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How to calculate for selecting a heat sink of a semiconductor device (1)

With the heat sink attached, calculate the permissible junction temperature of the semiconductor device by using junction-to-ambient thermal resistance (Rth(j-cf) + Rth(cf-a)).
( Tj - Ta ) = ( Rth(j-cf) + Rth(cf-a) ) × P

Select heat sink of thermal resistance so that the junction temperature of the semiconductor device is within the allowable range of value.
Rth(cf-a) = ((Tj - Ta) / P) - Rth(j-cf)

For example, Tj = 120 [deg.C], Ta = 50 [deg.C], P = 10 [W], Rth(j-cf) = 0.5 [deg.C/W] (When the contact resistance between a semiconductor device and a heat sink is disregarded)
Rth(cf-a) = 6.5 [deg.C/W]
To use a heat sink of a thermal resistance value lower than 6.5 deg.C/W, you can obtain the result to be expected.

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