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AN50 OptimizingPowerDeliveryinPWMMotorDriverICs

Thermal Behavior of Single-Phase, Three-Phase Single-ChipPWM Amplifiers SA57-IHZ, SA306-IHZ

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Device Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Optimizing Thermal Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 OptimizingMotorCommutationMethods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 BalancingtheElectricalLoad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 DefiningaMovementProfile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Cooling Options and Results with the 64-pin QFP Package . . . . . . . . . . . . . . . . . . . . . . . . . . 7 SMTMountingwithoutHeatsinkinLow-PowerApplications . . . . . . . . . . . . . . . . . . . . . . . . . 7 SMTMountingwithThermalPadandHeatsinkforMid-RangePowerApplications . . . . . . . 9 Flipped-OverSMTMountingwithHeatsinkforHigh-PowerApplications . . . . . . . . . . . . . . . 11 ComparisonsofLow-Power,Mid-RangePowerandHigh-PowerTestBoards . . . . . . . . . . 13 HighPowerHeatSinking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 PowerDissipationDerating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Appendix Derivation of Maximum Power and Thermal Dynamic Values . . . . . . . . . . . . . . . 22 SMTMountingWithoutHeatsinkinLow-PowerApplications . . . . . . . . . . . . . . . . . . . . . . . . 22 SMTMountingwithaThermalPadandHeatsinkforMid-RangePowerApplications . . . . . 23 Flipped-OverSMTMountingwithHeatsinkforHigh-PowerApplications . . . . . . . . . . . . . . . 24

AN50Optimizing Power Delivery in PWM Motor Driver ICs

Copyright © Apex Microtechnology, Inc. 2012(All Rights Reserved)www.apexanalog.com OCT 2012

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IntroductionTheApexSA306-IHZ isanadvanced3-phasesinglechipamplifier,designedforuseasan independentpowerstagetodrive3-phasebrushlessDCmotors .Itincorporatesa3-phasePWMamplifierthatacceptssixindependentcontrolsignals .Thisenablesadesignertochoosefromawidevarietyofcontrolsignalsasappropriateforthecom-mutationmethodchosen .ThisApplicationNotediscussesdifferencesinthethermalbehaviorofthedevicerelatingtomountingtechniques,methodsofmodulationofthepowerstageandexaminessometechniquestoimprovesystemperformanceandoverallefficiency .Inordertochoosetheappropriatethermaldesignforaparticularapplicationandforthebasicdimensioningoftheheatsink,thecalculationofthefirst-orderthermalapproximationisagoodstartingpoint .Therearesomeapplica-tionspecificaspectstobetakenintoaccountbeforestartingthebasicsystemdesign .ThisApplicationNotecoverssomeofthesebasicdesignsteps,dataandideastocarryforwardsothedesigncanachievethebestresults .

Device OverviewBoth theSA306-IHZandSA57-IHZdevices incorporateallof thesub-systemsnecessary todriveabrushedorbrushlessDCmotorunderthecontrolofanMCUorDSP .Atmaximumoutputthesedevicesarecapableofsafelyoperatingmotorsapproaching0 .5hp .Likemanytraditionalmotordriveproducts,SAdevicesintegratecontrol,pow-ersupply,gatedriveandpowerstageintoasingleIC .Unlikecompetingdevices,thisseriesisspecificallydesignedforMCUorDSPcontrolandintegratefeaturesthatenableimplementationofmoderncommutationtechniques,re-sultinginhigherefficiencydrivestages .BothSA306-IHZandSA57-IHZdevicesconsistofmajorblocks,asshowninFigure1 .Theprimarydifferencebetweenthetwodevicesisthenumberofphasessupported;theSA306-IHZsupportsthreephaseswhiletheSA57-IHZsupportstwo .Akey featureof thisseries is theability todirectlycontrol theoutputFETs .Thisenablesmoderncommutationschemessuchasfield-oriented,vector-oriented,andsinusoidaltobeimplementedwithinthecontrollerwithnoin-

SA306 Switching Amplifier

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Figure 1. System Block Diagram using the 3-Phase SA306-IHZ Device

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terveningcircuitry .ThisarchitecturesimplifiescircuitdesignandreducesEMC/EMI .ThesedevicestaketheinputsforeachgateandprovidesbufferinganddrivetothecompanionFET .Anothersignificantfeatureisbuilt-incurrentsense .Currentsenseisavailableforeachofthephasesfromasepa-rateoutputpin .Theoutputcurrentof thesensepins is~1/5000thof thephasecurrent .Thecurrentsensepinsprovidedirect,real-timefeedbacktothecontroller .Currentsenseisperformedatthehigh-sideonly .Sensingoflow-sidecurrentsunderbrakingorflybackconditionsisnotsupported .Over-temperature,short-circuitandcurrentlimitarealsoimplemented .Thesefaultsignalsprovideimportantfeed-backtothesystemcontrollerwhichcansafelydisabletheoutputdriversintheeventofafaultcondition .ThesetopicsarediscussedingreaterdetailinReference3 .Both theSA306-IHZandSA57-IHZ implementacycle-by-cyclecurrent limitscheme .Thisallowsvariableuser-definedcurrentlimitthresholdswithoutdamagetothedevice .However,eveninthismode,theusermustconsiderandplanforexcessivecurrentexcursionswhichcouldresultinexcessivepowerdissipation .ThearchitectureoftheSA306-IHZandSA57-IHZdevicesenablesthedesignertoconfiguredynamically-changingcurrentlimitinginthedeviceinresponsetochangingsystemoperatingconditions .TheSA306-IHZorSA57-IHZdatasheetsspecifyapeakcurrentof17amperes .However,ifdesiredthedesignercanchoosetolowerthislimitortodisablethefeaturealtogether .Inapplicationswherethecurrentinthemotorisnotdirectlycontrolled,boththeaveragecurrentratingofthemotorandtheinrushcurrentmustbeconsidered .Forexample,a1Acontinuousmotormightrequireadriveamplifierthatcandeliverwellover10APEAKinordertoprovidetherequiredstart-uptorque .Thecycle-by-cyclecurrentlimitfeatureenablestheSA57-IHZ/SA306-IHZtosafelyandeasilydriveawiderangeofbrushandbrushlessmotorsthroughastartupinrushcondition .Withlimitedcurrent,thestartingtorqueandaccelerationarealsolimited .TheplotsinFigure2illustratestartingcurrentandbackEMFwithandwithoutcurrentlimitenabled .

