatethat terminal,needsin order to betoit remainsdissipatedmaintain latched the within conduction. in the the device. ON This state Therefore, is and true it only does once if not thethe need deviceanode a continuous hascurrent been has switched-on supply exceeded of gatebythe the latching current gate terminal, incurrent order it(denotedto remains maintain by latched theIL in conduction. Figure in the 2).ON Thisstate isand true it does only ifnot the need anode a continuous current has supply exceeded of gate the current latching in current order to(denoted maintainAs long by the IasL in theconduction. Figure anode2). remains This is positively true only biased,if the anode it cannot current be switched has exceeded off until the the latching anode currentcurrent L (denotedfalls Asbelow long by theI as in theholding Figure anode 2).current remains (denote positively as I biased,H) and so it cannotthe voltage be switched across otheff until SCR the become anode zero current or negative.fallsAs below long the as holding the anode current remains (denote positively as IH) andbiased, so the it cannot voltage be across switched the SCR off becomeuntil the zero anode or negative. current falls Afterbelow the thethe current currentholding hashas current reached reached (denote natural natural as zero IH zero) point,and point, so a finitethe a finitevoltage amount amount across of time ofthe delay time SCR is delay become required is requiredzero before or negative.initiatingbefore initiating another another switch switchon procedure on procedure and during andduring this period this periodSCR must SCR retain must retainin off instate. off state.This minimumThisAfter minimum thetime current timedelay delay ishas called reached is called turn natural turnoff time off zero time(tQ). point, (tQ). a finite amount of time delay is required before initiatingTo have another right switch SCR gate on triggering,procedure followingfoll andowing during conditions this period need SCR toto bebe must provided:provided: retain in off state. This minimum time delay is called turn off time (tQ). Toanan haveappropriate appropriate right SCR gate gate gate to to cathode triggering, voltage following ( VGATE conditions) )is is needed needed need to to turn turn to be on on provided: the the device device while while the the SCR SCR is is forwardforward biased; biased; GATE angategate appropriate signal signal shall shall gate be be removed removedto cathode after after voltage the the device device (V under under) is needed test test is is toturned turned turn on on inthe in order order device to to reducewhile reduce the the the SCR losses losses is forwardandand avoid avoid biased; higher higher junction junction temperature; temperature;

gate signal shall be removed after the device under testGATE is turned on in order to reduce the losses whilewhile the the device device is is in in reverse reverse biased biased conditions, conditions, no no VVGATE signalsignal should should be be applied; applied; andwhen avoid the devicehigher isjunction in-off state, temperature; negative VGATE should be applied to improve the performance of when the device is in-off state, negative VGATE should be applied to improve the performance of while the device is in reverse biased conditions, no VGATE signal should be applied; thethe under-test under-test device. device. when the device is in-off state, negative VGATE should be applied to improve the performance of Taking into account the above considerations and noticing that the pulses must occur theTaking under-test into account device. the above considerations and noticing that the pulses must occur periodically periodically (considering the network frequency), it is evident that the gate is triggered by means of (considering the network frequency), it is evident that the gate is triggered by means of a pulsed a pulsedTaking waveform into account applied the across above the considerationsgate terminals andand willnoticing be able that to workthe pulsescorrectly must for alloccur the waveform applied across the gate terminals and will be able to work correctly for all the power switch periodicallypower switch (considering applications the and network with any frequency), load conditio it is ns.evident Therefore, that the it is gate very is importanttriggered byto knowmeans the of applications and with any load conditions. Therefore, it is very important to know the availability of aavailability pulsed waveform of SCR topologiesapplied across in the the market. gate terminals Based on and the will manufacturing be able to work technology correctly used, for all three the SCR topologies in the market. Based on the manufacturing technology used, three different topologies powerdifferent switch topologies applications of SCR and can with be realized—they any load conditio are ns.listed Therefore, below and it is arevery described important in tomore know detail the of SCR can be realized—they are listed below and are described in more detail in the literature, availabilityin the literature, of SCR e.g., topologies in [25]: in the market. Based on the manufacturing technology used, three differente.g., in [25 topologies]: of SCR can be realized—they are listed below and are described in more detail in thePlanarPlanar literature, type: e.g., TypicallyTypically in [25]: used used for for low low current current devices devices because because the junctions the junctions are obtained are obtained by diffusion by diffusionand come and to thecome same to the surface same on surface the cathode on the side.cathode The side. ratio The silicon ratio/ampere silicon/ampere is typically is typically high;  Planarhigh; type: Typically used for low current devices because the junctions are obtained by  diffusionMesa type: and It comeis used to thefor samehigh surfacecurrent on devices the ca thodebut low side. di/dt The ratio. ratio silicon/amperePower SCRs are is typicallytypically high;manufactured by this process;  MesaPress packtype: type:It is Widelyused for used high in current applications devices where but itlow is necessary di/dt ratio. to Powerhave SCR SCRs with are center typically gate manufacturedand large value by of this di/dt process;.  Press pack type: Widely used in applications where it is necessary to have SCR with center gate and large value of di/dt.

Energies 2020, 13, x FOR PEER REVIEW 4 of 20

Figure 3. Qualitative variation of Breakdown Voltage (VB0) vs. Gate Current (IG): the correct way to turn on a SCR device is to apply a fast current pulse with suitable amplitude on the gate terminal when a positive voltage is applied between anode and cathode to reduce the Breakover Voltage of the device.

Therefore, the gate current is usually made high enough to ensure that the SCR is switched to the ON state at the proper time and it is usually necessary for just an instant because a constant gate current is not required to trigger the SCR and it would only cause more power losses that needs to be dissipated within the device. Therefore, once the device has been switched-on by the gate terminal, it remains latched in the ON state and it does not need a continuous supply of gate current in order to maintain the conduction. This is true only if the anode current has exceeded the latching current (denoted by IL in Figure 2). As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (denote as IH) and so the voltage across the SCR become zero or negative. After the current has reached natural zero point, a finite amount of time delay is required before initiating another switch on procedure and during this period SCR must retain in off state. This minimumEnergies 2020 ,time13, 3539 delay is called turn off time (tQ). 5 of 20 To have right SCR gate triggering, following conditions need to be provided:

anMesa appropriate type: It isgate used to cathode for high voltage current (V devicesGATE) is butneeded low todi /turndt ratio. on the Power device SCRs while are the typically SCR is forwardmanufactured biased; by this process; gatePress signal pack shall type: be Widely removed used after in applications the device under where test itis is necessaryturned on to inhave order SCR to reduce with center the losses gate Energiesandand 2020 avoid large, 13, x FOR valuehigher PEER of junction diREVIEW/dt. temperature; 5 of 20 while the device is in reverse biased conditions, no VGATE signal should be applied; Usually, power SCRs use a Mesa process for manufacturing. It is important to note that the device Usually,when the power device SCRs is in-off use state, a Mesa negative process VGATE for should manufacturing. be applied It to is improve important the to performance note that the of deviceis prevalentlythe is under-testprevalently used indevice. used “back in to “back back” to mode back” in mode AC circuit in AC configurations, circuit configurations, see Figure see4 where Figure the 4 devicewhere theis turned device o isff turnedautomatically, off automatically, hence load hence current load is cuto currffentevery is cutoff zero crossingevery zero of crossing the signal. of Forthe thesignal. DC Taking into account the above considerations and noticing that the pulses must occur Forapplications, the DC applications, an ad hoc circuit an ad is hoc necessary circuit foris necess the useary of for SCR, the this use is of important SCR, this to is force important the device to force into periodically (considering the network frequency), it is evident that the gate is triggered by means of theturn device off state. into However, turn off state. the SCR However, application the SCR in DC application systems is in not DC further systems developed is not further in this developed paper as it a pulsed waveform applied across the gate terminals and will be able to work correctly for all the infocuses this paper only as on it AC focuses applications. only on AC applications. power switch applications and with any load conditions. Therefore, it is very important to know the availability of SCR topologies in the market. Based on the manufacturing technology used, three different topologies of SCR can be realized—they are listed below and are described in more detail in the literature, e.g., in [25]:

 Planar type: Typically used for low current devices because the junctions are obtained by diffusion and come to the same surface on the cathode side. The ratio silicon/ampere is typically high;  Mesa type: It is usedFigureFigure for high4. 4. TheThe current AC AC “Back “Back devices to to Back” Back” but Switch Switch low configuration. configuration.di/dt ratio. Power SCRs are typically manufactured by this process;  AsPressAs it it can canpack be be type: seen seen Widely from from Table Tableused 1 1,in, didifferentapplicationsfferent comme commercial wherercial it devicesis devices necessary available available to have ha have SCRve different di withfferent center latching latching gate currentscurrentsand I IlargeLL (in(in Table Tablevalue1 theof1 thedi holding/dt holding. currents currentsIH are IH alsoare also reported), reported), therefore, therefore, it is very it is important very important to evaluate to evaluatethe minimum the minimum pulse time pulse duration time toduration be given to to be an given SCR forto an its ignition.SCR for its This ignition. duration This is fundamental duration is fundamental to using SCRs to asusing switching SCRs as devices switching in di devicesfferent industrialin different applications, industrial applications, because having because knowledge having knowledgeonly on latching only currentson latching (IL) currents is not su ffi(ILcient) is not to assuresufficient proper to assure operation proper of SCR operation in all conditions. of SCR in all conditions. Table 1. Examples of SCR characteristics. Table 1. Examples of SCR characteristics. Device Manufacturer IF (A) VR (V) IH (A) IL (A) DeviceVSK135 Manufacturer Vishay IF135 (A) V 1600R (V) IH 0.2 (A) IL 0.4 (A) VSK135VSK142 Vishay Vishay 135 140 1600 1600 0.2 0.40.4 VSK142VSK162 Vishay Vishay 140 160 1600 1600 0.2 0.40.4 SKKT132VSK162 /16Vishay Semikron 160 220 1600 1600 0.150.2 0.30.4 SKKT162/16 Semikron 250 1600 0.15 0.3 SKKT132/16SK45KQ16 Semikron Semikron 220 46 1600 1600 0.08 0.15(a) 0.15 0.3(a) SKKT162/16SK70KQ16 Semikron Semikron 250 71 1600 1600 0.1 0.15(b) 0.2 0.3(b) SK45KQ16TN25 Semikron ST 46 25 1600 1000 0.050.08(b) (a) 0.090.15(b) (a) SK70KQ16TN12 Semikron ST 71 12 1600 1000 0.005 0.1 (b)(b) 0.0060.2 (b)(b) (b) TN25PO10 ST ST 25 0.8 1000 600 0.05 0.2 (b) 0.0060.09 (b) TN12(a) Max value can beST sometimes very high12 such as 1A1000 for SKKT162 /0.00516. (b) Max(b) value.0.006 (b) PO10 ST 0.8 600 0.2 0.006 (b) Indeed,(a) the Max minimum value can value be sometimes of IL depends very high on such the adopted as 1A for delaySKKT162/16. angle control (b) Max value.α, load impedance and on the RMS value of the applied voltage. Considering these points, in most of industrial solutions,

