energies

Technical Note A Mutual Blocking Technology Applied to Dual Switching Control

Hsin-Chuan Chen 1, Ping-Huan Kuo 2 and Chiou-Jye Huang 3,*

1 School of Computer Engineering, University of Electronic Science and Technology of China, Zhongshan Institute, Zhongshan 528402, China; [email protected] 2 Computer and Intelligent Robot Program for Bachelor Degree, National Pingtung University, Pingtung 90004, Taiwan; [email protected] 3 School of Electrical Engineering and Automation, Jiangxi University of Science and Technology, Ganzhou 341000, China * Correspondence: [email protected]

 Received: 15 December 2018; Accepted: 4 February 2019; Published: 13 February 2019 

Abstract: In many industries and medical power system applications, dual power source design is often used to ensure that the equipment runs continuously, even when the main power supply is shut down. However, the feedback between two independent power supplies and low loss output are critical issues for the system energy dissipation. Without using a dedicated chip, a new mutual blocking control technology is proposed in this paper to effectively solve the problem of voltage feedback existing in the conventional dual power system. In addition, without adding much hardware cost, the proposed dual power switch design can completely avoid voltage feedback and achieve a low voltage loss of about 30 mV when the load current is less than 0.5 A.

Keywords: dual power source switch; mutual blocking control technology; low loss output; voltage feedback

1. Introduction Since it is common for equipment to work for 24 hours a day in industry applications, the equipment must therefore record the current conditions when it is shut down, and initiate the necessary processing procedures. Hence, a dual power source switch circuit is necessary to turn on the backup battery power source to ensure that the system can continue to work at the moment of the main power failure. Generally, the simplest method is to connect the main power supply and the backup power supply to the anodes of two respectively, and the cathodes of the two diodes are coupled together to the load [1,2]. This method is simple and the problem of mutual feedback between the two power supplies can be avoided. However, in addition to the output voltage loss due to the voltage drop, when the power of the dual power supplies are relatively close, both power sources will supply power to the load simultaneously. Therefore, it is not suitable for dual power switching applications. In Figure1, we show the basic schema of the dual power source switch. At the moment of the main power failure, the equipment should not be restarted due to the power source being switched to backup power. The switch shown in Figure1 cannot be the mechanical type like relay, which has a longer switching time. Since MOSFET switches have a faster switching speed, and their on-resistance is extremely low, the power PMOS is often used as a switching transistor [3].

Energies 2019, 12, 576; doi:10.3390/en12040576 www.mdpi.com/journal/energies Energies 2019, 12, x FOR PEER REVIEW 2 of 11 Energies 2019, 12, 576 2 of 10 Energies 2019, 12, x FOR PEER REVIEW 2 of 11 V1 VO Switch Equipment V V21 VO Switch Equipment V2 Control

Control Figure 1. Schema of dual power source switching.

One reference [4] pointedFigure out 1.that Schema Pavio of etdual al. powerproposed source dual switching. DC source switching technology, whichOne was reference an effective [4] pointedmethod outfor switching that Pavio between et al. proposed the power dual that DC was source supplied switching from the technology, One reference [4] pointed out that Pavio et al. proposed dual DC source switching technology, andwhich the was secondary an effective power method source. for switchingChen et al. between [5] proposed the power dual that AC was source supplied switching from thetechnology fuel cell which was an effective method for switching between the power that was supplied from the fuel cell whichand the was secondary one of powerthe key source. technologies Chen et al.to [ensure5] proposed power dual supply AC source for important switching loads. technology During which the and the secondary power source. Chen et al. [5] proposed dual AC source switching technology switchingwas one of process, the key the technologies two power to ensuresupplies power usually supply have for asynchronous important loads. characteristics, During the so switching that the which was one of the key technologies to ensure power supply for important loads. During the zero-secondprocess, the switching two power affects supplies the load usually and have interferes asynchronous with its normal characteristics, operation so and that even the burns zero-second down. switching process, the two power supplies usually have asynchronous characteristics, so that the Fromswitching the previous affects the description, load and interferes it is understood with its normal that the operation circuit design and even for burns automatic down. switching From the zero-second switching affects the load and interferes with its normal operation and even burns down. betweenprevious the description, main power it is understoodsupply and thatthe backup the circuit po designwer supply for automatic must meet switching the following between functional the main From the previous description, it is understood that the circuit design for automatic switching requirements:power supply and the backup power supply must meet the following functional requirements: between the main power supply and the backup power supply must meet the following functional 1.requirements: WhenWhen the the main main power power is is present, present, the the equipment equipment is is supported supported by by the the main main power power supply; supply; when when thethe main main power power supply supply is is shut shut down, down, the the automati automaticc switch switch is is applied applied to to allow allow the the backup backup power power 1. When the main power is present, the equipment is supported by the main power supply; when supplysupply to to support support the the equipment. equipment. the main power supply is shut down, the automatic switch is applied to allow the backup power 2.2. TheThe two two sets sets of of power power sources sources must must be be comp completelyletely independent independent and and cannot cannot mutually give supply to support the equipment. feedbackfeedback to to each each other. other. 3.2. VoltageThe two loss sets due of topower switching sources should must be be reduced comp letelyas much independent as possible, and regardless cannot ofmutually whether give the 3. Voltage loss due to switching should be reduced as much as possible, regardless of whether the powerfeedback supply to each is switched other. to the main or backup power source. 3. Voltagepower supplyloss due is to switched switching to theshould main be or reduced backup as power much source. as possible, regardless of whether the powerTheThe major major supply contributions contributions is switched of to this the paper main include: or backup (a) (a) power the the design design source. of of an an innovative innovative dual dual power power source source switchswitch circuit circuit with with aa low low loss loss output; output; (b) (b) a mutual a mutual blocking blocking control control technology technology proposed proposed to solve to solve the The major contributions of this paper include: (a) the design of an innovative dual power source voltagethe voltage feedback feedback problem problem existing existing in inthe the conven conventionaltional dual dual power power switching switching design design and and (c) (c) the the switch circuit with a low loss output; (b) a mutual blocking control technology proposed to solve the demonstrationdemonstration of the proposed dual power sour sourcece switch circuit implemented and analyzed. voltage feedback problem existing in the conventional dual power switching design and (c) the ThisThis paperpaper isis organized organized as as follows: follows: Section section2 reviews two reviews the control the methodscontrol methods of dual power of dual switching power demonstration of the proposed dual power source switch circuit implemented and analyzed. switchingtechniques; techniques; Section3 introduces section three the proposed introduces dual the power proposed switching dual using power mutual switching blocking using technology; mutual This paper is organized as follows: section two reviews the control methods of dual power blockingSection4 illustratestechnology; the section experimental four illustrates results andthe experimental analysis; and Sectionresults 5and concludes analysis; the and experimental section five switching techniques; section three introduces the proposed dual power switching using mutual concludesresults of thisthe paper.experimental results of this paper. blocking technology; section four illustrates the experimental results and analysis; and section five 2.2.concludes Control Control Methodsthe Methods experimental of of Dual Dual resultsPower Power ofSwitching Switching this paper.

