Theoretical and experimental determination of the time response of junction temperature of a power transistor

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The time response of collector junction temperature of a power transistor to a square-wave power pulse is comprised of the response of the elements of the transistor with respect to the mounting base, and the response of the mounting base and case with respect to the heat dissipating facility. The former is much faster and is, therefore, more important in considering the collector junction temperature rise under short, high power pulse conditions. The collector junction temperature response of a power transistor differs considerably from the response of a typical grown junction transistor because of the dissimilarity of geometry of the elements. The thermal response of a typical grown junction transistor was analyzed by Mortenson. Both types have a p-n-p or n-p-n sandwich structure, but in a power transistor the collector is mounted in intimate, contact with a large copper base, providing a good thermal path, while the emitter contributes thermal inertia but no conductive path. In a grown junction transistor the bar is suspended at both ends, which can be assumed to be at constant temperature during short power pulses.