Drive circuit for fast switching of darlington-connected transistors

Abstract

Prior circuits for rapidly switching Darlington-connected transistors between on and off states have accomplished relatively fast switching by applying reverse base drive to the transistors to quickly sweep the excess carriers therefrom. However, such circuits have not accomplished the required degree of reduction of turn off time and have induced localized 'hot spots' in the base-collector junctions of the transistors. In order to overcome these problems, a drive circuit according to the present invention includes first and second power sources of first and second polarities, a turn-on network coupled between the first power source and the control electrode of a driver transistor of a Darlington-connected pair for selectively turning on the transistor pair, turn-off capacitors coupled between the turn-on network and the control electrodes of each of the transistors and a turn-off network coupled between the capacitors and the second power source for coupling the second power source to the capacitors to rapidly turn off the transistors. Means are included for providing a dwell interval between operation of the turn-on network and operation of the turn-off network to allow recombination of excess carriers in the transistors to minimize the incidence of localized hot spots.