2. ControlAsAs seen seen Methods in in Figure of 22, Dual, thethe switchesswitchesPower Switching inin aa dualdual powerpower switchingswitching designdesign oftenoften useuse twotwo powerpower PMOSPMOS transistors,transistors, which which their their source source nodes nodes are are connected connected to together.gether. The The purpose purpose of of this this is is to to achieve faster faster As seen in Figure 2, the switches in a dual power switching design often use two power PMOS switchingswitching and and reduce reduce conductive voltage loss. The The switching switching control control can can be be either a conventional transistors, which their source nodes are connected together. The purpose of this is to achieve faster controlcontrol method usingusing thethe main main power power detection, detection, or or a controla control method method using using a dedicated a dedicated chip. chip. These These two switching and reduce conductive voltage loss. The switching control can be either a conventional twomethods methods are describedare described in detail in detail below. below. control method using the main power detection, or a control method using a dedicated chip. These

two methods are described in detail below. Main Power (VCC)

Detector Main Power (VCC)

Q Detector 1 Control V Equipment Logic O Q1 Control V Q2 Equipment Logic O

Q2 Battery (VBAT) FigureFigure 2. 2. SwitchingSwitching control control architecture architecture using using main main power detection. Battery (VBAT)

Figure 2. Switching control architecture using main power detection.

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2.1.2.1. Control Control Method Method Using Using Main Main Power Power Detection Detection ItIt is is simplest simplest to to directly directly control control the the switching switching of of dual dual power power sources sources based based on on detecting detecting the the presencepresence or or absence absence of of main main power. power. In In this this traditional traditional design design architecture architecture shown shown in in Figure Figure 2,2, only a commoncommon voltage voltage detector detector IC IC such such as as TC54 TC54 [6] [6] or or MAX6376 MAX6376 [7] [7] is is needed needed to to detect detect if if the the main main power power supplysupply from from the the utility utility is is shut shut down. down. In In addition addition,, two two low low power power NMOS NMOS transistors transistors can can be be used used as as thethe control control logic logic switches switches [8] [8] as as seen seen in in Figure Figure 33.. According to to the the state state of of the main power detected, detected, 1 2 1 2 thethe gate gate voltages voltages of of Q Q and1 and Q can Q2 canbe controlled, be controlled, where where Q and Q 1Qand areQ connected2 are connected to main to power main supply power CC BAT O (supplyV ) and (V batteryCC) and (V battery), respectively. (VBAT), respectively. Essentially, Essentially, the output the V output to theV equipmentO to the equipment is given by: is given by: rr VV=×+×rDSDS2 21rDS DS1 V VO =O++× VCC CC+ × V BAT (1)(1) rDSrrDS1 +12rDS DS2 r rrDS DS1 12+ rDS DS2 where,where, rrDSDS1 1andand rDSrDS2 are2 are conductive conductive resistances resistances of oftwo two PMOS PMOS (Q (Q1 and1 and Q2 Q), 2respectively.), respectively.

From output voltage RD2 RD1

To Q2’s Gate QN2 From output of QN1 voltage detector To Q1’s Gate

FigureFigure 3. 3. AA control control logic logic circuit circuit consisting consisting of of two two NMOS NMOS transistors. transistors.

