Thyristor (diode) transient thermal impedance modeling and verification for inductive load applications

Circuit modeling is an essential tool in the design of power electronic applications. The necessity of these models to be closely connected to the manufacturers´ specifications and rating systems is self-evident. This paper addresses one of the most difficult parameters of thyristor and diode modeling; that of transient thermal impedance. The development of a model from the device physical dimensions and materials is described, along with how the transient solution of this thermal equivalent circuit can be obtained via PSPICE. To arrive at the simple four or five term equivalent circuit for modeling, a nonlinear regression method is described. This model is then verified against the ratings calculated with the STARS rating program for large area thyristor inductive load applications. The resulting simple series equivalent thermal circuit will permit design engineers to hard code and use SPICE circuit modeling to monitor the waveform of virtual junction temperature of the thyristor or diode in the proposed application