MoreinformationontheseandotherfeaturesoftheSA306-IHZandSA57-IHZcanbefoundintheproductdatasheets,aswellasanyassociatedApplicationNoteslistedintheReferencesectionfoundattheendofthisdocu-ment .

Optimizing Thermal PerformanceAllpowerdevices,fromFETsandIGBTstolargemicroprocessors,dissipatepowerintheformofheat .Fordeviceswhoseprimarypurposeispowerconversionorpowertransmissionappropriatemanagementofheatisthekeytodetermininghowmuchpowercanbesafelyprocessedbythedevice .Thisfactappliestospecificationlimitsaswell .Adevicemayberatedfor5Acontinuousbutthesystemdesignermustalwaystakecarethatthepowerlostintheformofheatdoesnotresultindamagetothedevice .Itmustbenotedthattotaloutputpowerandtotalpowerdissipationaredependentonboththeoutputcurrentandoperatingvoltage .ForallmountingtechniquesdiscussedinthisApplicationNotetheeffectofvoltageandcurrentlevelswillproducevariationsinpowerdissipation .Forexample,graphsoflowpoweroperationshowheatdissipa-tionforagivenoutputpower .BecausepowerdissipationfollowstheformulaP=I2R,increasingoperatingvoltagetothehighestpracticallimitwillresultingreateroutputpoweratagivencurrent(internalpowerdissipation) .Con-verselypowerdissipationcanbereduced foragivenoutputpowerby increasingvoltageandreducingcurrent .

Time, Milliseconds

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Figure 2. Motor Current Behavior at Startup — (a) Without cycle-by-cycle current limit. (b) With cycle-by-cycle current limit

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Graphsaregenerallynotprovidedformultiplecombinationsofcurrentandvoltage .Rathertheemphasisisonthecalculationofpowerdissipationandthermalresistance,thusenablingthede-signertocalculatedietemperatureforthesystem'scombinationofvoltage,current,andheatsinking .ShowninFigure3isaninfraredimageofaSA57-IHZdrivingabrushedDCmotorwithacontinuouscurrentof3Aatasupplyvoltageof24V .Thenumbersmarkthelocaltemperaturesofthedesign .Thehighandlow-sideFETsoftheactivehalf-bridgecanclearlybeseenaswhitedots,representingatemperatureofap-proximately110°C .Thispictureshowsthetemperaturegradientsacrossthesilicon .Theheatgeneratedatthetwohot-spotsspreadsoverthesiliconarea .The result isa core temperatureofapproximately90°C .TheredarearepresentstheheatslugoftheIC .Thistemperatureisnearlyequaltothecoretemperatureofthesilicon .Theheatistransferredtotheheatsinkonthebackside(yellow)andinthiscasetothetopandbottomlayer(green)ofthePCB,thendis-sipatedintotheambientair .(Notethattheapparenttemperaturedifferenceofthescrewsistheresultofemissivitydifferencesbe-tweenthescrewsandtherestofthetestsystem .Insteady-stateoperationthescrewsareapproximatelythesametemperatureastheheatsink) .

Optimized Motor Commutation MethodsTheON-resistanceofthepowerFETsistheprimarycontributortoheatgenerationwithinamotordrive .Incalculat-ingthepowerlosttotheONresistanceweusethestandardpowerformulaP=I2R,whereI=thecurrentflowingthroughthemotorandR=theON-resistanceoftheIC .AstheSA57-IHZandSA306-IHZofferdirectaccesstoeachsingleFET,itispossibletousealternativecommutationtechniquestoincreasethesystemperformanceandtoreducethermalloads .Thebestexampleforasimplesystemoptimizationissinusoidalcommutationofa3-phasebrushlessDCmotor .Thismethodoffersincreasedefficiencyandreducedpowerdissipation .Alternatetypesofcommutationaremorefeasiblenowthatvirtuallyeverymotorcontrolsystemhassometypeofmicro-controllerorDSP/DSCavailable .

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Figure 3. Infrared picture of a SA57 running at 3 amperes at 24 volts.

Figure 4. Power and voltage behavior in the case of 3-phase sinusoidal commutation.

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Sinusoidalcommutationofa3-phaseBLDCmotorspreads thegeneratedheatoverawiderareaof thesiliconthanwithblockcommutation .Thisreduceslocalizedheatinginthedie .Duringsinusoidalcommutationthecurrentthroughthemotorisdeliveredbytwohigh-sideFETsandasinglelow-sideFETor, inthenextstep,byusingasinglehigh-sideFETandtwolow-sideFETs .ThismeansthattherearealwaystwoactiveFETsinparallelwhichdecreasestheONresistanceofthetwoparallelconductingFETs .Theresultingresistanceisnotlinearbecausetheon-timeofeachPWMsignalofthetwoFETsconnectedinparallelfollowsasinωt/cosωtrelation .(SeeyellowareasinFigure4)TheeffectiveONresistanceacrossanelectriccommutationcyclecanbecalculatedbydividingtheON-resistancevalueoftheparallelhighorlowsideFETsby√2 .Usingthiscommutationtechniquecan,inadditiontoseveralotheradvantages,reducethethermalloadbyapproximately10%to15% .Theundissipatedpowercanbeusedtodrivethemotorortoreducethetotalpowerconsumption .Ineithercasetheoverallefficiencyofthesystemisincreased .