WhenWhen the the main power source isis present,present, the the output output of of the the voltage voltage detector detector can can directly directly turn turn Q1 Qon1 onby by the the reverse reverse of anof an NMOS NMOS (Q N1(Q);N1 and); and the the reverse reverse output output of thisof this NMOS NMOS can can turn turn Q2 Qoff2 off through through the thereverse reverse of another of another NMOS NMOS (QN2 (Q).N2 Therefore,). Therefore,rDS 1rDSapproaches1 approaches 0 and 0 andrDS r2DSis2 almostis almost infinite infinite such such that thatVO VequalsO equalsVCC V.CC Conversely,. Conversely, when when only only the the battery battery power power source source is present, is present, that that causes causes Q1 toQ1 turn to turn off andoff andQ2 toQ2 turn to turn on (i.e.,on (i.e.,rDS 1rDS≈1 ≈∞ ∞and andr DSrDS22 ≈ 0),0), and and thus thus the the power power supplied supplied to to the the equipment equipment is is switched switched toto battery battery power power (i.e., (i.e., VVOO == VVBATBAT). ). TheThe operation operation principle principle of of this this circuit circuit is is as as follows: follows: 1.1. WhenWhen the the main main power power supply supply and and the the battery battery are both present,present, oror onlyonly the the main main power power supply supply is is present,present,Q Q11 isis onon andand Q Q22 isis off,off, at at this this time time the the main main power power supply supply provides provides a a low low loss loss output output to tothe the equipment. equipment.

2.2. WhenWhen only only battery battery power power supply supply is is present, present, Q Q2 2isis on on and and Q Q1 1isis off, off, at at this this time, time, the the battery battery power power supplysupply provides provides a alow low loss loss output output to to the the equipment. equipment. 3.3. WhenWhen the the main main power power supply supply and battery and battery power power are both are present both presentbut the main but thepower main supply power is suddenlysupply isshut suddenly down, due shut to down,the characteristics due to the of characteristics MOSFET delay of transition MOSFET [9], delay the battery transition power [9], 1 supplythe battery is still power fed back supply to the is still main fed po backwer to supply the main terminal power via supply the terminaldelayed Q via. theTherefore, delayed the Q1 . voltageTherefore, detector the voltagestill regardsdetector the stillmain regards power the supply main as power active supply so that as the active controller so that still the controllerkeeps Q1 2 onstill and keeps Q off. Q1 Aton this and time, Q2 off. the At battery this time, power the batterysupplyi powerng the supplying equipment the will equipment suffer an will internal suffer D 2 1 diodean internal voltage diode drop voltage(V ) of Q drop, and (V anotherD) of Q 2power, and another discharge power path dischargethrough Q path will through be formed. Q1 will Accordingbe formed. to the description of operation state (3), the output voltage should be modified as: According to the descriptionrr of operation state (3), the output voltage should be modified as: VV=×+××+−×−DS12 DS  VFVVF()()1 O++ BAT CC BAT D (2) rrDSr 12 DS rr DS 12r DS V = DS1 × V + DS2 × [V × F + (V − V ) × (1 − F)] (2) O r + r BAT r + r CC BAT D where, F denotes theDS 1presenceDS2 factor of mainDS1 powerDS2 supply, F = 1 indicates presence, otherwise F = 0. Ofwhere, course,F denotes if the sources the presence of Q1 and factor Q2 ofare main connected power in supply, a seriesF =with 1 indicates the diodes, presence, respectively,otherwise andF the= 0 . cathodesOf course, of if two the diodes sources are of Qconnected1 and Q2 areto the connected equipment, in a seriesthen the with power the diodes, supply respectively, feedback problem and the cancathodes be avoided. of two However, diodes are in connectedthis condition, to the whethe equipment,r the main then power the power or thesupply battery feedbackpower is problempresent, thecan output be avoided. voltage However, to the equipment in this condition, will drop whether by passing the mainthrough power another or the diode. battery Figure power 4 shows is present, the power supply paths of the three different operating conditions mentioned above.

Energies 2019, 12, 576 4 of 10 the output voltage to the equipment will drop by passing through another diode. Figure4 shows the powerEnergies supply2019, 12, x paths FOR PEER of the REVIEW three different operating conditions mentioned above. 4 of 11

VCC VCC (remove) VCC (shut down)

ON OFF ON

VG1 Q1 VG1 Q1 VG1 Q1

Equip. Equip. Equip.

VG2 Q2 VG2 Q2 VG2 Q2 OFF ON OFF

VBAT VBAT VBAT (a) Only main power or both (b) Only battery (c) Main power shut down

