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848 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 27, NO. 2, FEBRUARY 2012Analysis and Design of a Bidirectional IsolatedDCDC Converter for Fuel Cells andSupercapacitors Hybrid SystemZhe Zhang, Member, IEEE, Ziwei Ouyang, Student Member, IEEE, Ole C. Thomsen, Member, IEEE,and Michael A. E. Andersen, Member, IEEEAbstractElectrical powersystemsinfutureuninterruptiblepower supplies or electricalvehiclesmay employ hybrid energysources, such as fuel cells and supercapacitors. It will be necessaryto efciently draw the energy from these two sources as well asrecharge the energy storage elements by the dc bus. In this paper,a bidirectional isolated dcdc converter controlled by phase-shiftangle and duty cycle for the fuel-cell hybrid energy system is ana-lyzed and designed. The proposed topology minimizes the numberof switches and their associated gate driver components by usingtwo high-frequency transformers that combine a half-bridge cir-cuitandafull-bridgecircuittogetherontheprimaryside.Thevoltagedoublercircuitisemployedonthesecondaryside. Thecurrent-fed input can limit the input current ripple that is favor-able for fuel cells. The parasitic capacitance of the switches is usedfor zero voltage switching (ZVS). Moreover, a phase-shift and duty-cyclemodulationmethodisutilizedtocontrol thebidirectionalpower ow exibly and it also makes the converter operate undera quasi-optimal condition over a wide input voltage range. Thispaper describes the operation principle of the proposed converter,the ZVS conditions, and the quasi-optimal design in depth. The de-sign guidelines and considerations regarding the transformers andother key components are given. Finally, a 1-kW 3050-V-input400-V-output laboratory prototype operating at 100-kHz switch-ing frequency is built and tested to verify the effectiveness of thepresented converter.IndexTermsBidirectional dcdc converter,current-fed, fuelcell (FC), phase shift, supercapacitor (SC).I. INTRODUCTIONTHEhybridsystembasedonfuelcells(FCs)andsuper-capacitors (SCs) as an environmentally renewable energysystem has been applied in many elds, such as hybrid electricvehicle, uninterruptible power supply (UPS), and so on [1][4].As an example, a block diagram of extended-run time battery-less double-conversion UPS system powered by FCs and SCs isillustrated in Fig. 1. Compared to diesel generators and batter-ies, FCs are electrochemical devices that convert the chemicalManuscript received November 9, 2010; revised February 13, 2011 and April13, 2011;acceptedMay29, 2011. DateofcurrentversionJanuary9, 2012.Recommended for publication by Associate Editor S. Choi.TheauthorsarewiththeDepartmentofElectricalEngineering, TechnicalUniversityofDenmark, KongensLyngbyDK-2800, Denmark(e-mail:;;; versions of one or more of the gures in this paper are available onlineat Object Identier 10.1109/TPEL.2011.2159515Fig. 1. Block diagram of a dual-conversion UPS system based on FC and SC.potential of the hydrogen into electric power directly with con-sequent high conversion efciency, so it has the possibility toobtaintheextendedruntimerangewiththecombustiblefeedfrom the outside. But one of the main weak points of the FCis its slow dynamics because of the limited speed of hydrogendeliverysystemandthechemical reactioninthemembraneswith a slow time constant [5]. Hence, during the warming-upstage or load transient, SCs [6], [7] are utilized as the auxiliarypower source for smoothing the output power. In addition, thefuel-cell output voltage is varied widely, almost 2:1, dependingon the load condition, and the terminal voltage of the SC bank isalso variable during charging and discharging periods. Thus, itis very important for the conversion systemto be capable of har-vesting power fromthese two different power sources efcientlyin widely input voltage range and load conditions.Inrecentyears, manycongurationsofahybriddcpowerconversion system relating to FCs and SCs have been proposed.ConnectingFCsandSCsbytwoindividualdcdcconvertersseparately to a mutual dc voltage bus is the most common cong-uration [8], [9], which offers many advantages, especially, fasterand more stable system response. However, it increases the sys-temcost and power losses. Amultiple dc voltage bus, which con-nects FCs and SCs to different cascaded voltage buses throughconverters,isalso awidelyusedconguration[10],[11],butthedisadvantagesarehighpower lossesandlowreliability.0885-8993/$26.00 2011 IEEEZHANG et al.: ANALYSIS AND DESIGN OF A BIDIRECTIONAL ISOLATED DCDC CONVERTER 849Fig. 2. Proposed hybrid bidirectional dcdc converter topology.Moreover, FCs and the SCs cannot keep the bus voltage con-stant except if they are oversized. The simplest conguration isto parallel FCs and SCs directly as one power source but theiroutput currents cannot be controlled independently. In addition,a multiport conguration was introduced [12], [13]. For the ap-plications where