Application Report SLVA February Calculating Motor Driver Power Dissipation Peter Millett - PDF document

ApplicationReport SLVA504–February2012 CalculatingMotorDriverPowerDissipation PeterMillett.................................................................................................MotorDriveBusinessUnit ABSTRACT WhenselectingamotordriverICforaparticularapplication,considerthemaximumamountofcurrentthat mustbedriven.Thethermalcharacteristicsofthe currentagivenmotordriverprovides.Tocalculatethemaximumallowablecurrentinagivenapplication, anestimationofthetotalmotordriverpowerdissipationisneeded.Thisapplicationreportshowshowto estimatethispowerdissipationinDCbrushmotorandsteppermotorapplications. 1SourcesofPowerDissipationinaMotorDriver ThereareanumberofsourcesofpowerdissipationinsideamotordriverIC.Someareobvious,likethe powerdissipatedintheFETON-resistance,othersaremoresubtle.Tomakeanaccurateassessmentof thetotalpowerdissipation,allsourcesmustbeconsidered. 1.1RDS(ON)Dissipation ThebiggestsourceofpowerdissipatedinsideamotordriverICisthepowerdissipated resistance,orRDS(ON).ThepowerdissipatedinanH-bridge(consistingofahigh-sideFETandalow-side FET)iscalculatedwiththefollowing: PRDS=(HS-RDS(ON)×(IOUT(RMS))2)+(LS-RDS(ON)×(IOUT(RMS))2) wherePRDSisthepowerdissipatedintheoutput LS-RDS(ON)istheresistanceofthelowsideFET,andIOUT(RMS)istheRMSoutputcurrentbeingappliedto themotor. NotethatRDS(ON)increaseswithtemperature,asthedeviceheats,thepowerdissipationincreases.This mustbeconsideredwhencalculatingthetotaldevicepowerdissipation. If eachH-bridge. 1.2SwitchingLosses Whenanoutputtransitionsfromhightoloworlowtohigh,theoutputdevicestraversealinearregion wheretheyaredissipatingsignificantlymorepowerthanwhenfullyturnedon.Thispowerdissipationis referredtoasswitchingloss. •riseandfalltimesoftheoutput,howquicklytheoutputswingsfromoneextremetotheother •supplyvoltage •outputcurrent •frequencythattheoutputisswitching Insomecases,suchasaDCmotordriverthatisonlyturnedonoroff(notsubjectedtoPWMspeed controlorcurrent asteppermotordriverusingcurrentcontrol,theoutputsarealwaysbeingswitchedatsomePWM frequencytomaintaincurrentregulation. 1 SLVA504–February2012 CalculatingMotorDriverPowerDissipation SubmitDocumentationFeedback Copyright©2012,TexasInstrumentsIncorporated SourcesofPowerDissipationinaMotorDriverwww.ti.com Asanapproximation,switchinglossesforeachoutputarecalculatedasfollows: PSW=PSW_RISE+PSW_FALL wherePSWisthetotalswitchingloss(inwatts)foroneoutput,andPSW_RISEisthepowerdissipatedduring therisingedge,andPSW_FALLisdissipatedduringthefallingedge. Expandingonthis: PSW_RISE=½×VM×IOUT×tR×fSW and PSW_FALL=½×VM×IOUT×tF×fSW whereVMisthesupplyvoltage(involts),IOUTistheoutputcurrent(inamps),tRistherisetime(in seconds),tFisthefalltime(inseconds),andfSWistheswitchingfrequency(inHz). ThevaluesoftRandtFaregenerallylistedinthemotordriverdatasheet.fSWmaybetheinternalPWM frequencyofthemotordriver,ifthemotordriverisperformingcurrentregulation(aswhendrivinga steppermotor),ormaybeanexternallyappliedPWMfrequency(aswhendoingDCmotorspeedcontrol fromaprocessor). InanH-bridge,itispossiblethatonesideorbothsidesoftheH-bridgearebeingswitched.Forexample, withaDCmotordriverorasteppermotorusingslowdecaymode,onlyonesideoftheH-bridgeisbeing pulse-widthmodulated,whiletheothersideremainsatafixedstate.Asteppermotordriverrunninginfast decay,however,reversesstateateachPWMcycle.BothsidesoftheH-bridgeswitch,causingtwiceas muchswitchingloss. 