Figure 4. Power supply paths under differentdifferent operatingoperating conditions.conditions. 2.2. Control Method Using Dedicated Chip 2.2. Control Method Using Dedicated Chip In order to solve the above-mentioned problem of the battery voltage being fed back to the main In order to solve the above-mentioned problem of the battery voltage being fed back to the main power supply caused by the sudden power failure, using the operating amplifiers (OPA) with a fast power supply caused by the sudden power failure, using the operating (OPA) with a fast response as the decision control when switching, is an often-used method [8,10]. In addition, they are response as the decision control when switching, is an often-used method [8,10]. In addition, they are integrated into the dedicated chips. Nowadays, one of dedicated control chips, such as LTC4414, integrated into the dedicated chips. Nowadays, one of dedicated control chips, such as LTC4414, has has been developed by Linear Technology Semiconductor [11], and it can be applied for dual power been developed by Linear Technology Semiconductor [11], and it can be applied for dual power switching control shown in Figure5[ 11,12], where VCC is the main power supply and VBAT is the switching control shown in Figure 5 [11,12], where VCC is the main power supply and VBAT is the battery power. battery power. The circuit operation principle of Figure5 is as follows: The circuit operation principle of Figure 5 is as follows: 1. When only VCC is present and Q1 is on, but the GATE output voltage is not high enough that Q2 1. Whenis not only completely VCC is present turned offand and Q1 partialis on, butVCC theis GATE fed back output to the volt batteryage is terminal. not high enough that Q2 CC 2. isWhen not completelyVCC and V turnedBAT are off both and present partial and V V isCC fed> VbackBAT, to and the thus battery Q1 is terminal. on and Q 2 is off, the main CC BAT CC BAT 1 2 2. Whenpower VVCC andhas V a low are lossboth output present to and the V equipment. > V , and thus Q is on and Q is off, the main power VCC has a low loss output to the equipment. 3. When only VBAT is present, and thus Q2 is on and Q1 is off, the battery power VBAT has a low 3. Whenloss output only V toBAT the is present, equipment. and thus Q2 is on and Q1 is off, the battery power VBAT has a low loss output to the equipment. 4. When VCC and VBAT are both present, but VCC suddenly shuts down, Q1 turns off and Q2 turns 4. When VCC and VBAT are both present, but VCC suddenly shuts down, Q1 turns off and Q2 turns on, on, and the battery power VBAT still maintains low loss output to the equipment. and the battery power VBAT still maintains low loss output to the equipment. 5. When Vcc is restored, then power supply returns to state two. However, if VCC-VBAT < 0.5 V, Q2 5. When Vcc is restored, then power supply returns to state two. However, if VCC-VBAT < 0.5 V, Q2 is is still on and Q1 is still off, the output VO cannot be switched back to VCC from VBAT. still on and Q1 is still off, the output VO cannot be switched back to VCC from VBAT. According to the above operation principles, if the main power supply is suddenly shut down, According to the above operation principles, if the main power supply is suddenly shut down, the LTC4414 indeed can reduce output loss, when switching to battery power and not incur feedback the LTC4414 indeed can reduce output loss, when switching to battery power and not incur feedback to the main power terminal. If the main power supply is restored and the output is switched back to to the main power terminal. If the main power supply is restored and the output is switched back to the main power again, the limiting condition is that the main power supply voltage must be more the main power again, the limiting condition is that the main power supply voltage must be more than 0.5 V higher than the battery voltage. In addition, if the battery voltage is much lower than the than 0.5 V higher than the battery voltage. In addition, if the battery voltage is much lower than the main power voltage, some of the voltage of the main power supply will feed back to the battery power main power voltage, some of the voltage of the main power supply will feed back to the battery terminal, and the equipment will not be able to obtain full supply voltage of the main power. power terminal, and the equipment will not be able to obtain full supply voltage of the main power.

Energies 2019, 12, 576 5 of 10 Energies 2019, 12, x FOR PEER REVIEW 5 of 11

Main Power Q1 （VCC）

VO To Load Q2 COUT Battery LTC4414 （VBAT） 7 6 VIN SENSE 3 8 GND GATE 47K

2 CTL STAT 1 Status Output

Figure 5. Typical application circuit of LTC4414. Source: Linear Technology Corporation.Corporation.

3. Dual Power Switching Using Mutual Blocking Technology The conventional dual power switching design has the problem of voltagevoltage feedback,feedback, which is whenwhen thethe mainmain powerpower isis shutshut downdown andand thethe batterybattery powerpower notnot onlyonly providesprovides thethe equipmentequipment with power, butbut alsoalso feedsfeeds voltagevoltage backback toto the main powerpower supply terminal. Therefore, a battery discharge path forms,forms, but this also shortensshortens the battery supplysupply hours. In this paper, we propose anan innovativeinnovative design architecturearchitecture shown shown in in Figure Figure6. We 6. stillWe applystill apply the voltage the voltage detector detector IC to determine IC to determine the presence the ofpresence a main of power a main source, power and source, separately and separately control PMOS control (Q1 )PMOS connected (Q1) toconnected the main to power the main supply power and PMOSsupply (Qand2) connected PMOS (Q to2) theconnected battery byto thethe NMOSbattery control by the logic. NMOS However, control unlike logic. traditional However, designs, unlike Qtraditional1 and Q2 aredesigns, connected Q1 and to Q the2 are respective connected power to the inputs respective with power their sources inputs insteadwith their of sources their drains instead [2]. Inof addition,their drains two [2]. PMOS In addition, (Q3 and two Q4 )PMOS with the (Q3 gate and cross Q4) with control the andgate source cross control docking and are source added docking to form aare mutual added blocking to form controla mutual circuit blocking as shown control in Figurecircuit9 asto shown avoid voltage in Figure feedback. 7 to avoid The voltage circuit operationfeedback. principleThe circuit designed operation in principle this paper design is as follows:ed in this paper is as follows:

