A Current-To-Voltage DC-DC Converter for Powering Backbone Devices of Scientiﬁc Cabled Seaﬂoor Observatories

energies

Article A Current-to-Voltage DC-DC Converter for Powering Backbone Devices of Scientiﬁc Cabled Seaﬂoor Observatories

Jiayu Zhu 1,2 and Feng Lyu 1,2,3,*

1 State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China; [email protected] 2 School of Ocean and Earth Science, Tongji University, Shanghai 200092, China 3 Center for Marine Science and Technology, Tongji University, Shanghai 201306, China * Correspondence: [email protected]; Tel.: +86-21-65981631

Received: 9 May 2019; Accepted: 10 June 2019; Published: 13 June 2019

Abstract: In scientific cabled seafloor observatories, branching units and optical repeaters are essential backbone devices, in which zener diodes are commonly used in their power supply circuits. However, the low efficiency of zener-diode-based power feeding modules under large currents makes for a significant threat to the long-term reliability of backbone devices. In this paper, a novel full-bridge DC-DC converter with a duty-cycle-overlap control (DCOC) strategy is proposed to achieve high-efficiency current-to-voltage conversion. The circuit design of the converter and the principle of the DCOC strategy are analyzed. A prototype of the converter is implemented to demonstrate the feasibility of the proposed power feeding approach for large-scale cabled seafloor observatories.

Keywords: cabled seaﬂoor observatories; power supply; current-to-voltage; DC-DC converters; duty-cycle-overlap control (DCOC)

1. Introduction With the development of ocean science and technology, cabled seafloor observatories (CSOs) have become a powerful tool for oceanography research, realizing long-term and real-time observation of complex ocean processes [1,2]. CSOs can provide abundant power and high data bandwidth to support a number of in-situ scientific experiments in deep sea [3]. As illustrated in Figure1, CSOs mainly consist of shore stations, primary nodes, junction boxes, instrument platforms, and single-conductor electro-optic submarine cables, as well as branching units (BUs) and optical repeaters as backbone devices [4,5]. Each BU connects backbone cables with a spur cable linking a primary node. In case a backbone or spur cable fault occurs, relevant power switches of the BU nearest to the fault must be opened to isolate the fault cable segment, so that the system can remain in normal operation. Moreover, the repeaters are used to compensate the loss of optical signals for the need of long-haul communication. Thus, these backbone devices are essential components of CSOs. Generally, constant current (CC) and constant voltage (CV) direct current (DC) power systems are two types of CSOs [6], as illustrated in Figure2. The CSOs powered by CV power sources can supply more power and have higher power transmission efficiencies than CC power-feeding CSOs, so that most scientific CSOs use CV power-feeding systems. As the backbone cables have only one conductor, the seawater is used as the current returning path.

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Figure 1. The system structure of seafloor observatories [4]. Figure 1. The system structure of seaﬂoor observatories [4]. Figure 1. The system structure of seafloor observatories [4].

Figure 2. Two types of power systems in cabled seaﬂoor observatories (CSOs). (a) Constant-current Figure 2. Two types of power systems in cabled seafloor observatories (CSOs). (a) Constant-current power-feeding system. (b) Constant-voltage power-feeding system. Figurepower-feeding 2. Two typessystem. of (powerb) Constant-voltage systems in cabled power-feeding seafloor observatories system. (CSOs). (a) Constant-current Inpower-feeding the traditional system. method, (b) Constant-voltage the power-supply power-feeding modules system. of backbone devices are based on zener diodes.In the Connected traditional in series method, with the backbone power-supply cables, twomodu back-to-backles of backbone zener devices diodes are can based form aon voltage zener regulationdiodes.In theConnected unit.traditional A zenerin series method, diode withconducts the backbone power-supply when cables, the DCmodutwo voltage back-to-backles of reachesbackbone zener its devices reverse diodes breakdownare can based form aon voltage zener anddiodes.regulation then Connected the unit. reverse A in zener bias series voltage diode with remainsbackboneconducts stable. cables,when This twothe method back-to-backDC voltage is compact zenerreaches in di circuit odesits reverse can conﬁguration form breakdown a voltage with highregulationvoltage reliability, and unit. then which A thezener is suitablereverse diode forbias conducts systems voltage poweredwhen remain the bys DC lowstable. voltage CC This power reachesmethod sources its is around reversecompact 0.65 breakdown Ain tocircuit 1.1 A. voltageconfiguration and thenwith thehigh reverse reliability, bias which voltage is suitable remain fors stable. systems This powered method by lowis compact CC power in sourcescircuit configuration with high reliability, which is suitable for systems powered by low CC power sources