the galvanic isolation is required, an isolatedmultiport converter family was investigated in [14]. Based onthe traditional half-bridge topology, a novel four-port converterwith bidirectional ability was presented in [15] and [16]. A mul-tiport current-fed dcdc converter based on the ux additivitywas proposed in [17]. The boost-type input port can limit thecurrent ripple and this characteristic is helpful to increase thelifetimeofFCs, but thediodeconnectedinserieswitheachMOSFETsmakesreversiblepowerowimpossible.Toover-comethisdrawback, twocurrent-feddual-input bidirectionalconverters were proposed and investigated in [18] and [19]. Thesolutions based on the dual-active-bridge converter using mag-netic coupling transformer were presented in [20][23], wherethe bidirectional power can be regulated by phase-shift controlscheme. Convertersusingresonant tankorinterleavedtrans-former windings were reported in [24] and [25], respectively.However, the control strategy for the multiport type is not easyto implement [25].Based on the boost-half-bridge (BHB) circuit [26], [27] andthe hybrid full-bridge structure [28], a novel hybrid bidirectionaldcdc converter was derived and presented in [29]. In this paper,characteristics of the proposed converter in [29] will be analyzedin depth. As shown in Fig. 2, a fuel-cell bank as the main inputpower source is connected to the BHB circuit which can limittheinput current ripple; anSCbankastheauxiliarypowersourcecandeliverpowertotheloadthroughthefull-bridgecircuit. The proposed converter can draw power from these twodifferent dc sources individually and simultaneously. Moreover,using the phase-shift plus duty-cycle control scheme [30], thebidirectionalpowerowcanberegulatedexiblyandtheaccurrent root-mean-square (RMS) value can be reduced over awide input voltage range.This paper is organized as follows. Section I introduces theresearch background and the contribution of this study. SectionII gives the operation principles and the theoretical calculations.Section III presents the quasi-optimal design method. To verifythe validity of the theoretical analysis and the design approach,experimental results fromthe laboratory prototype are presentedin Section V. Finally, the conclusion is given in Section VI.II. OPERATION PRINCIPLES OF THE HYBRIDBIDIRECTIONAL DCDC CONVERTERAs shown in Fig. 2, a BHB structure locates on the primaryside of the transformerT1and it associates with the switchesS1andS2that are operated at 50% duty cycle. The SC bankas an auxiliary energy source is connected to the variable low-voltage (LV) dc bus across the dividing capacitors, C1and C2.BidirectionaloperationcanberealizedbetweentheSCbankand the high-voltage (HV) dc bus. Switches S3 and S4 are con-trolled by the duty cycle to reduce the current stress and ac RMSvalue when input voltage VFCor VSCare variable over a widerange. The transformersT1andT2with independentprimarywindingsaswellasseries-connectedsecondarywindingsareemployedtorealizegalvanicisolationandboostalowinputvoltage to the HV dc bus. A dc blocking capacitor Cbis addedin series with the primary winding ofT2to avoid transformersaturation caused by asymmetrical operation in full-bridge cir-cuit. The voltage doubler circuit utilized on the secondary sideis to increase voltage conversion ratio further. The inductor L2on the secondary side is utilized as a power delivering interfaceelementbetweentheLVsideandtheHVside.Accordingtothedirectionofpowerow,theproposedconverterhasthreeoperation modes that can be dened as boost mode, SC powermode, and SC recharge mode. In the boost mode, the power isdelivered from the FCs and SCs to the dc voltage bus. In the SCpower mode, only the SCs are connected to provide the requiredload power. When the dc bus charges the SCs, the power owdirection is reversed which means the energy is transferred fromthe HVside to the LVside, and thereby the converter is operatedunder the SC recharge mode.A. Boost ModeIn the boost mode, the timing diagram and typical waveformsare shown in Fig. 3, where n1and n2are the turn ratios of thetransformers. The current owing in each power switch on theprimary side is presented, but the voltage and current resonantslopes during the switching transitions are not shown here forsimplicity. To analyze the operation principles clearly, the fol-lowing assumptions are given: 1) all the switches are ideal withantiparallel body diodes and parasitic capacitors; 2) the induc-tance L1 is large enough to be treated as a current source; 3) theoutput voltage is controlled well as a constant; 4) the leakageinductance of the transformers, parasitic inductance, and extrainductance can be lumped together as L2 on the secondary side.The half switching cycle can be divided into eight intervalsand the corresponding equivalent circuits are shown in Fig. 4.1) Stage 1 (t0t1): It can be seen that at any time, the volt-age across L2is always VT 1b+ VT 2b VCO, but VT 1b,VT 2b, andVCOhavedifferent val