1. Considering that the main power supply and the battery power supply are both present, Main Power (VCC) by detecting the presence of the main power supply, Q1 and Q3 will be on and Q2 and Q4 will be off. Q 2. When the main power supply is shut down (i.e.,1 its supply current ICC = 0), the control logic Detector prepares to turn on Q2 and turn off Q1, although Q1 has not changed its operating state immediately, the voltage VG4 from the drain of Q1 is quickly lowered to zero, which will cause Mutual Q4 to turn on. Control VO Blocking Equipment 3. At this time, the voltage LogicV from the drain of Q rises to the battery voltage, which causes Q ’s G3 Circuit2 3 gate voltage to rise and turn off Q3. By the diode inside Q3, the battery voltage would not be fed back to the main power supply terminal, and thus the battery power supply can provide a low loss output for the equipment. This situation is shown in Figure8. Q2 4. When the main power is restored, the control logic switches Q1 on and Q2 off, and VG3 is lowered due to the battery supply current IBAT = 0, thus causing Q3 to turn on. 5. At this time, V is raised to the main powerBattery supply (VBAT voltage) to turn off Q . As shown in Figure7, G4 4 the internal diode of Q4 prevents the main power from being fed back to the battery terminal, Figure 6. Proposed switching architecture using mutual blocking. and the main power supply provides the equipment an output with low loss again. 6. In addition, if only the main power supply is present, or if the battery voltage is much lower than that of the main power supply, the design will not allow the main power supply voltage to give feedback to the battery terminal.

Energies 2019, 12, x FOR PEER REVIEW 5 of 11

Main Power Q1 （VCC）

VO To Load Q2 COUT Battery LTC4414 （VBAT） 7 6 VIN SENSE 3 8 GND GATE 47K

2 CTL STAT 1 Status Output

Figure 5. Typical application circuit of LTC4414. Source: Linear Technology Corporation.

3. Dual Power Switching Using Mutual Blocking Technology The conventional dual power switching design has the problem of voltage feedback, which is when the main power is shut down and the battery power not only provides the equipment with power, but also feeds voltage back to the main power supply terminal. Therefore, a battery discharge path forms, but this also shortens the battery supply hours. In this paper, we propose an innovative design architecture shown in Figure 6. We still apply the voltage detector IC to determine the presence of a main power source, and separately control PMOS (Q1) connected to the main power supply and PMOS (Q2) connected to the battery by the NMOS control logic. However, unlike traditional designs, Q1 and Q2 are connected to the respective power inputs with their sources instead of their drains [2]. In addition, two PMOS (Q3 and Q4) with the gate cross control and source docking Energies 2019are, 12 added, 576 to form a mutual blocking control circuit as shown in Figure 7 to avoid voltage feedback. 6 of 10 The circuit operation principle designed in this paper is as follows:

Main Power (VCC)

Q1 Energies 2019, 12, x FOR PEER REVIEWDetector 6 of 11

Q1 connected Mutualto main power Control VO Blocking Equipment Logic Circuit

VG3 Q3 Q2 R3 Equipment Q Battery4 (V ) VG4 BAT R4 Energies 2019, 12, Figurex FOR PEERFigure 6. REVIEWProposed 6. Proposed switching switching architecturearchitecture using using mutual mutual blocking. blocking. 7 of 11

Q1 ON Q2 connected to battery ICC Q1 ON Figure 7. Mutual blocking circuit. ICC

1. Considering that the mainVG3 power supply and the battery power supply are both present, by Q3 Q3 ON detecting the presence of the main power supply, Q1 and Q3 will be on and Q2 and Q4 will be off. R3 Equipment Equipment 2. When the main power supply is shut down (i.e., =its supply current ICC = 0), the control logic VG4 Q4 OFF prepares to turn on Q2 and Qturn4 off Q1, although Q1 has not changed its operating state immediately, the voltageI4 R4 VG4 from the drain of Q1 is quicklyIBAT = lowered 0 to zero, which will cause Q4

to turn on. Q2 OFF 3. At this time, the voltage VG3 from the drain of Q2 rises to the battery voltage, which causes Q3’s IBAT = 0 gate voltage to rise and turn off Q3. By the diode inside Q3, the battery voltage would not be fed Q2 OFF back to the main power supply terminal, and thus the battery power supply can provide a low loss outputFigure for theFigure 7. equipment.Mutual 9. Mutual blocking Thisblocking situation when when theis shown main main power in power Figure supply supply 8. is restored. is restored.

Therefore, from the equivalentQ1 OFF circuit on the right side of Figures 8 and 9, the roles of Q3 and Q4

are hereby equivalent to twoI CCapproximately = 0 ideal dynamic diode switches. When they are turned on, Q1 OFF there is only a very low voltage drop, and when they are turned off, their internal diodes can block

the voltage feedback. Assume that the conductive resistancesICC r=DS 0 of Q3 and Q4 are zero in the ideal case, the output voltage to theVG3 equipment can be given by: Q3 Q3 OFF I R 3 3 rrEquipment = Equipment V=−×+−×DS21() V VSt() DS () V VSt () OVG4 ++ CC D12  BAT D (3) rrDS12 DSQ4  rr DS 12Q4 DS ON R4 where, S1(t) is the control function of Q3 to determine whetherIBAT Q 3 is on or off, depending on I3 from

BAT 2 4 the battery supply current I . Similarly, S (t) is used as the controlQ2 ON function for Q and is controlled

by I4 from the main power supplyIBAT current ICC. Equations (4) and (5) represent the control functions: S1(t) and S2(t), respectively, whereQ2 ONVth is the threshold voltage of PMOS. Here, R3 and R4 should be

