Originally Posted by alien
Has anyone done any calculations of this sort?
Hundreds (I had 3 summer jobs working at a factory that built electroplating rectifiers when I was in college). The general idea is simple. There is a 'thermal impedance' between the die in the semiconductor (diode, SCR, triac, SSR....) and the ambient which is the sum of the thermal impedances of the individual components (heat sink, heat sink compound, die to device case etc). The temperature rise is the product of the heat being dissipated times the sum of the thermal impedances. Thus, if for example, the thermal impedance is 0.5°/W and the device is dissipating 10 W then the rise is 5 °. If putting a fan on/near the heatsink decreases the thermal impedance to 0.1°/W then the rise is 1 °.
You will have to go to device manufacturers' specifications to get thermal impedance numbers. You can calculate the power dissipated by multiplying the rms voltage drop across the device at load by the rms load current. For example, if a device is carrying 50 amps and the voltage drop across it is 1 volt then the power dissipated is 50 W.