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around 0.65 A to 1.1 A. However, in large-scale CSOs with CV power-feeding systems, the However,backbone incurrents large-scale rise CSOs along with with CV the power-feeding increment in systems, undersea the payloads backbone [7]. currents Thus, rise the along efficiency with the of incrementzener-diode-based in undersea (ZDB) payloads modules [7]. decline Thus, the rapidly, efficiency and of large zener-diode-based amount of power (ZDB) is dissipated modules decline in the rapidly,form of andheat, large leading amount to over-temperature of power is dissipated of electronic in the components form of heat, in leading backbone to over-temperaturedevices. Hence, it ofis necessary electronic to components design a high-efficiency in backbone devices. power-su Hence,pply module it is necessary for backbone to design devices a high-e of large-scalefficiency power-supplyCSOs using CV module power-feeding for backbone systems. devices of large-scale CSOs using CV power-feeding systems. Similarly, inin terrestrialterrestrial DCDC powerpower transmissiontransmission systems,systems, severalseveral methodsmethods havehave beenbeen proposedproposed toto taptap powerpower fromfrom thethe high-voltagehigh-voltage DCDC line,line, i.e.,i.e., thethe current-fedcurrent-fed capacitor-switchedcapacitor-switched converter [8], [8], thethe current-fedcurrent-fed inductor-switchedinductor-switched converterconverter [[9],9], andand thethe H-bridgeH-bridge DC-DCDC-DC converterconverter [[10].10]. A new current-to-currentcurrent-to-current converter isis proposedproposed forfor CCCC power-feedingpower-feeding CSOsCSOs [11[11].]. InIn addition,addition, variousvarious current-fedcurrent-fed step-upstep-up conversionconversion topologies topologies have have been been proposed proposed for for some some specific specific applications applications such such as electricas electric vehicles, vehicles, servo-drive servo-drive systems, systems, uninterruptible uninterruptible power power supplies supplies (UPS) and (UPS) photovoltaic and photovoltaic systems, wheresystems, low where DC inputlow DC voltages input voltages must be must converted be conv intoerted higher into DChigher output DC output voltages. voltages. There There are three are basicthree current-fedbasic current-fed topologies, topologies which, arewhich push-pull are push-pull [12], full-bridge [12], full-bridge [13], and half-bridge[13], and half-bridge [14]. However, [14]. theseHowever, current-fed these current-fed converters converters need constant need DC constant voltage DC inputs voltage with inputs unidirectional with unidirectional current, while current, the backbonewhile the cablebackbone voltages cable of CSOsvoltages vary ofwith CSOs payloads vary with and payloads the cables and only the have cables one conductoronly havewith one bidirectionalconductor with current. bidirectional current. InIn thisthis paper,paper, wewe proposedproposed aa novelnovel current-to-voltagecurrent-to-voltage full-bridgefull-bridge DC-DCDC-DC converterconverter basedbased onon high-frequencyhigh-frequency pulse-width-modulationpulse-width-modulation (PWM) (PWM) power power switching switching technology technology for for backbone backbone devices devices of large-scaleof large-scale CSOs, CSOs, with with high high power power conversion conversion efficiency efficiency and low and heat low dissipation. heat dissipation. An earlier An version earlier ofversion this paper of this was paper presented was presented at the International at the International Conference Conference on OCEANS on OCEANS 2018. 2018. -

2.2. Startup Operation of the Proposed Current-to-Voltage ConverterConverter The purposepurpose ofof thethe current-to-voltagecurrent-to-voltage converterconverter designdesign is toto supplysupply powerpower forfor innerinner functionalfunctional loadsloads ofof backbonebackbone devices.devices. As the converter needs to startstart upup beforebefore itsits normalnormal operation,operation, aa compactcompact startupstartup circuitcircuit isis designeddesigned asas shownshown inin FigureFigure3 3..

Figure 3. Startup operation of the current-to-voltage converter. Figure 3. Startup operation of the current-to-voltage converter. The switch SA is normally-closed which is series connected with two back-to-back zener diodes. WhenThe the switch power S feedingA is normally-closed equipment (PFE) which in the is series shore stationconnected comes with into two operation, back-to-back the DC zener line currentdiodes. ﬂowsWhen throughthe powerSA andfeeding a steady equipment voltage (PFE) drop isin generatedthe shore acrossstation the comes diodes into which operation, can be the used DC by line the DC-DCcurrent flows isolation through module SA and to supply a steady power voltage for drop the inner is generated circuits across of the the current-to-voltage diodes which can converter. be used Onceby the the DC-DC converter isolation starts module up, the switchingto supply controlpower circuitfor the will inner turn circuits oﬀ SA ofand the isolate current-to-voltage zener diodes fromconverter. DC line. Once the converter starts up, the switching control circuit will turn off SA and isolate zener diodes from DC line.

3. Circuit Topology The circuit topology of the proposed current-to-voltage converter is illustrated in Figure 4. Unlike conventional full-bridge DC-DC converters, there are eight solid-state switches in the power

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3. Circuit Topology Energies 2019, 12, x FOR PEER REVIEW 4 of 13 The circuit topology of the proposed current-to-voltage converter is illustrated in Figure4. Unlike conventional full-bridge DC-DC converters, there are eight solid-state switches in the power conversion conversion circuit, of which four switches (Sa~Sd) are connected in reversal with the remains (S1~S4), respectively.circuit, of which The four purpose switches of (thisSa~S designd) are connected is to guarantee in reversal the withnormal the remainsoperation (S1 ~ofS 4the), respectively. proposed converterThe purpose under of thisbidirectional design is current. to guarantee the normal operation of the proposed converter under bidirectional current.

