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1、散热器扩散热阻的计算Accident?Considerthescenariowhereadesignerwishestoincorporateanewlydevelopeddeviceintoasystemandsoonlearnsthataheatsinkisneededtocoolthedevice.Thedesignerfindsaratherlargeheatsinkinacatalogwhichmarginallysatisfiestherequiredthermalcriteria.Duetooth
2、erconsiderations,suchasfannoiseandcostconstraints,anattempttouseasmallerheatsinkprovedfutile,andsothelargerheatsinkwasacceptedintothedesign.Aprototypewasmadewhich,unfortunately,burned-outduringtheinitialvalidationtest,theproductmissedthenarrowintroductiontim
3、e,andtheprojectwascanceled.Whatwentwrong?Thereasonscouldhavebeenmulti-fold.But,underthisscenario,themainculpritcouldhavebeenthespreadingresistancethatwasoverlookedduringthedesignprocess.Itisveryimportantforheatsinkuserstorealizethat,unlesstheheatsinkiscustom
4、developedforaspecificapplication,thermalperformancevaluesprovidedinvendor'scatalogsrarelyaccountfortheadditionalresistancescomingfromthesizeandlocationconsiderationsofaheatsource.Itisunderstandablethatthevendorsthemselvescouldnotpossiblyknowwhatkindofdevices
5、theuserswillbecoolingwiththeirproducts.Figure1-Normalizedlocaltemperaturerisewithheatsourcesofdifferentsize;fromLtoR,sourcearea=100%,56%,25%,6%,ofheat-sinkareaIntroductionSpreadingorconstrictionresistancesexistwheneverheatflowsfromoneregiontoanotherindiffere
6、ntcrosssectionalarea.Inthecaseofheatsinkapplications,thespreadingresistanceoccursinthebase-platewhenaheatsourceofasmallerfootprinfootprintareaismountedonaheatsinkwithalargerbase-platearea.Thisresultsinahigherlocaltemperatureatthelocationwheretheheatsourceisp
7、laced.Figure1illustrateshowthesurfacetemperatureofaheatsinkbase-platewouldrespondasthesizeoftheheatsourceisprogressivelyreducedfromlefttorightwithallotherconditionsunchanged:thesmallertheheatsource,themorespreadinghastotakeplace,resultinginagreatertemperatur
8、eriseatthecenter.Inthisexample,theeffectoftheedgesurfacesoftheheatsinkisignoredandtheheatsourceisassumedtobegeneratinguniformheatflux.Incaseswherethefootprintofaheatsinkneednotbemuchlargerthanth
