Page - 219 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2016,9, 410 To increase the voltage gain of the converter, the capacitors are switched and itwill act as a charge-pump. Themain advantage of the switched capacitor-based boost converter is that there isnoneedofa transformeror inductors. Themaindrawbacksof this topologyare the complexity of the topology, high cost, low power level and high pulsating current in the input side [11,21]. Inorder to increase theconversionefficiencyandvoltageconversionratio,multilevelcombinedthe switched-capacitor techniqueshavebeenproposedtoachieve lowerstressonpowerdevices [20–23]. AsshowninFigure3, in [22,23] twoconverters regulated thereasonablevoltageconversionratiowith asimplepulse-width_modulation(PWM)control.However, ifahighvoltageconversionratiomust beprovided,morepowerswitchesandcapacitorsare indeedrequired. Furthermore, although the extremedutycyclecanbeavoided, the inputcurrent ripple is largedueto their single-phaseoperation whichrenders theseBDCsunsuitable forhighcurrentandlowrippleapplications. CL L1+ VL - S2 + VH - S3 CH2 CH1 S4 S1 Discharge StateCharge State CL L1+ VL - S3 S1 + VH - L2 S2 C S4 CH Discharge StateCharge State (a) (b) Figure 3. Twomultilevel combined the switched-capacitor topologies: (a) circuit structure in [22]; (b) circuit structure in [23]. The objective of this paper is to study and develop a novel BDC for applications involving EVs connected to dc-microgrids. Tomeet the high current, low current ripple, and high voltage conversion ratiodemands, the studied topology consists of anunregulated level converter (ULC) cascadedwitha two-phase interleavedbuck-boost charge-pumpconverter (IBCPC). Indischargestate, the topologyactsasa two-stagecascadedtwo-phaseboostingconverter toachieveahighstep-upratio. Inchargestate, the topologyactsas two-stagecascadedtwo-phasebuckingconverter toachieveahigh step-downratio. Theextremedutycycleofpowerdeviceswillnotoccur forbidirectionalpowerflow conditions, thusnotonlycan theoutputvoltage regulation rangebe further extendedbutalso the conduction lossescanbereduced. Inaddition, the two-stagestructurebenefitsreducingthevoltage stressofactive switches,whichenablesone toadopt the low-voltage ratingandhighperformance devices, thus theconversionefficiencycanbe improved. Theremainderof thispaper isorganizedas follows: first, theconverter topologyandtheoperationprinciplesof thestudiedBDCare illustrated in Section2. Then, steady-statecharacteristicanalyzesarepresented inSection3.A500Wlaboratory prototypeisalsoconstructed,andthecorrespondingsimulationresults,aswellasexperimentalresults, areprovidedtoverify the feasibilityof thestudiedBDCinSection4. Finally, someconclusionsare offered in the last section. 2. ProposedBDCTopologyandOperationPrinciples The systemconfiguration for the studiedBDC topology is depicted in Figure 4. The system contains twoparts, includingaULCandatwo-phase IBCPC.Themajorsymbolrepresentationsare summarizedas follows:VHandVLdenote thehigh-sidevoltageandlow-sidevoltage, respectively. L1 andL2 represent two-phase inductorsof IBCPC.CBdenotes thecharge-pumpcapacitor.CH andCL are thehigh-sideandlow-sidecapacitors, respectively. Thesymbols,Q1~Q4, andS1~S4, respectively, are thepowerswitchesof the IBCPCandULC. 219