Balancing the Electrical LoadAswehavejustdiscussed,themainheatgeneratingcontributor isthecurrentflowingthroughthepowerFETs .BecausetheSA57-IHZandtheSA306-IHZarehigh-voltagedevicessuitableforsupplyvoltagesupto60V,anin-creaseinthesupplyvoltagecanleadtoadecreaseofthecontinuouscurrentforagivenpowerlevelandthereforetoadecreaseofthethermalload .Fromthispointofview,thebasicideaistoincreasethesupplyvoltagewhilekeepingthecontinuouscurrentcon-stanttoreducethethermalload .Inapulse-drivensystemwithaninductiveload,therelationbetweenvoltageandcurrentisnotlinearsowehavetotakeacloserlookatswitchinglossesandthecurrentcharacteristicsundervari-ousoperationconditions .An application is balanced when the for-wardandbackwardmagnetizationofthein-ductorisfullyrealizedwithinasinglePWMcycle .Thisshouldbethenormaloperatingconditioninapplicationswithconstantrota-tionspeedandlowdynamics,suchasfans,pumpsorunidirectionaldrives .Intheseap-plications,onceaccelerated,themotorrunsat a constant speed with a constant me-chanicalload .Shown in Figure 5 are the characteristicsof the voltage and current during a PWMpulseinabalancedoperatingsituation .Therisingand fallingedgesof thevoltagearenearlylinearwithintheperiodgivenbythetimevaluefortheraisingandfallingedges– typically 200 nanoseconds . The currentcharacteristicdependsonthetechnicalpa-rameters of themotor winding . Thewind-ings produce a reverse voltage when therisingedgeoccurs .Thecurrentreachesitsmaximumwhenthesupplyvoltageandthereversevoltagearemoreor lessequal .Theresultingphaseshift isrepresentedbytVI .Whenthesupplyvoltagev3isbeingincreased,thephaseshifttVIbecomeswiderasshownbyarrowv1andtherisingedgeofthecurrentv2appearslater .Theterminationofthedutycycleshutsdownthecurrentimmediatelyandalwaysatthesamepointintime .Sowhenthesupplyvoltageincreases,bothcurveschangeinthedirectionsindicatedbythearrowslabeledv1,andv2 .ShowninFigures6and7arevoltageandcurrent(overshootnotvisible)plotsresultingfromaPWMpulseappliedtophaseA .Both10Vand30Vsupplyvoltagesareshown .WhenwelookatthefallingedgeofthePWMpulse,weseethatthecurrentdropswiththesameslewrateatasup-plyvoltageofboth10Vand30V,butforalongertimeintervalbecauseofthehigherpeakcurrent .Thismeansthattheswitchinglossesincreaseonthefallingedge .AttherisingedgeofthePWMpulsethecurrentisless,sothattheswitchinglossesonthissidehavedecreased .

Voltage

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v2v3

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Figure 5: Characteristic of voltage and current during a PWM pulse in a balanced system

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Whenwecomparethetotalaveragecurrentovertime,wefindsimilarvaluesatasupplyvoltageof10Vand30V .Althoughthepeakcurrentat30Visapproximately30%higher,thepulsewidthis20%shorterandtheslopeofthepulsetopissteeperthanat10V .Inanoverallcomparisontheaveragecurrentofbothpulsesisnearlyconstantwithslightlyincreasedswitchinglosses .Becausetheaveragecurrentremainssimilar,theaveragetemperatureduetoI2RlosseswithintheFETsisalsosimilar .However,becausevoltagedeliveredtothemotordoesincrease,thereisgreaterpoweravailabletothemo-torwindings .Thisfactresultsinincreasedefficiencybydrivingmotorsathighervoltages .

Defining a Movement Profile Insomeapplications,suchasservodrivesforpositioningorrobotics,wefindawidevarietyofspecificoperationmodes, includingperiodicaccelerationor stalledmotoroperationwithhigh torqueand low rotational speed . Intheseoperatingmodesthesystemisnot“balanced”sincethemotorwindingsarenotallowedtoachievecompleteforwardandbackwardmagnetizationwithinasinglePWMcycle .Intheseconditionsthemotorwindingscanbecomeoverenergized,orsaturatedwithpower .Asthisbeginstooccur,increasedmotorcurrentnolongerproducesproportionalincreasesintorqueandincreasedheatingcanoccurintheFETsofthedevice .Insomecasesthemotorwindingsareoverenergizedtoobtainthetorquerequiredtobrakethemechanicalload,resultinginamajorchangeofthecurrentwaveformcharacteristic .

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Figure 6. Voltage (green) and current (blue) during a PWM pulse on phase A at a supply voltage of 10 V DC, a 25 kHz switching frequency and a 50% duty cycle.

Figure 7. Voltage (green) and current (blue) during a PWM pulse on phase A at a sup-ply voltage of 30 V DC, a 25 kHz switching frequency and a 50% duty cycle.

