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INTRODUCTIONThe SA306-IHZ is a fully-integrated switching amplifier designed primarily to drive three-phase brushless DC(BLDC)motors.Threeindependenthalfbridges,eachcomprisingaP-FETandaN-FETinaconfiguration,providemorethan15AofPEAKoutputcurrentunderdigitalcontrol.Thermalandshortcircuitmonitoringisprovided,whichgeneratesfaultsignalsforthemicrocontrollertotakeappropriateaction.AblockdiagramofthisICisprovidedinFigure1.
Figure 1. Polarity is Easily Switched–DC-to-DCconverter,threeterminalmodulecanbeswitchedfromaposi-tivetoanegativeconverterbysimplyinterchangingthejumpersidentifiedbyNote2.
DRIVING BRUSHLESS MOTORSBrushlessmotorsofthesamehorsepowerastheirbrushcounterpartsaresmallerandlighter.Whatisabsentintheformeristhefamiliarbrush–commutatorarrangementthathasbeenattheheartofsingle-phaseDCbrushmotorsformorethanacentury.Becausetheylackthisbrush-commutatorinterface,brushlessmotorsexhibitloweracous-ticnoise;arevirtuallymaintenancefree;andabrushlessmotorwillexhibitalongerlifecycle.Asrecentlyas2004,brushlessmotorswereconsideredtobesignificantlymoreexpensivethanbrushmotors.Atthetimeofthiswritingin2008,brushlessmotorshavebenefitedfromadecreaseincostsothattodaythepricedifferentialisaslittleasa10%whenshoppedagainstanequivalentbrushmotor.
CYCLE-BY-CYCLE CURRENT LIMIT – THE BENEFITSTraditionally,inapplicationswherethecurrentflowtoabrushlessmotorisnototherwisedirectlycontrolled,thein-rushcurrenthadtobeconsideredwhenselectingaproperdriveramplifier.Thisisinadditiontotheaveragecurrentthatwillflow.Asanexample,a1Acontinuousmotormightrequireadrivethatcandeliverwellover10APEAKinordertodelivertheinitialinrushcurrentthatflowsduringstartup.Manydiscretemotordrivesuseover-sizedFETstowithstandstartupconditionswhichresultsinhighersystemcostsandlargerpackagesizes.Howeverwiththeuniqueandrobustcycle-by-cyclecurrentlimitschemedesignedintotheSA306-IHZ,theinrushcurrentrequirementsofthemotorarenolongeranissuewhenselectingthedrive.Currentlimitschemesinherentlyreduceaccelerationofthemotor;however,theaveragecurrentdeliveredbytheSA306-IHZduringstartupishigher
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Copyright © Apex Microtechnology, Inc. 2012(All Rights Reserved)www.apexanalog.com OCT 2012
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thanwouldbedeliveredbyothercurrentlimitschemes.ByusingtheSA306-IHZ,themotorwillreachitsoperatingRPMfaster.ThustheSA306-IHZisabletosafelyandeasilydrivevirtuallyanybrushlessmotorwhichrequires5Acontinuouslyorless,throughitsstartupinterval—withoutregardtowhatitsin-rushrequirementsare.(Upto8AcontinuouslyinthecaseoftheSA306A-FHZ).
SA306-IHZ APPLICATIONSTheSA306-IHZ isdesignedprimarily todrive three-phasemotors.However, it canbeused foranyapplicationrequiringthreehighcurrentoutputs.ThesignalsetoftheSA306-IHZisdesignedspecificallyto interfacewithaDSPormicrocontroller.AtypicalsystemblockdiagramisshowninFigure1.Asexplainedbelow,over-temperature,short-circuitandcurrentlimitfaultsignalsprovideimportantfeedbacktothesystemcontrollerthatcansafelydisabletheoutputdriversinthepresenceofafaultcondition.High-sidecurrentsensorsmonitortheoutputcurrentofallthreephases,providingperformanceinformationthatcanbeusedtoregulateorlimittorque.
A SYSTEM OVERVIEW OF THE SA306-IHZWiththeintroductionoftheSA306-IHZ,designersnowhaveanoff-the-shelfsolutionfordrivingBLDCmotorsver-susdevelopingdrivercircuitsbyconfiguringthreediscretegatedriversandsixFETs.Theperformancespecifica-tionsfortheSA306-IHZareunusualinthatitcandeliverover15APEAKwithupto60VappliedtoitsFETs1.Imparting Rotation–ThreeindependentDMOSFEThalfbridgespro¬videtheoutputcurrent.Inoperation,asthemotorrotorrevolves,thecontrollercausesonemotorterminaltobedrivenHIGH,asecondLOWandthethirdtoFLOATinahighimpedancestate,asdepictedinFigure2.Thiscausesthemagneticfieldtorotateinsixstepsperelectricalrevolutioninthesimplestcase,impartingrotationtothepermanentmagnetsintherotor.Propersynchro-nizationofthissequenceisassuredbythefeedbackfromeitherHallsensorsorasensorlesscontrolsystemthatkeepsthemicrocontrollercontinuouslyinformedofthepositionoftherotorwithregardtothestatorwindings.
