The thermal characteristic of a power semiconductor device is defined by its transient thermal impedance (TTZ) curve. This curve is the only thermal information normally available to the equipment designer. The TTZ curve, however, is only valid for a device mounted on an infinite heat sink with no thermal interface resistance. If heat flow across the interface is not constant, the semiconductor and heat sink form a composite thermal system and the TTZ curve is no longer applicable. The authors present a technique for modeling the semiconductor and heat sink as distributed R/C networks, the semiconductor network being chosen so as to match the TTZ curve for the device over the time period of interest. The model has a theoretically sound basis and it allows a TTZ curve to be developed for the system. The model is especially valuable for applications in which the semiconductor current is of pulse durations spanning the transition range of the TTZ curve.
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