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In thesemodes of operation, tVI becomes smaller and a highcurrentpeakappearsattheraisingedgeofthePWMpulseasshowninFigure8 .Themorethesupplyvoltagev3increasesthehigherthecurrentpeakv2willbe .Thiscurrentpeakcanreachmultiplesofthecontinuouscurrent .TheplotsinFigure9depicttherelationshipbetweendutycycle,continuouscurrentandtemperatureatasupplyvoltageof20VDC .ByseparatingtheperformancegraphinFigure9intotwoparts,thebalancedoperation(A)andtheoverenergizedoperation(B)canbebetteranalyzed .Duringbalancedoperation,thecurrentandvoltagewaveformsfollowtherelationshipsshowninFigure5 .Thephaseshift tVI increasesso that thecontinuouscurrentandtemperatureincreaseonlyslightly .Whenthemotorwindingstartsbeingoverenergized(B),tVIdecreasesrapidlyasshownin Figure 8 .The current peak at the rising edge of thePWMcycleincreasesthetotalaveragecurrentandthethermalload .Overenergizedconditionsnormallyhappeninthe60-70%dutycyclerange .ThisrangecorrespondswiththekneeofthecurvesinFigure9 .Theexactpointisdependentonboththequalityofthemotorandtheoperatingcondition .Theoperatingconditionwith theexpectedmaximal thermal loadand itsmaximal timeintervalisthebasisfortheselectionofthebestmountingtech-niqueandheatsinkingmethod .The64-pinPowerQFPpackageused for theSA306-IHZandSA57-IHZ lends itself to different kinds ofmounting and heatsinking options which can be used under different operatingconditionstorealizethebestresultsaccordingtoperformanceandtotalsystemcost .Inthefollowingsectionssuggestionsareprovidedonhowtoselectthebestsolutionforyourapplication .

Cooling Options and Results with the 64-pin QFP PackageThe size and orientation of the heatsinkmust be selected tomanagetheaveragepowerdissipationofthedriverICs .Applica-tionsvarywidelyandvariousthermaltechniquesareavailabletomatchtherequiredperformance .Thissectiondiscussesseveraltechniquesthatareappropriatefordifferentpowerlevels .Thefollowingexamplesweredevelopedtoevaluatedifferentmountingandcoolingoptionsandtodefinetheirbasiccapabilities .DerivationofmaximumpowerandthermalsystemdynamicsisshowninAppendixA .Twodifferentsetsof testswereconductedon theSA306-IHZ .For thefirstsetof tests thepowerdevicesweremountedonaPCBwithaDigitalSignalController(DSC) .ThisDSCoffersanenhancedmotioncontrolinterfaceandallowsadesignertocommutatethemotorindifferentways .Forthesecondsetoftestsapowertestbenchwasusedtoshowtheeffectofsteadystatecurrentsonpowerdissipationandthermalperformance .

SMT Mounting without Heatsink in Low-Power Applications ThemostcosteffectivewayofmountingeitheroftheSA306-IHZorSA57-IHZdevicesistosolderthemdirectlyonaPCBwithoutanyfurtherheatsinkingcomponents .Theheatslugofthe64-pinQFPpackageoffersoptimizedheattransfertothePCB(asshowninFigure10) .TheICpackage,includingtheheatslug,canbesolderedtothetoplayerofthePCB .Beneaththeheatslug,severalviascanbeusedtooptimizeheattransfertothebacksideoftheboard .Thecopperareausedforthepowergroundisalsousedforbetterthermalcouplingtotheambientair .Aspartofthetests,semiconductortemperaturesensorwasplacedinthemiddleoftheheatslugareatomeasurethebackside(maximum)heatslugtemperature .Atanoutputpowerof20W,atemperaturegradientof99 .6°Kwasobtained .Thisindicatesthatthejunctiontemperaturewillbeapproximately130°Catanambienttemperatureof30°C .TheplotsinFigure11denoteavalueof136°Catanambienttemperatureof27°Cwithatotalthermalresis-tanceforthesystemofapproximately5 .343K/W .

Voltage

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Figure 8. Voltage (green) and current (blue) during a PWM pulse over energizing a mo-tor winding

Figure 9. Temperature and continuous cur-rent vs. PWM duty cycle during a stalled motor operation

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Using the thermal resistance value, the maxi-mumoutputpowercanbecalculatedforanappli-cationcoveringtheindustrialtemperaturerangeupto+85°Catapproximately9W .Thisresultisvalid for static operation under the specific setofvoltageandcurrentconditions .Inapplicationswith frequentaccelerationsanddecelerations itisimportanttoknowthethermalresponsetime .Thisvalueprovidesanindicationforthetimere-quiredtotransferaspecificamountofheatfromthedietothebacksideoftheboard .Derivationsforcalculatingthermalresistanceandmaximumpowerdissipation canbe found inAppendixA .ShowninFigure12isthetemperaturecharacter-isticofthegroundplanefromtheambienttothemaximumtemperaturewhenthemotorisdrivencontinuouslyatanoutputpowerof9W .

ConclusionAlthoughthemaximumoutputpowerratingof8Wto9Wseemslow,thesedevicesarestillcapableofdeliveringPEAKcurrentsupto15Aforseveralseconds .Inaddition,increasedvoltageorreducedambienttemperaturemayresultinincreasedpowercapability .ThismakestheSA57-IHZandSA306-IHZunusuallywellsuitedforapplicationsusinggearswithhightransmissionrateswherelargemechanicalloadshavetobeacceleratedthroughstart-upphaseandendathighspeedswithlowerpowerrequirements .Thismountingtechniqueisappropriateforsmaller,low-powerorhigh-speeddrivesincostsensitiveapplicationssuchasfans,pumps,scanners,surveillancecameras, labelingmachinesorpaperfeeders–allwheresizeandproductioncostsarecrucial .

