Wiblocks --- FAQ

Question

How do I calculate the safe operating temperature of a MOSFET or BJT?

Answer

To determine the temperature rise you need to calculate the junction to ambient thermal resistance (Tja) ---
where

thermal resistance between the IC junction and IC case. This is from the IC datasheet.
thermal resistance between the IC case and your heatsink. This should also be in the IC datasheet. Usually you place a thermal compound between the IC case and the heat sink to decrease the thermal resistance. The decrease occurs because the thermal compound fills the air gaps between the two surfaces and conducts heat better than the air.
thermal resistance between the heatsink and ambient air. This is in the heatsink datasheet. Most heatsink datasheets specify Tsa with and without airflow. You can determine if you need a fan by reviewing this value.

For a single IC heat-source the junction temperature would be
where P is the power dissipated in the IC and Tamb is the ambient temperature.
For a MOSFET
For a BJT

Make sure to use the Rds(on) and Vce(sat) values at the device operating temperature not at 25°C.

The maximum juction temperature is listed in the datasheet (usually in the "Absolute Maximum" section). A conservative design limit would be a maximum junction temperature less than 80% of the absolute maximum rating.

Unless you have very short on-times pulse-width modulation (PWM) is not going to significantly reduce the power dissipation. To see how much look at the transient thermal response graphs in the datasheet.