Driver circuit for high-voltage power transistors providing protection to the driver transistors

Claim 1

Driver circuit means for an even plurality of high voltage power switching transistors each having a switchable path between collector and emitter thereof and a base electrode for switching control of said path and each having a free wheeling diode connected antiparallel to said switchable path of said power switching transistor, said even plurality of power switching transistors having their respective switchable paths connected in cascade between positive and negative poles of a d.c. voltage source, and a load for said power switching transistors being connected between a midpoint of said cascade connection of power switching transistors and a conductor maintained at a potential midway between the respective potentials of said positive and negative poles of said source of d.c. voltage, comprising: an individual driver circuit and an individual driver power supply unit for each said power transistor, each said driver circuit comprising a final driver stage (18, 38) a control logic stage (17, 48) and a first galvanic isolation input device (16), which is interposed between a source (4, 5) of switching control signals and an input of said control logic stage; each driver power supply unit having a rectifier and a second galvanic isolation input device (15) interposed between said rectifier and a source of alternating current, said rectifier providing a reference potential output connection and at least one output connection providing a d.c. supply voltage with respect to said reference potential output connection, said reference potential output connection being connected galvanically to the emitter of the power switching transistor for which the power supply unit containing the rectifier providing said reference potential output is individual; each said final driver stage being powered by a said power supply unit individual to the same power switching transistor and having an output galvanically connected to the base electrode of said same power switching transistor for which the driver circuit containing the final driver stage is individual; each said driver circuit further comprising: a first settable and resettable bistable circuit (35) in said control logic stage of each said driver circuit, connected so as to store said switching control signals and to determine and time switched-on and switched-off conditions of the power transistor for which the driver circuit is individual in accordance respectively with set and reset conditions of said first bistable circuit; a second settable and resettable bistable circuit (51, 52) for registering error conditions and circuits associated therewith for setting said second bistable circuit in response to at least one of two voltage differences which are, namely, a first voltage difference between the emitter and collector of said power transistor exceeding a first predetermined value and a second voltage difference between base and emitter of said power switching transistor exceeding a second predetermined value while said first bistable circuit is set, and a blocking circuit including a diode connected to the collector of the said power switching transistor for which the driver circuit is individual for preventing the setting of said first bistable circuit so long as said free wheeling diode connected antiparallel to the said power switching transistor conducts current.