P2c7-DC-DC开关模式变换器

Chapter7：DC-DCSwitch-ModeConverters:3-1Introduction3-2Controlofdc-dcConverters3-3Step-Down(Buck)Converter3-4Step-Up(Boost)Converter3-5Buck-BoostConverter3-6Cukdc-dcConverter3-7FullBridgedc-dcConverter3-8dc-dcConverterComparison3-1InductionThedc-dcconvertersarewidelyusedinregulatedswitch-modedcpowersuppliesandindcmotordriveapplications.UncontrolleddioderectifierFiltercapacitorDC-DCconverterLoadDCunregulatedDCregulatedDCunregulatedACBatteryAdc-dcconvertersystemAssumptions:TheconverteroperatinginsteadystateTheswitchesareidealThelossesintheinductiveandcapacitiveelementsareneglectedThedcinputvoltagetotheconvertersispurelydcvoltageandhaszerointernalimpedanceTheoutputloadsareresistance(incaseofswitch-modedcpowersupplyload)orresistance-inductance(incaseofdcmotorload)3-2Controlofdc-dcConverterAbasicdc-dcconverterR+Vo-+Vd-VdVotontoffTsvotOperationprinciple:Theoutputvoltagevodependsontonandtoff.SwitchdutyratioD=ton/Ts.Controlmethods:PWM(pulse-widthmodulation)control:Switchingataconstantfrequencyfs(Ts)andadjustingtheondurationton.PFM(pulse-frequencymodulation)control:Switchingataconstanttonandadjustingthefrequencyfs(Ts).Othercontrol:BothSwitchingfrequencyfs(Ts)andtheondurationtonarevaried.PWM(pulse-widthmodulation)controlComparator+Amplifier-VrefVovctrvstSwitchcontrolsignalononoffoffVctrvstVctrvstSwitchcontrolsignalvctrvst)17(ˆstctrsonVvTtD7-3Step-Down(Buck)Converter7-3BUCKCONVERTERStep-downconverterismainlyappliedinregulateddcpowersuppliesandmotorspeedcontrol.7-3BUCKCONVERTERR+Vo-+Vd-VdVotontoffTsvotConceptually,Abasicdc-dcconverter(asbelow)constitutesastep-downconverterforpurelyresistiveload.Butithastwodrawbacks:(1)hasno“freewheels”pathforinductiveenergyflow;(2)theoutputvoltagefluctuatesbetweenzeroandVd.PracticalBuckConverter7-3BUCKCONVERTERSimplifyingAssumptions1.Transistors,diodesandotherpassivecomponentsareallideal.2.TheinputisapuredcvoltageVd.3.Theoutputvoltageispurelydcwithoutanyripple,thatisvo(t)≈Vo,4.Continuousconductionmode,CCMvoiVdOtonttoffsSfT1(b)+Vd-C+vL-R+vo=Vo-+voi-(a)LidiLioVo+Vd-C+vL-R+vo=Vo-+voi-(a)LidiLiott0ABTstont0t0iD(b)(c)(d)(e)iLidvL(Vd-Vo)(-Vo)(IL=Io)toffOperationprinciple7-3BUCKCONVERTER1)Turningontheswitchincreasetheinductorcurrent.2)Whenthetransistoristurnedoff,theinductorcurrent“freewheels”throughthediode.)37()()0DTtVVtTVtVVVsondoonsoonodL（Inthedcsteadystate:7-3BUCKCONVERTERVocanbecontrolledbyvaryingthedutyratioD.Inthedcsteadystate:7-3BUCKCONVERTERtconsVVkkvVVvVstdctrdstctrotanˆˆVovarieslinearlywiththecontrolvoltagevctr.Inthedcsteadystate:RVIIIooLC0)()(,titirippleLC)47(1::DIIandIVIVThereforePPdoooddod7-3BUCKCONVERTERNeglectingpowerlossesassociatewithallthecircuitelements.Inthedcsteadystate:RVIIIooLC0)()(,titirippleLC7-3BUCKCONVERTER)()(,tiItirippleLLL）（227)1(1)(1sosodLTDVLDTVVLiBuckConvertersinDCM7-3BUCKCONVERTERExample7-1IntheBuckdc-dcconverterofFig.3-5a,L=24μH.Itisoperatingindcsteadystateunderthefollowingconditions:Vin=20V,D=0.6,Po=12W,andfs=200kHz.Assumingidealcomponents,calculateanddrawthewaveformsshownearlierinFig.3-5d.7-3BUCKCONVERTERSolutionWithfs=200kHz,Ts=5μsandTon=DTs=3μs.Vo=DVin=12V.TheinductorvoltagevLfluctuatesbetween(Vin−Vo)=8Vand(−Vo)=−12V,asshowninFig.3-6.ADTVVLisoinL1245.608)(1AVPIIoooL11212Therefore,iL=IL+iL,ripple,asshowninFig.3-6.Whenthetransistorison,iin=iL,otherwisezero.TheaverageinputcurrentsisIin=DIo=0.6A.7-3BUCKCONVERTERPSpiceModeling:C:\FirstCourse_PE_Book03\Buckconv.sch7-3BUCKCONVERTERSimulationResults7-3BUCKCONVERTER7-4Step-Up(Boost)Converter7-4BOOSTCONVERTERStep-upconverterismainlyappliedinregulateddcpowersuppliesandregenerativebrakingofdcmotors.+-LVLiDLDi+oV-RC+-dVsiOperationprinciple1)TurningontheswitchappliesthevoltageVdacrosstheinductor,vL=Vd,andiLlinearlyrampsup,increasingtheeneryintheinductor.7-4BOOSTCONVERTERtt0(Vd-Vo)Tstont0t0iD(b)(c)(d)(e)vLiLis(Vd)ILtoff+-LVLiDLiD+oV-RC+-dVsi(a)2)turningofftheswitchforcestheinductorcurrenttoflowthroughthediode,someofthestoredenergyistransferredtotheoutputstage.Inthedcsteadystate:)267(110)(DtTVVtVVtVoffsdooffoinondAssumingalosslesscircuit,)277()D1::（doooddodIIandIVIVThereforePP7-4BOOSTCONVERTERtt0(Vd-Vo)Tstont0t0iD(b)(c)(d)(e)vLiLis(Vd)ILtoff+-LVLiDLiD+oV-RC+-dVsi(a)odioderipplediodeCIititi)()(,7-4BOOSTCONVERTERsdosdLTDVVLDTVLi)1)((1)(1RVDIIodL11tt0(Vd-Vo)Tstont0t0iD(b)(c)(d)(e)vLiLis(Vd)ILtoff+-LVLiDLiD+oV-RC+-dVsi(a)Boostconverter:voltagetransferratioThevoltageconversionratioofBoostconvertersoperatinginCCMdependsonD,butisindependentoftheoutputload.IftheoutputloaddecreasestotheextentthatiLbecomesdiscontinuous(DCM),therelationship(7-26)isnolongervalid.）267(11DVVino7-4BOOSTCONVERTERExample7-2InaBoostconverterofFig.3-8a,theinductorcurrenthasΔiL=2A.Itisoperatingindcsteadystateunderthefollowingconditions:Vin=5V,Vo=12V,Po=10W,andfs=200kHz.Assumingidealcomponents,calculateLanddrawthewaveformsasshowninFig.3-8c.7-4BOOSTCONVERTERSolutionTheduty-ratioD=0.583.Withfs=200kHz,Ts=5μsandTon=DTs=2.917μs.vLfluctuatesbetweenVin=5Vand−(Vo−Vin)=−7V.HDTiVLsLin29.