如何计算散热器的散热功率

1、如何计算散热器的散热功率CalculationCornerEstimatingParallelPlate-FinHeatSinkThermalResistanceRobertE.Simons,AssociateEditor,IBMCorporationAsnotedpreviouslyinthiscolumn,thetrendofincreasingelectronicmodulepowerismakingitmoreandmoredifficulttocoolelectronicpackageswithair.Asaresultthereareanincreasingnumberofappl

2、icationsthatrequiretheuseofforcedconvectionair-cooledheatsinkstocontrolmoduletemperature.AnexampleofawidelyusedtypeofheatsinkistheparallelplateconfigurationshowninFigure1.Figure1.Parallelplatefinheatsinkconfiguration.Inordertoselecttheappropriateheatsink,thethermaldesignermustfirstdeterminethemaximu

3、mallowableheatsinkthermalresistance.Todothisitisnecessarytoknowthemaximumallowablemodulecasetemperature,T,themodulepowerdissipation,P,andthethermalcasemodresistanceatthemodule-to-heatsinkinterface,R.Themaximumintallowabletemperatureattheheatsinkattachmentsurface,T,isgivenbasebyThemaximumallowablehea

4、tsinkresistance,R,isthengivenbymaxwhereT,isthetemperatureofthecoolingairattheinlettotheheatair-insinkpassages.Atthispointmanythermalengineerswillstartlookingatheatsinkvendorcatalogs(ormorelikelytodaystartsearchingvendorsontheinternet)tofindaheatsinkthatwillfitintheallowablespaceandprovideaheatsinkth

5、ermalresistance,R,lessthanRatsomehsmaxspecifiedflowrate.Insomecases,itmaybeusefultodoasizingtoestimateRforvariousplate-finheatsinkdesignstodetermineifahsfeasibledesignconfigurationispossible.Theremainderofthisarticlewillprovidethebasicequationstodothis.Thethermalresistanceoftheheatsinkisgivenbywhere

6、histheconvectiveheattransfercoefficient,Aistheexposedbasebasesurfaceareabetweenfins,Nisthenumberoffins,isthefinfinfinefficiency,andAisthesurfaceareaperfintakingintoaccountbothfinsidesofthefin.Toproceedfurtheritisnecessarytoestablishthemaximumallowableheatsinkvolumeintermsofwidth,W,height,H,andlength

7、intheflowdirection,L.Itisalsonecessarytospecifyafinthickness,t.Usingtheseparametersthegap,b,betweenthefinsmaybedeterminedfinfromTheexposedbasesurfaceareamaythenbedeterminedfromandtheheattransferareaperfinfromAtthispointitisnecessarytospecifytheairflowrateeitherintermsoftheaveragevelocity,V,betweenth

8、efinsoravolumetricflowrate,G.Ifavolumetricflowrateisused,thecorrespondingairvelocitybetweenthefinsisTodeterminetheheattransfercoefficientactinguponthefins,anequationdevelopedbyTeertstraetal.1relatingNusseltnumber,Nu,toReynoldsnumber,Re,andPrnumber,Pr,maybeemployed.ThisequationisThePrandtlnumberiswhe

9、reisthedynamicviscosityofair,cthespecificheatofairatpconstantpressure,andkisthethermalconductivityofair.TheReynoldsnumberusedin(8)isamodifiedchannelReynoldsnumberdefinedaswhereisthedensityofair.Equation(8)isbaseduponacompositemodelspanningthedevelopingtofullydevelopedlaminarflowregimesandwaswheremis

10、givenbyvalidatedbytheauthors1bycomparingwithnumericalsimulationsoverabroadrangeofthemodifiedchannelReynoldsnumber(0.26Re175)bandwithsomeexperimentaldataaswell.UsingtheNusseltnumberobtainedin(8)theheattransfercoefficientisgivenbyliNli-cmNote:KfinshouldbeK.20051017wherekisthethermalconductivityofthehe

11、atsinkmaterial.Thefinefficiencyofthefinsmaybecalculatedusing(12)Usingtheseequationsitispossibletoestimateheatsinkthermalperformanceintermsofthethermalresistancefromthetemperatureatthebaseofthefinstothetemperatureoftheairenteringthefinpassages.ItmaybenotedthattherelationshipforNusseltnumber(8)include

