Heat Pipe Reliability In HighPower ApplicationsWord文件下载.docx

Heat Pipe Reliability In HighPower ApplicationsWord文件下载.docx

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Useofheatpipesinhigh-power（>

150W）coolingapplicationshasbeenlimitedtocustomapplicationsrequiringeitherlowthermalresistanceorhavingaseverelyrestrictedenclosurearea.Thecostoftheselargerdiameterheatpipeswashighduetoalimitednumberofmanufacturersandhandmadeassemblytimes.

Fig.1.Theextremeheatloadsandfluxdensitiesexperiencedbythelatestgenerationofsemiconductorpowermodulesdemandtheuseofheatpipestoenhancebase-plateheatspreading.WhenheatpipesaremountedintheZ-axisofthe,perpendiculartothebase-mountingsurface（asshownabove）,theycanboorstfinefficienciesbynearly100%.

EnternowthelatestgenerationofIGBTandothersemiconductorpowermodules.Thesemodulesofferhighpoweroutputsandevenmorechallengingpowerdensities-coolingofthemodulesatfullratedoutputpowerisvirtuallyimpossible.Asinmodernmicroprocessors,theremovalofwasteenergyintheformofheathasarguablybecomethemostchallengingengineeringtaskofthemechanicaldesigneffort.

Theseheatloadsandfluxdensitiesaresohighthatinmanycasesconventionalair-cooledaluminumextrusionandevenbondedfinheatsinkswillnotprovidesufficientcooling.Usingforcedair-cooling,theycannotremoveheatfastenoughtokeepthemodulefromexceedingitsmaximumrecommendedjunctiontemperatures.Theintroductionofasolidcopperheatspreader（copperhas2Xtheconductivityofextrudedaluminum）intothebaseofanextrusionalsowillnotsuffice.

Historically,somehigh-powersystemshaveusedheatpipestoenhancebase-plateheatspreadingindifferentmodesasthesolutiontokeepsystemsusingair-coolingandkeepthemawayfromliquidcooling（Fig.1）.Aheatpipehasanapparentconductivitymanytimesgreaterthancopperandreliesonthelatentheatofvaporizationofaworkingliquidinsideaheatpipetooperate.

Reliabilityandlongevityoftheseclosed-loopcoolersandthesystemtheyareusedinnowbecomealargeissue.Whatwillhappeniftheheatpipestopsworking?

WhatMTBFcanbeexpectedfromtheselargediameterheatmovers?

Isthereanywaytoensuretheykeepworkinguptoandbeyondtheexpectedlifeofthesystem?

HeatPipeOperation

Basicallyaheatpipeisapartiallyevacuated,closedvesselthatrecirculatesasmallamountofworkingfluid,whichthroughtheadditionofheat,changesfromliquidtogas.Condensingthatgasbacktoaliquidandreleasingtheabsorbedheatrequiresadditionalcoolingsurfaceorothermeansofheatremoval.

Inoperation,aheatpipeabsorbssignificantamountsofheatintheevaporatorsectionasitreachesasettemperature.Theworkingfluidandthepartialpressureinsidethepipesetthistemperature.Heatofvaporizationofliquidallowsforhighquantitiesofheattobeabsorbedatgiventemperature.Thisissimilartoliquidwaterat100℃vs.steamat100℃.Theadditionalheatabsorptionisrequiredtochangephase.

Heatedgasmovestothecoldendoftheheatpipeatnearlythespeedofsoundandundernearlyisothermalconditions.Atthecondensersectionthegascoolsslightly,releasingtheheatgain,revertingwithlessthana5℃differentialfromoneendofthepipetoanother.

Operationtakesplaceatvirtuallythesametemperatureanddoesnotdependonwheretheheatentersorleavestheheatpipe.Dependingonpipediameter,thisprocesscanmovehundredsofwattsadistanceofmanyinches,offeringanapparentthermalconductivityofthousandsofW/mK（Fig.2）.

Heatpipesoffermanyadvantagesintheiruseandoperation.First,aheatpipebyitselfdoesnotremoveordissipateheat.Itisonlyaconduitthroughwhichheatcanbemovedfromonepointtoanotherwithalowthermalresistance.Tomakeitoperational,aheatpipemusthaveasmuchcoolingsurfaceareainformoffinsadtheequivalent-sizeair-coolingheatsinkwithoutaheatpipe.Itmustalsohaveahighconductivitythermalcontacttotheheatsourcetobringheatintotheheatpipe.

Heatpipesareorientation-sensitiveinrelationshiptogravity.Heatpipeswillcarrylargeamountsofheatwhentheyoperateinaheat-down,cooling-upattitude.Thisorientationallowsfortherapidreturnofcooledliquidtoevaporator.Heatpipesalsowilloperatewuthlittlelossofconductivityinahorizontalattitude.However,operationinaheat-up/cooling-downorientationmustbecarefullyengineered.Dependingonthestyleofwickorliquidreturncapillary,mostheatpipeswilllosesomeefficiency.

