Abstract: The phenomenon of slow thermal runaway is studied and verified with thermal circuits to describe the rise and fall characteristics of lattice temperature during continuous pulses of applied electric power which produces thermal effects.Our results show that a high-frequency switch can operate in the thermally safe operation area if the delay time of the applied pulses is longer than the thermal delay time during the fall process of lattice temperature.We show that LDMOS will switch in the thermally safe operation area with its thermal resistance of 3.5K/W and thermal capacitance of 5μs·W/K if the frequency of the applied pulses is less than 7.14kHz with a duty cycle of 0.5,or if the duty cycle of the applied pulses is less than 0.3 with a frequency of 10kHz.A criterion is given for power devices in the thermally safe operation area.

Key words: thermal circuits, formulas, fall, slow thermal runaway