Combined transient thermal impedance estimation for pulse-power applications

High voltage high pulse-power switches are used in many applications. In many of these applications switches are realized using thyristors. Heatsinks are used to facilitate the heat transfer and limit the junction temperature below the maximum specified value. In many cases thermal time constant of semi-conductor and heatsink are much longer than the time for which the thyristor conducts. Hence, the temperature profile in the thyristor package and heatsink will be in transient conditions, and use of static models will not provide proper design outcome. This demands the use of transient thermal impedance curve for the thermal calculations, instead of steady state thermal resistance. But when the subsystems such as thyristor and heatsink are physically connected together, estimation of combined thermal impedance of the overall system is difficult due to the influence of the individual subsystems on each other. In this paper a systematic procedure is formulated for the estimation of transient thermal impedance of the combined system from the known thermal impedance parameters of the individual subsystems. The procedure is validated by simulations and experimental results.