Overload protection circuit - limits collector-emitter voltage of power transistor switch by using auxiliary transistor to form quasi-darlington pair

摘要

The protection circuit has an auxiliary transistor (4) connected by its emitter to the power transistor's (5) base and by its collector via a diode (7) to the power transistor's collector. The bases of the two transistors are connected via separate resistors to the same control current source (1). The two transistors act as a quasi-Darlington circuit in overload mode so that the control current is small (w.r.t. load current) during overload and the loss through the pair is small during normal operation. The diode prevents inversion. The auxiliary transistor is off in normal operation.