the drivingIndeed, pulses the minimum length are value oversized of IL depends in time. on Hence, the adopted the triggering delay angle pulse control widthis α, much load impedance longer than andthe actualon the width RMS neededvalue of to guaranteethe applied the voltage. device turningConsidering on with these a driving points, signal in most and toof achieveindustrial the solutions,desired control the driving function. pulses The length aim ofare the oversized paper is in to time. investigate Hence, andthe triggering propose an pulse optimum width solution,is much longerwhich than can improvethe actual overall width systemneeded etoffi guaranteeciency and th functionalitye device turning and on can with be adopteda driving in signal wide and range to achieveof applications. the desired control function. The aim of the paper is to investigate and propose an optimum solution,Here, which it is alsocan improve worth understanding overall system a efficien very importantcy and functionality aspect in certain and can industrial be adopted applications. in wide rangeThe conduction of applications. angle (sometimes also denoted as ignition angle) is very important as it depends on the valueHere, ofit is the also consequent worth understanding current’s peak a very value. important In some aspect applications, in certain for industrial example inapplications. the case of Thepermanent conduction magnet angle systems, (sometimes the state also ofdenoted magnetization as ignition of theangle) system is very depends important precisely as it depends on the peak on the value of the consequent current’s peak value. In some applications, for example in the case of permanent magnet systems, the state of magnetization of the system depends precisely on the peak value of the magnetizing current. To increase this peak value, with the same system, it is possible to act on the ignition angle, making it small. Therefore, the possibility of having a system that as a whole (hardware and software) is able to turn on the SCR at low conduction angles is certainly an advantage. The advantages are multiple: in terms of ampere-turns, it is therefore possible to either obtain more from the same system or to decrease ampere-turns, obtaining an undeniable advantage

Energies 2020, 13, 3539 6 of 20 value of the magnetizing current. To increase this peak value, with the same system, it is possible to act on the ignition angle, making it small. Therefore, the possibility of having a system that as a whole (hardware and software) is able to turn on the SCR at low conduction angles is certainly an advantage. EnergiesThe advantages 2020, 13, x FOR are PEER multiple: REVIEW in terms of ampere-turns, it is therefore possible to either obtain6 more of 20 from the same system or to decrease ampere-turns, obtaining an undeniable advantage in economic in economic terms by saving on material and dimensions. All of these are obviously of completely terms by saving on material and dimensions. All of these are obviously of completely general validity general validity even if, in the aforementioned application, it appears to be of fundamental even if, in the aforementioned application, it appears to be of fundamental importance to influence the importance to influence the competitiveness or otherwise of the adopted solution. competitiveness or otherwise of the adopted solution. The obtained results could be also influenced by the type of application, the type of driver circuit, The obtained results could be also influenced by the type of application, the type of driver circuit, the adopted control method and, obviously, by the characteristics of the SCR used (and with this the the adopted control method and, obviously, by the characteristics of the SCR used (and with this the presence and also the importance of the above discussion). presence and also the importance of the above discussion). Finally, the importance of the considerations and evaluations (numerical and experimental) that Finally, the importance of the considerations and evaluations (numerical and experimental) that are discussed in the following sections (Sections 3 and 4) are very evident. are discussed in the following sections (Sections3 and4) are very evident.

3. Problem Problem Formulation: Formulation: General General and and Practical Practical Discussions Discussions

3.1. General General Discusion Discusion EnergiesThe 2020 equivalentequivalent, 13, x FOR PEER circuit,circuit, REVIEW in in Figure Figure5, shows5, shows the typicalthe typical representation representation of the of SCR the application, SCR application, where4 of 20 wheretwo SCRs two in SCRs back in to back back to configuration back configuration are connected are connected to an ohmic-inductive to an ohmic-inductive (R-L) load. (R-L) load.

Vgat e

LOAD R SCR L v(t)

Vgat e

Figure 5. Equivalent circuit of SCR application for R-L load: two SCRs in back to back configuration configuration Figureare connected 3. Qualitative to an ohmic-inductive variation of Breakdown (R-L) load. Voltage (VB0) vs. Gate Current (IG): the correct way to turn on a SCR device is to apply a fast current pulse with suitable amplitude on the gate terminal whenThe lagging a positive load voltage angle, is applied φϕ, is the between phase anode displacement and cathode between to reduce load the voltage Breakover and Voltage current, of the and its expressiondevice. is given by: X ϕ = arctg (1) =R (1) Therefore, the gate current is usually made high enough to ensure that the SCR is switched to thewhere ONX stateis the at reactance the proper and timeR is and the it resistance is usually of necessary the load. for The just second an instant Kirchho becauseff law permitsa constant to write gate where X is the reactance and R is the resistance of the load. The second Kirchhoff law permits to write currentthe expression is not required illustrate to below: trigger the SCR and it would only cause more power losses that needs to the expression illustrate below: be dissipated within the device. Therefore, once the device has been switched-on by the gate terminal, di((t)) dL(t()) it remains latched in the ON statev((t) )and=∙= itR does(i(t))++ notL(( t)need)∙ a continuous++i((t)) ∙ supply of gate current in order(2)(2) to maintain the conduction. This is true only· if the ·anodedt current· dt has exceeded the latching current (denotedwhere L is by the IL inductive in Figure 2).part of the load. Note Note that, that, in in many many applications, applications, the the last last term term of of Equation(2) Equation(2) can be As discarded long as the becausebecause anode ititremains representsrepresents positively thethe dependency dependency biased, it ofcannot of the the inductance inductancebe switched to to theoff the until time time variation,the variation, anode whichcurrent which it fallsitis eitheris beloweither very verythe low holding low in magnitude in currentmagnitude or (denote very or slowvery as I (diHslow) ffanderent (different so time the domain)voltage time domain)across comparing the comparing SCR to electrical become to electrical current’s zero or negative.current’sdynamic. dynamic. Ignoring Ignoring the last the term last and term solving and solv theing previous the previous equation, equation, the current the current expression expression can can beAfter obtained: the current has reached natural zero point, a finite amount of time delay is required before be obtained: α V V t ω initiating another switch on procedure and during this period SCR must− retain in off state. This i(t) = √2 sen(ω t ϕ) √2 sen(α ϕ) e− τ ⁄ (3) Z Z minimum time delay is( called) = √2 turn∙ ∙ off· ·time(∙− (t·Q−). ) −−√2 ∙ · ∙· (−− )· ∙ (3) where:To have right SCR gate triggering, following conditions need to be provided: where: ani(t) appropriate is the sum of gate a regimen to cathode term voltage (first term) (VGATE and) is a needed transient to one turn (second on the term),device which while depends the SCR onis  forwarditime(t) is constantthe biased; sum τof equal a regimen to L/R term; (first term) and a transient one (second term), which depends gateon time signal constant shall be τ equalremoved to L after/R; the device under test is turned on in order to reduce the losses  andV is theavoid rms higher voltage junction value temperature;applied by the voltage source, v(t);  Z is the load impedance; while the device is in reverse biased conditions, no VGATE signal should be applied;  α is the control phase angle evaluated starting from the voltage zero crossing and it can be when the device is in-off state, negative VGATE should be applied to improve the performance of themanaged under-test by the device. gate driver circuit often triggered through a supervisory microcontroller. Taking into account the above considerations and noticing that the pulses must occur periodically (considering the network frequency), it is evident that the gate is triggered by means of a pulsed waveform applied across the gate terminals and will be able to work correctly for all the power switch applications and with any load conditions. Therefore, it is very important to know the availability of SCR topologies in the market. Based on the manufacturing technology used, three different topologies of SCR can be realized—they are listed below and are described in more detail in the literature, e.g., in [25]:

 Planar type: Typically used for low current devices because the junctions are obtained by diffusion and come to the same surface on the cathode side. The ratio silicon/ampere is typically high;  Mesa type: It is used for high current devices but low di/dt ratio. Power SCRs are typically manufactured by this process;  Press pack type: Widely used in applications where it is necessary to have SCR with center gate and large value of di/dt.

Energies 2020, 13, x FOR PEER REVIEW 4 of 20

Figure 3. Qualitative variation of Breakdown Voltage (VB0) vs. Gate Current (IG): the correct way to turn on a SCR device is to apply a fast current pulse with suitable amplitude on the gate terminal when a positive voltage is applied between anode and cathode to reduce the Breakover Voltage of the device.