1.3OperatingSupplyCurrentDissipation SomecurrentisconsumedbythemotordriverIC.OnTImotordriverdatasheets,thisisusuallycalledVM operatingsupplycurrent. Powerdissipatedbytheoperatingsupplycurrentiscalculatedusing: PIVM=VM×IVM 1.4OtherPowerDissipation Motordriversmaycontainothercircuitsthatdissipatepower.ManyTImotordriverscontainanLDO regulatorthatisusedtoprovidesomereferencecurrent,orcurrenttopowerexternalloads.Youmust accountforthispowerdissipationaswell. ForanLDO,powerdissipationiscalculatedasfollows: PLDO=ILDO_OUT×(VM–VOUT) 2 CalculatingMotorDriverPowerDissipation SLVA504–February2012 SubmitDocumentationFeedback Copyright©2012,TexasInstrumentsIncorporated www.ti.comExampleCalculation 2ExampleCalculation Section2.2showsanexamplepowerdissipationcalculationforaDRV8825motorcontrollerdrivinga bipolarsteppermotoroperatedwithcurrentregulation.Similarcalculationsaremadeusingotherstepper andbrushDCmotordrivers. 2.1ConditionsandAssumptions Whendrivingasteppermotorusingcurrentcontrol,twoH-bridgesarealwaysconductingcurrent,andare alwaysbeingswitched(toregulatecurrentusingPWM).Inthisexample,usingaDRV8825steppermotor controller,weusethefollowingoperatingparameters: •VM(supplyvoltage)=24V •IOUT=1.5-ARMSperphase •Onlyslowdecayisused(soonlyonesideofeachH-bridgeisswitching) •2-mAcurrentisdrawnfromtheLDOregulator(V3P3OUT) Forestimationpurposes,usethetypical85°CRDS(ON)number,andtheworst-case(maximum)riseandfall timesfromthedatasheet.Otherdatasheetparametersusedarethetypicalvalues. 2.2PowerCalculation Thetotalpowerdissipationiscalculatedasfollows: PTOT=(2×PRDS)+(2×PSW)+PIVM+PLDO Thefactorof2onthePRDSandPSWtermscomesfromthefactthattherearetwoH-bridgesinthe DRV8825,eachdrivingonephaseofthesteppermotor,andonesideofeachH-bridgeisbeingpulse- widthmodulatedtoregulatewindingcurrent. Calculatingthecomponentsstep-by-step,usingdatafromtheDRV8825datasheet: PRDS=(HS-RDS(ON)×(IOUT(RMS))2)+(LS-RDS(ON)×(IOUT(RMS))2) PRDS=(0.25\r×(1.5A)2)+(0.25\r×(1.5A)2) PRDS=1.125W PSW=PSW_RISE+PSW_FALL PSW=(½×VM×IOUT×tR×fSW)+(½×VM×IOUT×tF×fSW) PSW=(½×24V×1.5A×200ns×30kHz)+(½×24V×1.5A×200ns×30kHz) PSW=0.216W PIVM=VM×IVM=24V×5mA PLDO=ILDO_OUT×(VM–VOUT)=2mA×(24V–3.3V)=0.04W Finally, PTOT=(2×PRDS)+(2×PSW)+PIVM+PLDO PTOT=2×1.125W+2×0.216W+0.12W+0.04W=2.84W ThisistheapproximateamountofpowerdissipatedintheIC. 3 SLVA504–February2012 CalculatingMotorDriverPowerDissipation SubmitDocumentationFeedback Copyright©2012,TexasInstrumentsIncorporated Referenceswww.ti.com 2.3DieTemperatureEstimation Onceyouknowthepowerdissipatedinthedriver,youcanestimatethedietemperature.Calculatethedie temperatureusingthethermalresistanceJAandtheambienttemperatureasfollows: TDIE=TAMB+(JA×PTOT) Inthisexample,iftheDRV8825weremountedonaJEDEC-standardPCB,theJAvalueis31.6°C/W.The resultingdietemperatureata25°Cambienttemperatureis: TDIE=TAMB+(JA×PTOT) TDIE=25°C+(31.6°C/W×2.86W)=115.4°C Thistemperatureiswellbelowtheovertemperatureshutdowntemperatureof150°C. However,anambienttemperatureof70°Cproducesthefollowing: =70°C+(31.6°C/W×2.8614W) =160.4°C Atanambienttemperatureof70°C,thedevicewouldenterovertemperatureshutdown. NOTE:WhenmountedontheapplicationPCB,theactualJAvalueisprobablydifferentthantheJA valuelistedonthedatasheetforaJEDEC-standardboard.Pleasereferto www.ti.com/thermalformoreinformationonhowtodeterminetheJAvalueonyourPCB. 3References 1.UnderstandingMotorDriverCurrentRatings(SLVA505) 2.www.ti.com/thermal 4 CalculatingMotorDriverPowerDissipation SLVA504–February2012 SubmitDocumentationFeedback Copyright©2012,TexasInstrumentsIncorporated IMPORTANTNOTICE TexasInstrumentsIncorporatedanditssubsidiaries(TI)reservetherighttomakecorrections,modifications,enhancements,improvements, andotherchangestoitsproductsandservicesatanytimeandtodiscontinueanyproductorservicewithoutnotice.Customersshould obtainthelatestrelevantinformationbeforeplacingordersandshouldverifythatsuchinformationiscurrentandcomplete.Allproductsare soldsubjecttoTI¶stermsandconditionsofsalesuppliedatthetimeoforderacknowledgment. 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