Power Loss Calculation With CSI Consideration for ...

1 ApplicationReportSLPA009A June2011 RevisedJuly2011 PowerLossCalculationWithCommonSourceIndu ctanceConsiderationforSynchronousBuckCon vertersDavidJauregui,BoWang, PowerStageABSTRACTT hesynchronousbuckconverterisa widelyusedtopologyinlow-voltage, (CSI)is theinductancesharedbythemaincurrentpatha ndthegatedriverloopina (HS)FETCSI hassignificantimpactonconverterperforman ce,especiallyswitchingpowerloss:thegreat ertheHSCSI, PowerBlock, ,a complementarydesigncalculator(SLPC015) is a a June2011 RevisedJuly2011 PowerLossCalculationWithCommonSourceIndu ctanceConsiderationforSynchronousBuckCon vertersSubmitDocumentationFeedbackCopyri ght 2011,TexasInstrumentsIncorporated 2co nd(HS)ds(ON)HSRMS(HS)P= RI 2cond(L S)ds(ON)LSRMS(L S)P= RI = +++ -+- 22ripp leripplerippleripp leRMS(HS )

2 Oneofthemostpopulartopologiesfortoday s a singleprocessorcontinuetoincrease,variou schallengesariseforlow-voltage, synchronousbuckconvertersis synchronousbuckconverterincludesseveralp arts:MOSFET loss ,inductorloss,printed-circuitboard(P CB) loss , ,MOSFET lossis showstheschematicofa asynchronousbuckconverteris thesumofhighside(HS)andlowside(LS) ,LSFET switchingloss,gatedriveloss,LSbodydiodel oss, ,MOSFET switchinglossis determinedbythetransitiontimeandinfluenc edbyseveralparametersin (CSI)is ,CSIhasa significantimpactonthesystemperformance, a discussedandquantifiedin ,theconductionlossforHSFET andtheLSFET canbecalculatedbyEquation1 andEquation2, respectively.

3 (1)(2)(3)2 PowerLossCalculationWithCommonSourceIndu ctanceConsiderationforSLPA009A June2011 RevisedJuly2011 SynchronousBuckConvertersSubmitDocumenta tionFeedbackCopyright 2011,TexasInstrumentsIncorporated - = +++ -+- 22ripplerippleripplerippleRMS(LS)OUTOUTO UTOUTIIII1 DIIIII32222-DriverPLggdriverVVI =R + R swswOUTswgQP= V (4)NOTE:SeeAppendixB composedofseveralparts:MOSFET switchingloss(HSandLS),MOSFET gatedriveloss,LSbody-diodeloss, ,duetotheLSclampingeffects,theHSdeviceis affectedbybothhighcurrentandhighvoltagea tthesametime, , drain-sourcevoltageVDSanddraincurrentIDS waveformsduringHSturnonforthesynchronous buckconvertershownin ,theHSgatecurrentandtherelatedswitchingl ossareestimatedbyEquation5 andEquation6:(5)(6)NOTE.

4 FETplateauvoltageVPLcanbefoundin , , June2011 RevisedJuly2011 PowerLossCalculationWithCommonSourceIndu ctanceConsiderationforSynchronousBuckCon vertersSubmitDocumentationFeedbackCopyri ght 2011,TexasInstrumentsIncorporated-- DSgs(eff)gswcsidIV= VVLdt- rippleIOUTDSLcsi(t1)csicsiIdI2V= L= Ldt1dt1--DriverPL(HS)Lcsi(t1)g1(on)g(HS) driverVVVI=R+ Rgs2(HS)g1(on)Qdt1 =IrippleDriver PL(HS)g1(on)IOUTg(HS) driver csigs2(HS)VVI=I2R + R + LQ-- rippleIgs2(HS)sw(t1)OUTswg1(on)QP= VIN If2I - Figure3 asanexample,whentheHSFET drain-currentIDSbeginstoincreaseatturnon ,a voltagedropproportionaltodIDS/dtis ,theeffectivegate-sourcevoltageis reducedbythisCSIvoltageasshownin Equation7:(7)ToaccountfortheeffectofCSIi n powerloss,Equation5 andEquation6 periodt1 First,analyzeareaQGS2in Figure2.

5 Whenthegate-sourcevoltagereachesthethres holdvoltageVTH,thedevicedraincurrentIDSb eginstoincreasefrom0 A untilit reaches(IOUT Iripple/2)attheendoft1,whereIrippleis voltageVLcsi(t1)onHScommonsourceinductan ceLcsi. VLcsi(t1)andthegatecurrentIg1(on)duringt imet1thencanbecalculatedbyEquation8 andEquation9:(8)(9)(10)SolvingEquation8, Equation9, andEquation10, thegatecurrentis obtainedandshowninEquation11:(11)Therela tedswitchingpowerlossis thencalculatedasfollows,(12) periodt2 Next,whenenteringareaQGD, theLSFET outputcapacitanceCossis chargedfrom0 V toVIN, andthechargecurrentflowsthroughLcsi. Similarly,thiscurrentdecreasestheeffecti vegate-sourcevoltageof4 PowerLossCalculationWithCommonSourceIndu ctanceConsiderationforSLPA009A June2011 RevisedJuly2011 SynchronousBuckConvertersSubmitDocumenta tionFeedbackCopyright 2011,TexasInstrumentsIncorporated oss(LS )Cossoss(L S)oss(LS)QdVVINI= C= C=dt2dt2dt2gd(HS)g2(on )Qdt2 =I()2csioss(LS)g2(on)csioss(LS)CossLcsi( t2)csi22gd(HS)LQILQdIV= L=dt2 Qdt2 =()( )()csioss(LS )2g 2(on)g(HS)d riverDriverP L(HS)g2 o n2g d(HS)LQI+ R+ RIVV= 0Q --DriverPL(HS)Lcsi(t2)g2 (on)g(HS)driverVVVI=R+ R-- csioss(LS )2gd(HS)LQa =Q( )drive rg HSb = R+ R()--DriverPL (HS )