Figure 4. Circuit topology of the current-to-voltage converter. Figure 4. Circuit topology of the current-to-voltage converter. It’s worth mentioning that the DC line current of CSOs is variable both in direction and value. In a CCIt’s power-feedingworth mentioning system, that the the current DC line is alwayscurrent constantof CSOs inis normalvariable operation. both in direction However, and in casevalue. of Inashunt a CC power-feeding fault to seawater, system, shore the stations current will is reversealways theconstant polarity in normal to prevent operat theion. corrosion However, of exposed in case ofconductor a shunt [15fault]. Similarly, to seawater, the DC shore line currentstations ofwill a CV reverse power-feeding the polarity system to prevent will also the alter corrosion its direction of exposedand amplitude conductor along [15]. with Similarly, the operating the DC modes line curre [16].nt Therefore,of a CV power-feeding it is necessary system for the will converter also alter to itsoperate direction under and the amplitude DC line bidirectional along with current.the operating modes [16]. Therefore, it is necessary for the converter to operate under the DC line bidirectional current. The capacitor C1 is a key component in the process of power conversion. If the DC line current The capacitor C1 is a key component in the process of power conversion. If the DC line current flows from up to down in Figure4, the switches ( Sa~Sd) will be passed by with some voltage drop flows from up to down in Figure 4, the switches (Sa~Sd) will be passed by with some voltage drop across the diodes. Then the switches S1 to S4 operate at high frequency to alter the direction of the across the diodes. Then the switches S1 to S4 operate at high frequency to alter the direction of the current flowing into C1 and the coupled transformer T1. At the beginning of a switching period, current flowing into C1 and the coupled transformer T1. At the beginning of a switching period, switches S1 and S4 are closed while S2 and S3 are open. The current flows into and charges C1. During switches S1 and S4 are closed while S2 and S3 are open. The current flows into and charges C1. During the charging process, the voltage across C1 increases, so does the primary winding of T1. When the the charging process, the voltage across C1 increases, so does the primary winding of T1. When the voltage reaches a preset value, S2 and S3 are switched on while S1 and S4 are switched off. The current voltage reaches a preset value, S2 and S3 are switched on while S1 and S4 are switched off. The current flowing in C1 reverses its direction and then charges it in an opposite direction. Alternate repetition of flowing in C1 reverses its direction and then charges it in an opposite direction. Alternate repetition this process will cause the generation of an alternating voltage across C1, and then the voltage can be of this process will cause the generation of an alternating voltage across C1, and then the voltage can transferred to the secondary side of T1, which then realizes the electrical isolation from the DC line. be transferred to the secondary side of T1, which then realizes the electrical isolation from the DC line.4. Analysis of the Operating Principle

4.4.1. Analysis Steady-State of the Analysis Operating Principle To analyze the steady state of the current-to-voltage converter, the equivalent model of the 4.1. Steady-State Analysis proposed converter is built, as shown in Figure5. It is assumed that the switching frequency of switches and theTo capacitanceanalyze the ofsteadyC1 have state been of determined,the current-to-v the turnoltage ratio converter,N2/N1 of the the transformerequivalent modelT1 is 1:1, of andthe proposedthe initial voltageconverter across is built,C1 is as zero. shown in Figure 5. It is assumed that the switching frequency of switches and the capacitance of C1 have been determined, the turn ratio N2/N1 of the transformer T1 is 1:1, and the initial voltage across C1 is zero.

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Figure 5. The equivalent power conversion circuit: (a) Under no-load condition; (b) under loading condition. Figure 5. The equivalent power conversion circuit: (a) Under no-load condition; (b) under loading 4.1.1.condition. No-Load Condition Figure5a shows the equivalent power conversion circuit under the no-load condition. When the 4.1.1. No-load Condition proposed converter operates, the current ﬂows into C1 and T1, and then charges C1. Thus, the voltage acrossFigureC1 increases 5a shows gradually. the equivalent After power the voltage conversion across circuitC1 reaches under thethe maximumno-load condition. value, C When1 starts the to proposeddischarge converter and releases operates, the stored the current energy. flows The into following C1 andtime-domain T1, and then charges equations C1. canThus, be the derived voltage in acrossthis process: C1 increases gradually. After the voltage across C1 reaches the maximum value, C1 starts to discharge and releases the stored energy. The followingdI Ttime-domain equations can be derived in UC = UT = L (1) this process: dt dU d2I C dI T IC = C=== LCT (2) UULCTdt dt (1) dt 2 d IT IM = IC +dUIT = LC dI2 + IT (3) IC==C LCdt T (2) C dt dt where C is the capacitance of C1, L is the inductance of the primary winding of T1, UC is the voltage across C , U is the voltage across the primary winding,dII2 is the DC current, I is the charging current 1 T =+=M T + C I M IILCCT I T (3) of C1, and IT is the current ﬂowing into the primary winding.dt Solving the simultaneous equations, we can get the following results: where C is the capacitance of C1, L is the inductance 1of the primary winding of T1, UC is the voltage across C1, UT is the voltage across the IprimaryC = 2 cos winding,( t)I MIM is the DC current, IC is the charging(4) √LC current of C1, and IT is the current flowing into the primary winding. Solving the simultaneous 1 equations, we can get the following results:IT = ( 1 2 cos( t))IM (5) − √LC = 1 ICMr2cos(tI ) (4) L LC 1 UC = 2 sin( t)IM (6) C √LC =− 1 ITM(1 2 cos(tI )) (5) where t is smaller than π/ √LC. If t exceeds π/ √LC,LCIT will be equal to IM, while IC and UC will be zero. In that case, the power conversion circuit reaches a steady state until IM alters its direction. = L 1 UtICM2sin() (6) C 4.1.2. Loading Condition LC π π whereFigure t is smaller5b illustrates than the/ equivalentLC . If t exceeds power conversion/ LC , IT will circuit be equal of the to converter IM, while under IC and the UC loading will be zero.condition. In that In case, this the process, power the conversion following circuit equations reaches can bea steady built: state until IM alters its direction.

