Power-MOSFET-losses-in-DC-DC-buck-converter--ver2

PowerMOSFETlossesinDC-DCBuckconverterVer:2AssistantProductMarketingManagerRegionalSalesOrganization/AsiaPacificAug./15”03StanleyYCLiuStanleyYCLiuJan/9”072BasicVRMFormulas•Theseformulasarebasicguidetopowerloss–Willnotbeaccurateenoughtopredictactualloss–Willmissthesubtledifferencesindevicesthatcangivelargevariationsinpowerloss–ConvenienttoanalysepowerlossesandtohaveacommentsenseaboutpowerMOSFET.•Mainerrors–Noeffectoftemperatureonrds(on)•errorinconductionloss–Novoltagedependencyoncapacitances•errorinQgdloss,especiallywithhighVin–Noeffectofinductancesongatedrive•errorinQgs2loss(especiallyforDpak&D2Pak)–NoeffectofSyncFETQrronControlFET•errorinControlFETturnonlossQG(total)(nC)VGS(V)510152025303540455012345678910QGIDVDSQGDQGS2VGS(th)t1t2t3VGS(plateau)GateChargeDiagramandCrossoverThispageprovidedbyJamesWang/SAC34PowerLossFormulas-ControlFETDutyCycleVoutVinEfficiency´IgonVDrive_ContVplateau-()RgdriverRgControlFET+IgoffVplateauRgdriverRgControlFET+ControlFETConductionIout()2Rdson()´DutyCycle´ControlFETTurn_OfffswVinVdiode+()Iout´2´Qgs2Qgd+Igoff÷÷´ControlFETCossfswVinVdiode+()2´CossSync2´CossCont2+ControlFETTurn_OnfswVinVdiode+()Iout´2´Qgs2Qgd+Igon÷÷´)(BLPower-PowerManagementPillRutter23/06/20035ControlFETdutycycleDutyCycleVoutVinEfficiency´InenergypointofviewSTonSTonDKHzFreqestimateVVoVVinAssumeDTTonVinVoTVoTonVin625.0%5.12125.058.0122.1200.),(8.0%,2.1,12:%%APRSO–StanleyYCLiuAug./14”036IgonVDrive_ContVplateau-RgdriverRgControlFET+•Oneendoftheresistoristiedtothegatedriver,whichbythistimehasreachedVdrive.•TheotherendoftheresistoristheinstantaneousVgsinsidetheMOSFET:1.DuringtheQgs2period:VgsisrampingfromVgs(th)toVgs(plateau)2.DuringtheQgdperiod:ItisconstantatVgs(plateau).3.BytheendofQgd:a.Ig(on)valuewillindeeddecreasebecauseVgsisrisingtowardsVdrive.b.Thisperiodisnotpartoftheswitchinglosscalculationperiod.c.VdshasfallentothesteadystatevalueVds(on)[=Rdson*Iout],andofcoursed.thecurrenthasbeenatIoutforsometimealready.e.AfterQgdperiodhasfinished,switchinglosseshavealsofinished.Ig(on)canbecalculatedasthepotentialdroponthetotalgateresistanceRgdividedbyresistor.ControlFETaveragegatedrivecurrent(turnon)-1BLPower-PowerManagementDr.EdHuang13/08/20037IgonVDrive_ContVplateau-RgdriverRgControlFET+•ItisthereforeassumeVgs(plateau)forthewholeoftheswitchinglossesperiodisstrictlynottotallyaccurate.•Butgiventheapproximatenatureofthewholecalculations,thelinearrampalreadyintroducesmuchbiggerinaccuracies,andgiventhattheQgs2periodisusuallyshorterthantheQgdperiod.•EvenduringtheQgs2period,thevalueofVgsissomewhereinbetweenVgs(th)andVgs(plateau),itisareasonablyaccurateapproximationtosimplifythingsandsayVgsisatVplateauthroughoutthewholeswitchinglossperiod.Ig(on)canbecalculatedasthepotentialdroponthetotalgateresistanceRgdividedbyresistor.ControlFETaveragegatedrivecurrent(turnon)-2BLPower-PowerManagementDr.EdHuang13/08/20038ControlFETaveragegatedrivecurrent(turnoff)IgoffVplateauRgdriverRgControlFET+•Ig(on)andIg(off)comeintotheformulaincalculatingthetime.•Ifweknowthegatecurrent,thenthetimeissimplytherelevantQg(whetherQgs2orQgd)dividedbyIg.Duringturn-off:•Thesamereasoningapplies,butnowoneendoftheRgresistoristiedtoground.•Thegatedrivershouldhavegoneto0Vbythistime,whiletheotherendisatVplateau.Therefore:Ig(off)=(Vplateau-0V)/totalRg.Ig(off)canbecalculatedasthepotentialdroponthetotalgateresistanceRgdividedbyresistor.BLPower-PowerManagementDr.EdHuang13/08/20039ControlFETturnon/offswitchinglossControlFETTurn_OnfswVinVdiode+()Iout´2´Qgs2Qgd+Igon÷÷´•DuringtheQgs2period:VoltageonthecontrolFETisconstantathigh,andthecurrentrampslinearlyfromzerotoIout.•DuringtheQgdperiod:CurrentisconstantatIout,andvoltagerampslinearlyfromhightozero.•Theassumptionoflinearrampinbothcasesisonlyaveryroughapproximation,especiallyFortheQgdperiod.Inbothperiodswehaveatriangleofpowerloss,theaverageofwhichisIpeak´Vpeak/2.Henceintheformulawehavethismultipliedbythetimetogettheactualpowerloss.ControlFETTurn_OfffswVinVdiode+()Iout´2´Qgs2Qgd+Igoff÷÷´•Turnoffswitchinglossbasicallyhasthesameassumptionasturnonswitchingloss.BLPower-PowerManagementDr.EdHuang13/08/200310ControlFETconductionlossControlFETConductionIout()2Rdson()´DutyCycle´•Qgtotalisthetotalgatechargeweneedtoputinviathegateterminaltotakethedevicefromtheoff-state(sayVds=12V,ID=0,Vgs=0)toafullyturnedonstate(sayID=20A,Vgs=5Vor10V,Vdspositiveandverylow).•Thisissimilartothetransitionofthehighsidedevicefromitsoffstatetoitson-stateinaVRMcircuit,soQgtotal´Vgsisagoodestimateofthedriverlosses.BLPower-PowerManagementDr.EdHuang13/08/200311ControlFETCosslossControlFETCossfswVinVdiode+()2´CossSync2´CossCont2+)(•WhenH/Sturnsonturnsoff,L/Sturnsoff:ControlFET:Vds=0V12V,Vgs=12V0V,Vgd=12V-12VSyncFET:Vds=-0.7V(=Vdiode),Vgs=0V,Vgd=0.7V•WhenH/Sturnoff,L/Sturnsoffturnson:ControlFET:Vds=12V,Vgs=0V,Vgd=-12VSyncFET:Vds=-0.7V0V,Vgs=0V12V,Vgd=0.7V12V•WhenH/Sturnsoff,L/Strunsonturnsoff:ControlFET:Vds=12V,Vgs=0V,Vgd=-12VSyncFET:Vds0V-0.7V,Vgs=12V0V,Vgd=12V0.7V•WhenH/Sturnsofftruns