SMT Mounting with Thermal Pad and Heatsink for Mid-Range Power ApplicationsForapplicationswherehigheroutputpowerisrequiredforalongerperiodoftime,buteaseofproductionisstillamaindesignissue,anadditionaltestboardwasdesignedwheretheSA306-IHZisstillmountedintheconventionalmanner .However,beneaththeICisa100mm²cutout .AthermalpadisusedtotransfertheheatfromtheheatslugdirectlytotheHS33heatsink .Thethermalpadismadefromapolymerthatcontainsmaterialschosenforlowthermalresistance,suchassilver .Thesepadsremainsoftforyearsandthereforecanaccommodateanyshiftingthatmayoccurtoequalizebetweenmaterials

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Figure 10. Bottom layer of the PICtail™ plus test board for low power applications

Figure 11. Temperature characteristic of the SA306-IHZ mounted on test board for low-power applications

Figure 12. Thermal response time at MAX output power of 9 W

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withdifferentcoefficientsofexpansionthatislikelytooccurduetotemperaturechanges .Onthemid-powertestboarda2mmthermalpadisusedtofillthegapbetweentheheatslugandtheheatsink(seeFigure13) .ThethermalpadchosenisaBerquistTMGapPad5000S35 .Thecutoutareais10mmx10mm .

Thisboarddesignprovidesthreeadvantages:• TheSA306-IHZisstillmountedlikeaconventionalSMTpart,includingtheside-wingsoftheheatslug .• Thermalcouplingisalwaysconstant,evenifthethicknessofthePCBsvariesfrom1 .5mmto1 .7mm .• Accurateplacementoftheheatsinkisnotrequired .

ShowninFigure14isthebackofthemid-powertestboard .The100mm²heatslugarea(red)hasbeencutouttoaccom-modatethethermalpad .ThetemperaturesensorhasbeenmovedtoanaturalcutoutoftheHS33heatsink .Theheightofthesensorpackageisexactlytheheightofthecutout .Thethermalcouplingcomesaboutduetotheintimatecontactofthepackagetopwiththeheatsink .Theheatsinkismountedwithtwoscrews .Notethatthelightgreenareaoftheheatsinkhasdirectcontactwiththedark-greengroundplanearea .Withthismountingtechniquethetotalthermalresistanceisreducedwhilethecontactareaisincreasedtoambientair .AlthoughtheHS33heatsinkisusedonthistestboard,itisnotessential .Athermalpadcouldalsobeusedtocoupletheheatslugtotheenclosuretoimprovethethermalpath .The100mm²x1 .5mmthickthermalpadwithitsthermalconductivityof86W/mKhasathermalresistanceof0 .174K/W .Whenthisvalueiscomparedtothe

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Figure 13. Simplified cross sectional view of the thermal system of the mid-power test board

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Figure 14. Bottom layer of the test board for mid-range power applications

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Figure 16. Thermal response time at output power of 20 W

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previousmodelwithvias,thissolutionprovideslessthana10thofthethermalresistance .Usingthethermalpad,approximately51Wofcontinuousoutputpowerisachievedwithdietemperaturesofapproximately+155°Catanambienttemperatureof+26°Cusingblockcommutation .Themaximumpoweroutput,up to+85°Cfor thisconfiguration isap-proximately20Watthechosenvoltageandcurrent .Withthisso-lution it ispossible todouble theoutputpowerat similar systemtemperatures .Whatismostimportantistheimprovedperformanceovertheshortheatpathtotheheatsinkdueinparttothelowther-malresistancethroughthethermalpad .

ConclusionThethermalpadofferseasysystemassemblyandgoodthermalperformanceatmoderatepowerratings .Theshortthermalpathaf-fordsbenefitsinacceleratingheavymechanicalloadsforalongertime .Thisisanappropriatesolutionforawidevarietyofindustrialapplicationsusinggearsandwithproductionrunsinmid-rangevol-umes .

Flipped-Over SMT Mounting with Heatsink for High-Power ApplicationsThe third test boarduses the sameheat sinkingmethodas theApex Microtechnology Evaluation Boards DB63R and DB64R .ThistechniqueisalsodescribedintheHigh-Power Heatsinkingsection .This time the SA306-IHZ is flipped over and mounted . TheheatsinkisthenmounteddirectlyontotheheatslugoftheIC .ThisshortensthethermalpathfromtheICtotheambientairandreducesthetotalthermalresistanceofthesystemtoamin-imumof1 .674K/W .ShowninFigure17istheassemblyoftheICandheatsinkonthetestboardforhigh-powerapplications .Thepositionofthetemperaturesensor,themountingmethodoftheheatsinkandthethermalcouplingtothegroundplanearestillthesame .Theonlydifference is that theSA306-IHZ isflippedoverandsol-deredinaprecisecutoutfromthebacksideoftheboard .Apic-tureoftheactualmountingtechniquecanbeseeninFigure20 .As a result of thismounting technique, the characteristics ofthetemperatures inFigure18becomequite linearacrossthewholepowerrange .Theoutputpoweratadietemperatureof135°Cisapproximately52Wat+25°Candapproximately29Wat+85°C .Thissolutionprovidesonlyaslightlyhighermaxi-mumoutputpower than the thermal pad .However, the shortthermalpathhas itsadvantages indynamicapplications .Thefastresponsetimebetweenthedieandtheheatslugprovidesbenefitsinapplicationswheremultipleaccelerationsoccurovershorttimespans(seeFigure19) .

ConclusionTheprincipaladvantageoftheflipped-overmountingisthat itprovidesashortheatpathforsupportinghighdynamicapplica-tions .Thefreeaccesstotheheatslugoffersawidevarietyofheatsinkingmethodsandmoreeffectivecoolingoptions .

