Patent ID: 7333904

Claim:
A method of determining a junction temperature (T J )of a field effect transistor (FET) comprising the steps of; measuring the temperature (T th ) adjacent to the FET, measuring the drain-to-source voltage (V DS ) of the FET, determining the temperature T J at the junction of the FET in a stable region of operation where T J −T th is nearly constant accordance with a first set of equations, determining a step change in successive measurements of V DS indicative of a lag of T th relative to T J thereby rendering T th unusable in the first set of equations, immediately determining an initial FET current I FET in response to the step change using the most recent value of R DS in accordance with I FET =V DS /R DS , and determining the temperature T J by resolving T J based upon a second set of equations until T J −T th re-stabilizes.