Figure 2. Imparting Rotation–BymonitoringtheHallsensors–orbymonitoringBackEMFinasensorlesscon-figuration–thestatorwindingfieldscanbemadetorotatesothattheresultantfieldofthetwoenergizedstatorwindingsandthepoleofthepermanentmagnetrotorremainatrightangles,therebymaximizingtheinstantaneous
torque.
Shoot-Through Protection–Theshoot-throughprotectionfeatureofthisICidentifiesthestateinwhichboththeupperandlowerportionsofahalfbridgeareONatthesametime.Shootthroughmustbeavoided,forifitweretooccur,itwouldshortthesupplytoground,overloadthecircuitanddestroytheFETs.Consequently,a‘deadtime’isprogrammedtoallowaFETtoturnfullyoffbeforeitscompanionFETisturnedon.Duringdeadtime,inductivewindingcurrentscontinuetoflow,orcommutate,throughinternalorexternalreversebiaseddiodes.Faultstatus
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indicationandcurrentlevelmonitorsareprovideddirectlytothecontroller.Outputcurrentsaremeasuredusinganinnovativelow-losstechniquediscussedinalatersection.TheSA306-IHZalsoofferssuperiorthermalperformancewithaflexiblefootprint.Controlling Brushless Motor Drivers–Mostbrushlessmotordriversarecontrolledbymicrocontrollersorsomeotherintelligentsystem.AnumberofmanufacturersincludingAnalogDevices,Freescale,MicrochipandTexasIn-strumentsmarketmicrocontrollersformotioncontrol–andmorespecificallyfordrivingbrushlessmotors.Choosing a Brushless Motor–Althoughthereareanumberofsourcesforassistanceinchoosingamotor,brush-lessorotherwise,agoodstartingpointisReference2.Astheauthorpointsout,choosingamotorrequireslookingatawholelistofissuesincludingefficiency,torque,powerreliabilityandcost.Whatcanbesaidcategoricallyisthatabrushlesspermanentmagnetmotoristhehighestperformingmotorintermsoftorqueversusefficiency.AlsoallthreestatorwindingscanbecontrolledwhichisnotthecaseinatraditionalDCmotorwherecommutationreliesonbrushes.
SENSOR VERSUS SENSORLESS COMMUTATIONHallSensorsarenotrequiredinsensorlesscommutation.InsteadtheinstantaneouspositionoftherotorrelativetothestatorisdeterminedbytheBackEMF(BEMF)developedinthestatorwindings.TheabsenceofboththeHallSensorsandtheattendantwiringlowersthemotor’scostandincreasesreliability,thoughderivingthelostinforma-tionfromtheBEMFrequiressomewhatmorecomplexcontrol.ThisapproachisattractiveinapplicationssuchasrefrigerationorHVACsystemswhichgenerateheatthatcouldacceleratefailuresoftheHallSensors.Ontheotherhand,startingpresentsaprobleminsensorlesscommutationsimplybecausethereisnoBEMFwhenthemotorisatrest.Secondly,abruptchangesinthemotorloadcancauseaBEMFdrivelooptogooutofsynchro-nization.Centraltobothsensorless-andsensor-basedcontrolsystemsisthepresenceofpulsewidthmodulation(PWM)whichisdiscussedintheAppendix.
CYCLE BY CYCLE CURRENT LIMITING – THE FUNDAMENTALSInapplicationswherethecurrentinthemotorisnotdirectlycontrolled,boththeaveragecurrentratingofthemotorandthein-rushcurrentmustbeconsideredwhenselectingaproperdrive.Forexample,amotorthatrequires1Awhenrunningatconstantspeed,mightrequireadrivethatcandeliverwellover10APEAKinordertosurvivetheinrushconditionatstartup.Butasthisdiscussionwillmakeclear,thisisnotthecasewhenusingtheSA306-IHZ.DepictedinFigure3aisthebehaviorinatraditionalmotorwherethereisnocycle-by-cyclecurrentlimit.(ThisisdiscussedinmoredetailintheAppendix)
Inthiscase,whentherotorisnotturning(noBEMF),thecurrentislimitedonlybytheresistanceoftherotorofthemotorplusanyseriesresistancethatmaybepresent.Asthemotoraccelerates,thebackEMFbuildsup,graduallyreducingthecurrentsothatitdiminishestothesteady-statecurrent.Figure3illustratesmotorbehaviorwhencycle-by-cyclecurrentlimitisappliedatstart-up.Inthiscase,thecurrentislimitedbycircuitrywithintheSA306-IHZ–notbytheimpedanceoftherotor.Asthemotorreachesitssteady-statespeed,thecurrenttailsoff.
