During the recess time of the repetitive power pulses, the measuring pulses of Icbo (reverse saturation current) which can be arbitrarily varied in their phase, width and height are put into the collector of the transistor under test. Then, the transient temperature of the junction at any instant during the recess interval can be measured successively. With this method, we could know the period from switch-in of the power pulse up to thermal steady state of the junction and the behavior of the temperature decay during the recess of the power pluses. Also we could estimate the maximum temperature rise of the junction right after the power pulses are off in the duration.
Consequently, it is found that there occur local spots in the junction more highly heated than expected.
At first, for the temperature measurements, we must make a calibration curve with Icbo vs. Tj (junction temperature) corresponding to each one of transistors using a constant temperature bath.