Therefore, the gate current is usually made high enough to ensure that the SCR is switched to the ON state at the proper time and it is usually necessary for just an instant because a constant gate current is not required to trigger the SCR and it would only cause more power losses that needs to be dissipated within the device. Therefore, once the device has been switched-on by the gate terminal, it remains latched in the ON state and it does not need a continuous supply of gate current in order to maintain the conduction. This is true only if the anode current has exceeded the latching current (denoted by IL in Figure 2). As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (denote as IH) and so the voltage across the SCR become zero or negative. After the current has reached natural zero point, a finite amount of time delay is required before initiating another switch on procedure and during this period SCR must retain in off state. This minimumEnergies 2020 ,time13, 3539 delay is called turn off time (tQ). 7 of 20 To have right SCR gate triggering, following conditions need to be provided:

anV isappropriate the rms voltage gate to value cathode applied voltage by the (VGATE voltage) is needed source, tov( tturn); on the device while the SCR is forwardZ is the loadbiased; impedance; gateα is signal the control shall be phase removed angle after evaluated the device starting under from test is the turned voltage on in zero order crossing to reduce and the it canlosses be Energiesandmanaged avoid2020, 13 by,higher x FOR the PEER gatejunction REVIEW driver temperature; circuit often triggered through a supervisory microcontroller.7 of 20 while the device is in reverse biased conditions, no VGATE signal should be applied; Figure6 decomposes the current expression in (3). Here, it is possible to observe both the initial whenFigure the 6device decomposes is in-off the state, current negative expression VGATE in should (3). Here be, appliedit is possible to improve to observe the both performance the initial of transienttransientthe under-test component component device. and and the theregime regime (steady-state) (steady-state) components components of the of current. the current. During During the first the instants, first theinstants, two components the two components have close values,have close but values considering, but considering the negative the polarity negative of polarity the transient of the componenttransient Taking into account the above considerations and noticing that the pulses must occur in (3),component the results in (3 of), thethe Equationresults of the (3) areEquation about (3) null are at about starting null point, at starting which point, can bewhich in the can range be in ofthe the periodicallydriverrange pulse of the (considering duration. driver pulse In thisthe duration. network situation, In frequency), this the SCRsituation, ignition it is the evident canSCR be ignition that very the di canffi gatecult, be is very andtriggered difficult, it is possible by and means it not is of to a pulsed waveform applied across the gate terminals and will be able to work correctly for all the reachpossible required not toIL reachand fail required to gain IL theand control fail to gain of SCR. the control of SCR. power switch applications and with any load conditions. Therefore, it is very important to know the availability of SCR topologies in the market. Based on the manufacturing technology used, three different topologies of SCR can be realized—they are listed below and are described in more detail in the literature, e.g., in [25]:

 Planar type: Typically used for low current devices because the junctions are obtained by diffusion and come to the same surface on the cathode side. The ratio silicon/ampere is typically high;  Mesa type: It is used for high current devices but low di/dt ratio. Power SCRs are typically manufactured by this process;  Press pack type: Widely used in applications where it is necessary to have SCR with center gate and large value of di/dt.

FigureFigure 6. 6.Simulated Simulated shape of the the current. current.

ConsideringConsidering this this problem,problem, it it should should be be possible possible to evaluate to evaluate the necessary the necessary time (tα+ time) for the (tα+ current) for the to reach the latching current IL by inversing (3). Given the complexity and nonlinearity of the Equation current to reach the latching current IL by inversing (3). Given the complexity and nonlinearity of the(3), Equation it is necessary (3), it to is simplify necessary it to to obtain simplify the desired it to obtain result. the In desiredparticular result. simplified In particular expressions simplified of the 푡−훼⁄ − 휔 t α 푒 휏 푠푒푛(휔−∙ ω푡 − 휑) expressionselements of the elements and e τ and senas ( reportedω t ϕ) as in reported the following in the expressions following expressions (obtained from (obtained the − · − fromMacLaurin the MacLaurin series expansion). series expansion). 푡−훼⁄ 2 3 푛 − 휔 푥 푥 푥 푒 휏 = 푒−푥 = 1 − 푥 + + … + (−1)푛 ∙ + ⋯ t α 2! 3! 푛! − ω x2 x3 n xn e τ = e x = 1 x +3 +5 ... + ( 1) 2∙푛++1 ... (4) − − 푥 2!푥 3! 푛 푥 n! 푠푒푛(휔 ∙ 푡 − 휑) = 푠푒푛(푥) = 푥−− + 3 … +5 (−1) −∙ n· 2 n+1 + ⋯ (4) ( ) = ( ) = 3! x 5!+ x + ( (2) ∙ 푛 x+· 1)! + sen ω t ϕ sen x x 3! 5! ... 1 (2 n+1)! ... · − − − 푡−훼·⁄ − 휔· It can be seen that when the conduction starts, the element 푒 휏 can be simplified by the first t α two elements of the MacLaurin series, while the term 푠푒푛(휔 ∙ 푡 −− ω휑) can be simplified, by the first It can be seen that when the conduction starts, the element e− τ can be simplified by the first two elementselement of of the the MacLaurin MacLaurin series, series, whileonly when the term the senvalue(ω oft itϕ is) closecan be to simplified,zero. by the first element of · − the MacLaurinConsidering series, only only the when first theelements value of of the it is MacL closeaurin to zero. series and using a time shift of α (t’ = t- 훼⁄ ), the following simple expression of the current reported in (5) is obtained. Considering휔 only the first elements of the MacLaurin series and using a time shift of α (t = t- α ), 푉 푉 푡′ 0 ω the following simple푖(푡 expression′) = √2 ∙ ∙ of(휔 the∙ 푡′ current+ 훼 − 휑 reported) − √2 ∙ in∙ 푠푒푛 (5)(훼 is− obtained.휑) ∙ (1 − ) (5) 푍 푍 휏 Expression (5) provides the possibility to evaluate the time t’, which is the !time required to reach V V t0 i(t0) = √2 (ω t0 + α ϕ) √2 sen(α ϕ) 1 (5) a specified current i(t’). Simply replacing· Z · · i(t) −equal− to latching· Z · current− · IL −it isτ possible to obtain the time required by the system to reach IL current and it will result in Expression (6): Expression (5) provides the possibility푍 to evaluate the time t , which is the time required to reach 퐼퐿 ∙ + 푠푒푛(훼 − 휑) − 훼 + 휑0 √2 ∙ 푉 a specified current i(t0). Simply푡′ replacing= i(t) equal to latching current IL it is possible to obtain(6) the 푠푒푛(훼 − 휑) 휔 + time required by the system to reach IL current and it휏 will result in Expression (6): It is to be noted that expression (6) is applicable only when the conduction starts close to the Z ( IL) + sen(α ϕ) α + ϕ point where the elements 푠푒푛 휔 ∙ 푡 − 휑 · √is2 closeV to zero.− In− the case where this element value is far t0 = · (6) from zero, it is necessary to adopt a different approachsen(α ϕto) compute the desired time value. In this ω + − study, in order to obtain this time value, Expressionτ (3) is simulated in a dedicated simulation software and corresponding time required to reach the latching current (IL) is extracted. In order to represent the proposed approach, the values of delay angle α is varied between 0 and 10 ms considering a load with R = 10 Ω, L = 55 mH (typical values for the considered application), V = 230 V, f = 50 Hz (standard industrial values for Europe), and IL = 0.4 A (typical value for high current

Energies 2020, 13, 3539 8 of 20

It is to be noted that expression (6) is applicable only when the conduction starts close to the point where the elements sen(ω t ϕ) is close to zero. In the case where this element value is far from · − zero, it is necessary to adopt a different approach to compute the desired time value. In this study, inEnergies order 20 to20, obtain 13, x FOR this PEER time REVIEW value, Expression (3) is simulated in a dedicated simulation software8 andof 20 corresponding time required to reach the latching current (IL) is extracted. SCRIn devices, order to as represent described the in proposed Table 2), approach, in order the to values evaluate of delay the minimum angle α is theoretical varied between gate 0 pulse and 10dur msation considering as reported a load in Table with R2. =It 10canΩ be, L noticed= 55 mH that, (typical for an values example for thepulse considered duration application),of 200 μs, in Vsome= 230 cases V, f denoted= 50 Hz with (standard “/” the industrial strategy valuesworks and forEurope), the pulse and durationIL = 0.4 is Aenough (typical to valueignite forthehigh SCR currenthowever SCR in some devices, other as describedcases denoted in Table by 2“),X” in the order pulse to evaluateduration theof 200 minimum μs is not theoretical enough to gate ignite pulse the durationSCR. as reported in Table2. It can be noticed that, for an example pulse duration of 200 µs, in some cases denoted with “/” the strategy works and the pulse duration is enough to ignite the SCR however in someTable other 2. Value cases of denoted the minimum by “X” time the impulse pulse duration to get the of leaching 200 µs iscurrent not enough for R = to10ignite Ω and the L=55mH SCR. (휑 = 3.3 푚푠). Table 2. Value of the minimum time impulse to get the leaching current for R = 10 Ω and L = 55 mH α [ms] Minimum tpulse [μs] Injection Problem with a tpulse = 200 μs (ϕ = 3.3 ms). 0 671 X Injection Problem with a 0.2 α [ms] 500Minimum t [µs] X pulse t = 200 µs 0.4 379 pulseX 0.6 0298 671X X 0.2 500 X 0.8 0.4243 379X X 1 0.6204 298X X 2 0.8114 243/ X 1 204 X 3 284 114 / / 4 372 84 / / 5 469 72 / / 5 69 / 6 672 72 / / 7 786 86 / / 8 8120 120 / / 9 257 X 9 257 X

IfIf similar similar analysis analysis is applied is applied to di tofferent different load configurations, load configurations, it is easy it to is identify easy to critical identify situations critical assituations reported as in reported Figure7 where,in Figure the 7 loadwhere, value the highlyload value influence highly the influence minimum the valueminimum of the value necessary of the gatenecessary pulse gate and inpulse particular and in whenparticular it has when correspondence it has correspondence with the zero with of the the zero voltage. of the It voltage. is also clear It is thatalso highclear voltagethat high values voltage allow values the allow current the to current quickly to gain quickly to the gain latching to the currentlatching of cur therent SCR. of the For SCR. the sameFor the situation, same situation, a lower valuea lower of value the voltage of the leadsvoltage to oppositeleads to opposite effects. effects.

FigureFigure 7. 7.Minimum Minimum ignition ignition time time variation variation for for di differentfferent loads loads at atV V= =230 230V V andandf f == 50 Hz.

3.2. Detail and Practical Discussions In order to optimally design an SCR control system, it is necessary to size the control system to operate on different applications without a change in its hardware topology and/or software paradigm. To achieve this, it is fundamental to know the application and corresponding load characteristics.