~MΩ to minimize these extra incurred current as possible. Energies 2019, 12,Figure x FOR PEERFigure 8. Mutual REVIEW 8. Mutual blocking blocking when when thethe main main power power supply supply is shut is off. shut off. 6 of 11 0 if v=× I R − V < V ()= GS333 O th St1  (4) Q connected to main=× power − ≥1 2 G3 4. When the main power is restored,1 the1 controlif vGS logic333 Iswitches R V OQ Von th and Q off, and V is lowered due to the battery supply current IBAT = 0, thus causing Q3 to turn on. 5. At this time, VG4 is raised to the main0 powerif vsupply=× I voltage R − V to< turn V off Q4. As shown in Figure 9, St()= GS444 O th the internal diode of Q4 prevents2 the main power =×from being − fed ≥ back to the battery terminal, and(5) 1 if vGS444 I R V O V th the main power supply provides theV equipmentG3 an output with low loss again. Q3 6. InAlthough addition, this if only mutual the mainblocking power design supply adds is present,two more or PMOS, if the battery and the voltage voltage is lossmuch of lowerthe power than R3 Equipment supplythat to of the the equipmentmain power slightly supply, increases the design by will one not PMOS allow voltage the main drop, power the supplyvoltage voltage loss has to beengive

effectivelyfeedback reduced, to the batterycompared terminal. with the general diQode4 voltage drop. Particularly for equipment with VG4 a large load, the improvement in voltageR4 and power loss is more obvious when the power PMOS with ultra low on-resistance is chosen. Meanwhile, it can effectively solve the problem of voltage feedback in the conventional design of dual power switching.

Q2 connected to battery 4. Experimental Results and Analysis Basically, no matter whichFigure MOSFET-basedFigure 9. 7.Mutual Mutual dual blocking blocking power circuit. circuit.switching design is used, when power supply is switched, it will not cause the equipment to reset at the moment of switching. In order to 1.verify Considering the operation that of the different main typespower of supply dual po andwer thswitchinge battery design power and supply compare are theboth performances present, by amongdetecting them, theunder presence the same of the PMOS main specifications,power supply, weQ1 andhave Q implemented3 will be on and three Q2 anddifferent Q4 will types be off. of 2.dual When power the switching main power circuits supply including is shut downthe co (i.e.,nventional its supply control current type ICC using = 0), the controlmain power logic detection,prepares the to control turn ontype Q 2usin andg dedicatedturn off Q chip,1, although and the Q proposed1 has not mutual changed blocking its operating control statetype. Figureimmediately, 10 shows the voltagecircuit prototype VG4 from theof thedrain prop of Qosed1 is quicklydual power lowered source to zero, switch, which where will two cause other Q4 lowto power turn on.NMOS transistors are used as the control logic to turn on or turn off the two PMOS 3.switches At this (Q time,1 and the Q2 ),voltage and the V G3control from logicthe drain circuit of isQ 2the rises same to the as thebattery circuit voltage, shown which in Figure causes 3. HereQ3’s gate voltage to rise and turn off Q3. By the diode inside Q3, the battery voltage would not be fed back to the main power supply terminal, and thus the battery power supply can provide a low loss output for the equipment. This situation is shown in Figure 8.

Q1 OFF

ICC = 0 Q1 OFF

ICC = 0 VG3 Q3 Q3 OFF I3 R3 Equipment = Equipment VG4 Q4 Q4 ON

R4 IBAT

Q2 ON

IBAT Q2 ON

Figure 8. Mutual blocking when the main power supply is shut off.

4. When the main power is restored, the control logic switches Q1 on and Q2 off, and VG3 is lowered due to the battery supply current IBAT = 0, thus causing Q3 to turn on. 5. At this time, VG4 is raised to the main power supply voltage to turn off Q4. As shown in Figure 9, the internal diode of Q4 prevents the main power from being fed back to the battery terminal, and the main power supply provides the equipment an output with low loss again. 6. In addition, if only the main power supply is present, or if the battery voltage is much lower than that of the main power supply, the design will not allow the main power supply voltage to give feedback to the battery terminal.

Energies 2019, 12, 576 7 of 10

Therefore, from the equivalent circuit on the right side of Figures7 and8, the roles of Q 3 and Q4 are hereby equivalent to two approximately ideal dynamic diode switches. When they are turned on, there is only a very low voltage drop, and when they are turned off, their internal diodes can block the voltage feedback. Assume that the conductive resistances rDS of Q3 and Q4 are zero in the ideal case, the output voltage to the equipment can be given by:     rDS2 rDS1 VO = (VCC − VD × S1(t)) + (VBAT − VD × S2(t)) (3) rDS1 + rDS2 rDS1 + rDS2 where, S1(t) is the control function of Q3 to determine whether Q3 is on or off, depending on I3 from the battery supply current IBAT. Similarly, S2(t) is used as the control function for Q4 and is controlled by I4 from the main power supply current ICC. Equations (4) and (5) represent the control functions: S1(t) and S2(t), respectively, where Vth is the threshold voltage of PMOS. Here, R3 and R4 should be ~MΩ to minimize these extra incurred current as possible. ( 0 i f vGS3 = I3 × R3 − VO < Vth S1(t) = (4) 1 i f vGS3 = I3 × R3 − VO ≥ Vth

( 0 i f vGS4 = I4 × R4 − VO < Vth S2(t) = (5) 1 i f vGS4 = I4 × R4 − VO ≥ Vth Although this mutual blocking design adds two more PMOS, and the voltage loss of the power supply to the equipment slightly increases by one PMOS voltage drop, the voltage loss has been effectively reduced, compared with the general diode voltage drop. Particularly for equipment with a large load, the improvement in voltage and power loss is more obvious when the power PMOS with ultra low on-resistance is chosen. Meanwhile, it can effectively solve the problem of voltage feedback in the conventional design of dual power switching.

