开关电源回路控制设计参考资料

1®TB417.1CAUTION:Thesedevicesaresensitivetoelectrostaticdischarge;followproperICHandlingProcedures.1-888-INTERSILor321-724-7143|Intersil(anddesign)isaregisteredtrademarkofIntersilAmericasInc.Copyright©IntersilAmericasInc.2003.AllRightsReservedAllothertrademarksmentionedarethepropertyoftheirrespectiveowners.DesigningStableCompensationNetworksforSinglePhaseVoltageModeBuckRegulatorsAssumptionsThisTechnicalBriefmakesthefollowingassumptions:1.Thepowersupplydesignerhasalreadydesignedthepowerstageofthesinglephasebuckconverter.Thelaststeptothedesignisthecompensationnetwork.2.Thedesignerhasatleastabasicunderstandingofcontrolsystemstheory.3.ThedesignerhasabasicunderstandingofBodeplots.IntroductionSynchronousandnon-synchronousbuckregulatorshavethreebasicblocksthatcontributetotheclosedloopsystem.Theseblocksconsistofthemodulator,theoutputfilter,andthecompensationnetworkwhichclosestheloopandstabilizesthesystem.ModulatorThemodulatorisshowninFigure2.Theinputtothemodulatoristheoutputoftheerroramplifier,whichisusedtocomparetheoutputtothereference.TheoutputofthemodulatoristhePHASEnode.Thegainofthemodulatorissimplytheinputvoltagetotheregulator,VIN,dividedbythepeak-to-peakvoltageoftheoscillator,∆VOSC,or:ThepeaktopeakvoltageoftheoscillatorcanbeobtainedfromthedatasheetforthecontrollerIC.OutputFilterTheoutputfilterconsistsoftheoutputinductorandalloftheoutputcapacitance.ItisimportanttoincludetheDCresistance(DCR)oftheoutputinductorandthetotalEquivalentSeriesResistance(ESR)oftheoutputcapacitorbank.TheinputtotheoutputfilteristhePHASEnodeandtheoutputistheregulatoroutput.Figure3showstheequivalentcircuitoftheoutputfilteranditstransferfunction.ThetransferfunctionfortheoutputfiltershowsthewellknowndoublepoleofanLCfilter.ItisimportanttonotethattheESRofthecapacitorbankandtheDCRoftheinductorbothinfluencethedampingofthisresonantcircuit.ItisalsoimportanttonoticethesinglezerothatisafunctionoftheoutputcapacitanceanditsESR.OpenLoopSystemFigure4illustratestheopenloopsystemandpresentsthetransferfunction.REFERENCEOUTPUT+_FIGURE1.BASICBLOCKSOFTHEBUCKREGULATORERRORAMPLIFIERMODULATOROUTPUTFILTERCOMPENSATIONNETWORKFIGURE2.THEMODULATORVIN∆VOSCPWMDRIVERCOMPARATORDRIVERPHASE+-OSCOUTPUTOFERRORAMPLIFIERGAINMODULATORVIN∆VOSC----------------------=VOUTLODCRESRPHASECOFIGURE3.THEOUTPUTFILTERGAINFILTER1sESRCOUT⋅⋅+1sESRDCR+()COUT⋅⋅s2LOUTCOUT⋅⋅++----------------------------------------------------------------------------------------------------------------------------------------=VOUTLODCRESRCO+-FIGURE4.THEOPENLOOPSYSTEME/AOUTPUTGAINOPENLOOPVIN∆VOSC----------------------1sESRCOUT⋅⋅+1sESRDCR+()COUT⋅⋅s2LOUTCOUT⋅⋅++---------------------------------------------------------------------------------------------------------------------------------------⋅=TechnicalBriefDecember2003Author:DougMattingly2Figure5showstheasymptoticBodeplotoftheopenloopsystemgain.Figure5representsagenericopenloopsystem.SpecificsystemswillhavedifferentdoublepoleandESRzerofrequencies.ForsystemswithverylowDCRandESRparameters,thephasewillexperienceaverysharpslopedownwardatthedoublepolewhilethegainwillhavearatherhighpeakatthedoublepole.Systemsthathavesuchresonantoutputfilterswillbemoredifficulttocompensatesincethephasewillneedanextraboosttoprovidethenecessaryphasemarginforstability.SystemssuchasthiswilltypicallyneedaTypeIIIcompensation,whichwillbediscussedlaterinthisbrief.ClosingTheLoop-TheCompensationNetworkClosingthecontrolloopallowstheregulatortoadjusttoloadperturbationsorchangesintheinputvoltagewhichmayadverselyaffecttheoutput.Propercompensationofthesystemwillallowforapredictablebandwidthwithunconditionalstability.Inmostcases,aTypeIIorTypeIIIcompensationnetworkwillproperlycompensatethesystem.TheidealBodeplotforthecompensatedsystemwouldbeagainthatrollsoffataslopeof-20dB/decade,crossing0dbatthedesiredbandwidthandaphasemargingreaterthan45oforallfrequenciesbelowthe0dBcrossing.Forsynchronousandnon-synchronousbuckconverters,thebandwidthshouldbebetween20to30%oftheswitchingfrequency.TypeIICompensationFigure6showsagenericTypeIIcompensation,itstransferfunctionandasymptoticBodeplot.TheTypeIInetworkhelpstoshapetheprofileofthegainwithrespecttofrequencyandalsogivesa90oboosttothephase.Thisboostisnecessarytocounteracttheeffectsoftheresonantoutputfilteratthedoublepole.Iftheoutputvoltageoftheregulatorisnotthereferencevoltagethenavoltageprogrammingresistorwillbeconnectedbetweentheinvertinginputtotheerroramplifierandground.Thisresistorisusedtooffsettheoutputvoltagetoalevelhigherthanthereference.Thisresistor,ifpresent,hasnoeffectonthecompensationandcanbeignored.Figure7showstheclosedloopsystemwithaTypeIIcompensationnetworkandpresentstheclosedlooptransferfunction.Thefollowingguidelineswillhelpcalculatethepolesandzeroes,andfromthosethecomponentvalues,foraTypeIInetwork.1.ChooseavalueforR1,usuallybetween2kand5kΩ.2.Pickagain(R2/R1)thatwillshifttheOpenLoopGainuptogivethedesiredbandwidth.Thiswillallowthe0dBcrossovertooccurinthefrequencyrangewheretheTypeIInetworkhasaflatgain.ThefollowingequationwillcalculateanR2thatwillaccompli