Energies 2020, 13, 3539 9 of 20

3.2. Detail and Practical Discussions In order to optimally design an SCR control system, it is necessary to size the control system to operate on different applications without a change in its hardware topology and/or software paradigm. To achieve this, it is fundamental to know the application and corresponding load characteristics. In some cases, especially when the load current needs to be unidirectional, as happens in electromagnetic applications, the control phase angle α can be higher or lower compared to the load angle ϕ, as in the example of Figure6. Meanwhile, in other cases, such as for AC protection systems, the control phase angle α can only take values higher than that of the load angle ϕ. Moreover, in some applications, there is a strong variation of the inductance during the activation/deactivation cycles. This situation is common in applications where the load changes its magnetic state. An example of aforementioned situation occurs in electrical motor during the startup, where the current rise to a very high value and reaches the steady state in the end. Many such cases of the described situation can be also found in equipment with permanent magnets. Many different applications which uses permanent magnets are found in the literature, in particular concerning the permanent magnet motors and related applications [26–33]. An interesting load, where it is possible to find both the previously mentioned problems is the electro permanent magnet (EPM). An EPM is a device realized by permanent magnets that can be switched -on or -off by a small amount of unidirectional current applying to its internal winding coil. The main applications of the EPMs are in handling of heavy ferromagnetic loads, such as iron bars and steel plates for manipulations over extended period of time, and for fixing the position of different mechanisms. In this application, the driving current is necessary only for few seconds during the activation (EPM is magnetized starting from an initial state of demagnetization) and deactivation cycles (initially magnetized EPM needs to be demagnetized). Thanks to this functionality, EPM can be considered as a system with zero electrical energy losses during working phases as described in [34]. As described, EPM needs unidirectional current, and during the activation/deactivation cycles the inductance value widely changes due to the variable nature of the magnets and the ferromagnetic circuit (function of the locked ferromagnetic load). This phenomenon leads to a variation in current patterns, pulse after pulse, with particular regard to the transient current. In these cases, the theory and the results as described above, could be no longer adequate to define the phenomenon (due to L variation). In particular, in some cases, where it is possible to predict strong variations in the value of the inductance (L), the current may vanish, or in some cases, during the initial stage of conduction of the device, a very strange and/or unexpected shape of the current can be found. This may lead to a failure of the ignition strategy of the SCR control system. Even though a very strong variation of inductance is not so common and important in many other applications, in the loads described here, this aspect is a very important element. Normally there can be also a huge variation of the load resistance due to the very high temperature variation (around 125 ◦C) and it can vary the resistance up to 50% of its nominal value. In order to understand the impact on minimum pulse time duration due to several factors involved in the problem definition, in the following the sensitivity analysis results based on the procedure described in (3)–(6) has been performed with a dedicated simulation software. The presented aspects in this section are mainly general and applicable to wide range applications (at least theoretically). According to the analytical procedure (3)–(6), four factors have an impact on minimum pulse duration time. These are applied voltage RMS value V, load impedance Z, load power angle ϕ and control pulse angle α. In particular, the nominal voltage V = 230 V, impedance Z and power angle ϕ equal to 10 Ω and 0.64 radiant (R = 8 Ω, L = 19 mH) respectively, are considered. The analysis is performed varying one element, around its rated value, while the other elements are fixed to the nominal ones. In this analysis the value of alpha is fixed and is equal to 12.5 µs. This value is considered as reference for all the tests because the value of alpha doesn’t affect the direction of the variation trend but only the value of the minimum pulse duration time. The effect of impedance variation is evaluated considering impedance Energies 2020, 13, 3539 10 of 20 as one concrete parameter (varying amplitude and phase) and then its resistance (R) and inductance (L) are varied independently. In this way, the dependency of the minimum pulse time duration for SCR correct conduction is studied. The obtained results are reported in Figure8 and they show that: a. Supply voltage variation: applied voltage magnitude has linear effect on minimum pulse duration. For standard European voltage of 230 V RMS, this dependency is shown in Figure8a with 10% variation around nominal value in compliance with the international standard IEC ± 50160. It can be noticed that voltage variation has minor effect on minimum pulse time duration; b. Impedance variation: Figure8b shows the load impedance variation on the minimum pulse time duration with constant ϕ and V. The variation range is 50%. An increase in Z means ± a lower current and a decrease in Z means a higher current. Therefore, it can be seen that, by increasing the load, the minimum pulse time duration required will increase, and with lower load current, wider pulses are required to guarantee the conduction; c. Load power angle variation: with fixed Z and V, effect of load power angle variation has been studied. Figure8c shows the e ffect of this variation on minimum pulse duration while the load changes from more resistive to more inductive absorbing the same RMS current. The variation range also in this case is 50%. It can be seen that increasing the load power angle will cause ± increase in minimum pulse duration time. More inductive load needs pulses with longer width or in other word, with more resistive load the required pulse width for conduction become shorter; d. Resistance variation: Figure8d shows the e ffect of varying load resistance variation on minimum pulse time (t) required, with constants L and V. The variation range also in this case is 50%. ± It can be noticed that increasing or decreasing the load resistance does not significantly influence the pulse duration. Therefore, it can be seen from Figure8d that the variation in resistance doesn’t have a major influence on the required pulse duration; e. Inductance variation: Figure8e shows the e ffect of varying load inductance on minimum required pulse time (t), with R and V as constant values. The variation range is once again 50%. ± Increasing the load inductance results in lower load current and vice versa. Hence, it can be noticed that by increasing the load inductance the minimum pulse duration needed will also increase. An aspect to note is that the variation in inductance is of very important with respect to the other parameters considered.

As it can be seen from the aforementioned analysis, the variation in resistance has no practical influence on the pulse duration, while variation in both the inductance and the applied voltage must be considered as dominant factors. Starting from this point of view, in order to make the adopted control technique more efficient, pulse width to be considered in the gate signal should be at least equal to the sum of both variations of the inductance and voltage foreseen to the circuit under the test. As has been observed, the problem in the choice of the ignition instant can be critical in order to obtain the desired operation and the maximization of the duration of the control pulse could introduce additional losses into the device. When we consider the testbed consisting of the SCR circuit described in Figure5, we arrive to the practical problem of conduction delay, which is always present. When the SCR is turned off the load is not powered by the source voltage v(t). Despite fast turn-on and turn-off characteristics, due to the inherent nature, this application introduces a short conduction delay at each voltage/current zero crossing which can be considered as distortion on load supply voltage. An example of this conduction delay is shown in Figure9 for a resistive load. The conduction delay can have different side effects, it affects the performance of the power circuit, interfere electromagnetically with broadband over power lines (BPL) communication devices and also deteriorates control performance of the power electronic based system [35]. Conduction delay principally depends on two main aspects concerning power circuit and SCR driver circuit. Due to the intrinsic nature, the SCR require a natural time to properly turn on, in addition to this, when the driver Energies 2020, 13, x FOR PEER REVIEW 10 of 20 minimum pulse time duration for SCR correct conduction is studied. The obtained results are reported in Figure 8 and they show that: a. Supply voltage variation: applied voltage magnitude has linear effect on minimum pulse duration. For standard European voltage of 230 V RMS, this dependency is shown in Figure 8a with ±10% variation around nominal value in compliance with the international standard IEC 50160. It can be noticed that voltage variation has minor effect on minimum pulse time duration; b. Impedance variation: Figure 8b shows the load impedance variation on the minimum pulse time duration with constant φ and V. The variation range is ±50%. An increase in Z means a lower current and a decrease in Z means a higher current. Therefore, it can be seen that, by increasing the load, the minimum pulse time duration required will increase, and with lower load current, wider pulses are required to guarantee the conduction; c. Load power angle variation: with fixed Z and V, effect of load power angle variation has been studied. Figure 8c shows the effect of this variation on minimum pulse duration while the load changes from more resistive to more inductive absorbing the same RMS current. The variation range also in this case is ±50%. It can be seen that increasing the load power angle will cause increase in minimum pulse duration time. More inductive load needs pulses with longer width or in other word, with more resistive load the required pulse width for conduction become shorter; d. Resistance variation: Figure 8d shows the effect of varying load resistance variation on minimum pulse time (t) required, with constants L and V. The variation range also in this case is ±50%. It can be noticed that increasing or decreasing the load resistance does not significantly influence the pulse duration. Therefore, it can be seen from Figure 8d that the variation in resistance doesn’t have a major influence on the required pulse duration; e. Inductance variation: Figure 8e shows the effect of varying load inductance on minimum required pulse time (t), with R and V as constant values. The variation range is once again ±50%.

EnergiesIncreasing2020, 13, 3539 the load inductance results in lower load current and vice versa. Hence, it 11can of 20be noticed that by increasing the load inductance the minimum pulse duration needed will also increase. An aspect to note is that the variation in inductance is of very important with respect circuitto introduces the other parameters a time delay, considered. it should be considered. If the power circuit aspect is kept unchanged, the time delay introduced by driver circuit can be analyzed in detail.

Energies 2020, 13, x FOR PEER REVIEW 11 of 20 (a) (b) Figure 8. Minimum time duration in function of (a) main voltage V, (b) Z load, (c) φ load, (d) R load and (e) L load variation.

As it can be seen from the aforementioned analysis, the variation in resistance has no practical influence on the pulse duration, while variation in both the inductance and the applied voltage must be considered as dominant factors. Starting from this point of view, in order to make the adopted control technique more efficient, pulse width to be considered(c) in the gate signal should be at least equal to( dthe) sum of both variations of the inductance and voltage foreseen to the circuit under the test. As has been observed, the problem in the choice of the ignition instant can be critical in order to obtain the desired operation and the maximization of the duration of the control pulse could introduce additional losses into the device. When we consider the testbed consisting of the SCR circuit described in Figure 5, we arrive to the practical problem of conduction delay, which is always present. When the SCR is turned off the load is not powered by the source voltage v(t). Despite(e) fast turn-on and turn-off characteristics, due to the inherent nature, this application introduces a short conduction delay at each voltage/current zero Figure crossing 8. Minimum which can time be duration considered in function as distortion of (a) main on voltage load supplyV,(b) Z load, voltage. (c) ϕ Anload, example (d) R load of this conductionand (e) L dloadelay variation.is shown in Figure 9 for a resistive load.

FigureFigure 9. AnAn example example of of practical practical conduction conduction delay delay at zero at zero voltage voltage crossing crossing for ohmic for ohmic load with load pulse with pulsestrategy. strategy. 4. SCR Driver Circuits, Simulation and Experimental Results The conduction delay can have different side effects, it affects the performance of the power circuit,In orderinterfere to run electromagnetically an SCR properly, with a gate broadband signal needs over to power be applied lines for(BPL) suffi communicationcient time to allow devices the deviceand also to latch.deteriorates In a practical control application, performance this of is the achieved power throughelectron aic driver based circuit system to [ manage35]. Conduction the gate signal.delay principally The driver depends circuit can on be two realized main aspects either by concerning digital microcontroller power circuit orand by SCR an analogdriver circuit. Due to theTwo intrinsic main circuitnature, topologies the SCR require to control a natural an SCR time device to properly have been turn considered on, in addition in this study. to this, Both when of thethe considereddriver circuit driving introduces circuits a time are applied delay, it for should a constant be considered. system with If anthe SCR power switch circuit unchanged aspect is overkept theunchanged, tests in order the time to compare delay introduced their performance. by driver Moreover, circuit can to be compare analyzed the in experimentaldetail. results, both driving systems are supplied with the same voltage (equal to 15 V) and they have been controlled with the4. SCR same Driver microcontroller-based Circuits, Simulation board. and Experimental Results In order to run an SCR properly, a gate signal needs to be applied for sufficient time to allow the device to latch. In a practical application, this is achieved through a driver circuit to manage the gate signal. The driver circuit can be realized either by digital microcontroller or by an analog circuit. Two main circuit topologies to control an SCR device have been considered in this study. Both of the considered driving circuits are applied for a constant system with an SCR switch unchanged over the tests in order to compare their performance. Moreover, to compare the experimental results, both driving systems are supplied with the same voltage (equal to 15 V) and they have been controlled with the same microcontroller-based board.