6 C =VV-- 2g2(on )b + b4acI=2arippleIg d(HS)sw(t2)OUTswg2(on )QP= V INIf2I - rippleIgs2(HS)gd(HS)sw(HSon)OUTswg1(on)g 2(on)QQP= VIN If2II - + (on)byinducinga voltageVLcsi(t2) ,HSCSI voltage,andHSgatecurrentcanbeobtainedbyt hefollowingequations:(13)(14)(15)(16)Reo rganizingEquation14, Equation15, andEquation16, thefollowingEquation17is obtained:(17)Definingthreeparametersasfo llows,(18)(19)(20)Then,thegatecurrentwit hint2is givenbyEquation21:(21)WithIg2(on)solved, theswitchingpowerlossduringtimet2is shownin Equation22:(22) thesumoflossesin areaQgs2(t1)andareaQgd(t2),givenby:(23) ,theinductorcurrentreachesthepeakvalue(I OUT+ Iripple/2).

7 Then, showstheHSFETVGS, VDS, June2011 RevisedJuly2011 PowerLossCalculationWithCommonSourceIndu ctanceConsiderationforSynchronousBuckCon vertersSubmitDocumentationFeedbackCopyri ght 2011,TexasInstrumentsIncorporatedrippleP L(HS)g1(off)IOUTg(HS)drivercsigs2(HS)VI= I2R + R+LQ + -- 2g2(off)b + b4acI=2a csioss(LS )2gd(HS)LQa =Qg(HS)driv erb = R+ R-PL(HS)c =VrippleIgs2(HS)gd(HS)sw(HSoff)OUTswg1(o ff)g2(off)QQP= VIN If2II + + sw(HS)sw(HSon)sw(HSoff)P= P+ to3,theHSgatecurrentandswitchinglossattu rnoffcanbeobtainedin thefollowingequations:(24)(25)(26)(27)(2 8)(29) thesumofHSturnonandturnofflossesasshowni n Equation30.

8 (30) , ,bothLSturnonandturnoffaresoftswitchinga tnormaloperationswhentheinductorcurrenti s ,theLSswitchinglossis smallandthusneglectedin June2011 RevisedJuly2011 SynchronousBuckConvertersSubmitDocumenta tionFeedbackCopyright 2011,TexasInstrumentsIncorporated()g ateg ate (HS)gate(LS )g(HS)g(LS)DriverswP= P+ PQ+ QVf= - rip plerip pled eadtim eSDOU Tdea dtim e (r)OUTdeadtim e(f)swIIP= VIt+I+tf22 DRRrr(L S)swP= (31) (shoot-through),twoshortdead- ,boththeHSandtheLSFET sareoff, :dead-timeloss(diodeconductionloss) inducedbyLSbodydiodeconductionduringdead -timesandcanbecalculatedusingEquation32.

9 (32) ,whentheLSFET bodydiodeconductstheinductorcurrentdurin gtheriseedgedeadtime,minoritycarriersare injectedintoandstoredin ,a Figure5, duringthistime, , thisperiodis ,thediodereverse-recoverylossatHSturnonc anbecalculatedasfollows:(33)7 SLPA009A June2011 RevisedJuly2011 PowerLossCalculationWithCommonSourceIndu ctanceConsiderationforSynchronousBuckCon vertersSubmitDocumentationFeedbackCopyri ght 2011, = = synchronousbuckconvertersis FEToutputcapacitanceloss,whichis Equation34andEquation35, respectively.(34)(35)4 PowerLossCalculationAssumptionNotethatth eprecedingequationsareonlyaccurateforcon tinuousconductionmode(CCM)atwhichtheoutp utcurrentis greaterthanorequaltohalfoftheinductorpea k-to-peakcurrentripple(Iripple).

10 Inthisapplicationreport, Irippleis theoutputcurrentis smallerthanthecriticalcurrent,theconvert ermayworkattwodifferentmodes:forcedconti nuousconductionmode(FCCM)ordiscontinuous conductionmode(DCM).Foreachofthetwomodes ,someofthepowerlosscalculationsintroduce din ,whentheconverterworksin DCM,theinductorcurrentincreasesfrom0 A to(IOUT+ Iripple/2)atHSFETon-time,thendecreasesba ckto0 A ,theinductorcurrentremainsat0 A fora ,theRMScurrentcalculationsin Equation3 andEquation4 :whentheconverterworksin FCCM, ,theHSFET turnonis speededupinsteadofbeingsloweddownandtheH Sturnonswitchinglossthenis ,onlytheMOSFET spowerlossatoutputcurrentsabovethecritic alcurrentis ,twoexamplesaregivenin thisapplicationreport:Converter1 is a conventionalsynchronousbuckconverterwith twodiscreteMOSFETsasHSandLSswitches.