Figure 3. Motor Current Behavior at Startup–(a)Withoutcycle-by-cyclecurrentlimit.(b)Withcycle-by-cyclecurrentlimit.
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CYCLE-BY-CYCLE CURRENT LIMIT BEHAVIOR ShownalongthetopofFigure4isthecycle-by-cyclebehaviorofthePWMpulsetrainappliedtotheSA306-IHZbythemicrocontroller.Themotorcurrentisshowninblueatthebottomoftheillustration.Outlinedinred,andsuperim-posedoverthecurrentwaveform,istheactualPWMpulsetrain(‘PWMoutput’)deliveredbytheFETswhichhas,ineffect,beenmodulatedbythecurrentflow.Itshutsofftheoutputpulseshouldthecurrentexceedthe‘currentlimit’setexternallybytheuser.1st Pulse–Inthecaseofthe1stPWMpulse,nocurrentlimitingoccursbecausethepulseendsbeforetherisingcurrentreachesthecurrentlimitthreshold.Attheendofthe1stpulsethereisashortdecaybeforethePWMpulseturnsononcemoreandthecurrentresumesitsrise.Notethatbecausecurrentcannotchangeinstantaneouslyinaninductivereactance,whichis,infact,whattherotorofthebrushlessmotoris,thecurrentvalue,unlikethePWMvoltage,isalwayscontinuous.2nd Pulse–InthecaseofthesecondPWMpulse,thecurrentreachesthelimitvaluebeforethePWMinputpulseends.Consequently,thePWMoutputpulseisshutoffearlyinitscycle.Thenthemotorcurrentdecaysuntilthethirdpulseisappliedwhichonceagaincausesthecurrenttorise.Subsequent pulses–ThebehaviorjustdescribedcontinuesuntilthebackEMFrisestothepointwherethecurrentfallsbelowthecurrentlimitthreshold.Thisoccursasthemotorapproachesitsoperatingspeedandthecurrentde-scendstoitssteadystatevalue,asdepictedinFigure3b.Theratioofthepeak-to-averagemotorcurrentdependsontheinductanceofthemotorwinding,thebackEMFdevelopedinthemotor,mechanicalloadingofthesystemsandthewidthofthepulse.
Figure 4. Cycle by Cycle Current Limit
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CONTROL AND SENSE ARCHITECTUREAsdepictedinFigure5,theoutputcurrentoftheupperoutputFETU14forOUTAiscontinuallymeasured,(ThesameoccursforthecorrespondingoutputFETsforOUTBandOUTC).TheoutputofthecurrentsensecircuitisappliedtoacurrentmirrorcomprisingU1,U2andU3whichdevelopsavoltageacrosstheexternalcurrentlimitresistor.ThisvoltageiscomparedwiththecurrentlimitthresholdbyComparatorU6.
Figure 5. Control and Sense Architecture
Disabling Circuitry–IfthevoltageappliedtotheplusterminalofcomparatorU6exceedsthecurrentlimitthresholdvoltage(Vth),allthreeoutputsOUTA,OUTBandOUTCaredisabled.ThedisablingpathisviagatesU7andU8andgatesU12andU13.OncethevoltageappliedtoComparatorU6fallsbelowtheVththresholdvoltage,andthedisabledtopsideinputtothegatecontrolgoeslow,theoutputstagewillreturntoanactivestateontherisingedgeofanytopsideinputcommandsignal(At,Bt,orCt).NotethatthecorrespondingdisablesignalsfromphasesBandCarealsoappliedviagateU7andtheirbehaviorisexactlythesameasisthecaseforOUTA.AlsotheILIM/DIS1goesHIGHwhenanyofthethreecurrentsensecircuitsdetectsanovercurrentsituation.Thecycle-by-cyclecurrentlimitfeatureoftheSA306-IHZwillreseteachPWMcycle.ThusthePEAKcurrentislimitedineachphaseduringeachPWMcycle,asillustratedinFigure4.NoticealsothatthemomentatwhichthecurrentsensesignalexceedstheVththresholdisasynchronouswithrespecttotheinputPWMsignalbecausethiseventisgovernedbytheinter-sectionoftherisingcurrentwiththecurrentlimit–notthePWMdutycycle.ThedifferencebetweenthePWMperiodandthemotorwindingL/Rtimeconstantwilloftenresultinanaudiblebeatfrequencysometimescalleda‘sub-cycleoscillation’.Thisoscillationcanbeviewedwithanoscilloscopebyapplyingaprobetopin7,ILIM/DIS1.Undervoltage Lockout–SeeTable1andPage7.Limitations at High and Low Speeds–InputsignalsappliedtothePWMandcommandinga0%-ora100%-dutycycle,maybeincompatiblewiththecurrentlimitfeatureduetotheabsenceofrisingedgesofAt,Bt,andCt–exceptatinstanceswhentherotationofthemotorrequirestheoutputFETstochangestates.Athighmotorspeeds,thismayresultinpoorperformanceandsignificantlyincreasedtorqueripple.WhereasatlowmotorspeedsthemotormaystallifthecurrentlimittripsandthemotorcurrentreacheszerowithoutacommutationedgethatwouldchangethestateoftheoutputFETsandnormallyresetthecurrentlimitlatch.