4.1.2. Loading Condition dI U = U = U = L T (7) C T R dt Figure 5b illustrates the equivalent power conversion circuit of the converter under the loading 2 condition. In this process, the following equations cand beIT built:L dIT IM = IC + IR + IT = LC + + IT (8) dt dI R dt === T UUULCT R (7) where R is the load resistance, UR is the load voltage,dt and IR is the load current. To solve the simultaneous equations, the results are as follows: dI2 dI =++=TT +L + I M IIILCCRT I T (8) 2 s1t dt2 Rs2t dt IC = LC k1s1e + k2s2e (9) where R is the load resistance, UR is the load voltage, and IR is the load current. To solve the simultaneous equations, the results are as follows:

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s1t s2t IT = k1e + k2e + IM (10) s1t s2t UC = L k1s1e + k2s2e (11)

The values of s1, s2, k1 and k2 can be expressed as:

L + √L2 4LCR2 s = − − (12) 1 2LCR

L √L2 4LCR2 s = − − − (13) 2 2LCR ! L IM k1 = 1 (14) √L2 4LCR2 − 2 − ! L IM k2 = 1 (15) − √L2 4LCR2 − 2 − The above time-domain models suggest that the voltage UC is always generated across C1 under both no-load and loading conditions. When the input current ﬂows into C1 and T1, it charges C1 and the capacitor voltage increases gradually until reaching the limited value. After the stored energy on C1 is released, the power conversion circuit reaches a steady state. Due to the switching operation, the current which ﬂows into C1 alters its direction, and it charges C1 in an opposite direction and generates a voltage with the reverse polarity. During two charging processes, an AC voltage is generated across C1. And the input power Pin of the converter over one switching period can be expressed as follows: Pin = UrmsIin (16) where Urms is the root-mean-square (RMS) value of the AC voltage and Iin is the DC current.

4.2. Operating Principles As a constant-voltage-output converter, the closed-loop control is the premise of its normal operation. For conventional full-bridge converters, the control strategies have the dead time to avoid the short-circuit fault owing to the synchronous conduction of the leading and lagging switch arms. However, the current-to-voltage converter is a series connected with the DC line, and thus the synchronous turn-on of the leading and lagging arms rarely affects the normal operation of DC systems. Then the synchronous turn-off of the arms must be avoided. Otherwise, the voltage stress of the switches would instantly increase to the high voltage on the DC line, which leads to the switch breakdown and the converter failure. In our design, a novel duty-cycle-overlap control (DCOC) strategy is proposed based on adjusting the overlap region of duty cycles with constant switching frequency. During the dead time of conventional full-bridge DC-DC converters, four switches are open. On the contrary, during the overlap region of duty cycles, all switches are closed and there is almost no voltage drop caused by the converter in the DC line which means no power is tapped in this process. When the current-to-voltage converter operates in a steady state, the operating modes can be divided into three stages. Figure6 shows the modes of the converter over one switching period, in which the red lines are the flowing paths of the input and output currents. Energies 2019, 12, 2261 7 of 13 Energies 2019, 12, x FOR PEER REVIEW 7 of 13

(a) (b)

(c)

FigureFigure 6. 6.Three Three operating operating modes modes of of the the current-to-voltage current-to-voltage converter. converter. ( a()a) Mode Mode 1 1 (charging). (charging). ( b(b)) Mode Mode 22 (no-charging). (no-charging). ( c()c) Mode Mode 3 3 (reverse (reverse charging). charging).

ModeMode 1 1(charging): (charging): The The switches switchesS S1 1and andS S44are areclosed, closed, while while the the switches switchesS S2 2and andS S33 areare stillstill open.open. ThenThen the the input input current current flows flows into into the the capacitor capacitorC 1Cand1 and the the transformer transformerT 1T,1 and, and charges chargesC C1.1. During During the the chargingcharging process, process, the the voltage voltage across acrossC 1Cincreases1 increases gradually, gradually, and and causes causes the the generation generation of theof the voltage voltage at theat the secondary secondary side side of T of1. T1. ModeMode 2 2(no-charging): (no-charging): WhenWhen thethe output output voltage voltage reaches reaches the the preset preset value, value, the the converter converter stops stops to to taptap power power from from the the DC DC line. line. The The switches switchesS S1~1~SS44 areare allall closed,closed, andand thethe currentcurrentflows flows through through the the switchesswitches without without charging chargingC C1.1. InIn thisthis process, process, the the converter converter enters enters into into a a no-charging no-charging mode mode and and the the loadload is is powered powered by by the the filter filter capacitor capacitorC Co.o. ModeMode 3 3(reverse (reverse charging): charging): In In this this mode, mode, the the switches switchesS 1S1and andS S4 4are are open, open, while while the the switches switchesS 2S2 andandS S3 3are are still still closed. closed. Then Then the the current current flows flows into intoC C1 1through throughS S2,2, and and also also charges charges it. it. The The di differencefference withwith the the previous previous charging charging process process in inMode Mode 1 1is is that that the the charging charging direction direction of ofC C1 1is is reversed, reversed, so so is is the the polaritypolarity of of the the voltage. voltage. In Mode 2, all switches are closed and the voltage across C1 is always zero. The closed-loop controller of the converter can adjust the width of Mode 2 to regulate the output voltage on specific load, ignoring the voltage drop on rectifier diodes, which can be estimated as:

