INTRODUCTION
R-C thermal model parameters for Vishay power MOSFETs available under the product information menu offer a simple means to evaluate thermal behavior of the MOSFET under a defined transient operating condition.
Steady state values of thermal impedance, Rth(j-a) and Rth(j-c) / Rth(j-f), along with normalized thermal transient impedance characteristics published in a power MOSFET datasheet, are adequate to analyze the thermal behavior of a part under a regular wave-shaped, single pulse or the periodic power dissipation of known duty cycle.

However, thermal analysis for transient or irregular wave-shaped power profiles requires extending the thermal characterization offered in the datasheet. What is really needed is a thermal model emulating the thermal transient behavior of the power MOSFET on a suitable software platform. The thermal transient impedance characteristics published in a datasheet are a net effect of the thermal resistance and thermal capacitance of the physical structure of a device.