LLC半桥谐振资料AN2450

October2007Rev51/32AN2450ApplicationnoteLLCresonanthalf-bridgeconverterdesignguidelineIntroductionThegrowingpopularityoftheLLCresonantconverterinitshalf-bridgeimplementation(seeFigure1)isduetoitshighefficiency,lowlevelofEMIemissions,anditsabilitytoachievehighpowerdensity.SuchfeaturesperfectlyfitthepowersupplydemandofmanymodernapplicationssuchasLCDandPDPTVor80+initiativecompliantATXsilverbox.Oneofthemajordifficultiesthatengineersarefacingwiththistopologyisthelackofinformationconcerningthewaytheconverteroperatesand,therefore,thewaytodesignitinordertooptimizeitsfeatures.Thepurposeofthisapplicationnoteistoprovideadetailedquantitativeanalysisofthesteady-stateoperationofthetopologythatcanbeeasilytranslatedintoadesignprocedure.ExactanalysisofLLCresonantconverters(see[1.])leadstoacomplexmodelthatcannotbeeasilyusedtoderiveahandydesignprocedure.R.Steigerwald(see[2])hasdescribedasimplifiedmethod,applicabletoanyresonanttopology,basedontheassumptionthatinput-to-outputpowertransferisessentiallyduetothefundamentalFourierseriescomponentsofcurrentsandvoltages.Thisiswhatiscommonlyknownasthefirstharmonicapproximation(FHA)technique,whichenablestheanalysisofresonantconvertersbymeansofclassicalcomplexac-circuitanalysis.Thisistheapproachthathasbeenusedinthispaper.ThesamemethodologyhasbeenusedbyDuerbaum(see[3])whohashighlightedthepeculiaritiesofthistopologystemmingfromitsmulti-resonantnature.Althoughitprovidesananalysisusefultosetupadesignprocedure,thequantitativeaspectisnotfullycompletesincesomepracticaldesignconstraints,especiallythoserelatedtosoft-switching,arenotaddressed.In(see[4])adesignprocedurethatoptimizestransformer'ssizeisgivenbut,again,manyothersignificantaspectsofthedesignarenotconsidered.Theapplicationnotestartswithabriefsummaryofthefirstharmonicapproximationapproach,givingitslimitationsandhighlightingtheaspectsitcannotpredict.Then,theLLCresonantconverterischaracterizedasatwo-portelement,consideringtheinputimpedance,andtheforwardtransfercharacteristic.TheanalysisoftheinputimpedanceisusefultodetermineanecessaryconditionforPowerMOSFETs'ZVStooccurandallowsthedesignertopredicthowconversionefficiencybehaveswhentheloadchangesfromthemaximumtotheminimumvalue.Theforwardtransfercharacteristic(seeFigure3)isofgreatimportancetodeterminetheinput-to-outputvoltageconversionratioandprovidesconsiderableinsightintotheconverter'soperationovertheentirerangeofinputvoltageandoutputload.Inparticular,itprovidesasimplegraphicalmeanstofindtheconditionfortheconvertertoregulatetheoutputvoltagedowntozeroload,whichisoneofthemainbenefitsofthetopologyascomparedtothetraditionalseriesresonantconverter.(all-primary-referred)model.................................19Figure11.Transformerconstruction:E-coresandslottedbobbin............................20Figure12.LLCresonanthalf-bridgeconverterelectricalschematic..........................25Figure13.Circuitefficiencyversusoutputpoweratvariousinputvoltages.....................27Figure14.Resonantcircuitprimarysidewaveformsatnominaldcinputvoltageandfullload......28Figure15.Resonantcircuitprimarysidewaveformsatnominaldcinputvoltageandlightload.....28Figure16.Resonantcircuitprimarysidewaveformsatnominaldcinputvoltageandno-load......29Figure17.Resonantcircuitprimarysidewaveformsatnominaldcinputvoltageandlightload.....29Figure18.+200Voutputdiodevoltageandcurrentwaveforms.............................30Figure19.+75Voutputdiodevoltageandcurrentwaveforms.................