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In Mode 2, all switches are closed and the voltage across C1 is always zero. The closed-loop controller of the converter can adjust the width of Mode 2 to regulate the output voltage on speciﬁc Energiesload, ignoring 2019, 12, x theFOR voltage PEER REVIEW drop on rectiﬁer diodes, which can be estimated as: 8 of 13

UUnDUload ==−n((11 D ))Ucrms (17) load− crms (17) wherewhere Uloadload isis output output voltage voltage across across the the load, load, nn isis equivalent equivalent to the turn ratioratio NN22/N1 ofof TT11, ,DD isis the the ratio ratio ofof ModeMode 2 2 toto one one switching period, and Ucrms isis the the RMS RMS value value of of the the voltage voltage in in Mode 1 1.. Then, Then, it it is is tenabletenable that that UUrmsrms== (1(1 − D)DU)crmsUcrms, and, and thus thus UloadU cancan be expressed be expressed as as − load UnU= (18) Uloadload= nUrms rms (18) Based on (16) and (18), the efficiency η of the converter can be obtained as follows: Based on (16) and (18), the efficiency η of the converter can be obtained as follows: nP η = out nPout (19) η = IU (19) IinU loadload where Pout is the output power of the converter. Thus, it can be seen, that turn ratio n forms a direct where Pout is the output power of the converter. Thus, it can be seen, that turn ratio n forms a direct proportional relationship with η. Generally, Pout and Uload are preset according to actual demands. In proportional relationship with η. Generally, Pout and Uload are preset according to actual demands. orderIn order to improve to improve the the conversion conversion efficiency, efficiency, the the most most effective effective solution solution is is to to adjust adjust the the turn turn ratio ratio of of thethe transformer transformer along along with with input input current current increase. increase. As As shown shown in in Figure Figure 77,, there are several taps on the secondarysecondary side side of of the the transformer transformer and and these these taps taps ar aree connected connected to to the the rectifie rectifierr through through a a relay relay group, group, whichwhich can can ensure ensure that only one tap can be turned on at any time when the converter operates.

Figure 7. The design to adjust the turn ratio of the transformer of the proposed converter. Figure 7. The design to adjust the turn ratio of the transformer of the proposed converter. To illustrate the dynamic characteristics of the converter, the transfer function of the duty-cycle to the outputTo illustrate voltage the is dynamic analyzed characteristics by the small signal of the method. converter, The the transfer transfer function function of theof the output duty-cycle filter is as follows: to the output voltage is analyzed by the small signal1 method. The transfer function of the output Ho = (20) filter is as follows: 2 Lo LoCos + R s + 1 = 1 On the basis of (11) and (20), the transferHo function of the duty-cycle to the output voltage can be 2 L (20) LCs++o s 1 obtained as follows: oo R On the basis of (11) and (20), the transfer functionnIm(Ls +of2 theR) duty-cycle to the output voltage can be Gvd = (21) 2 R 2 Lo Rd obtained as follows: 2(s + s + L )(LoCos + ( R + RdCo)s + R + 1) nI(2) Ls+ R = m 5. Experiment Results Gvd 22++R +LRod + + + (21) 2(ss )( LCsoo ( RCs do ) 1) The block diagram and the photographL of theRR experiment setup are shown in Figure8a,b respectively. High frequency metal-oxide-semiconductor field-effect transistors (MOSFETs) are used as 5.power Experiment switches, Results and the control circuit is based on analog integrated circuits (ICs). Meanwhile, there are ten taps at the secondary winding of the transformer, which are connected in the load loop through The block diagram and the photograph of the experiment setup are shown in Figure 8a,b single-pole double-throw (SPDT) relays. And the control of relays is based on sample logical circuits respectively. High frequency metal-oxide-semiconductor field-effect transistors (MOSFETs) are used as power switches, and the control circuit is based on analog integrated circuits (ICs). Meanwhile, there are ten taps at the secondary winding of the transformer, which are connected in the load loop through single-pole double-throw (SPDT) relays. And the control of relays is based on sample logical circuits and a current sensor. This design is to realize the adjusting of turn ratio N2/N1 of the transformer by the control of relays.

Energies 2019, 12, 2261 9 of 13 Energies 2019, 12, x FOR PEER REVIEW 9 of 13 andEnergies a current 2019, 12 sensor., x FOR PEER This REVIEW design is to realize the adjusting of turn ratio N2/N1 of the transformer9 of by 13 the control of relays.

(a) (b)