GroundPlane

HeatSlugTemperatureSensor

Heat Slug Temperature vs Electrical Output Power

0

20

40

60

80

100

120

140

160

180

Hea

t Slu

g Te

mpe

ratu

re [°

C]

- Flipped-Over SMT w. Heatsink HS33 -

0 10 20 30 40 50 60 70Total Electrical Power [W]

Block Commutation

Sinusoidal Commutation

Figure 17. Bottom layer of the test board for high-power applications

Figure 18. Temperature characteristic of the SA306-IHZ mounted flipped over on the test board for high power applications using the HS33 heatsink

Figure 19. Thermal response time at a output power of 25 W

Thermal Response Time at MAX Output Power- Common SMT Mounting Technique -

00 20 40 60 80 100 120 140 160

Time [sec]

20

40

60

80

100

120

140

Hea

t Slu

g Te

mpe

ratu

re[°

C] t90

T90

T30

t30

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Comparisons of the Low-Power, Mid-Range Power and High-Power Test BoardThefollowingtablecomparesthebenchmarkvaluesforeachmountingtechniqueinaside-by-sidecomparison .Pleasenote that thecolumns for themaximumoutputpowerare rated forambient temperaturesof+25°Cand+85°C .ThereisamoredetailedchartforthepowerdissipationderatingacrossambienttemperaturesinthePower Dissipation Deratingsection .

Table 1. Temperature characteristics of the SA306-IHZ mounted flipped over on the PICtail™ Plus test board for high-power applications using the HS33 heatsink

MountingTechnique

Calculated MAX Output Power@ +85°C (W)

MAX Output Power at

Test Voltage@ +25°C (W)

MAX Continuous Current at Test

Voltage@ +25°C (A)

Total Thermal Resistance RTH (K/W)

Average Ther-mal Response

Time k90(K/sec)

CommonSMT 9 .0 19 0 .93 2 .789-5 .343 1 .323CommonSMTwithPad&Heatsink 19 .8 38 2 .73 0 .628-2 .529 1 .869

FlippedOverwithHeatsink 23 .9 52 4 .12 0 .801-1 .726 2 .637

AllvaluesinthisSectionaretheresultofactualtestsunderlaboratoryconditionsandshouldprovideanindicationofthecapabilitiesofseveraldesignvariants .ThebehavioroftheseICsmaydifferunderdifferentdesignandopera-tionconditions .

High-Power Heat SinkingSincetheSA306-IHZisdesignedforhigherpowerapplications,itisalsonecessarytodetermineheatsinkingfac-torsthatallowforhigherpowerdissipation .Becauseoftheconcentrationofheatasaresultofpowerdensity,itisimportanttouseheatsinktypesthatprovideshortthermalpaths .Forthisreasonlargeextrusionswithfinsonwidecentershavenotbeenincludedinthisevaluation .Dissipationcapacitywithtwotypesofaluminumextrusionswereevaluated .TheHS33isasmallheatsink,0 .4"x0 .4"x1 .5",thathasfourfinson0 .118-inchcenters .ThisconfigurationplacesallfourfinsdirectlyundertheheatslugoftheSA306-IHZpackage .TheHS33heatsinkisshowninFigure20 .ThisheatsinkisusedontheApexMicrotech-nologyDB64RevaluationboardfortheSA306-IHZ .Thesecondheatsink inFigure21 isapinconfigurationwithdimensionsof1"x2"x2" .Thepinsare0 .070" indiameteron0 .137"centers .Thishighlyeffectiveheatsinkprovidesmultipleheatpathsdirectlyunderthesilicon .Becauseofitssize,itiscapableofsafelydissipatingmuchhigherpowerlevelsthantheHS33 .Thetestset-upfortheHS33usedtheApexMicrotechnologyDB64Revaluationboardwithan8Ωresistiveload .Toincreasepower,theDCvoltageacrosstheboardandloadwereincreased .Thisset-updoesnotfullysimulatetheoperationofamotor .However,itdoesallowstabletemperaturestobeachieved,thusfacilitatingmeasurement .Thisset-upmetthegoalofallowingpowerdissipationanddietemperaturemeasurementswithvaryingheatsinks .

Figure 20. HS33 Finned Heatsink with SA306-IHZ Figure 21. Pin Heatsink with SA306-IHZ

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Table 2. Results of Thermal Testing

Configuration Orientation Convection Dissipated Power (W)

Output Power @ 85% Effi-

ciency (W)

Heatsink Temp MAX

(ºC)

Junction Temp MAX

(ºC)

Thermal Resis-tance (ºC/W)

