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1、DesigningwiththeTL431byDr.RayRidleyDesignerSeriesXVCurrent-modecontrolisthebestwaytocontrolconverters,andisusedbymostpowersupplydesigners.Forthistypeofcontrol,theoptimalcompensationnetworkisaTypeIIampli-fier,anexampleofwhichisshowninFigure1.Inthisconfig-uration,aconventionaloperationala
2、mplifierisusedtoampli-fythedifferencebetweentheoutputvoltageofthepowersupplyandafixedreferencevoltage.Powersuppliesareconstantlystrivingtoreducecost.Pricesarenowbeingdrivendowntoinsanelylowlevels,andcom-promisesmustoftenbemadetomeetperformanceandcostgoals.Oneofthetargetsforcostreduction
3、isalwaysthecontrolloop.Designersareoftensofocusedonpowerprocessingthatcontroldesignisoftenanafterthought,andnotviewedascrucialtoperformance.Onlylaterinthedesignprocessdoestheimportanceofagoodcontrollerbecomeapparent.OneoftheearlywaystoreducethecostofthefeedbackloopwastoutilizetheTL431co
4、ntroller.Whilenotofferingasgoodagain-bandwidthasastandaloneamplifier,thisthree-terminalpartincludesareference,takesuplittleboardspace,andbecamewidespreadintheindustryasawaytoachievereasonableperformanceatareducedcost.Figure1a:TypeIICompensationFeedbackInthisdesignerseriesarticle,we'lllo
5、okatthecomplicationsinvolvedinusingtheTL431,especiallywhenitisconfiguredTheamplifierissuppliedbyaseparateVcc,andtheoperationwithanoptocouplertoprovideisolationinthefeedbackloop.isnotaffectedbyvariationsinthesupplyvoltageduetogoodpowersupplyrejectionratio.1.OperationalAmplifierFeedbackFi
6、gure1bshowsthetypicalcompensationcurveforaTypeIIForthebestperformance,thepreferredapproachtofeedbackamplifier.Atlowfrequencies,thecircuitactslikeanintegra-controlcompensationisanerroramplifierandaprecisiontor,utilizingcomponentsC1andR1toprovidehighgain.reference.Fornonisolatedconverters
7、,theamplifierandrefer-ResistorRbprovidesthecorrectdcregulationlevel,butduetoencemaybeincludedinthePWMcontrolchip,andtheyarethevirtualgroundattheinputoftheerroramplifier,itdoesusuallyofsufficientqualitytomeetdemandingstandards.notappearinanyofthegainequations.©Copyright2005Switc
