Fast turn-off circuit for power transistor

Claim 1

In a system for energizing a load from a voltage source having first and second terminals, the combination comprising: a. a power transistor having an emitter, base and collector with its collector-to-emitter path connected in series with the load and between the terminals of the voltage source to provide a low impedance path for the load when a drive signal is received at said base; and b. regenerative feedback means coupled between said collector and base of said power transistor for only feeding back to said base a signal responsive to an increasing potential sensed at said collector to reduce the time necessary to turn off said power transistor, said feedback means comprising: i. a second transistor having an emitter, base and collector, said collector of said second transistor being electrically coupled to said base of said power transistor; ii. transient circuit means including a series circuit combination of a resistor and a capacitor connected between said collector of said power transistor and said base of said second transistor for causing saturation of said second transistor when the voltage across the collector to emitter of said power transistor is in the process of increasing; and iii. a diode connected directly across the base-to-emitter of said second transistor in a reverse polarity relationship to protect a base-to-emitter junction of said second transistor from being destroyed by a reverse biasing feedback signal responsive to a decreasing potential at said collector of said power transistor when said power transistor is being driven into saturation by a signal applied to said base of said power transistor.