DC-DC-SEPIC-Converter原理设计

EE462L,PowerElectronics,DC-DCSEPICConverterVersionMarch1,2013Page1of13OverviewSEPICconvertersmakeitpossibletoefficientlyconvertaDCvoltagetoeitheralowerorhighervoltage.SEPICconvertersareespeciallyusefulforPVmaximumpowertrackingpurposes,wheretheobjectiveistodrawmaximumpossiblepowerfromsolarpanelsatalltimes,regardlessoftheload.TheoryofOperationRelationBetweenVoutandVininContinuousConductionTheidealizedSEPICconvertercircuitisshownbelowinFigure1.Undernormaloperation,thecircuitisin“continuousconduction”(i.e.,iL1andiL2arealwaysgreaterthanzero).ThefirstimportantrelationshipcomesfromthefactthatcapacitorC1shouldbelargeenoughsothatvoltage1Cvhaslowripple.ApplyingaverageKVLaroundtheloopformedbyVin,L1,C1,andL2,andrecognizingthattheaveragevoltagesacrossL1andL2areeachzero,yieldsinCVv1.(1)ThesecondimportantrelationshipcomesbyapplyingKCLintheaveragesenseatthenodeatopL2.SincetheaveragecurrentsinC1andCarebothzero,thenoutdavgavgLIii2.(2)Withcontinuousconduction,thecircuithastwostates–switchclosed,andswitchopen.ThesestatesareshowninFigures2aand2b.Figure1.DC-DCSEPICConverterRemember–neverconnectavariacdirectlytoaDBR!Variac(Optional,seeStep3)120/25VacTransformerDBRVin+Vout–CiCIoutidC1L2iL2iC11CviL1L11Lv+vL2–Iin0.01ΩLeavethe10µFcapacrosstheinputterminalsEE462L,PowerElectronics,DC-DCSEPICConverterVersionMarch1,2013Page2of13Whentheswitchisclosed(Figure2a),thediodeisreversebiasedandopen,current1Liincreasesattherateof11LVdtdiinL,DTt0,(3)sothatL1is“charging.”Whentheswitchisopen(Figure2b),thediodeisforwardbiased,andLidecreasesattherateof11LVdtdioutL,TtDT,(4)sothatL1is“discharging.”ThevoltageacrossL1isshowninFigure3.Becauseofthesteady-stateinductorprinciple,theaveragevoltageacrossL1iszero.Since1Lvhastwostates,bothhavingconstantvoltage,theaveragevalueof1LvisFigure3.InductorL1VoltageinContinuousConductionoutV0inVFigure2b.SwitchOpenfor(1-D)TSecondsVinL1+Vout–CC1L2Vin+–L1dischargingC1chargingL2dischargingCchargingFigure2a.SwitchClosedforDTSecondsVinL1+Vout–CC1L2–Vin+L1chargingC1dischargingL2chargingCdischarging(Vout+Vin)–+EE462L,PowerElectronics,DC-DCSEPICConverterVersionMarch1,2013Page3of130)1)((TTDVDTVoutin,sothat0DVVDVoutoutin.(5)Simplifyingtheaboveyieldsthefinalinput-outputvoltageexpressionDDVVinout1.(6)Thus,theconverterisin“buck”modeforD0.5,andin“boost”modeforD0.5.Theassumptionofalosslesscircuitrequiresinputpowertoequaloutputpower,soDIDIinout1.(7)InductorCurrentsinContinuousConductionThegraphof1LiisshowninFigure4.ForPVapplications,itisobviouslydesirabletohavelowripplein1Litokeepthesolarpaneloperatingatthepeakofitsmaximumpowercurve.FromFigure4andEquation(3),whentheswitchisopen(i.e.,L1is“discharging”),11LVdtdioutL,sothatFigure4.InductorL1CurrentWaveformforContinuousConductionDT(1–D)TT211max1IiiavgLLinavgLIi1211min1IiiavgLL1IEE462L,PowerElectronics,DC-DCSEPICConverterVersionMarch1,2013Page4of13fLDVTDLVIoutout11111,(8)wherefistheswitchingfrequency.Theboundaryofcontinuousconductionfor1Liswhenmin1Li=0,asshowninFigure5.Thus,attheboundary,fLDVIboundaryoutin112,(9)sothatfIDVfIDDDVfIDVLinininininoutboundary2211211.(10)AsDapproachesunity,fIVLinin21(11)willguaranteecontinuousconduction.Notein(10)and(11)thatcontinuousconductioncanbeachievedmoreeasilywheninIandfarelarge.DT(1–D)TTFigure5.InductorL1CurrentattheBoundaryofContinuousConductionavgLLIi1max12inavgLIi10min1Li1IEE462L,PowerElectronics,DC-DCSEPICConverterVersionMarch1,2013Page5of13Thegraphof2LiisshowninFigure6.FromFigures2band6,whentheswitchisopen(i.e.,L2is“discharging”),TDILVdtdioutL1222,sothatfLDVLTDVIoutout22211,(12)wherefistheswitchingfrequency.Theboundaryofcontinuousconductionfor2Liswhenmin2Li=0,asshowninFigure7.Thus,attheboundary,fLDVIboundaryoutout212,(13)DT(1–D)TTFigure7.InductorL2CurrentattheBoundaryofContinuousConductionavgLLIi2max22outavgLIi20min2Li2IFigure6.InductorL2CurrentWaveformforContinuousConductionDT(1–D)TT222max2IiiavgLLoutavgLIi2222min2IiiavgLL2IEE462L,PowerElectronics,DC-DCSEPICConverterVersionMarch1,2013Page6of13sothatfIDVLoutoutboundary212.(14)Sincethemaximumvalueof(14)occursatD→0,fIVLoutout22(15)willguaranteecontinuousconductionforL2forallD.Notein(14)and(15)thatcontinuousconductioncanbeachievedmoreeasilywhenoutIandfarelarge.CurrentRatingsforContinuousConductionOperationContinuouscurrentwaveformsfortheMOSFET,thecapacitors,andthediodeincontinuousconductionareshowninFigure8onthefollowingpage.CorrespondingwaveformsfortheinductorswereshownpreviouslyinFigures4and6.Followingthesameformulasandreasoningusedforthebuckconverter,conservativecurrentratingsforcomponentsL1,L2,theMOSFET,andthediodefollow.ForL1,usingFigure5,31121212222max,,1inininrmsLIIII,sothatinrmsLII32max,,1.(16)Similarly,forL2,usingFigure7,outrmsLII32max,,2.(17)EE462L,PowerElectronics,DC-DCSEPICConverterVersionMarch1,2013Page7of13Figure8.CurrentWaveformsforMOSFET,Capacitors,andDiodeinContinuousConduction0MOSFET21IIoutinII0Diode21LLii01LiC11IinI2IoutI2Li21IIinI0CoutLLIii21outIDT21