How to choose heatsink for power devices (transistor, LED, regulator)?
The process of selecting an appropriate heat sink is pretty straightforward.


Determine the maximum power dissipation, \$P_{MAX}\$, of the transistor or IC.
Determine the maximum junction (or die) temperature of the transistor/IC. This is usually given in the data sheet as a maximum \$T_J\$. This value can range from 70 C to over 125 C.
Determine the maximum ambient temperature, \$T_A\$, where the device will be used.
Calculate the maximum temperature increase of the device from ambient, \$T_J - T_A\$
Divide the maximum temperature change by the maximum power consumption. This will be the maximum allowable thermal resistance, \$\Theta_{MAX}\$, from the junction to ambient, in C/W (or the equivalent K/W). In other words, \$\Theta_{MAX} = (T_J - T_A)/P_{MAX}\$
From the device data sheet, find the value of the device's thermal resistance from the junction to the outside surface of the device case (i.e. the package). This is usually called \$\Theta_{JC}\$
Subtract \$\Theta_{JC}\$ from \$\Theta_{MAX}\$. The result is the maximum allowable thermal resistance \$\Theta_{SA}\$ from the sink to ambient for the selected heatsink. You may also need to subtract the thermal resistance between the device case and the heatsink, \$\Theta_{CS}\$, for high power devices. You can greatly reduce \$\Theta_{CS}\$ by using thermally conductive grease or a thermally conductive pad between the device and the sink, but 1.0 C/W is a reasonable estimate for \$\Theta_{CS}\$.