4. Experimental Results and Analysis Basically, no matter which MOSFET-based dual power switching design is used, when power supply is switched, it will not cause the equipment to reset at the moment of switching. In order to verify the operation of different types of dual power switching design and compare the performances among them, under the same PMOS specifications, we have implemented three different types of dual power switching circuits including the conventional control type using the main power detection, the control type using dedicated chip, and the proposed mutual blocking control type. Figure 10 shows the circuit prototype of the proposed dual power source switch, where two other low power NMOS transistors are used as the control logic to turn on or turn off the two PMOS switches (Q1 and Q2), and the control logic circuit is the same as the circuit shown in Figure3. Here the main power voltage (VCC = 12.2 V) comes from the utility power adapter, while the other power source is supplied by a lithium battery pack (VBAT = 11.8 V). According to the various switching conditions, the performance of the three types of switching circuits are measured and compared. The maximum supply current to the equipment depends on the power PMOS used as a switch. In our experiments, all power PMOS transistors are the same component of IRF9540, and the maximum continuous drain current of IRF9540 [13] is about 19 A.

4.1. Measured Waveforms In the practical measurement, we try to observe the waveforms shown in Figure 11 during switching, where CH1 represents the main power voltage, CH2 represents the battery voltage, and CH3 indicates the output voltage. For clearer observation, the battery voltage is adjusted to 11 V temporarily. When the main power voltage starts to drop, the output to the equipment is not switched to the battery supply immediately; that is because the main power voltage does not drop to the desired threshold Energies 2019, 12, x FOR PEER REVIEW 8 of 11 the main power voltage (VCC = 12.2 V) comes from the utility power adapter, while the other power source is supplied by a lithium battery pack (VBAT = 11.8 V). According to the various switching conditions, the performance of the three types of switching circuits are measured and compared. The maximum supply current to the equipment depends on the power PMOS used as a switch. In our experiments, all power PMOS transistors are the same component of IRF9540, and the maximum continuous drain current of IRF9540 [13] is about 19 A.

Energies 2019, 12, 576x FOR PEER REVIEW 8 8of of 11 10 the main power voltage (VCC = 12.2 V) comes from the utility power adapter, while the other power voltage of the voltage detector IC. This threshold voltage depends on the voltage that ensures the source is supplied by a lithium battery pack (VBAT = 11.8 V). According to the various switching conditions,equipment the works performance correctly, of where the three the types threshold of switching voltage circuits is set to are 9.27 measured V in the and proposed compared. design. The maximumWhen the mainsupply power current is restored, to the eq theuipment output depends to the equipment on the power can bePM quicklyOS used switched as a switch. back In to our the experiments,main power supply.all power The PMOS switching transistors time of are the th proposede same component design is veryof IRF9540, short, only and aboutthe maximum a few µs, continuousand it depends drain on current the used of PMOS.IRF9540 [13] is about 19 A.

Figure 10. Circuit implementation of the proposed dual power source switch.

4.1. Measured Waveforms In the practical measurement, we try to observe the waveforms shown in Figure 11 during switching, where CH1 represents the main power voltage, CH2 represents the battery voltage, and CH3 indicates the output voltage. For clearer observation, the battery voltage is adjusted to 11 V temporarily. When the main power voltage starts to drop, the output to the equipment is not switched to the battery supply immediately; that is because the main power voltage does not drop to the desired threshold voltage of the voltage detector IC. This threshold voltage depends on the voltage that ensures the equipment works correctly, where the threshold voltage is set to 9.27 V in the proposed design. When the main power is restored, the output to the equipment can be quickly switched back to the main power supply. The switching time of the proposed design is very short, only about a fewFigureFigure μs, and 10. CircuitCircuitit depends implementation implementation on the used of PMOS.the pr proposedoposed dual power source switch.

4.1. Measured Waveforms In the practical measurement, we try to observe the waveforms shown in Figure 11 during switching, where CH1 represents the main power voltage, CH2 represents the battery voltage, and CH3 indicates the output voltage. For clearer observation, the battery voltage is adjusted to 11 V temporarily. When the main power voltage starts to drop, the output to the equipment is not switched to the battery supply immediately; that is because the main power voltage does not drop to the desired threshold voltage of the voltage detector IC. This threshold voltage depends on the voltage that ensures the equipment works correctly, where the threshold voltage is set to 9.27 V in the proposed design. When the main power is restored, the output to the equipment can be quickly switched back to the main power supply. The switching time of the proposed design is very short, only about a few μs, and it depends on the used PMOS.

Figure 11. WaveformsWaveforms of switching operation.

