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1、0082-05.fmPage279Wednesday,August23,200010:09AM5RadiationHeatTransferinElectronicEquipment5.1INTRODUCTIONRadiationcoolingofelectroniccomponentsandboxesisnotusuallyaconcerntothermalengineers.Thethreefactorsthataffectcoolingbyradiationarethetemper-ature
2、differencebetweenanobjectanditssurroundings,thesurfacecharacteristicsoftheobjectanditssurroundings,andtheviewthattheobjecthasofitssurroundings.Typically,thetemperaturedifferenceisthedeltabetweenthedevicecaseandanouterchassis,oranouterchassisandthewall
3、sofaroom.Sinceradiationheattransferisbasedonthistemperaturedifference,whenthedeltabetweenacomponentanditssurroundingsbecomeshighenoughforradiationcoolingtomatter,thedeviceismostlikelyalreadyaboveitsmaximumjunctiontemperature.Atroomtempera-ture,thetota
4、lemissivepowerofaperfectemitterisabout450W/m2.Thisislessthan10%oftheheatthatconvectioninaircantransfer.Thecharacteristicsofthesurfacethatcoversthedeviceorthechassisinquestionisanotherimportantvariable.Materialsusedinelectronicsarenormallyopaquetoradia
5、-tion.Therefore,thesurfacecharacteristicsareimportanttoadepthofonlyabout2.5106mformetalsandabout0.5103mfornonmetals.Intherangeoftemperaturesusedforelectronics,thecolorofthesurfacedoesnotaffecttheradiationemittance.Thethirdvariableistheviewfactor.T
6、hisisthefractionoftheradiationthatleavesonesurfaceandisinterceptedbyanothersurface.Thiscanbeashighas1.0forthecaseofasphereinsidealargersphere,orquitelow,suchastwoplatesatanangleapproaching180°.Algebraicequationscancalculatetheviewfactor,andthisisusual
7、lydonewiththeaidofacomputer.Althoughradiationisnotusuallyaconcernwhentryingtocoolanelectronicpackage,thepackagemayabsorbradiationheatbybeingnearahigh-temperaturesource.Suchacasemayoccurintheenginecompartmentofacar,whereelectronicmodulesareexposedtothe
8、radiativeheatofhotenginecomponentsandexhaustmanifolds.Althoughthecolorofanobjectisnotimportantinradiativecooling,colorisimportantwhentheobjectmayabsorbheatenergyfromawide-bandradiationsource.Thiscasemayoccurwhenweexposeanelectronicpackagetothe
