控制环路资料大全(UNITRODE研讨会资料)1

1、LloydDixonisnearlyequivalenttoaverageinductorcurrentcontrol.Inaconventionalswitchingpowersupplyemployingabuckderivedtopology,theinduc-torisintheoutput.Currentmodecontrolthenisactuallyoutputcurrentcontrol,resultinginmanyperformanceadvantages.Ontheotherhand,inahighpowerfactorpreregulatorusingtheboosttopology,theinductorisintheinput.Currentmodecontrolthencontrolsinputcurrent,allowingittobeeasilyconformedtothedesiredsinusoidalwaveshape.PeakCurrentModeControlProblemsPoornoiseimmunity.Thepeakmethodof。

2、inductorcurrentcontrolfunctionsbycomparingtheupslopeofinductorcurrent(orswitchcurrent)toacurrentprogramlevelsetbytheouterloop-seeFig.1.Thecomparatorturnsthepowerswitchoffwhentheinstantaneouscurrentreachesthedesiredlevel.Thecurrentrampisusuallyquitesmallcomparedtotheprogramminglevel,especiallywhen~Nislow.Asaresult,thismethodisextremelysuscepti-bletonoise.Anoisespikeisgeneratedeachtimetheswitchturnson.Afractionofavoltcoupledintothecontrolcircuitcancauseittoturnoffimmediately,resultinginasubhar-mon。

3、icoperatingmodewithmuchgreaterripple.Circuitlayoutandbypass-v.'(1'(1'(1ingarecriticallyimportantJUULtosuccessfuloperation.VISlopecompensationGATEnnnrequired.Thepeakcur-~,DRIV£jUULIII---=iLATCHPWN,rentmodecontrolmethod-L1-LsR..h1blClOCKV.rrorISmerentyunstaeatdutyratiosexceeding0.5,Fig.1-PeakCulTentModeControlCircuitandWavefonnsAbstractCurrentmodecontrolasusuallyimplementedinswitchingpowersuppliesactuallysensesandcontrolspeakinductorcurrent.Thisgivesrisetomanyseriousproblems,includingpoornoiseimmu。

4、nity,aneedforslopecompensation,andpeak-to-averagecurrenterrorswhichtheinherent-lylowcurrentloopgaincannotcorrect.Averagecurrentmodecontroleliminatestheseproblemsandmaybeusedeffectivelytocontrolcurrentsotherthaninductorcurrent,allowingamuchbroaderrangeoftopologicalapplication.GeneralPerspectiveCurrentmodecontrolisatwo-loopsystemasshowninthesimpleexampleofFig.1.Theswitchingpowersupplyinductorishiddenwithintheinnercurrentcontrolloop.Thissimplifiesthedesignoftheoutervoltagecon-trolloopandimprovespow。

5、ersupplyperfor-manceinmanyways,includingbetterdynamics.Theobjectiveofthisinnerloopistocontrolthestate-spaceaveragedinductorcurrent,butinpracticetheinstantaneouspeakinductorcurrentisthebasisforcontrol.(Switchcurrent--equaltoinductorcurrentduringtheontime--isoftensensed.)Iftheinductorripplecurrentissmall,peakinductorcurrentcontrolol.~LjLOADI@JVI~E/AI5-1AverageCurrentModeControlresultinginsub-harmonicoscillation.Acom-pensatingramp(withslopeequaltotheinduc-torcurrentdownslope)isusuallyappliedtotheco。

6、mparatorinputtoeliminatethisinstability.Inabuckregulatortheinductorcurrentdown-slopeequalsVolL.WithVoconstant,asitusuallyis,thecompensatingrampisfIXedandeasytocalculate-butitdoescomplicatethedesign.Withaboostregulatorinahighpowerfactorapplication,thedownslopeofinductorcurrentequals(VIN-VO)/Landthusvariescon-siderablyastheinputvoltagefollowstherecti-tiedsinewaveform.AfIXedrampprovidingadequatecompensationwillovercompensatemuchofthetime,withresultingperformancedegradationandincreaseddistortion.Pea。

7、ktoaveragecurrenterror.Thepeaktoaveragecurrenterrorinherentinthepeakmethodofinductorcurrentcontrolisusuallynotaseriousprobleminconventionalbuck-derivedpowersupplies.Thisisbecauseinduc-torripplecurrentisusuallymuchsmallerthantheaveragefullloadinductorcurrent,andbe-causetheoutervoltagecontrolloopsoonelimi-natesthiserror.Inhighpowerfactorboostpreregulatorsthepeak/avgerrorisveryseriousbecauseitcausesdistortionoftheinputcurrentwaveform.Whilethepeakcurrentfollowsthedesiredsinewavecurrentprogram,theave。

8、ragecurrentdoesnot.Thepeak/avgerrorbecomesmuchworseatlowercurrentlevels,especiallywhentheinduc-torcurrentbecomesdiscontinuousasthesinewaveapproacheszeroeveryhalfcycle.Toachievelowdistortion,thepeak/avgerrormustbesmall.Thisrequiresalargeinductortomaketheripplecurrentsmall.Theresultingshallowinductorcurrentrampmakesthealreadypoornoiseimmunitymuchworse.Topologyproblems.Conventionalpeakcurrentmodecontrolactuallycontrolsinductorcurrent.Asnormallyusedforoutputcurrentcontrol,itismosteffectivewhenapplie。

9、dtoabuckregulatorwheretheinductorisintheoutput.Butforflybackorboosttopologiestheinductorisnotintheoutput,thewrongcurrentiscontrolled,andmuchoftheadvantageofcurrentmodecontrolislost.Likewise,theboosttopologywithitsinduc-torattheinputiswellsuitedforinputcurrentcontrolinahighpowerfactorpreregulator,butbuckandflybacktopologiesarenotwellsuitedbecausetheinductorisnotintheinputandthewrongcurrentiscontrolled.AverageCurrentModeControlPeakcurrentmodecontroloperatesbydirectlycomparingtheactualinductorcurre。

10、ntwaveformtothecurrentprogramlevel(setbytheouterloop)atthetwoinputsofthePWMcomparator.Thiscurrentloophaslowgainandsocannotcorrectforthedeficienciesnotedabove.ReferringtoFig.2,thetechniqueofaveragecurrentmodecontrolovercomestheseprob-lemsbyintroducingahighgainintegratingcurrenterroramplifier(CA)intothecurrentloop.AvoltageacrossRp(setbytheouterloop)representsthedesiredcurrentprogramlevel.ThevoltageacrosscurrentsenseresistorRsrepresentsactualinductorcurrent.Thedifference,orcurrenterror,isamplifiedan。