Figure 8. (a) The block diagram of the experimental setup; (b) the photograph of the experimental (a) (b) setup. FigureFigure 8. 8.(a ()a The) The block block diagram diagram of the of experimentalthe experimental setup; setup; (b) the (b photograph) the photograph of the experimentalof the experimental setup. setup.And the specifications of the prototype are illustrated in Table 1. In the design, the input current canAnd vary the from speciﬁcations 1 A to 10 ofA the to prototypemeet the arerequirem illustratedent of in scientific Table1. In CSOs the design, with CV the inputpower-feeding current cansystems. varyAnd from Moreover,the 1specifications A to 10 the A design to of meet the of theprot the requirementotype output are volt illustrated ofage scientiﬁc and in rated Table CSOs output 1. with In the CVpower design, power-feeding adapts the input to the systems. current actual Moreover,candemands vary thefromof BUs design 1 andA ofto repeaters. the10 A output to meet voltage the andrequirem ratedent output of scientific power adapts CSOs towith the actualCV power-feeding demands of BUssystems. and repeaters. Moreover, the design of the output voltage and rated output power adapts to the actual demands of BUs and repeaters. Table 1. Parameters of the prototype. Table 1. Parameters of the prototype. Parameters Values Table 1. Parameters of the prototype. InputParameters current Values 1–10 A OutputParametersInput currentvoltage 1–10 A Values 12 V RatedInputOutput output current voltage power 12 V 1–10 10 W A Rated output power 10 W OutputCapacitor voltage C1 0.47 12 V μ F Capacitor C 0.47 µF Rated output power1 10 W SwitchingSwitching frequency frequency 40 kHz 40 kHz CapacitorTapsTaps number number C1 10 0.47 10 μ F SwitchingMinimumMinimum frequency turn turn ratio 1.2 40 1.2kHz MaximumMaximumTaps number turn turn ratio 12 10 12 Minimum turn ratio 1.2 FigureFigure9a 9a depicts depicts the the AC AC voltageMaximum voltage across across turnC ratio1Cwhen1 when the the prototype prototype operates operates 12 at at the the normal normal operation operation underunder 1 A1 A current. current. The The curve curve approximates approximates a sinusoidal a sinusoidal wave wave and and the the unsmooth unsmooth regions regions are causedare caused by theby overlap theFigure overlap of 9a duty depicts of duty cycles. the cycles. AC voltage across C1 when the prototype operates at the normal operation under 1 A current. The curve approximates a sinusoidal wave and the unsmooth regions are caused by the overlap of duty cycles.

(a) (b)

Figure 9. Cont. (a) (b)

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Figure 9. (a) The AC voltage across C1. (b) The voltage stress across the switch. (c) The output voltage across the load when the prototype starts up. (d ) The ripple of the output voltage. (c) (d) In Figure 9b, the voltage stress across the switch is illustrated. The voltage stress is irrelevant to FigureFigure 9. 9.( a(()a) The) TheThe AC ACAC voltage voltagevoltage across acrossacross C C1C.(11.. b((b))) The TheThe voltage voltagevoltage stress stressstress across acrossacross the thethe switch. switch.switch. ( c (()c) The) TheThe output outputoutput voltage voltagevoltage the acrossDCacross line the the voltage, load load when when and the the only prototype prototype influenced starts starts up. byup. (dthe ()d The)) TheACThe ripple rippleripplevoltage of ofof the thetheacross output outputoutput C1 voltage.. voltage.voltage.As shown in this figure, the maximum value of the voltage stress is approximately 40 V. When the prototype starts up, the rising trajectoryInIn Figure Figure of the9 b,9b, output the the voltage voltage voltage stress stress across across across the load the the switch switchis depictedis isillustrated. illustrated. in Figure The 9c. To voltage illustrate stress stress the is is quality irrelevant irrelevant of the to tooutputthe the DC DC power,line line voltage, voltage, Figure and 9d and showsonly only influenced the inﬂuenced ripple byof by thethe the ou ACtput AC voltage voltagevoltage. across across The Cpeak-peak11C.. As1As. As shownshown shown value inin this inthisof thisthe figure,figure, ripple ﬁgure, thethe is theaboutmaximum maximum 100 mV. value value of the of thevoltage voltage stress stress is approximatel is approximatelyy 40 V. 40 When V. When the prototype the prototype starts starts up, the up, rising the risingtrajectoryThe trajectory dynamic of the of output the voltage output voltage and voltage current across across thewaveforms load the is load dep of is ictedthicted depictede output inin FigureFigure in diode Figure 9c.9c. and ToTo9c. illustrate illustratethe To inductor illustrate thethe qualityqualityare the shown quality ofof thethe in ofFigureoutput the output 10.power, The power, waveformsFigure Figure 9d shows are9d showsmeasured the ripple the under ripple of the 2 of ouA the tputinput output voltage. current. voltage. The peak-peak The peak-peak value of value the ripple of the is ripple about is 100 about mV. 100 mV. TheThe dynamic dynamic voltage voltage and and current current waveforms waveforms of of the the output output diode diode and and the the inductor inductor are are shown shown in in FigureFigure 10 10.. The The waveforms waveforms are are measured measured under under 2 2 A A input input current. current.

(a) (b)

Figure 10. The dynamic voltage and current waveforms of the output diode (a) and the inductor (b).

The dynamic characteristics(a) of the prototype are depicted in Figure (11.b) When the input current jumpsFigureFigure from 10. 10. 2 TheA The to dynamic dynamic5 A, the voltage voltagemaximum and and current currentovershoot waveforms waveforms is approximately of of the the output output 1 diode diodeV. However, (a ()a and)) andand the thethe the inductor inductorinductor decrease (b (().b ).).of the current has little impact on the output voltage. Figure 11b shows that when the load leaps from 50% to 100%TheThe dynamicof dynamic the rated characteristics characteristics output power, of of the thethe prototype overshootprototypeare ofare the depicted depicted output in involtage Figure Figure is11 11.about. When When 0.8 the theV. input input current current jumpsjumpsjumpsFigure from fromfrom 2 11c22 A AA todepicts toto 5 55 A, A,A, the experimentalthethe maximum maximummaximum overshootwaveforms overshootovershoot is isofis approximately approximatelythe output voltage 1 1 V. V. However, withHowever, the alteration the the decrease decrease of the of of theturn the currentratio.current Generally, has has little little impact outputimpact on voltageon the the output output is almost voltage. voltage. impervious Figure Figure 11 to b11b showsthe shows variation that that when ofwhen the the turnthe load load ratio. leaps leaps It from can from 50%be seen50% to 100%thatto 100% when of the of the ratedthe turnrated output ratio output power,decreases power, the from overshootthe overshoot6 to 1.2, of the the of overshootoutput the output voltage of voltage the is output about is about voltage 0.8 V. 0.8 V.is less than 0.8 V. Figure 11c depicts experimental waveforms of the output voltage with the alteration of the turn ratio. Generally, output voltage is almost impervious to the variation of the turn ratio. It can be seen that when the turn ratio decreases from 6 to 1.2, the overshoot of the output voltage is less than 0.8 V.