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Disabling Cycle-by-Cycle Current Limit–ThecurrentlimitfeaturemaybedisabledbypullingtheILIM/DIS1pintoGND.ThecurrentsensecircuitryidentifiedinFigure5,willcontinuetoprovidetopFEToutputcurrentinformation.External Current Control Options–Typicallythecurrentsensepinssourcecurrentintogroundedresistorswhichprovidevoltagesto thecurrent limitcomparators,asshown inFigure1. If insteadthecurrent limit resistorsareconnectedtoavoltageoutputDAC,thecurrentlimitcanbecontrolleddynamicallybythesystemcontroller.Thistechniqueessentiallyreducesthecurrentlimitthresholdvoltageto(Vth-VDAC).Duringexpectedconditionsofhightorquedemand,suchasstart-uporreversal,theDACcanadjustthecurrentlimitdynamicallytoallowperiodsofhighcurrent.Innormaloperationwhenalowcurrentisexpected,theDACoutputvoltagecanincrease,reducingthecurrentlimitsettingtoprovidemoreconservativefaultprotection.Thisisdiscussedindetailunder‘CurrentSense–AnAdvantage’.Three Degrees of Freedom–Theappliedvoltage,theswitchingfrequencyandthePWMdutycyclearethreecru-cialparametersthatcanbeprogrammedindependently.Howthesevariablesareselectedwillaffectthebehaviorofthemotorwithregardtohowfastitwillaccelerate,andconsequentlyhowfastitsspeedandtorquewillrise.
CONTROL AND SENSE PINSAsummaryofthecontrolandsensepins,andtheirfunctions,issuppliedinTable1.
Table 1. Control and Sense FeaturesNomenclature Pin # Function Description Remarks
DIS2 23 Control
When a HIGH is applied to thisSchmitttriggeredlogiclevel,itplac-esOUTA,OUTB,andOUTCinahighimpedancestate.
Pin DIS2 (23) has an internal12kΩpull-downresistorconnect-ed to ground and thereforemaybeleftunconnected.(SeeFigure5.)
ILIM/DIS1 7 Control/Sense
Control: Pulling this pin to logicHIGH placesOUTA, OUT B, andOUTC inahigh impedancestate.Pulling this pin to a logic LOWef-fectivelydisablesthecycle-by-cyclecurrentlimitfeature.Sense: This pin is also connectedinternallytotheoutputofthecurrentlimit latch through a 12kΩ resistorand can be monitored to observethe function of the cycle-by-cyclecurrentlimitfeature.
SC 3 Sense
GoesHIGH ifashort circuit isde-tected or an output occurs that isnot in accordance with the inputcommands,
TheSCsignal isblankedforap-proximately200nsduringswitch-ingtransitionsbutinhighcurrentapplications, short glitches mayappear on the SC pin. A highstate on the SC output will notautomaticallydisable thedevice.The SC pin includes an internal12kΩseriesresistor,asshowninFigure5.
TEMP 25 Sense
GoesHIGH if the SA306 die tem-perature reaches approximately135ºC.
ThispinWILLNOTautomaticallydisablethedevice.TheTEMPpinincludes a 12kΩ series resistor.SeeFigure5.
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Nomenclature Pin # Function Description RemarksUVLO (Under-voltage LockOut)
None Control/SenseDisablesalloutputFETsuntilVS isabove theUVLO thresholdvoltagewhichistypically8.3V.
SeediscussioninSection3.4.
Vth None Control
Ifthevoltageofanyofthethreecur-rentsensepinsexceedsthecurrentlimitthresholdvoltage(Vth),whichistypically 3.75 volts, all outputs aredisabled.AfterallcurrentsensepinsfallbelowtheVththresholdvoltageand theoffendingphase’s topsideinputgoeslow,theoutputstagewillreturntoanactivestateontheris-ingedgeofANYtopsideinputcom-mandsignal(At,Bt,orCt).