Energies 2020, 13, x FOR PEER REVIEW 12 of 20 EnergiesEnergies 20202020,, 1133,, x 3539 FOR PEER REVIEW 1212 of of 20 20 4.1. Driver Circuit 1 4.1. Driver Circuit 1 4.1. DriverIn Figure Circuit 10, 1the first driver circuit is shown. In this configuration, an impulse transformer is In Figure 10, the first driver circuit is shown. In this configuration, an impulse transformer is required to control the SCR. The input (V1) has to be a high frequency pulse in order to permit the requiredIn Figureto control 10, thethe firstSCR. driver The input circuit (V1) is shown.has to be In a this high configuration, frequency pulse an impulsein order transformerto permit the is impulse transformer to work correctly with a variable signal. impulserequired transformer to control the to work SCR. Thecorrectly input with (V1) a has variable to be signal. a high frequency pulse in order to permit the impulse transformer to work correctly with a variable signal.

(a) (b) (a) (b) Figure 10. Typical circuit used for SCR drive with impulse transformer. (a) schematic circuit diagram FigureFigure 10. 10. TypicalTypical circuit circuit used used for for SCR SCR drive drive with with impulse impulse transformer. transformer. ( (aa)) schematic schematic circuit circuit diagram diagram (b) practical realization. ((bb)) practical practical realization. realization. It is important also to note that the pulse width duration cannot be too short, because with short ItIt is is important important also also to to note note that that the the pulse pulse width width duration duration cannot cannot be be too too short, short, because because with with short short pulse width, the minimum value of the latching current IL cannot be guaranteed for wide range of pulsepulse width, width, the the minimum minimum value value of of the the latching current IIL cannotcannot be be guaranteed guaranteed for for wide wide range range of of given load (see Table 1) and cannot be too long either L because the impulse transformer works givengiven load (see (see Table Table1) and 1) and cannot cannot be too be long too either long either because becau the impulsese the impulse transformer transformer works properly works properly only with variable signals. properlyonly with only variable with signals.variable signals. In general, to satisfy every load condition, it is very common to control the circuit by a series of InIn general, general, to to satisfy satisfy every every load load condition, condition, it itis isvery very common common to tocontrol control the the circuit circuit by bya series a series of pulses (pulse train). In this case, it is mandatory to have a minimum delay time between two pulsesof pulses (pulse (pulse train). train). In thisIn this case, case, it isit is mandatory mandatory to to have have a a minimum minimum delay delay time time betw betweeneen two two successive pulses in order to de-energize the impulse transformer properly. Considering the pulses successivesuccessive pulses pulses in in order order to to de de-energize-energize the the impulse impulse transformer transformer properly. properly. Considering Considering the the pulses pulses shown in Figure 11, with a pulse command of 200 μs, it is possible to observe in Figure 11a that only shownshown in in Figure Figure 1111,, with with a a pulse pulse command command of of 200 200 μµs,s, it it is is possible possible to to observe observe in in Figure Figure 1111aa that that only only the first pulse can energize the impulse transformer (it can manage the ignition of the SCR). While thethe first first pulse cancan energizeenergize thethe impulse impulse transformer transformer (it (it can can manage manage the the ignition ignition of theof the SCR). SCR). While While the the second and third pulses cannot reach their peak value (those cannot manage the ignition of the thesecond second and and third third pulses pulses cannot cannot reach reach their their peak peak value value (those (those cannot cannot manage manage the ignitionthe ignition of the of SCRthe SCR correctly) because the pulses are too close to each other (switching frequency about 3 kHz). SCRcorrectly) correctly) because because the pulses the pulses are too are close too to eachclose other to each (switching other (switching frequency aboutfrequency 3 kHz). about Considering 3 kHz). Considering Figure 10b, all the pulses with the same pulse command of 200 μs can energize the CFigureonsidering 10b, all Figure the pulses 10b, withall the the pulses same pulsewith command the same of pulse 200 µ commands can energize of 20 the0 μ impulses can energize transformer the impulse transformer and consequently can run the SCR (switching frequency about 1.5 kHz). impulseand consequently transformer can and run consequently the SCR (switching can run frequency the SCR (switching about 1.5kHz). frequency about 1.5 kHz).

(a) (b) (a) (b) Figure 11. Impulses with a frequency of (a) around 3 kHz–problems on the second and third pulses FigureFigure 11. 11. ImpulsesImpulses with with a a frequency frequency of of ( (aa)) around around 3 3 kHz kHz–problems–problems on on the the second second and and third third pulses pulses (b) around 1.5 kHz–both pulses represented has similar behavior. ((bb)) around around 1.5 1.5 kHz kHz–both–both pulses represented has similar behavior. InIn thisthis work,work, pulsespulses ofof 200200µ μss widthwidth with with a a frequency frequency of of 1.5 1.5 kHz kHz is is used used to to supply supply Driver Driver Circuit Circuit 1 In this work, pulses of 200 μs width with a frequency of 1.5 kHz is used to supply Driver Circuit and1 and control control the the SCR SCR module. module. 1 and control the SCR module. AA digitaldigital microcontrollermicrocontroller isis requiredrequired toto generategenerate thethe pulsespulses forfor DriverDriver CircuitCircuit 1,1, inin certaincertain casescases A digital microcontroller is required to generate the pulses for Driver Circuit 1, in certain cases analoganalog circuitscircuits cancan bebe used,used, butbut inin generalgeneral thisthis topologytopology cancan generategenerate only a seriesseries of pulsespulses due toto analog circuits can be used, but in general this topology can generate only a series of pulses due to thethe didifficultyfficulty inin generatinggenerating thethe pulsespulses atat thethe desireddesired momentmoment constrainedconstrained byby thethe limitationlimitation ofof thethe the difficulty in generating the pulses at the desired moment constrained by the limitation of the impulseimpulse transformer.transformer. impulse transformer.

EnergiesEnergies 20202020,, 1133,, x 3539 FOR PEER REVIEW 1313 of of 20 20

4.2. Driver Circuit 2 4.2. Driver Circuit 2 The second driver circuit considered in this study adopts an opto-isolator to generate the pulses and manageThe second the necessary driver circuit gate considered current to incontrol this study the SCR adopts properly. an opto-isolator The circuit to schema generate is theshown pulses in Figureand manage 12. The the main necessary difference gate between current the to controlDriver Circuit the SCR 2 and properly. Driver The Circuit circuit 1, is schema in the gate is shown current in controlFigure 12philosophy.. The main As diff thereerence is betweenno pulse thetransformer, Driver Circuit it is not 2 and necessary Driver Circuitto provide 1, is a in pulse the gate or a current series ofcontrol pulses. philosophy. This also Asenables there isto nouse pulse a constant transformer, DC voltage it is not to necessary ignite the to provideSCR. The a pulsesimplicity or a series of this of drivingpulses. Thissystem also is enablesone of the to usemain a constantadvantages DC of voltage this topology. to ignite theWith SCR. this, The a simple simplicity driving of this circuit driving (as simplesystem as is onea push of thebutton) main can advantages be used ofto thisrun topology.the circuit. With Moreover, this, a simple the amount driving of circuitpower (as required simple to as energizea push button) the circuit can beis quite used tolow run and the hence circuit. normally Moreover, the thepower amount loss is of reduced. power required to energize the circuit is quite low and hence normally the power loss is reduced.

(a) (b)

FigureFigure 12. 12. TypicalTypical circuit circuit used used for forSCR SCR drive drive with withopto- opto-isolator.isolator. (a) schematic (a) schematic circuit diagram circuit diagram (b) the practical(b) the practical realization. realization.

4.3.4.3. Driver Driver C Circuitircuit L Lossesosses C Comparisonomparison i ToTo evaluate evaluate the power power losses losses of of the the driver driver circuits, circuits, the the absorbed absorbed current current icc fromfrom the the control control power power sourcesource has has to to be be measured measured in in both both circuits. circuits. A A s simpleimple test test circuit, circuit, as as shown shown in in Figure Figure 5,, isis used in order toto evaluate evaluate thethe performanceperformance of of the the two two driver driver circuits. circuits. The The grid grid voltage voltagev(t )v is(t) 230 is 230 V at V 50 at Hz. 50 TheHz. loadThe loadis 1 kVAis 1 kVA R-L load.R-L load. Here, Here the, aimthe aim is to is evaluate to evaluate the the losses losses of theof the driver driver circuits. circuits. The The power power circuit circuit is iskept kept the the same same during during both both driver driver topology topology tests tests (to(to relaxrelax anyany possiblepossible eeffect),ffect), and the driver circuits circuits are supplied with 15 V (V ) voltage source. are supplied with 15 V (푉퐷퐶DC) voltage source. TheThe absorbed absorbed current current from from voltage voltage source source ( (iicc)) has has been been measured measured in in both both circuits circuits in in order order to to evaluateevaluate the the absorbed absorbed power power to to manage manage the the driver driver circuits. circuits. In In order order t too measure measure this this current, current, a a small small resistanceresistance is is inserted inserted in in series series to to the driver circuit. This This inserted inserted resistance resistance poses poses no no change change in in the the mainmain circuit circuit behavior. behavior. The The measured measured voltage voltage across across the the resistor resistor (given (given that that the the ohmic ohmic value value is is known) known) representsrepresents the the cur currentrent in the driver circuit. ConsideringConsidering that that there there is is an an impulse impulse transformer transformer in in SCR SCR Driver Driver 1, 1, the the current current value value follows follows the the pulsespulses and hashas raisingraising and and falling falling edges. edges. In In order order to evaluateto evaluate the the power power for thisfor driverthis driver circuit, circuit, the mean the meanvalue value of supply of supply voltage voltageVDC multiplied VDC multiplied with thiswith current this current (ic) is ( consideredic) is considered (7). (7). 1 Z 푃푙표푠푠푒푠 = ∙ 1∫(푉퐷퐶 × 푖푐)푑푡 (7) P =푇 (V ic)dt (7) losses T · DC × In (7) T is the effective average period and ic is the current absorbed from the voltage source VDC. InIn case (7) T ofis SCR the eDriverffective 2, average there is periodno need and to supplyic is the the current driver absorbed circuit with from pulses, the voltage instead source constantVDC . VDC =In 15 case V has of SCRbeen Driver used. 2,Therefore, there is no the need current to supply can be the a driverconstant circuit DC value. with pulses, However, instead in order constant to avoidVDC = the15 Veffect has of been measurement used. Therefore, noises the in the current evaluation can be of a constantabsorbed DC power, value. same However, analysis in as order in the to previousavoid the case effect has of been measurement followed. noises in the evaluation of absorbed power, same analysis as in the previousTable case 3 reports has been the followed.losses for the two driver circuits under test. As it can be noticed, SCR Driver 2 has Tableabout3 reports25% fewer the losses forwith the respect two driver to SCR circuits Driver under 1, because test. As the it SCR can be Driver noticed, circuit SCR 2 Driverneeds a2 veryhas about low amount 25% fewer of current losses with ic. Moreover, respect to considering SCR Driver 1,the because low peak the SCRcurrent Driver value circuit absorbed 2 needs by a SCR very Driverlow amount 2 with of respect current toic .the Moreover, SCR Driver considering 1, the components the low peak size current in Driver value 2 absorbed can be reduced by SCR Driverand the 2 overallwith respect driver to circuit the SCR size Driver can be 1, decreased. the components This can size also in Driver lead to 2 more can be efficient reduced and and cheaper the overall industrial driver designcircuit sizeof the can final be decreased.product. As This the opto can also isolator lead can to more work e ffiwithcient very and low cheaper voltage industrial (5 V instead design of of15 theV)