(a) (b)

Figure 11. Cont.

(a) (b)

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(c) Figure 11.11(a) The output voltage under the alteration of input current. (b) The output voltage under Figure . (a) The output voltage under the alteration(c) of input current. (b) The output voltage under thethe alteration alteration of of the the load. load. (c ()c The) The output output voltage voltage under under the the adjusting adjusting of of the the turn turn ratio. ratio. Figure 11. (a) The output voltage under the alteration of input current. (b) The output voltage under FiguretheTo alteration further 11c depicts clarifyof the load. experimentalthe interrelation(c) The output waveforms betweenvoltage under of the the efthe outputficiency adjusting voltage and of thethe with turnturn the ratio. ratio, alteration we have of theadjusted turn ratio.the circuit Generally, of the output prototype voltage and is the almost turn impervious ratio. Then to the the efficiency variation of of the the prototype turn ratio. under It can bedifferent seen thatcurrents whenTo further theand turnturn clarify ratio ratio the decreases conditionsinterrelation from is 6betweendescribed to 1.2, the the in overshoot ef Figureficiency 12. of and theIt the outputcan turn be voltage seenratio, that we is lesshaveas the than adjusted current 0.8 V. theincreases, circuitTo further ofthe the clarifyefficiency prototype the significantly interrelation and the turn is between reduced.ratio. Th the enHowever, ethefficiency efficiency the and efficiency theof the turn prototype increases ratio, we under haveobviously adjusteddifferent with thecurrentsthe circuit increment and of theturn in prototype the ratio turn conditions rati ando. theThe turnmaximumis described ratio. Thenvalue in theFigureis 0.9. effi ciency12. It can of the be prototype seen that underas the di currentfferent currentsincreases, and the turn efficiency ratio conditions significantly is described is reduced. in Figure However, 12. It canthe beefficiency seen that increases as the current obviously increases, with thethe eincrementfficiency significantly in the turn rati is reduced.o. The maximum However, value the e ffiis ciency0.9. increases obviously with the increment in the turn ratio. The maximum value is 0.9.

Figure 12. Three-dimensional curves of the efficiency under different current and turn ratio Figureconditions. 12. Three-dimensional curves of the efficiency under different current and turn ratio conditions. Figure 12. Three-dimensional curves of the efficiency under different current and turn ratio Toconditions.To illustrate illustrate the the advantage advantage of of the the proposed proposed design, design, the the zener zener diodes diodes typed typed 1N3311 1N3311 are are used used to to comparecompare with with the the prototype. prototype. TheThe reversereverse biasbias voltagevoltage ofof thethe diodesdiodes isis 1212 V,V, and and the the rated rated power power dissipationdissipationTo illustrate is is 50 50 W. the W. To advantage avoidTo avoid the overheatingofthe the overheating proposed or damage desi or gn,damage of the diodes, zener of diodes, threediodes diodes threetypedconnected diodes1N3311 connected are in parallelused to in arecompareparallel used toarewith form used the a to ZDBprototype. form power-supply a ZDB The power-supplyreverse module. bias volt Then module.age a resistiveof Thenthe diodes a load resistive isis parallel12 loadV, and is connected parallel the rated connected with power the unitdissipationwith as the a load unit is and 50as W. thea load To load avoidand power the the consumptionload overheating power co isornsumption 10 damage W. Then ofis the 10diodes, comparison W. Then three the ondiodes comparison the econnectedfficiencies on ofthein theparallelefficiencies ZDB are module usedof the andto ZDB form the module prototype a ZDB and power-supply under the prototype different module. un currentsder Thendifferent is illustrated a resistive currents in load Figureis illustrated is parallel 13. It canin connected Figure be seen 13. thatwithIt can the the be e ffiunit seenciency as that a of loadthe the efficiency ZDBand the module loadof the decreasespower ZDB moduleco obviouslynsumption decreases with is 10 obviously the W. increasing Then with the of thecomparison the increasing input DC on of linethe the current.efficienciesinput DC By line contrast,of the current. ZDB the module eByffi ciencycontrast, and of the thethe prototype prototypeefficiency un onlyofder the decreasesdifferent prototype currents slightly only decreases withis illustrated the increasing slightly in Figure with of the 13.the inputItincreasing can current.be seen of Thethethat input conversionthe efficiency current. effi The ofciency the conversion ZDB deceasing module effi ofciency thedecreases prototype deceasing obviously is of less the thanwith prototype 10%the increasing even is less though than of the10%the inputinputeven currentDCthough line varies thecurrent. input in a By widecurrent contrast, range, varies the and inefficiency the a wide efficiency raofnge, the is aboveandprototype the 80% efficiency only under decreases 10 is Aabove current, slightly 80% which under with is 10 thethe A worstincreasingcurrent, case. which of the is input the worst current. case. The conversion efficiency deceasing of the prototype is less than 10% even though the input current varies in a wide range, and the efficiency is above 80% under 10 A current, which is the worst case.