FAULT INDICATIONSIn thecaseofeitheranover-temperatureorshort-circuit fault, theSA306-IHZwill takenoaction todisable theoutputs.However,theSCandTEMPsignalscanbefedtoanexternalcontroller,asdepictedinFigure6,whereadeterminationcanbemaderegardingtheappropriatecourseofaction.Inmostcases,theSCpinwouldbecon-nectedtoaFAULTinputontheprocessor,whichwouldimmediatelydisableitsPWMoutputs.TheTEMPfaultdoesnotrequiresuchanimmediateresponse,andwouldtypicallybeconnectedtoaGPIO,orKeyboardInterruptpinoftheprocessor.Inthiscase,theprocessorwouldrecognizetheconditionasanexternalinterrupt,whichcouldbeprocessedinsoftwareviaanInterruptServiceRoutine.Theprocessorcouldoptionallybringallinputslow,orassertahighleveltoeitherofthedisableinputsontheSA306-IHZ.Figure6depictsanexternalSRflip-flopwhichprovidesahardwiredshutdownofalloutputsinresponsetoafaultindication.AnSCorTEMPfaultsetsthelatch,pullingtheDIS2pinHIGH.TheprocessorclearsthelatchedconditionwithaGPIO.Thiscircuitcanbeusedinsafetycriticalapplicationstoremovesoftwarefromthefault-shutdownloop,orsimplytoreduceprocessoroverhead.Inapplica-tionswhichmaynothaveavailableGPIO,theTEMPpinmaybeexternallyconnectedtotheadjacentILIM/DIS1pin.Ifthedevicetemperaturereaches~135ºCalloutputswillbedisabled,de-energizingthemotor.TheSA306-IHZwillre-energizethemotorwhenthedevicetemperaturefallsbelowapproximately95ºC.TheTEMPpinhysteresisiswidetoreducethelikelihoodofthermaloscillationsthatcangreatlyreducethelifeofthedevice.
Figure 6. External Latch Circuit
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UNDER-VOLTAGE LOCKOUTWithoutsufficientsupplyvoltage, theSA306-IHZcontrol circuit cannotsufficientlydrive thegatesof theoutputFETs.Theundervoltagelockoutconditionresults intheSA306-IHZunilaterallydisablingalloutputFETsuntilVSisabovetheUVLOthreshold indicated in thespecification table in theDataSheet1.There isnoexternalsignalindicatingthatanundervoltagelock-outconditionisinprogress.TheSA306-IHZhastwoVSconnections:oneforphaseA,andanotherforphasesB&C.Thesupplyvoltagesonthesepinsneednotbethesame,buttheUVLOwillengageifeitherisbelowthethreshold.HysteresisontheUVLOcircuitpreventsoscillationswithtypicalpowersupplyvariations.
CURRENT SENSE — AN ALTERNATIVE ExternalpowershuntresistorsarenotrequiredwiththeSA306-IHZ.Forwardcurrentineachtop,P-channeloutputFETismeasuredandmirroredtotherespectivecurrentsenseoutputpin,Ia,IbandIc,asdepictedinFigures1and5.Byconnectingaresistorbetweeneachcurrentsensepinandareference,suchasground,avoltagedevelopsacrosstheresistor that isproportional to theoutputcurrent for thatphase.AsanalternativethecurrentsIa, Ib,andIccanbefedtoasingleresistorandthevoltagedevelopedmonitoredbyahigh-impedanceA/Dconverter,asshowninFigure7.Asdepictedinthe‘CurrentSense’plotonpage4oftheSA306-IHZproductdatasheet,themaximumcurrentperphaseisslightlylessthan2milliamperes.Consequently,choosearesistorthatwilldevelopthevoltageappropriatefortheA/DConverterchosen.Thefullscalevoltagewillbethevoltagedevelopedacrosstheresistorbythesumofthethreecurrents—6milliamperes.Alsoallowaheadroomofapproximately0.5V.
Figure 7. Current Monitoring Circuit
EXTERNAL FLYBACK DIODESExternalfly-backdiodes(D1throughD6),depictedinFigure8,willoffersuperiorreverserecoverycharacteristicsandlowerforwardvoltagedropthantheinternalback-bodydiodes.Inhighcurrentapplications,externalflybackdiodescanreducepowerdissipationandheatingduringcommutationofthemotorcurrent.Reverserecoverytimeandcapacitancearethemostimportantparameterstoconsiderwhenselectingthesediodes.Ultra-fastrectifiersofferbetterreverserecovery timeandSchottkydiodestypicallyhave lowcapacitance. Individualapplicationre-quirementswillbetheguidewhendeterminingtheneedforthesediodesandforselectingthecomponentwhichismostsuitable.