Energies 2020, 13, 3539 14 of 20

final product. As the opto isolator can work with very low voltage (5 V instead of 15 V) the losses of the SCR Driver 2 can be further decreased by adopting the same pulses strategy necessary for Driver 1.

Table 3. Driver circuits losses.

Ic max (A) Ic min (A) Ic ave (A) VDC (V) Losses (W) Driver 1 0.658 0 0.041 14.75 0.605 Driver 2 0.031 0.031 0.031 14.75 0.457

4.4. Driver Conduction Delay Comparison The conduction delay depends on control circuit, control technique, SCR topology, supply voltage RMS V, load power angle ϕ, (resistive, inductive or capacitive or any mix of these types), load impedance magnitude Z, and control pulse angle α. To compare the different control system, the same SCR switch for both driver circuits (so the contribution of the SCR on conduction delay is kept constant around 50 µs in both cases), and the same power circuit, as shown in Figure5, are used. Moreover, both driving systems are supplied by the same voltage, equal to 15 V as mentioned previously, and managed with the same microcontroller board. Here, the total conduction delays for the two different driver systems and the three different control strategies are analyzed. These different solutions can be classified according to the type of power supply: a SCR Driver 1 and b SCR Driver 2; and/or according to the implementation control strategy:

I gate pulse, II series of gate pulses, III constant gate signal.

These classifications are not completely independent because, as it has been discussed, it is advisable to couple a) with I) or II), and b) with III). Only in some cases is a combination of b) and I) adopted, while the combination of a) and III) cannot work. When the gate pulse strategy (and in particular for SCR Driver 1) is adopted, the conduction delay could be more higher than the natural time delay of the SCR, especially in an EPM application that needs a unidirectional current for its proper operation and they are controlled with a control phase angle α less than the load power angle ϕ and very close to zero. This can happen due to the variable nature of the load as described in the Table4, where a load with resistance R = 1 Ω, inductance L = 50 mH, mains voltage of V = 230 V, f = 50 Hz and an SCR with IL = 0.4 A, has been considered. Cases denoted with “/” mean that the strategy works and the pulse duration is enough to ignite the SCR however in other cases denoted by “X” the pulse duration of 200 µs is not enough to ignite the SCR. Following the theory analyzed before, the results show that the resistance variation has no practical influence on the necessary pulse duration while both the inductance variation and the applied voltage have to be considered as dominant factors. Starting from this point of view, in order to make this control technique efficient, it is mandatory to supply the gate circuit for a time at least equal to the sum of the contributions due to the variation in both the inductance and the voltage. Therefore, in some cases, the gate pulse has to be wide enough to correctly manage the SCR, and hence this strategy can be adopted only with SCR Driver 2 and cannot be adopted by SCR Driver 1, because the impulse transformer, present in the control circuit can be saturated. The series gate pulses strategy is used for SCR Driver 1 only, in order to increase the probability to turn-on the SCR. In some cases, the pulses are automatically generated by an analog circuit when a Energies 2020, 13, x FOR PEER REVIEW 15 of 20

0.4 345 346 0.3 468 35.6 375 8.7 X 0.8 219 219 0.0 343 56.6 241 10.0 X 1 184 184 0.0 266 44.6 202 9.8 X 1.2 158 158 0.0 231 46.2 174 10.1 X Energies 2020, 1.413, 3539 139 139 0.0 204 46.7 153 10.1 X 15 of 20 1.6 124 124 0.0 183 47.6 137 10.4 / microcontroller2 is not present103 to manage103 the0.0 system. 153 In this case,48.5 the conduction114 10.6 delay could/ be higher than the natural delay time of the SCR especially in applications such as EPM that needs unidirectional currentFollowing and the thecontrol theory phase analyzed angle α before,should thestay results close to show the zero that as the mentioned. resistance variation has no practicalAs in influence the previous on the method necessary described, pulse duration when SCR while Driver both 1 the is inductance driven by a variation series of and pulses, the appliedthe conduction voltage delayhave to is notbe considered constant, and as dominant it shows a factors. periodic Starting behavior. from This this is point due to of the view, fact in that order the topulse make signal, this whichcontrol is technique able to set efficient, the SCR init is conduction mandatory state, to supply cannot the always gate occur circuit at for the a same time moment. at least equalTherefore, to the the sum conduction of the contributions delay time due varies to the between variation the minimumin both the conduction inductance delayand the (few voltage.µs which dependsTherefore, on SCR) in andsome the cases, maximum the gate one pulse (as ahas function to be wide of the enough delay timeto correctly used to manage correctly the manage SCR, and the henceSCR Driver this strategy 1, usually can cannotbe adopted be less only than with 700 SCRµs). Driver This time 2 and can cannot increase be adopted for some by load SCR typologies, Driver 1, becausewhen the the starting impulse current transformer, is very close present to zero in the and control the SCR circuit cannot can bebe switched-on saturated. with the first pulse. So inThe the series worst gate case pulses scenario, strategy a time is delay used betweenfor SCR theDriver two 1 pulsesonly, in can order be necessary to increase (so the the probability delay will toreach turn around-on the 1.4SCR. ms), In assome example cases, a the practical pulses test are isautomatically reported in Figure generated 13, where by an theanalog distance circuit between when atwo microcontroller pulses has been is not fixed present higher to than manage 700 µ thes, equal system. to 1120In thisµs, case, in order the toconduction eliminate delay any saturation could be higherproblems than of the impulse natural delaytransformer. time of the SCR especially in applications such as EPM that needs unidirectional current and the control phase angle α should stay close to the zero as mentioned. As in the previousTable method 4. Conduction described, delay when due SCR to load Driver and voltage 1 is driven variation. by a series of pulses, the conduction delay is not constant, and it shows a periodic behavior. This is due to the fact that the Reference R = 1.5 Ω L = 75 mH V = 207 V min Injection pulse signal, whichValue is able to+ 50% set the SCR in conduction+50% state, cannot10% always occur at the same α ms − Problem with a moment. Therefore,Minimum the MinimumconductionVariation delay timeMinimum varies Variationbetween theMinimum minimumVariation conductiontpulse of delay 200 µs (few µs which dependstpulse µ ons SCR)tpulse andµs the maximum% tpulse oneµs (as a function% tpulse of theµs delay% time used to correctly manage0 the SCR 628 Driver 1, 629usually cannot 0.2 be less 770 than 700 22.6 μs). This 663 time can 5.6 increase for X some load 0.2 460 460 0.0 670 45.6 492 6.9 X typologies,0.4 when 345 the starting 346 current 0.3 is very close 468 to zero 35.6 and the 375 SCR cannot 8.7 be switched X -on with the first0.8 pulse. So 219 in the worst 219 case scenario, 0.0 a 343 time delay 56.6 between 241 the two 10.0pulses can be X necessary 1 184 184 0.0 266 44.6 202 9.8 X (so the 1.2delay will 158 reach around 158 1.4 ms), 0.0 as example 231 a practical 46.2 test is 174 reported 10.1 in Figure 13 X, where the distance1.4 between 139 two pulses 139 has been 0.0 fixed higher 204 than 700 46.7 μs, equal 153 to 1120 10.1 μs, in order to X eliminate any saturation1.6 problems 124 of 124 the impulse 0.0 transformer. 183 47.6 137 10.4 / 2 103 103 0.0 153 48.5 114 10.6 /

(a) (b)

Figure 13. ExampleExample conduction conduction delay delay response response with with pulses pulses control control changing changing the the pulse pulse time time ( (aa)) far far to to thethe natural natural voltage voltage zero zero crossing, crossing, ( (b)) close to the natural voltage zero crossing (both pulses happen with positive voltage and the distance between two pulses is 1.12 ms).

Thus, applying this strategy to SCR Driver 1, the conduction delay will be variable, and this can produce unexpected unexpected (and (and unwanted) unwanted) effects effects on other on parts other of parts the power of the circuit power (reducing circuit performance (reducing performanceof the connected of the power connected electronic power devices, electronic compromising devices, compromising power quality power level quality and it level can alsoand it aff canect other electronic boards and control systems). Moreover, the variable nature of load and mains voltage may increase the uncertainties in the behavior of the SCR turn-on. The above-mentioned conduction delay can contribute to load voltage as well as in current distortion and it can contribute to power quality issues at their connection point. Figure 14 shows an Energies 2020, 13, x FOR PEER REVIEW 16 of 20

also affect other electronic boards and control systems). Moreover, the variable nature of load and mains voltage may increase the uncertainties in the behavior of the SCR turn-on. EnergiesThe2020 , above13, 3539-mentioned conduction delay can contribute to load voltage as well as in current16 of 20 distortion and it can contribute to power quality issues at their connection point. Figure 14 shows an exampleexample of of the the distorted distorted load load voltage voltage used used for for total total harmonic harmonic distortion distortion (THD) (THD) analysis analysis and and the the load load currentcurrent in in the the case case of of a a 1 1 kVA kVA load load with with a a power power factor factor equal equal to to 0.6 0.6 managed managed by by a a SCR SCR in in AC AC back back to to backback configuration configuration controlled controlled by by a a delay delay of of around around 200 200µ μs.s.