HS33andPCB

Horizontal,HSdown Natural 7 .5 42 .5 139 145 16

HS33andPCB

Vertical,HSvertical Natural 9 51 138 146 13

HS33andPCB

Horizontal,HSdown Forced 17 96 .3 127 140 6 .8

PinHeatsink Horizontal,HSup Natural 16 90 .7 125 141 7 .3

PinHeatsink Horizontal,HSdown Forced 39 221 .0 79 113 2 .3

Fivedifferentthermalconfigurationsweretested,andtheresultsareshowninTable2 .Notethatconsistentwithrealworldapplications,thedifferencesintestset-upandmeasurementtechniquesbetweenthistestandtheprevioussectionproducedifferenttestresultsandpowercapabilities .Thisistobeexpectedandshowsthatdesignersmusttakeintoaccounttheirspecificoperatingenvironmentwhenevaluatingthermalperformance .ThreerowsofHS33dataareshowninTable2 .Thefirsttwousedconvectioncoolingonly .Asonemightassume,placingtheheatsinkinahorizontalpositiondoesnotproduceaseffectiveacoolingsurfaceasplacingtheheatsinkvertically .Withaverticalheatsinkunderconvectionconditionstheheatsinkdissipates9W,allowing50Wofmotorpowerata+140°Cjunctiontemperature .Thermalresistanceresultswiththeverticalheatsinkmeasured13°C/Wjunctiontoair .The thirdHS33testwasperformedusingasmall fanblowingacross theboard .Theobjectwas tosimulateanenclosedhousingwithafanprovidingoutsideair,similartoaPC .Inthiscase,dissipationimprovedto17Wandallowingalmost100Watthemotorat85%efficiency .Thermalresistancejunction-to-casealsoimproveddramati-callyto6 .8°C/W .Subsequent testingwith this configuration suggests that use of a small fan, in this case a 1" Sunon unit (PNGM0502PFV1-8,similartoMPUorgraphicschipFAN),directedontotheHS33,allowsheatsinkdissipationinex-cessof21Wandsuggestingmotorpowerofapproximately120W .Undertheseconditions,dietemperatureshouldnotexceed+140°C .For full-power applications, the pin heatsink was tested . The pin heatsink was tested under both convectionandforced-airconditions .ThetestboardandfancanbeseeninFigure22 .Thefanusedwasa2"Sunon(PNKDE1205PHV2)capableof595linearfeetperminuteofairflow .Theretainingscrewsforthefanalsoholdtheheatsinkandthedeviceinplaceontheboard .ThetestconfigurationforthepinheatsinkusedamodifiedPCBlayoutanda4Ωresistiveload .Undernormalconvectionconditions,thisheatsinkwasabletodissipate16Wofpower,allowingmotorpowerof91W,similartotheHS33withairflow .However,whenusingforcedairwiththisheatsink,powerdissipationwithintheheatsinkincreasedtoalmost40W,allowingforgreaterthan220Wofpowertothemotor .Inaddition,thejunctiontemperaturedroppedto+113°C,sug-gestingthisisahighreliabilityconfigurationwithroomforincreasedpoweroutput .The junction to air thermal resistance also improved dramatically,measuring2 .3°C/W .Withsomeincreaseindietemperatureandawellde-signedsystem,thepinheatsinkinaforcedairconfigurationwillallowthefull48Wand5Aratingofthedevice .Useofaheatsinkdoesincreasesystemcost .Insomecases,thedesignermayfindthatasmallamountofairflowacrossanHS33-typeheatsinkpro-videssufficientdissipationandlowercostsversusalargerheatsink .How-ever,forfullpowerdissipation,useofalargeheatsinkwithforcedairmaybenecessary . Figure 22. Pin heatsink and test

board with fan attached

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Power Dissipation DeratingToprovidehigh-reliabilityoperation,thedietemperaturemustbekepttoasafelevel .Thedesignermustconsidertotalpower,heatsinkcapabilityandambienttemperatureinordertomakeappropriatesystemchoices .ThegraphinFigure23showsderatingcurvesforpower .Curvesaregivenforloadpowerversusheatsinkandambienttem-peratures .Inaddition,curvesaregivenforsingle-sidedanddoublelayerPCBs .Figure23alsoshowsthatbelow+25°CtheSA306-IHZcanoperateabove300W,assumingdietemperaturesareheldtoareliablelevel .Above+30°Cimprovedheatsinkingmustbeemployedorthedevicemayneedtobede-rated .Again,actualpowercapabilityisafunctionofoperatingvoltage,efficiency,ambienttemperatureandheatsinkcapability .Actualderatingwillvarybasedonthesefactors .Itshouldalsobenotedthatthederatingofheatsinksthemselveschangewithtemperatureandthereforewithpowerdissipation .AsshowninFigure24fortheHS33,bothairflowandambienttemperatureplayaroleinaccuratelymodelingthedissipationcapabilityoftheheatsink .

ConclusionApplicationsvarywidelyandvarious thermal techniquesareavail-able tomatch the required performance .The size and orientationoftheheatsinkmustbeselectedtomanagetheaveragepowerdis-sipationoftheIC .StandardPCBmountingtechniquesenablethesedevicestodissipateasmuchas9W .Theuseofaheatpadorthepatent-pending mounting technique shown in Figure 20, with theSA306-IHZinvertedandsuspendedthroughacutoutinthePCB,isadequateforpowerdissipationexceeding20W .Infreeair,mountingthePCBperpendiculartotheground,sothattheheatedairflowsupwardalongthechannelsofthefins,canprovideimprovedcooling .Inapplicationsinwhichhigherpowerdissipationor lowerjunctionorcasetemperaturesarerequired,alargerheat-sink or circulated air can significantly improve performance .A pinheatsink can successfully dissipate the thermal energy generatedwhendrivingmotors inexcessof200W .Byusing the techniquesinthisApplicationNote,theSA57-IHZandSA306-IHZcanprovidelong-term,reliablemotorcontrolinaverycompactpackage .

References1 . SA57-IHZ Pulse Width Modulation AmplifierDataSheet,www .apexanalog .com2 . SA306-IHZ Pulse Width Modulation AmplifierDataSheet,www .apexanalog .com3 . 3-Phase Switching AmplifierApplicationNoteSA306-IHZ,AN46,www .apexanalog .com

SA306-IHZ Output Power Dissipation Derating

0

50

100

150

200

250

300

350

0 20 40 60 80 100 120 140Ambient Temperature, Ta (°C)

Load

Pow

er, P

(W)

HS33 and PCB Horizontal/HSdown - Natural HS33 and PCB Vertical/HSvertical - Natural HS33 and PCB Horizontal/HSdown - Forced

PIN Heatsink Horizontal/HSdown - Forced

Note: Careful consideration of operating current isrequired so not to exceed maximum wire temp of +150° C.

