How to find a fitting cooler/heatsink for a Mosfet?
First you need to determine how much power is dissipated in the transistor. The second factor is the maximum ambient temperature that you expect the transistor to experience. Finally, decide what maximum temperature you want at the transistor chip itself...maybe 75&deg;C is a good target. Subtract the ambient temperature from the maximum transistor die temperature and you have the maximum temperature increase from the transistor chip to the outer surface of the heatsink. Divide the temperature difference by the power dissipated and you have a value for thermal resistance in &deg;C/W (degrees C per watt). This is the maximum thermal resistance you can allow from the transistor die to the ambient air. Find out the thermal resistance of the transistor package itself. For the transistor you list above, this is given in the data sheet as "Thermal Resistance, Junction to Case" and it has a value of 6.25&deg;C/W. Now subtract the thermal resistance of the transistor package from the total allowable thermal resistance you calculated earlier. Now go to the catalogs and look for a heatsink that fits the TO-220 package and has a thermal resistance, in still air, equal to or less than this value.