FigureFigure 14. 14. ExampleExample ofof loadload voltagevoltage andand currentcurrent (obtained(obtained withwith aa 11 kVAkVA load load withwith power power factorfactor PFPF equalequal to to 0.6) 0.6) a affectedffected by by a a conduction conduction delay delay in in the the AC AC back back to to back back device. device.

ItIt isisworth worth toto mention mention that,that, inin anan ideal ideal system, system, thethe interruptionsinterruptions inin thethe voltagevoltage isis expectedexpected toto appearsappears asas sharpsharp stepssteps (shown(shown byby dasheddashed lightlight blueblue lineslines inin the the Figure Figure 14 14).). However,However, inin termsterms ofof practicalpractical application, application there, there should should be be a snubbera snubber circuit circuit to to limit limitdv /dvdt/ondt on SCR. SCR. The The simulation simulation results results in Figurein Figure 14 are 14 are obtained obtained using using datasheet datasheet values values of commercial of commercial snubber snubber circuit circuit SKRC SKRC 440. 440. ConsideringConsidering thethe complexitycomplexity inin evaluatingevaluating thethe voltagevoltage andand current current distortions distortions in in the the load, load, signal signal datadata analysisanalysis isis mademade inin dedicateddedicated softwaresoftware toto extractextract thethe THDTHD valuesvalues forfor oneone case case considering considering SCRSCR DriverDriver 1. 1. TheThe load load connected connected as as mentioned mentioned previously previously is is of of 1 1 kVA kVA and and its its power power factor factor can can be be varied varied from from 1 to1 to 0.4. 0.4. The The signal signal data data has has been been elaborated elaborated to to find find the the THD THD of of the the load load voltage. voltage. Performing Performing the the FFTFFT analysis,analysis, the the THD THD value value has has been been evaluated. evaluated. The The voltage voltage THD THD varies varies between between the the range range 1.56–4.04% 1.56–4.04% as expectedas expected (the (theminimum minimum value value can be can considered be considered the contribution the contribution of the SCR of onthe conduction SCR on conduction (ignition) delay(ignition) that isdelay equal that to is 50 equalµs). These to 50 valuesμs). These are asvalues reported are as in reported Table5. in Table 5.

TableTable 5. 5.THD THD of of the the load load voltage voltage obtained obtained adopting adopting series series gate gate pulses pulses strategy strategy in inDriver Driver 1. 1.

VoltageVoltage THDTHD PowerPower Factor Factor 11 0.80.8 0.60.6 0.4 0.4 Range Range DriverDriver 1 1 1.56–1.99%1.56–1.99% 2.04–3.67%2.04–3.67% 2.19 2.19–3.94%–3.94% 2.27 2.27–4.04%–4.04% 1.56 1.56–4.04%–4.04%

Considering this strategy and Driver 1, it can be noticed that the THD is varying and, moreover, cannotConsidering be predicted. this Even strategy though and Driverthe power 1, it quality can be noticedindices thatare small the THD and istheir varying contribution and, moreover, always cannotstays within be predicted. standard Even [36,37]. though However, the power these quality are simulation indices are results small and and considers their contribution only SCR always effects, staysin a real within system standard this distortion [36,37]. However, may be higher. these areThe simulation harmonic propagation results and considers will deteriorate only SCR the eoverallffects, insystem a real systemperformance this distortion which occurred may be higher.as a consequence The harmonic of variable propagation conduction will deteriorate delay in thethe overallcase of systemDriver performance1. which occurred as a consequence of variable conduction delay in the case of DriverUnlike 1. SCR Driver 1, SCR Driver 2 can be managed by constant command, hence, the total conductionUnlike SCRdelay Driver is constant 1, SCR and Driver proportional 2 can be to managed the contribution by constant of the command, SCR only. hence,In this topology, the total conductiona constant DC delay signal is constant can be andadopted proportional as input tosignal the contributionfor the SCR gate of the signal. SCR only.For the In thisadopted topology, SCR, athis constant conduction DC signal delay can has be been adopted recorded as input by laboratory signal for test the (for SCR resistive gate signal. load) Forand theas it adopted is reported SCR, in thisFigure conduction 15. The conduction delay has been delay recorded is constant, by laboratory invariable, test and (for equal resistive to 50 µs load) (which and is as the it istime reported required in Figureby the 15 SCR).. The To conduction evaluate THD delay results, is constant, similar invariable, to previous and case, equal simulations to 50 µs (which are performed is the time requiredfor loads bywith the different SCR). To power evaluate factors THD and results, Table similar 6 reports to previousthe obtained case, THD simulations results. are performed for loads with different power factors and Table6 reports the obtained THD results. Energies 2020, 13, 3539 17 of 20 Energies 2020, 13, x FOR PEER REVIEW 17 of 20

Figure 15. Driver 2 example of practical conduction delay response for ohmic load. For the used SCR, Figure 15. Driver 2 example of practical conduction delay response for ohmic load. For the used SCR, this conduction delay has been recorded by laboratory test assuming a resistive load. Red circles show this conduction delay has been recorded by laboratory test assuming a resistive load. Red circles show the distortion at the zero-cross of the voltage. the distortion at the zero-cross of the voltage. Table 6. THD of the load voltage obtained adopting constant gate pulse strategy in Driver 2. Table 6. THD of the load voltage obtained adopting constant gate pulse strategy in Driver 2. Voltage THD Power Factor 1 0.8 0.6Voltage THD 0.4 Range Power Factor 1 0.8 0.6 0.4 Range Driver 2 1.56% 2.04% 2.19% 2.27% 1.56–2.27% Driver 2 1.56% 2.04% 2.19% 2.27% 1.56–2.27%

ComparingComparing the the two two diff differenterent circuit circuit topologies, topologies, SCR SCR Driver Driver 1 and 1 and gate gate pulse pulse or seriesor series of gateof gate pulsespulses strategy strategy leads leads to variableto variable conduction conduction delay. delay. This This can can cause cause a problem a problem with with respect respect to controlto control of theof the power, power, cause cause issues issues inpower in power quality, quality and, and can can also also diversely diversely interfere interfere in thein the performance performance and and operationoperation of otherof other systems. systems. SCR SCR Driver Driver 2 with 2 with constant constant gate gate signal signal has has a constant a constant conduction conduction delay delay whichwhich depends depends only only on theon the adopted adopted SCR. SCR. Hence, Hence, this this SCR SCR Driver Driver 2 does 2 do notes not have have a power a power control control problemproblem and and also also helps helps in reducing in reducing power power quality quality issues. issues.

5. Conclusions5. Conclusions TheThe aim aim of theof the paper paper is to is analyzeto analyze and and investigate investigate in detailin detail the the parameters parameters that that influence influence the the conductionconduction delay delay of anof SCRan SCR used used in anin ACan AC back back to backto back controlling controlling device device in orderin order to findto find a novel a novel andand generic generic solution solution enhancing enhancing the controlthe control capability capability of an of SCR an SCR for its for use its in use awide in a wide range range of loads of loads in industrialin industrial applications. applications. TheThe paper paper shows shows that that the the conduction conduction delay delay depends depends on supplyon supply voltage voltage magnitude magnitudeV, loadV, load power power angleangleϕ, loadφ, load impedance impedance magnitude magnitudeZ, and Z, and obviously obviousl controly control pulse pulse angle angleα. Inα. particular,In particular, it has it has been been demonstrateddemonstrated that thatcontrol control phase phase angle α angleand loadα and inductance load inductanceL, and supply L, and voltage supply are very voltage important are very withimportant respect to with the respect other parameters to the other on parameters minimum on pulse minimum duration pulse to duration get correct to conductionget correct conduction of SCR. Therefore,of SCR. theseTheref aspectsore, these are aspects considered are considered in applications in applications such as an such electro as permanentan electro permanent magnet (EPM) magnet that(EPM) need that a unidirectional need a unidirectional current in current which in the which control the phase control angle phaseα anglecan be αclose can be to close zero to and zero the and inductancethe inductance undergoes undergoes considerable considerable variation. variation. Moreover,Moreover, the the paper paper introduces, introduces, evaluates, evaluates and, and compares compares two two diff differenterent commonly commonly used used driver driver circuitscircuits for for SCR SCR application application through through simulation simulation and and practical practical laboratory laboratory tests tests (SCR (SCR Driver Driver 1-realized 1-realized adoptingadopting an impulsean impulse transformer transformer and SCRand DriverSCR Driver 2 is built 2 is usingbuilt anusing opto-isolator). an opto-isolator). Three diThreefferent different and widelyand usedwidely control used strategiescontrol strategies for these for two these driver two topologies driver topologies are presented, are presented, which include: which gateinclude: pulse, gate seriespulse, of gate series pulses, of gate and pulses, constant and gate constant signal gate strategies signal strategies respectively. respectively. ForFor comparison comparison between between the the two two driver driver circuits, circuits, three three aspects aspe arects are considered: considered: driver driver system system losses,losses, conduction conduction delay delay on loadon load voltage voltage due due to the to the adopted adopted control control circuit circuit strategy strategy and and power power quality quality indices.indices. SCR SCR Driver Driver 1 has 1 has higher higher losses losses and and needs needs a much a much more more complex complex driving driving signal signal which which can can be be generatedgenerated by digitalby digital microcontroller microcontroller or by or aby complex a complex analog analog circuit. circuit. However, However, SCR SCR Driver Driver 2 has 2 has lower lower operationaloperational losses, losses, which which can can be realizedbe realized and and run run by muchby much more more simple simple driving driving system. system. From conduction delay point of view, SCR Driver 1 leads to a long and a variable conduction delay, while SCR Driver 2 has a short and constant conduction delay.

