Turn Off Snubber Design for High Frequency Modules.pdf

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1、ApplicationNoteAPT0404December2004Turn-OffSnubberDesignforHighFrequencyModulesSergeBontempsR&DManagerAdvancedPowerTechnologyEurope,ChemindeMagret,33700,Merignac,FranceTurn-offsnubbersarepassivecircuitsmadeofEnergyinthetransistorisgivenbythediodes,res

2、istorsandcapacitorsdedicatedtorelationship:storingenergyforashortperiodto:-Decreaseswitchpowerlossesduringturn-tfVM⋅IM⋅tfoffEoff=∫V⋅i⋅dt=02-ModifyswitchingI-VdiagramtofitsafelywithintheswitchingsafeoperatingareaInpractice,thevoltageisrampingupwithati

3、meequaltotrv.Hard-switchedturn-offswitching1-Hard-SwitchedInductiveLoadTurn-Offenergyistheareaoftheproductofswitchvoltageandcurrent.SincetheoverlapinvoltageLAssumingthattheloadtimeconstantismuchandcurrentisshaped,likeatriangle,theareaisronehalfthebas

4、e(trv+tf)timesheight(buslongerthantheswitchingtime,currentinthevoltagetimesloadcurrent).(Seefig.2).loadcanbeconsideredconstantduringturn-off.Thesumofcurrentsinthediodeandinthettrv+fVIttMMr⋅⋅+()vftransistorremainsconstant.ForwardvoltageofEvoff=⋅∫MiM⋅d

5、t=02thefreewheelingdiodebeingnegligible,allsupplyvoltageisappliedtothetransistorduringturn-off.(Seefig.1.)iIMiIMtftvtftVMvVMtrvtFig.2ExperimentalturnoffbehaviourwithaninductiveloadtFig.1Switchturn-offbehaviourwithan2-Turn-OffSnubberTheoryofOperationi

6、nductiveloadFig.3describestheelectricaldiagramofaturn-offsnubber.CapacitorCisthekeyelementofwww.advancedpower.comAPTwebsite–http://www.advancedpower.com1/5ApplicationNoteAPT0404December2004thecircuit.ThediodeandresistorareonlyusedAttheendoftheinterva

7、ltftot2,thecapacitorCtopreventinstantaneousdischargeoftheenergyischargeduptothevoltageVM.Itisonlyduringstoredinthecapacitorwhiletheswitchturnson.thelastphase(t>t2)thatcurrentswitchesfromCtothefreewheelingdiode.LDuringturn-off,snubberdiodeDconducts,an

8、dthevoltageacrossthecapacitorequalsthevoltageacrosstheswitch(ignoringthesmallvoltagedropacrossthesnubberdiode).Duringrtheintervalt0totfthecapacitorcurrentisCItItMM⋅⋅iC==.tttf0−fThecapacitorandswitchvoltageisRD21IIMM⋅tvtC=⋅∫⋅dt+=+VCzeroVCzeroCtff2Ct⋅⋅