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Thermal Consideration
ThermaldResistancedofdLightingdLED
Thermaldresistance(RTH)disdonedofdthedprimarydtoolsduseddindthermaldmanagementddesign.dItdisddefneddasdthedratiodofd
temperatureddi erencedtodthedcorrespondingdpowerddissipation.dThedoveralldRTH,dJ-Ad(Junction-Ambient)dofdadLight-
ingdLEDdplusdMCPCBdisdillustrateddasdfollow:
R d(@C/W)d=dΔT /dPdd...........................................................................................................................d(1)
TH,dJ-A dJ-Ad
Whered
ΔTd =T T (@C)
J-A Junction Ambientd
Pd(Powerddissipated,dW)d=dForwarddcurrentd(I )dxdForwarddvoltaged(V )
d F F
Indaddition,dheatdgenerateddatdthedjunctiondofdsemiconductorddiedtravelsdalongdthedfollowingdpathdtodambientdenvi-
ronment:ddjunction-to-case,dcase-to-board,dboard-to-ambientdair.
TCase
R TH, J-C
TJunction
R TH, C-B
TBoard
R TH, B-A
TAmbient
Figure 1. Thermal Resistance in Series
Thedoveralldthermaldresistancedthereforedcandbedexpresseddasdthedsumdofdeachdindividualdresistancedalongdaboved
heat-traveldpath:
R d=dR +dR d+dR d.........................................................................................................................d(2)
TH,dJ-A TH,dJ-Cd TH,dC-B TH,dB-A Lighting LED > Application Notes
AsdΔTd d=d(dPdd)dxd(dR )dfromdEquationd1,dwedhave
J-A TH,dJ-Ad
T d=dT d+d(dPdd)dxd(dR )d........................................................................................................................d(3)
Junction Ambient TH,dJ-Ad
BaseddondEquationd3.dT dcandbedobtaineddifd ,dPddanddR dareddetermined.
Junction TAmbient TH,dJ-A
Todincreasedlightdoutputde ciencydofdLEDs,donedmustdincreasedtheddissipationdofdheatdgenerateddbydLEDs;dindotherd
words,dtodreducedthermaldresistancedfromdLEDsdtodambientdenvironmentdLite-Ondhasdoptimizeddthedjunction-to-
boarddthermaldpathdofdLightingdLEDdtodminimizedthedoveralldthermaldresistanced(RTH,dJ-B)ddowndtod8@C/Wd(e.g.p03d
series).
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