12、stheeffectofthetemperatureriseintheairasitflowsthroughthefinpassages.Toobtainthetotalthermalresistance,R,tothebaseofthetotheatsinkitisnecessarytoaddinthethermalconductionresistanceacrossthebaseoftheheatsink.ForuniformheatflowintothebaseRtotisgivenbyForpurposesofillustrationtheseequationswereusedtoes

13、timateheatsinkthermalresistancefora50 x50mmaluminumheatsink.TheeffectofincreasingthefinheightandthenumberoffinsisshowninFigure2foraconstantairvelocityandinFigure3foraconstantvolumetricflowrate.Inbothcasesitmaybeseenthattherearelimitstohowmuchheatsinkthermalresistancemaybereducedbyeitherincreasingfin

14、lengthoraddingmorefins.Ofcoursetodeterminehowaheatsinkwillactuallyperforminaspecificapplicationitisnecessarytodeterminetheairvelocityorvolumetricflowratethatcanbedeliveredthroughtheheatsink.Todothisitisnecessarytoestimatetheheatsinkpressuredropcharacteristicsandmatchthemtothefanorblowertobeused.This

15、isatopicforconsiderationinafuturearticle.Figure2.Effectoffinheightandnumberoffinsonheatsinkthermalresistanceatanairvelocityof2.5m/s(492fpm).0030cULfirihfinhcighi(nun)Figure3.Effectoffinheightandnumberoffinsonheatsinkthermalresistanceatavolumetricairflowrateof0.0024m3/s(5CFM).References1.Teertstra,P.

16、,Yovanovich,M.M.,andCulham,J.R.,AnalyticalForcedConvectionModelingofPlateFinHeatSinks,Proceedingsof15thIEEESemi-ThermSymposium,pp.34-41,1999.如何计算散热器的散热功率计算角估计平行板翅式散热器的热阻罗伯特E西蒙斯，副主编，IBM公司正如以前在本专栏中，增加电力电子模块的趋势正在使越来越多的困难与空气冷却电子封装。结果有一个应用程序，需要强制对流风冷散热器来控制模块的温度越来越多。的一个广泛使用的散热器类型的例子是平行板的配置如图1所示。要做到这一点，图1。

17、平行板翅式散热器的配置。允许的最大散热片电阻，R最大，然后给出在新台的，是冷却空气入口温度对散热器的通道。供应商目录（或更可能今天开始在互联网上搜索供应商）许的空间，并提供一个散热器热阻，RHS,小于它可能是有用的做大小来估计各种板翅式散热器设计置是可能的。本文的其余部分将提供基本方程来做到这一点。该散热器的热阻由下式给出在这一点上许多热工程师将开始在散热器希望找到一个散热器，将适合在允R最大在某些特定的流量。在某些情况下，RHS,以确定是否一个可行的设计配其中h是对流换热系数，侮是裸露的基地面积鳍片之间，是每鳍表面积考虑到双方的鳍。要继续进一步就必须建立在宽度，W，高度Nfin是数量的鳍，鳍

18、鳍效率，H,并在流动方向上所允许的最大长度散AFIN热器体积，L.还必须指定一个翅片厚度，tfin。使用这些参数的差距，B之间的鳍，可确定要知道最大允许模块外壳温度，TCASE,模块的功耗，PMOD,并在模块到散热片接口，RINT热阻。最大的散热片附着在表面，TBASE,允许温度由下式给出为了选择合适的散热器，热设计者必须首先确定最大允许散热器热阻。基面的暴露面积，然后确定可能从和每翅片换热面积从此时有必要指定的平均速度，V之间的鳍或体积流量，G.,条款空气流速或者如果体积流量使用，相应的鳍之间的空气流速是要确定传热系数后，鳍，由Teertstra等人开发一个公式行事。1与努塞尔数，怒族，以雷诺数，重，和Pr数，错，可受聘。这个方程普朗特数其中是空气动力粘度，CP的空气定压比热，k是空气的导热系