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Figure 8. Three-Phase Motor Drive Circuit
A SPECIFIC EXAMPLE Beginbychoosingamotorthatwillexhibitthemechanicalperformancerequired—whichistosaythetorque,theefficiency,andother issuesthatwill fulfill themotordriverequirementsofyoursystem.Wehavechosenasourexamplealow-inertia,brushlessmotorthatdelivers557mNmoftorqueat2000rpm.WeselectedaMaxonECFlatmotorwww.maxonmotor.com.Thecharacteristicsofthewindingsofthismotorarethatanystator-windingpairexhibitsaresistance(Rm)of2.3ohmsandaninductance(Lm)of2.5millihenries.Thetorqueconstant(Kt)ofthemotoris217mMmoz-in/A.
DEMONSTRATION BOARD FOR THE SA306-IHZIt isrecommendedthat thedesigneracquire theDB64DemonstrationBoard3andassembleaprototype. In theDB64 theSA306-IHZ inputPWMcontrol isachievedbymeansofaTexas Instrumenton-boardUCC3626PWmicrocontroller4.TheDB64isdesignedtodemonstratethecapabilitiesoftheSA306-IHZasa3-phasebrushlessDCmotordriverIC.FromthereonecaneasilyconvertthecircuitdevelopedwiththeDB64DemonstrationBoardintoacircuitsimilartotheoneshowninFigure8whichcanserveasthebasecircuitforaproductionversion.
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LAYOUT CONSIDERATIONSAsimpletwo-layerprintedcircuitboardconstructionissufficientbecauseoftheconvenientpinoutoftheSA306-IHZPowerQuadpackage.InputsignalsareroutedintoonesideoftheSA306-IHZpackageandhigh-poweroutputsig-nalsareroutedfromtheothersidein2ouncecopper.Thiseliminatestheneedtoroutecontrolsignalsnearmotorconnectionswherenoisemightcorruptthesignals.Fillingtopandbottomlayerswithcopperreducesinductivecou-plingfromthehighcurrentoutputs.Use1nFcapacitorswithexcellenthighfrequencycharacteristicstobypasstheVSmotorsuppliesateachphaseaswellasswitchinggradeelectrolyticcapacitors.Thesix100VSchottkydiodes(D1–D6)conductthecommutationcurrentvialowforwardvoltagepathswhichreducesthepowerdissipationintheSA306-IHZ.Thesediodesareratedfor5Acontinuous.MountthemclosetotheSA306-IHZtoreduceinduc-tanceinthecommutatingcurrentloop.Forapplicationswithcontinuouscurrentslessthan5A,theSchottkydiodesmaynotbenecessary,butonemustconsiderthehigherforwardvoltageinternalbodydiodesandtheassociatedpowerdissipationthatresults.Output Traces–OutputtracescarrysignalswithveryhighdV/dtanddI/dt.Properroutingandadequatepowersupplybypassingensuresnormaloperation.Poorroutingandbypassingcancauseerraticandlowefficiencyop-erationaswellasringingattheoutputs.Bypassing–TheVSsupplyshouldbebypassedwithasurfacemountceramiccapacitormountedascloseaspossibletotheVSpins.TotalinductanceoftheroutingfromthecapacitortotheVSandGNDpinsmustbekepttoaminimumtopreventnoisefromcontaminatingthelogiccontrolsignals.AlowESRcapacitorofatleast25μFperampereofoutputcurrentshouldbeplacedneartheSA306-IHZaswell.Capacitortypesratedforswitchingapplica-tionsaretheonlytypesthatshouldbeconsidered.NotethatphasesB&CshareaVSconnectionandthebypassrecommendationshouldreflectthesumofB&Cphasecurrent.ThebypassingrequirementsoftheVDDsupplyarelessstringent,butstillnecessary.A0.1μFto0.47μFsurfacemountceramiccapacitor(X7RorNPO)connecteddirectlytotheVDDpinissufficient.Ground Connections and Ground Planes–SGNDandPGNDpinsareconnectedinternally.However,thesepinsmustbeconnectedexternallyinsuchawaythatthereisnomotorcurrentflowinginthelogicandsignalgroundtracesasparasiticresistancesinthesmallsignalroutingcandevelopsufficientvoltagedropstoerroneouslytrig-gerinputtransitions.Alternatively,agroundplanemaybeseparatedintopowerandlogicsectionsconnectedbyapairofback-to-backSchottkydiodes.Thisisolatesnoisebetweensignalandpowergroundtracesandpreventshighcurrentsfrompassingbetweentheplanesections.UnusedareaonthetopandbottomPCBplanesshouldbefilledwithsolidorhatchedcoppertominimizeinductivecouplingbetweensignals.Thecopperfillmaybeleftuncon-nected,althoughagroundplaneisrecommended.