Z-axisHeatRemoval

Mosthigh-powerheatpipeapplicationshaveusedheatpipesorvaporchambers（aflatheatpipe）tohelpheatspreadingunderthebaseofapowermodule.Manytimesaseriesofroundheatpipesareembeddedinaheatsinkbasetohelpaveragethetemperatureofthealuminummountingplate.

Althoughthisisapositivestep,itstillleavesconventionalair-cooledfinsorextendedcoolingsurfacesoperatingatfinefficienciesoftenaslowas50%to60%.HeatpipesusedintheZ-axis,perpendiculartothebasemountingsurface,canofferfinefficienciesapproaching100%.

Fig.3.Acomparisonofefficiencyforahighratioextrusionheatsick（a）,abonded-finheatsink（b）,andaheatsinkwithheatpipeassemblyembeddedintheZ-axis（c）.Overallthermalperformancevariesfrom100%fortheextrusionto150&

forthebonded-finheatsinktogreaterthan200%fortheheatsinkwithheatpipeassembly.

Fig.3showsacomparisonoffinefficienciesandoverallthermalperformanceforthreetypesofheatsinks-ahighratioextrusion,abonded-finheatsink,andaheatsinkwithaheat-pipeassemblymountedintheZ-axis.

Foranextrusionwithafinareaof1X,thefinefficiencyis70%to90%,andtheoverallthermalperformanceoftheheatsinkofthesamesize,afinareaof1.5Xisachieved,whichproducesafinefficiencyof60%to80%andanoverallthermalperformanceof150%.Inthecaseoftheheatsinkwithheatpipeassembly,afinefficiencyof3Xispossible.Thisheatsinkachievesafinefficiencyashighas90%to95%andanoverallthermalperformancegreaterthan200%.

Heatpipeswiththeabilitytomoveheatwithnear-zerotemperatureriseareemployedasconduitstoeliminatethisfinefficiencyproblem.Z-axiscoolerdesignuseslargediameterheatpipesmountedthroughabaseplateheatspreader,perpendiculartotheheatsinkmountingsurface.Heatspreadersintegraltotheheatpipesarepositionedinthebaseplatetomakemaximumcontacttothehighheatfluxsites（diepositions）underthepowermodule.

Theheatpipesmoveheatawayfromthebaseplateanduseaseriesofthin,copperfinsattachedtothepipestodissipatethisheatintoaforcedairstream.Duetotheeffectsoftheheatpipe,thecopperfinfurthestfromthebaseplatewillhavevirtuallythesamefinefficiencyastheclosestfin.Thisallowssignificantincreasesinfincountandcoolingsurfaceoveranextrusiontypeheatsink.InmanycasesZ-axiscoolerscanincreasecoolingofapowermodule（IGBTorsimilardevice）byupto100%

HeatPipeReliability

Overthepast40yearsofheatpipedesignandmanufacture,reliabilityandconsistencyofperformancehavealwaysbeenissues.Doheatpipesleakoverlongperiodsoftime?

Cantheycontinuetooperateattheirlimitsforyears?

Whataretheirlimits?

Tounderstandheatpipelongevityandpotentialfailure,itisnecessarytounderstandthemanufacturingstepsanddesignforreliability.Virtually100%ofprematurelyfailedheatpipescomefrom:

●Impropercleaning/oxidationoftheinterior.

●Improperfillingorcharging.

●Poorsealingorpotentialleakageovertime.

●Incompatiblematerials.

●Overtemperatureduringassembly.

Failuresalsocanbeseenastheresultofdesignersnotunderstandingthelimitationsofheatpipesinapplicationandlong-termuse.Intermsoflongevity,howaheatpipeisappliedisjustasimportantashowitisassembled.Inshortthesefailuremodesare:

●Dry-out（highheatloads/heatfluxes）.

●Improperorientationtogravity.

●Sealing/crimpproblems.

●Flexfailureduetoshaping.

●Catastrophicfailureduetotoohightemperatureatassemblyorinoperation（Fig.4）

PredictingOperationalLife

Heatpipesaresimilartosemiconductorelectronicsinthattheydemonstratehigherfailureratesatstartup,duetoinitialinfantmortality,andattheendoflife,duetowear-out.Afterpassingthefirstfewhoursofoperation,aheatpipewillnormallyoperateformanytensofthousandsofhoursbeforefailureoccurs.

Inmanyreal-life,highduty-cycleapplications,largediameterheatpipeshavebeeninoperationformorethan20yearswithoutfailure.Theseapplicationsincludesteel-wheellocomotiveandtractiondrive,electricallypoweredpeoplemovers,aswellaswindpowergenerators,high-horsepoweracmotordrivesandregionbuses.

Onepredictorofpotentialheatpipelifeistheuseofshort-term,high-temperaturetestingtoinducefailureandmathematicallypredictwear-out.Theuseofcontrolledtemperaturechamberstoa