Abstract: A method for providing thermal protection for forced air cooled power electronic semiconductors without direct measurement of semiconductor temperature, the semiconductors being mounted on a heat sink over which cooling air is blown includes the steps of computing the cooling air mass flow rate, creating an electronic model of the semiconductor/heat sink combination and estimating the semiconductor junction temperature from the mass flow rate and model. The cooling mass flow rate is determined by measuring the temperature of the cooling air prior to passage over the heat sink, measuring atmospheric pressure of the cooling air, determining the volumetric air flow rate of the cooling air and computing, from the cooling air temperature, pressure and volumetric flow rate, the cooling air mass flow rate. Power dissipation in the semiconductors is then computed and heat sink and semiconductor temperatures are estimated from power dissipation and cooling air mass flow rate.