1、如何計算散熱器的散熱功率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.如何計算散熱器的散熱功率計算角估計平行板翅式散熱器的熱阻羅伯特E西蒙斯，副主編，IBM公司正如以前在本專欄中，增加電力電子模塊的趨勢正在使越來越多的困難與空氣冷卻電子封裝。結果有一個應用程序，需要強制對流風冷散熱器來控制模塊的溫度越來越多。的一個廣泛使用的散熱器類型的例子是平行板的配置如圖1所示。要做到這一點，圖1。

17、平行板翅式散熱器的配置。允許的最大散熱片電阻，R最大，然后給出在新臺的，是冷卻空氣入口溫度對散熱器的通道。供應商目錄（或更可能今天開始在互聯網上搜索供應商）許的空間，并提供一個散熱器熱阻，RHS,小于它可能是有用的做大小來估計各種板翅式散熱器設計置是可能的。本文的其余部分將提供基本方程來做到這一點。該散熱器的熱阻由下式給出在這一點上許多熱工程師將開始在散熱器希望找到一個散熱器，將適合在允R最大在某些特定的流量。在某些情況下，RHS,以確定是否一個可行的設計配其中h是對流換熱系數，侮是裸露的基地面積鰭片之間，是每鰭表面積考慮到雙方的鰭。要繼續進一步就必須建立在寬度，W，高度Nfin是數量的鰭，鰭

18、鰭效率，H,并在流動方向上所允許的最大長度散AFIN熱器體積，L.還必須指定一個翅片厚度，tfin。使用這些參數的差距，B之間的鰭，可確定要知道最大允許模塊外殼溫度，TCASE,模塊的功耗，PMOD,并在模塊到散熱片接口，RINT熱阻。最大的散熱片附著在表面，TBASE,允許溫度由下式給出為了選擇合適的散熱器，熱設計者必須首先確定最大允許散熱器熱阻。基面的暴露面積，然后確定可能從和每翅片換熱面積從此時有必要指定的平均速度，V之間的鰭或體積流量，G.,條款空氣流速或者如果體積流量使用，相應的鰭之間的空氣流速是要確定傳熱系數后，鰭，由Teertstra等人開發一個公式行事。1與努塞爾數，怒族，以雷諾數，重，和Pr數，錯，可受聘。這個方程普朗特數其中是空氣動力粘度，CP的空氣定壓比熱，k是空氣的導熱系