Energies 2020, 13, 3539 18 of 20

From conduction delay point of view, SCR Driver 1 leads to a long and a variable conduction delay, while SCR Driver 2 has a short and constant conduction delay. From power quality point of view, the system with SCR Driver 1 leads to variable THD on the load voltage with consideration of the variable nature of load. Instead, with SCR Driver 2, the THD value is almost constant. In both these cases, the THD value always lies within required value specified by the standards and the difference between these two configurations is very small. In conclusion, we can state that SCR Driver 2, which is based on opto-isolator, shows better performance with respect to SCR Driver 1, which is based on impulse transformer. Hence, SCR Driver 2 can be recommended, since its control and performance do not depend on its control circuit. We also recall what was previously discussed regarding the importance of minimizing the conduction angle, as minimization of the conduction angle leads to incredible advantages in all those applications in which the final effect (which is also appreciable in terms of better overall performance) is a function of the magnetizing current peak value, which is controlled by the SCR device. These considerations reported are fully general and applicable, in all applications controlled by SCR, at least in the theoretical aspects. To address the future research, considering at a higher level, the range of applications for an SCR can be divided into four broad fields, namely AC-AC systems (mostly the topics covered in this paper), AC-DC systems where the solutions offered in this paper should operate with no or minor changes, DC-AC systems, and DC-DC systems. In DC-AC and DC-DC systems, the application of SCR and the solutions proposed in this paper will need further investigation and research. Moreover, the novel driver circuit could be investigated. Those topics are left open for future studies.

Author Contributions: Conceptualization, Methodology, Investigation, Writing-Original Draft Preparation, Writing-Review & Editing, R.F., H.H., K.A. and M.L. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Isermann, R. Towards intelligent control of mechanical processes. In Control Engineering Practice; Elsevier: Delft, The Netherlands, 1993; Volume 1, pp. 233–252. ISSN 0967-0661. 2. Tan, K.K.; Tang, K.Z.; Dou, H.F.; Huang, S.N. Development of an integrated and open-architecture precision motion control system. Control. Eng. Pract. 2002, 10, 757–772. [CrossRef] 3. Godse, A.P.; Bakshi, U.A. Basic Electrical and Electronics Engineering; Technical Publications: Pune, India, 2007; ISBN 8184312733. 4. Huang, A.Q. Power Semiconductor Devices for Smart Grid and Renewable Energy Systems. Proc. IEEE 2017, 105, 2019–2047. [CrossRef] 5. Osornio-Rios, R.A.; Romero-Troncoso, R.d.J.; Herrera-Ruiz, G.; Castañeda-Miranda, R. The application of reconfigurable logic to high speed CNC milling machines controllers. Control Eng. Pract. 2008, 16, 674–684. [CrossRef] 6. Charbonnaud, P.; Carrillo, F.J.; Ladevèze, D. Monitored robust force control of a milling process. Control Eng. Pract. 2001, 9, 1047–1061. [CrossRef] 7. Naso, D.; Cupertino, F.; Turchiano, B. Precise position control of tubular linear motors with neural networks and composite learning. Control Eng. Pract. 2010, 18, 515–522. [CrossRef] 8. Faranda, R.; Lazzaroni, M. Industrial low cost temperature measurement in permanent electro-magnetic platens. Measurement 2012.[CrossRef] 9. Wang, H.; Liu, Y.; Yan, K.; Fu, Y.; Zhang, C. Analysis of static VAr compensators installed in different positions in electric railways. IET Electr. Syst. Transp. 2015, 5, 129–134. [CrossRef] 10. Singh, B.; Saha, R.; Chandra, A.; Al-Haddad, K. Static synchronous compensators (STATCOM): A review. IET Power Electron. 2009, 2, 297–324. [CrossRef] Energies 2020, 13, 3539 19 of 20

11. Liu, L.; Zhuang, J.; Wang, C.; Jiang, Z.; Wu, J.; Chen, B. A Hybrid DC Vacuum Circuit Breaker for Medium Voltage: Principle and First Measurements. IEEE Trans. Power Deliv. 2015, 30, 2096–2101. [CrossRef] 12. Raonic, D.M. SCR self-supplied gate driver for medium-voltage application with capacitor as storage element. IEEE Trans. Ind. Appl. 2000, 36, 212–216. [CrossRef] 13. Patel, Y.P. Mitigation of the Effects of Common-Mode Current on the Operation of SCR-Based Rectifiers for AC Drives. IEEE Trans. Ind. Appl. 2016, 52, 4827–4834. [CrossRef] 14. Eltamaly, A.M. Harmonic injection scheme for harmonic reduction of three-phase controlled converters. IET Power Electron. 2018, 11, 110–119. [CrossRef] 15. Chen, J.T.; Ker, M.D. ESD Protection Design with Diode-Triggered Quad-SCR for Separated Power Domains. IEEE Trans. Device Mater. Reliab. 2019, 19, 283–289. [CrossRef] 16. Liu, W.; Liou, J.J.; Yeh, H.C.; Wang, H.; Li, Y.; Yeo, K.S. Bidirectional diode-triggered silicon-controlled rectifiers for low-voltage ESD protection. IEEE Electron Device Lett. 2012, 33, 1360–1362. [CrossRef] 17. Junxiong, G.; Jizhi, L.; Rui, T.; Liu, Z.; Zhiwei, L. Novel Diode-Triggered Silicon-Controlled Rectifier (DTSCR) for High-Temperature Low-Voltage Electrostatic Discharge (ESD) Applications. In Proceedings of the 2016 IEEE International Nanoelectronics Conference (INEC), Chengdu, China, 9–11 May 2016; pp. 1–2. [CrossRef] 18. D’Antona, G.; Faranda, R.; Hafezi, H.; Accetta, G.; Della Giustina, D. Open UPQC: A possible solution for customer power quality improvement. Shunt unit analysis. In Proceedings of the Harmonics and Quality of Power (ICHQP), 2014 IEEE 16th International Conference, Bucharest, Romania, 25–28 May 2014; pp. 596–600. 19. D’Antona, G.; Faranda, R.; Hafezi, H.; Accetta, G.; della Giustina, D. Open UPQC: A possible solution for power quality. Series unit analysis. In Proceedings of the Power Electronics, Electrical Drives, Automation and Motion (SPEEDAM), 2014 International Symposium, Ischia, Italy, 18–20 June 2014; pp. 1104–1109. 20. D’Antona, G.; Faranda, R.; Hafezi, H.; Bugliesi, M. Experiment on Bidirectional Single Phase Converter Applying Model Predictive Current Controller. Energies 2016, 9, 233. [CrossRef] 21. Govindhasamy, J.J.; McLoone, S.F.; Irwin, G.W.; French, J.J.; Doyle, R.P. Neural modelling, control and optimisation of an industrial grinding process. Control Eng. Pract. 2005, 13, 1243–1258. [CrossRef] 22. Abele, E.; Altintas, Y.; Brecher, C. Machine tool spindle units. CIRP Ann. Manuf. Technol. 2010, 59, 781–802. [CrossRef] 23. Cardone, M.; Cosmai, G.; Faranda, R.; Giglio, A. Modular Magnetic Device for Clamping Ferromagnetic Workpieces. U.S. Patent 8,957,750, 21 August 2014. 24. Faranda, R.; Hafezi, H.; Karakasli, V.; Ingrao, M.; Lazzaroni, M. AC “Back to Back” Switching Protection in Inductive Application. In Proceedings of the 2018 IEEE International Conference on Environment and Electrical Engineering and 2018 IEEE Industrial and Commercial Power Systems Europe (EEEIC/I&CPS Europe), Palermo, Italy, 12–15 June 2018; pp. 1–6. 25. Bakshi, U.A.; Godse, A.P. Elements of Electronics Engineering; Technical Publications: Pune, India, 2008; p. 476. ISBN 818431292X. 26. Lee, T.-S.; Lin, C.-H.; Lin, F.-J. An adaptive H controller design for permanent magnet synchronous motor ∞ drives. Control Eng. Pract. 2005, 13, 425–439. [CrossRef] 27. Yousefi, H.; Hirvonen, M.; Handroos, H.; Soleymani, A. Application of neural network in suppressing mechanical vibration of a permanent magnet linear motor. Control Eng. Pract. 2008, 16, 787–797. [CrossRef] 28. Zhao, S.; Tan, K.K. Adaptive feedforward compensation of force ripples in linear motors. Control Eng. Pract. 2005, 13, 1081–1092. [CrossRef] 29. Wu, D.; Chen, K. Design and Analysis of Precision Active Disturbance Rejection Control for Noncircular Turning Process. IEEE Trans. Ind. Electron. 2009, 56, 2746–2753. 30. Yang, Y.; Gao, F. Adaptive control of the filling velocity of thermoplastics injection molding. Control Eng. Pract. 2000, 8, 1285–1296. [CrossRef] 31. Konrad, H. Fault detection in milling, using parameter estimation and classification methods. Control Eng. Pract. 1996, 4, 1573–1578. [CrossRef] 32. Tsai, C.-C.; Lin, S.-C.; Wang, T.-Y.; Teng, F.J. Stochastic model reference predictive temperature control with integral action for an industrial oil-cooling process. Control Eng. Pract. 2009, 17, 302–310. [CrossRef] 33. Sánchez, R.; Aisa, J.; Martinez, A.; Mercado, D. On the relationship between cooling setup and warpage in injection molding. Measurement 2012, 45, 1051–1056. [CrossRef] Energies 2020, 13, 3539 20 of 20

34. Cardone, M.; Faranda, R.; Giglio, A.; Foglia, G.; Moroni, E. Safety Device in Magnetic Locking System for Moulds; MACPLAS International (Technical Magazine for the Plastics and the Rubber Industry); Promaplast Srl: Assago, Italy, 2006; pp. 31–34. ISSN 0394-3453. 35. Faranda, R.; Dezza, F.C.; Mazzucco, I.; Redi, P.; Tironi, E. An interface converter for DG/storage system able to improve Power Quality of the load. In Proceedings of the IEEE Power Engineering Society PES, Montreal, QC, Canada, 18–22 June 2006. 36. Hafezi, H.; Faranda, R. Power quality and custom power: Seeking for a common solution in LV distribution network. In Proceedings of the 2017 IEEE International Conference on Environment and Electrical Engineering and 2017 IEEE Industrial and Commercial Power Systems Europe (EEEIC/I&CPS Europe), Milan, Italy, 6–9 June 2017. 37. IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems. In IEEE Std 519-2014 (Revision of IEEE Std 519-1992); June 2014. [CrossRef]

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