Table 2. Parts List for Figure 7ReferenceDesignation DescriptionR1,R2,R3 470ΩR4,R5,R6 1kΩR8 20kΩpotentiometer(tocontrolthePWMdutycycle)R9,R10,R14,R15,R16,R17 5kΩC1,C7,C8 1µFC2,C3,C4,C5,C6 1nFC9,C10,C11 2.2nFC14,C16 0.1µFD1,D2,D3,D4,D5,D6 PDS5100U1 ApexMicrotechnologySA306-IHZorSA306A-FHZU2 TexasInstrumentUCC3626U3 LM78L05
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POWER DISSIPATIONThethermally-enhancedpackageoftheSA306-IHZallowsforseveraloptionsformanagingthepowerdissipatedinthethreeoutputstages.PowerdissipationintraditionalPWMapplicationsisacombinationofoutputpowerdis-sipationandswitchinglosses.Outputpowerdissipationdependsonthequadrantofoperationandwhetherexternalflybackdiodesareusedtocarrythereverseorcommutatingcurrents.SwitchinglossesaredependentonthefrequencyofthePWMcycleasdescribedinthetypicalperformancegraphs.ThesizeandorientationoftheheatsinkmustbeselectedtomanagetheaveragepowerdissipationoftheSA306-IHZ.Applicationsvarywidelyandvarious thermal techniquesareavailable tomatch the requiredperformance.ThepatentpendingmountingtechniqueshowninFigure9,withtheSA306-IHZinvertedandsuspendedthroughacutoutinthePCB,isadequateforpowerdissipationupto17WwiththeHS33(a1.5-inchlongaluminumextrusionwithfourfins).Infreeair,mountingthePCBperpendiculartotheground,sothattheheatedairflowsupwardalongthechannelsofthefinscanprovideatotalθJ-Aoflessthan14ºC/W(9WmaxaveragePD).MountingthePCBparalleltothegroundimpedestheflowofheatedairandprovidesaθJ-Aof16.66ºC/W(7.5WmaxaveragePD).Forapplicationsinwhichhigherpowerdissipationisexpectedorlowerjunctionorcasetemperaturesarerequired,alargerheatsinkorcirculatedaircansignificantlyimprovetheperformance.AlsoseeReferences5and6.
Figure 9. HS33 HeatsinkAppendix
A Brief Overview of PWMThefirstpulsewidthmodulation(PWM)ICsappearedonthemarketsome40yearsago.SotheconceptofPWMisatleastasold.Thoughtheearliestapplicationswereinswitchingpowersupplies,itwasnotmuchlaterthatthetechniquewasfirstemployedtodrivebrushlessmotors.TheprincipalbenefitofPWMasacontroltechniquebe-comesclearbyexaminingFigureA1.Thetraditionallinearpowerdeliverytechniqueforlimitingpowersimplyem-ploysavariableresistanceasdepictedinFigureA1(a).Whenmaximumoutputiscommanded,thedriverreducesresistanceofthepasselementtoaminimum.Atthisoutputlevel,lossesinthelinearcircuitarerelativelylow.Whenzerooutputiscommandedthepasselementresistanceagainapproachesinfinityandlossesagainapproachzero.However,thedisadvantageofthelinearcircuitbecomesclearinthemidrangewhentheoutputlevelisinthevicin-ityof50%.Attheselevelstheresistanceofthepasselementisequaltotheloadresistancewhichmeanstheheatgeneratedintheamplifierisequaltothepowerdeliveredtotheload!Inotherwords,alinearcontrolcircuitexhibitsaworstcaseefficiencyof50%whendrivingresistiveloadsatmidrangepowerlevels.What’smore,whentheloadisreactive,thisefficiencydropsevenfurther.
PatentPending
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Figure A1. PWM versus Linear Control–PWMcontrolin(b)exhibitsfarlowerlossesthanthetraditionallinearcontroltechniquein(a)
NowconsiderPWMoperationasdepictedinfigureA1(b).InaPWMcontrolsystemananaloginputleveliscon-vertedintoavariable-duty-cycleswitchdrivesignal,Theprocessofswitchingfromoneelectricalstatetoanother,whichinthiscaseissimplybetweenOFFandON,iscalled‘modulation’,whichaccountsforwhythistechniqueiscalled‘pulsewidthmodulation’.Beginningatzerodutycycle,whichistosayOFFallthetime,thedutycycleisoftenadvancedasthemotorbeginstorotate,untilitisrunningatthespeedand/orthetorquerequiredbytheapplica-tion.InthecaseofaPWMcontrolcircuit,therathernegligiblelossesareprimarilyduetotheONresistanceoftheswitchingFETandtheflybackdiodewhichiswhyefficienciesashighas80%to95%areroutine.However,athighswitchingfrequenciestheenergyrequiredtoturntheFETsonandoffcanbecomesignificant.
