电子设备热设计(第七课时)Thermal_Design_of_Electronic_Equipment(Ralph_Remsburg)教材.pdf

《电子设备热设计(第七课时)Thermal_Design_of_Electronic_Equipment(Ralph_Remsburg)教材.pdf》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。

1、0082-07.fmPage343Wednesday,August23,200010:12AM7CombinedModesofHeatTransferforElectronicEquipment7.1INTRODUCTIONAlthoughwehaveexaminedthethreemodesofheattransferingreatdetail,inengineeringpracticeweusuallyseecaseswheremodesarecombinedsimultaneously.Radiationisnotus

2、uallyasignificantfactor,buttoachievethebestresultweshouldcalculatetheimportanceofeachmode.Inacomputerchip,forexample,heatisconductedinparallelpathsfromthejunctiontothecaseandleads.Heatisthenconductedfromtheleadstothecircuitboard,andfromthecasetoaheatsink.Simultaneou

3、sly,theheatintheleadsandintheheatsinkisconvectedtotheairandradiatedtotheambientenvironment.Table7.1isareminderoftheequationsusedforheattransferandthermalresistanceinthethreemodes.Thesimplestwaytosolvemostproblemsofcombinedmodesistosetuparesistancenetwork.Inthismann

4、erwecangraphicallyexaminethepathsofeachmodeforsimultaneous,parallel,andseriesheattransfer.7.2CONDUCTIONINSERIESANDINPARALLELBecausecombinedmodesofheattransferoftenoccurinparallel,reviewingthetheoryofseriesandparallelcombinationsmaybehelpfulasappliedtoconduction.Whe

5、nheatisconductedthroughasinglewallofasinglematerial,therateofheatconductionandthethermalgradientareconstant.However,whenheatisconductedinaseriespathofdifferentmaterials,thetemperaturegradientisdifferentforeachmaterial.Examiningthecompositewallofthreematerialsinseri

6、es,asshowninFigure7.1,therateofheatconductionthroughtheentireblock,qk,canbefoundbyT1T4q-----------------------------------------------------k------L------L------LkAAkABkAC©2001byCRCPRESSLLC0082-07.fmPage344Wednesday,August23,200010:12AMTABLE7.1Therm

7、alResistancesandHeatTransferRatesHeatTransferModeRateofHeatTransfer,qThermalResistance,Conductionk------Lqk()T1T2--L-kkAConvection1qchcAT()sTc---------hcARadiationT1T2qA()T4T4----------------------------------------------r112,12rA()T4T4112,12Ifw

8、ehaveanindeterminatenumber,N,ofindividualmaterials,n,inseries,therateofheatconductionisTT1TN1qk-----------L-------------------------------nN