4.2. Output Voltage and Power Loss Using PMOS PMOS as as a aswitching switching power power transistor, transistor, the the output output voltage voltage lossloss is extremely is extremely low in low a fully in a conductivefully conductive state. state.It can Itbe canseen be from seen Table from 1 Table that1 only that the only switching the switching circuit circuitusing usingthe main the power main powerdetection detection causes causesthe battery the battery output output voltage voltage to lose to loseabout about 0.6 0.6V when V when the the main main power power supply supply is momentarily shut down. Whether the other switching circuits switch to the main power or the battery, the output to the equipment is almost equal to the original supply voltage. However, since the main power supply voltage is not greater than the battery voltage by more than 0.5 V in our experiment, if the dedicated chip control is applied to the switching circuit, the output cannot be switched back to the main power supply from the battery when the main power supply is restored. Whatever the main power supply is greater than the battery or not, the switching circuit using a mutual blocking control does not appear in the aboveFigure situation. 11. Waveforms of switching operation. Considering the different current levels of the load, Table2 shows the power loss during the 4.2.turn-on Output caused Voltage by and the Power power Loss PMOS or the diode for different switching control methods. For the switching circuit using the main power detection control, the load current from the backup power Using PMOS as a switching power transistor, the output voltage loss is extremely low in a fully supply will pass through an internal diode of PMOS when the main power supply is shut down. conductive state. It can be seen from Table 1 that only the switching circuit using the main power detection causes the battery output voltage to lose about 0.6 V when the main power supply is

Energies 2019, 12, 576 9 of 10

Thus, there exists a power loss caused by this internal diode. Although the power loss of the mutual blocking control is twice as large as that of the dedicated chip control due to the adding of one power PMOS on the load current path, the power loss caused by the power PMOS is still reduced compared with the power loss of the diode. If the power PMOS with ultra low on-resistance is further chosen, like PMN50XP [14], the improvement in power loss will be more obvious even for large loads.

Table 1. Output voltage to equipment.

Main Power Dedicated Mutual Blocking Condition Detection Control Chip Control Control

Only main power is present 12.2 V (VCC) 12.2 V (VCC) 12.2 V (VCC) Only battery 11.8 V (VBAT) 11.8 V (VBAT) 11.8 V (VBAT) Main power and battery are present 12.2 V (VCC) 12.2 V (VCC) 12.2 V (VCC) Main power shut down 11.2 V (VBAT) 11.8 V (VBAT) 11.8 V (VBAT) Main power restored 12.2 V (VCC) 11.8 V (VBAT) 12.2 V (VCC)

Table 2. Power loss of power PMOS or diode under different load current.

Main Power Dedicated Mutual Blocking Load Current Detection Control Chip Control Control 0.1 A 0.06 W 0.002 W 0.003 W 0.5 A 0.37 W 0.043 W 0.085 W 1 A 0.72 W 0.17 W 0.34 W 1.5 A 1.22 W 0.42 W 0.83 W 2 A 1.82 W 0.71 W 1.42 W

4.3. Output Voltage Feedback A good design for dual power switching should consider that the two power supplies must be completely independent and does not mutually give feedback to each other. After the practical measurement shown in Table3, we find that the switching circuit using the main power detection control will feed battery voltage back to the main power supply terminal when the main power supply is momentarily shut down. When only the main power supply is present, or if the battery voltage is lower than that of the main power supply, the voltage feedback from the main power to the battery terminal also occurs in the switching circuit using a dedicated chip. However, if the mutual blocking control is adopted in the switching circuit, the two power supplies can be independent from each other under any condition, and they do not mutually give feedback to each other.

Table 3. Power voltage feedback situation.

Main Power Dedicated Chip Mutual Blocking Condition Detection Control Control Control

Only main power is present No Feedback to VBAT No Only battery is present No No No Main power and battery are present No No No Main power shut down Feedback to VCC No No Main power restored No No No

5. Conclusions In addition to reducing circuit complexity, the use of commercial chips (such as the LTC4414) also effectively solves the voltage feedback problem of the conventional dual power switching design; and its output voltage retains the advantage of low loss in PMOS conduction. However, if the main power supply voltage varies to be only slightly higher than that of the backup power, or even lower than the backup power voltage, when the main power supply is shut down and restored again, Energies 2019, 12, 576 10 of 10 the output voltage cannot be switched back to the main power supply from the backup power supply. Therefore, the use of commercial chips is not the best choice for practical applications that are intended to meet various power supply conditions. In this paper, a mutual blocking control technology is proposed to be applied to the dual power switching design, which can switch correctly and achieve a low loss output voltage. According to the experimental results, the voltage loss is about less than 0.3 V under a load current of 1 A, and if the power PMOS with ultra low on-resistance is further chosen, the voltage loss still keeps low even for a large load current. Moreover, the proposed dual power switching design can also completely avoid the voltage feedback problem under any condition, and it will satisfy the requirement of the equipment running continually and have a better performance.

Author Contributions: H.-C.C. planned this study, completed the circuit design, and presented the analysis of experiment results. P.-H.K. completed this circuit implementation and test. C.-J.H. handled this circuit measurement and the partial article writing. H.-C.C., P.-H.K. and C.-J.H. contributed in drafted and revised the manuscript. Funding: This work was performed under auspices of the University of Electronic Science and Technology of China, Zhongshan Institute, China, under Grants 418YKQN11. Conflicts of Interest: The authors declare no conflict of interest.

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