Figure A2. Linear versus PWM—Currentbehaviorwithasteady-stateexcitationin(a);CurrentbehaviorwithPWMexcitationin(b)
Pulse width controlLinear control(a)
V S
DriverLoad
Pass element
95%
50%
5%On
(b)
Off
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V
t
t
V
t
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On
OffOn On
V
tt
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Imax. I = VR
VR
RtLI = (1 - e )
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pere
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Ioff
toff(b)
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Inadditiontoenhancedefficiency,PWMcanplayadditionalroleswhichincludelimitingthestart-upcurrent,control-lingspeedandcontrollingtorque.Theoptimumswitchingfrequencywilldependoninertiaandinductanceofthebrushlessmotorchosenandtheapplication.Thechoiceoftheswitchingfrequencyaffectsbothlossesandthemag-nitudeoftheripplecurrent.AgoodruleofthumbisthatraisingtheswitchingfrequencyincreasesthePWMlosses.Ontheotherhand,loweringtheswitchingfrequencylimitsthebandwidthofthesystemandcanraisetheheightsoftheripplecurrentpulsestothepointthattheybecomedestructiveorshutdownthebrushlessmotordriverIC.TheripplecurrentpulsesaredepictedinFigureA2(a)andarediscussedbelow.
BRUSHLESS MOTOR BEHAVIOR – AN OVERVIEW Oneofthemostcriticalmomentswithregardtoabrushlessmotor–alsotrueforamotorwithbrushes–iswhenpowerisfirstappliedwhilethemotorisatrest.Atthistimetherotorisstationaryandisdeliveringno‘backEMF’(VBEMF).VBEMFcanbeexpressedas:
VBEMF=(Kb)(Speed) (Equation1)
Where:Kb=voltageconstant(volts/1000RPM) Speed=revolutionsperminute(expressedinthousands)
Onceavoltageisappliedtothemotor,therotorbeginsturning,generatingaVBEMFgovernedby(Equation1).IgnoreforthemomentthattheplanistodrivethemotorwithaPWMsource,andassumethemotorisdrivenbyasteady-statevoltage,thenwecanexpressthecurrentbythisequation:
I=[(V-VBEMF)/Rm][1-e-Rmt/Lm] (Equation2)
Where:V=theappliedvoltage VBEMF=backEMF Rm=statorresistance(windingpair) Lm=statorinductance(windingpair)
Notethatin(Equation2),thecurrent(I)atanymomentisafunctionofboththebackEMF(VEMF)andthetime(t).Thecurrentwhenthemotorisstopped(VBEMF=0)isillustratedinFigureA2(a)andisafamiliarwaveformforchar-acterizingthecurrentinanyL-RcircuitwithitsrisetimegovernedbythetimeconstantL/R.Nowlet’sexchangethesteady-stateexcitationvoltageforaPWMsource,asshowninFigureA2(b).ThecurrentrisesuntilthefirstONpulseends;whenthevoltageabruptlyfallstozeroattheendofthefirstappliedvoltagepulse,thecurrentbeginstodecaytowardszero.However,thenextpulsewillagaindrivethecurrentupwards,andsoforth,sothatthecurrentcontinuestorise.Asthemotoraccelerates,thecurrentwaveformwillexhibitasawtoothprofile.Thissawtoothcharacteristicisalsoknownasripple.Becausetorqueisdirectlyproportionaltocurrent,thesequenceof risingcurrentpulsesdrive themotor,whichdevelopsacorresponding torque thataccelerates themotor.Butthisisnotsointhecaseofcycle-by-cyclecurrentlimit.Becauseinthiscasethecurrentriseceasesim-mediatelyifthecurrentreachesthelimitvalueduringanyPWMpulseinterval.
REFERENCES1.SA306-IHZ Pulse Width Modulation Amplifier Data Sheet,www.apexanalog.com2.Motion Control PrimerbyDavidPalombo,www.aveox.com3.DB64 Demonstration Board For SA306EX,www.apexanalog.com4.UCC2626, UCC3626 Brushless DC Motor Controller,www.ti.com5.ApplicationNote22,SOA (Safe Operating Area) and Load Lines,www.apexanalog.com6.ApplicationNote11,Thermal Techniques,www.apexanalog.com
BIBLIOGRAPHY“Modeling,Simulation,andAnalysisofPermanent-MagnetMotorDrives,PartII:TheBrushlessDCMotorDrive”byPragasenandRamuKrishnan,IEEE Transactions on Industry Applications,Vol.25,No.2,March/April1989,pages274-279
“ANewSimulationModelofBLDCMotorwithRealBackEMFWaveform”byY.S.Jeon,H.S.Mok,G.H.Choe,D.K.Kim,J.S.Ryu,7th Workshop on Computers in Power Electronics, 2000;pages217-220
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