0

50

100

150

200

250

300

350HS33 and PCB Horizontal/HSdown - Natural HS33 and PCB Vertical/HSvertical - Natural HS33 and PCB Horizontal/HSdown - Forced PIN Heatsink Horizontal/HS up - Natural PIN Heatsink Horizontal/HSdown - Forced Typical SMT device on a Double-Sided BoardTypical SMT device on a Single-Sided Board

THE

RM

AL

RE

SIS

TAN

CE

(°C

/W)

AIR FLOW (LFM)200 1000800600400

0.00

20.00

15.00

10.00

5.00

1.0 5.14.13.02.0AIR FLOW (m/s)

Figure 23. Load power derating curves for ambient temperature, Ta (°C)

Figure 24. Heatsink thermal resistance versus air flow

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Appendix A – Derivation of Maximum Power and Thermal Dynamics Values for PCB Based TestsSection A.1 Result for Common SMT Mounting with Heatsink in Low-Power Applications

ThetotalthermalresistanceofthecommonSMTmodelforaSA306-IHZis4 .98K/W(Kelvinperwatt) .Tocalculatethemaximumoutputpowerforthismountingtechniquewithoutspecificvaluesforvoltageorcurrent,wehavetotransformtheequation:

(12)Where: T0=SA306-IHZorSA57-IHZjunctiontemperature T2=ambientairtemperature PE=electricaloutputpower

TheplotsinFigure7denoteavalueof+136°Catanambienttemperatureof+27°Cwithathermalresistanceofapproximately5 .343K/W .Thedifferenceisduetothefactthatthefactorincludesthethermalresistanceintotheambientairandisthereforeacombinedvaluefor(αA)-1+Rth .WhenweusethevalueforthethermalresistanceofthePICtailTMPlusTestBoardtocalculatethemaximumoutputpowerforanapplicationcoveringthewholeindustrialambienttemperaturerangeupto+85°C,themaximumcon-tinuousoutputpowerisapproximately9W:

(13)

Where: T0=SA306-IHZorSA57-IHZjunctiontemperature T2=ambientairtemperature RT=totalthermalconductivity PMAX=MAXelectricaloutputpower

ShowninFigure12isthetemperaturecharacteristicofthegroundplanefromtheambienttothemaximumtem-perature,whenthemotorisdrivencontinuouslyatamaximumpowerof9W .ForlatercomparisonsthefocusisonthetransferratioΔT30/Δt30andΔT90/Δt90,describedbyaquotientKfortheslewratefromthecoldstateto30%and90%,respectively,ofthemaximumtemperature .Theslewratequotientkisanindicationforthequalityofthedynamicbehaviourofthethermalsystem:

(14a)

(14b)Where:

k=slewratequotientinKelvinpersecond Δθ=temperaturedifferenceinKelvin Δt=durationinseconds T0=temperatureattimet0 T1=temperatureattimet1

T0 - T2 = ( + Rth) • RDS(ON) • IC21

αA

T0 - T2 = ( + Rth) • PE

1αA

PMAX =T0 - T2

Rt

PMAX = = 9.4W135°C - 85°C

5.343 KW

k = = ∆θ T1 - T0∆t t1 - t0

k30 = = 1.31754.5°C - 26.8°C21 sec

Ksec

k90 = = 0.988109.7°C - 26.8°C84 sec

Ksec

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Section A.2 SMT Mounting with a Thermal Pad and Heatsink for Mid-Range Power Applications

Thefirstorderapproximationofthethermalmodelgivesathermalresistanceof2 .678K/W .Thefollowingequationisusedtoverifytherealvalueofthecompletesystem:

(15)

Where:T0=SA306-IHZorSA57-IHZjunctiontemperatureT2=ambientairtemperaturePE=electricaloutputpowerRt=totalthermalconductivity

Nextistofindthemaximumpowerratingtocoverthetotalindustrialrangeforambienttemperaturesupto+85°Catamaximumdietemperatureof+135°C .

(16)

Where:T0=SA306-IHZorSA57-IHZjunctiontemperatureT2=ambientairtemperatureRt=totalthermalconductivityPMAX=Max .electricaloutputpower

ThechartinFigure12showstheimprovementsofthethermaldynamics .Theslewratequotientsk30andk90are:

(17a)

(17b)

Where:k=slewratequotientinK/sΔθ=temperaturedifferenceinKelvinΔt=durationinsecondsT0=temperatureattimet0T1=temperatureattimet1

= ( + Rth)1αA

T0 - T2 = ( + Rth) • PE

1αA

T0 - T2PE

Rt = = 2.529155°C - 26°C51W

KW

PMAX =T0 - T2

Rt

PMAX = = 19.8W135°C - 85°C

2.529 KW

k = = ∆θ T1 - T0∆t t1 - t0

k30 = = 1.93855.8°C - 26.8°C15 sec

Ksec

k90 = = 1.407114.7°C - 26.8°C63 sec

Ksec

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Section A.3 Flipped Over SMT Mounting with Heatsink for High-Power Applications

Thefollowingequationsareusedtocalculatetheestimatedmaximumoutputpowerforambienttemperaturesof+85°C:

(18)

Where:T0=SA306-IHZ/SA57-IHZjunctiontemperatureT2=ambientairtemperatureRt=totalthermalconductivityPMAX=MAXelectricaloutputpower

Usingthismountingtechnique,thefollowingvaluesareobtainedfork30andk90:

(19a) (19b)Where:k=slewratequotientinK/sΔθ=temperaturedifferenceinKelvinΔt=durationinsecondsT0=temperatureattimet0T1=temperatureattimet1

k = = ∆θ T1 - T0∆t t1 - t0

k30 = = 2.63858.5°C - 26.8°C12 sec

Ksec

k90 = = 1.792121.8°C - 26.8°C53 sec

Ksec

PMAX =T0 - T2

Rt

PMAX = = 28.97W135°C - 85°C

2.096 KW

= ( + Rth)1αA

T0 - T2PE

Rt = = 2.096135°C - 26°C52W

KW

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