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 (R_th(j-cf) ＋ R_th(cf-a)).
( T_j － T_a ) = ( R_th(j-cf) ＋ R_th(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.
R_th(cf-a) ＝ ((T_j － T_a) / P) － R_th(j-cf)

For example, T_j ＝ 120 [deg.C], T_a = 50 [deg.C], P ＝ 10 [W], R_th(j-cf) = 0.5 [deg.C/W] (When the contact resistance between a semiconductor device and a heat sink is disregarded)
R_th(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.