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Figure 13. Comparison on the efficiency of the zener-diodes-based module and the prototype under Figure 13. Comparison on the efficiency of the zener-diodes-based module and the prototype under different input direct current (DC) line currents. different input direct current (DC) line currents. 6. Conclusions 6. Conclusion This paper has proposed a novel current-to-voltage full-bridge DC-DC converter for feeding powerThis to backbonepaper has devicesproposed in large-scalea novel current-to-voltage cabled seafloor observatories.full-bridge DC-DC The detailedconverter model for feeding of the powerconverter to backbone has been mathematicallydevices in large-scale derived cabled and analyzed. seafloor Aobservatories. new duty-cycle-overlap The detailed control model strategy of the converterhas been proposed has been to realizemathematically the constant derived voltage and output analyzed. of the converter. A new Theduty-cycle-overlap experimental results control have strategyproved the has feasibility been proposed of the design to realize and the showed constant that voltage much higher output effi ofciency the converter. can be achieved The experimental under large resultscurrents have by the proved proposed the feasibility converter thanof the the design traditional and showed zener-diode-based that much powerhigher supply efficiency modules. can be achieved under large currents by the proposed converter than the traditional zener-diode-based powerAuthor supply Contributions: modules.Conceptualization, F.L.; methodology, J.Z. and F.L.; software, J.Z.; validation, F.L.; formal analysis, J.Z.; investigation, J.Z. and F.L.; resources, F.L.; writing—original draft preparation, J.Z.; writing—review Authorand editing, Contributions: J.Z. and F.L.; Conceptualization, visualization, J.Z.; supervision,F.L.; methodology, F.L.; funding J.Z. and acquisition, F.L.; software, F.L. J.Z.; validation, F.L.; formalFunding: analysis,This work J.Z.; was invest fundedigation, by Shanghai J.Z. and science F.L.; and reso technologyurces, F.L.; innovation writing—original action plan, grant draft number preparation, 16DZ1205000. J.Z.; writing—review and editing, J.Z. and F.L.; visualization, J.Z.; supervision, F.L.; funding acquisition, F.L. Acknowledgments: The authors acknowledge the support of Shanghai Science and Technology Commission for Funding:the fund of This Shanghai work Sciencewas funded and Technology by Shanghai Innovation science and Action technology Plan (16DZ1205000). innovation action The authors plan, alsogrant appreciate number 16DZ1205000.reviewers for the constructive comments that helped improve the quality of this manuscript. Conflicts of Interest: The authors declare no conflict of interest. Acknowledgments: The authors acknowledge the support of Shanghai Science and Technology Commission for the fund of Shanghai Science and Technology Innovation Action Plan (16DZ1205000). The authors also appreciateReferences reviewers for the constructive comments that helped improve the quality of this manuscript.

Conflicts1. Massion, of Interest: G. Ocean The Observing authors declare Systems: no Visionconflicts and of Details.interest. In Proceedings of the MTS/IEEE OCEANS 2006, Boston, MA, USA, 18–21 September 2006; pp. 1–6. References2. Lyu, F.; Zhou, H.; Peng, X.; Yue, J.; Wang, P. Technical preparation and prototype development for long-term cabled seaﬂoor observatories in Chinese marginal seas. In SEAFLOOR OBSERVATORIES: A New Vision of the

1. Massion,Earth from G. the Ocean Abyss ;Observing Favali, P., Beranzoli,Systems: L.,Vision Eds.; and Springer: Details. Berlin, In Proceedings Germany, 2015;of the pp. MTS/IEEE 503–529. OCEANS 3. 2006,Lyu, F.;Boston, Zhou, MA, H.; Yue, USA, J.; He,18–21 B. PowerSeptember system 2006; structure pp. 1–6. and topology reliability of cabled seaﬂoor observatory 2. Lyu,networks. F.; Zhou,J. Tongji H.; Univ.Peng, (Nat. X.; Yue, Sci.) 2014J.; Wang,, 42, 1604–1610. P. Technical preparation and prototype development for 4. long-termLyu, F.; Peng, cabled X.; Zhou,seafloor H.; observator Yue, J.; He,ies B.in DesignChinese of marginal a prototype seas. system In SEAFLOOR for cabled OBSERVATORIES: seaﬂoor observatory A Newnetworks. Vision Chin.of the J.Earth Sci. Instrum.from the 2012Abyss, 33; Favali,, 1134–1140. P., Beranzoli, L., Eds.; Springer: Berlin, Germany, 2015; pp. 5. 503–529.Howe, B.M.; Duennebier, F.K.; Lukas, R. The ALOHA Cabled Observatory. In SEAFLOOR OBSERVATORIES: 3. Lyu,A New F.; Vision Zhou, of H.; the Yue, Earth J.; from He, the B. Abyss Power; Favali, system P., structure Beranzoli, and L., topology Eds.; Springer: reliability Berlin, of cabled Germany, seafloor 2015; observatorypp. 439–463. networks. J. Tongji Univ. (Nat. Sci.) 2014, 42, 1604–1610. 4.6. Lyu,Howe, F.; B.M.;Peng, Kirkham, X.; Zhou, H. H.; Power Yue, SystemJ.; He, B. Considerations Design of a prototype for Undersea system Observatories. for cabled seafloorIEEE J. observatory Ocean. Eng. networks.2002, 27, 267–274. Chin. J. Sci. [CrossRef Instrum.] 2012, 33, 1134–1140.

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