Use of a step-response approximation for thermal transient modeling in power MOSFETs

In a previous ARFTG paper, we presented the measurement of the thermally-induced transient drain voltage response of a power Si MOSFET to a step excitation in gate voltage. The data was fit by assuming the drain voltage takes an exponential shape. This relies on the assumption that the drain current is a step function, which is actually only approximate due to the dependence of the drain current on temperature. In this work, we show that the exponential approximation actually possesses the same asymptotic behavior of a more accurate, model-based solution we have obtained that incorporates thermal feedback. We show that the more theoretically accurate solution and exponential approximation of the solution both provide excellent fits to measured data for the Si MOSFET.