Page - 220 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2016,9, 410 CH VH Bidirectional Power Flow Unregulated Level Converter (ULC) Two-Phase Interleaved Buck-Boost Charge-Pump Converter (IBCPC) CM1 CM2 S1 S2 CL S3 S4 La Lb L1 L2 Q4 Q3 Q2 Q1 CB + - VL + - VM + - High-Side StageLow-Side Stage Figure4.Systemconfigurationof thenovelBDCtopology. In this study,as the low-sidestage,ahighefficiencymagnetic-lessULCwithbidirectionalpower flowisadoptedtooutputafixedvoltageforagiven inputvoltage. Becauseonlyasmall sizedhigh frequency linefilter (La,Lb) is required, it cansubstantiallyboost thepowerdensityof the low-side stage. Furthermore,by leavingthevoltageregulationtoanotherhigh-sidestage, thestudiedBDCfor the low-sidestagewithfixed2:1underchargestateoperationor1:2conversionratiounderdischarge stateoperation, canachievehighefficiencywitha relatively low-sidevoltage inwhole load range. As to thehigh-sidestage, thestructureof two-phase IBCPCissimilar toaconventionalbuck-boost converter except twoactive switches in series anda charge-pumpcapacitor (CB) employed in the powerpath. Thecircuit structure issimpleandit canreachthehighvoltageconversionratiowitha reasonabledutycycle. Therefore, it canreduce theconduction lossof theswitch, to furtherupgrade theefficiencyof thewholebidirectionalconverter. ThestudiedBDCtopologycandeliverenergy inbothdirections.Whentheenergyflowsfrom VH toVL, it operates in charge state (i.e., buckoperation);Q1 andQ2 are controlled to regulate the output. Thus,Q1 andQ2 aredefinedas theactiveswitches,whileQ3 andQ4 are thepassiveswitches. Thepassiveswitchesworkassynchronousrectification(SR).WhentheenergyflowsfromVL toVH, it operates indischargestate (i.e., boostoperation);Q3 andQ4 arecontrolled toregulate theoutput. Thus,Q3 andQ4 aredefinedas theactiveswitches,whileQ1 andQ2 are thepassiveswitches. In thisstudy, the followingassumptionsaremadetosimplify theconverteranalyzesas follows: (1) theconverter isoperated incontinuousconductionmode(CCM); (2) capacitorsCH andCL is large enoughtobeconsideredasavoltagesource; (3) themiddle-linkvoltageVM=VM1 +VM2 is treated asapuredcandconsideredasconstant; (4) the two inductorL1 andL2 have thesame inductorLs; (5)allpowersemiconductorsare ideal; (6) thecharge-pumpvoltageVCB is treatedasapuredcand consideredasconstant. 2.1. ChargeStateOperation Figures5and6showthecircuit configurationandcharacteristicwaveformsof thestudiedBDC inchargestate, respectively. It canbeseen that switchesQ1 andQ2 aredrivenwith thephaseshift angleof180˝;Q3 andQ4workassynchronousrectification. Inchargestate,whenS1,S3 are turnedon andS2,S4 are turnedoff;orelseS2,S4 are turnedonandS1,S3 are turnedoff. The low-sidevoltageVL ishalf themiddle-linkvoltageVM, i.e.,VL=0.5VM. In this state,onecansee that,whendutyratioof Q1 andQ2 aresmaller than50%, thereare fouroperatingmodesaccordingto theon/off statusof the activeswitches. 220