Device including a thermally protected switching transistor

A device (1) for supplying electric power to a load (4) connected to the device (1) in a power-on mode comprises a driver stage (7), a control unit (9), adapted to supply turn-on pulses to the driver stage (7), and an output stage (12) with a switching transistor (1) which can be driven into its conductive state for a time interval (TEIN) by the driver stage (7) in the power-on mode when turn-on pulses occur, and which can be driven into its non-conductive state after expiry of the time interval (TEIN), the driver stage (7) being also adapted to monitor the operating temperature (TS) of the switching transistor (11) and to activate a power-off mode when the operating temperature (TS) of the switching transistor (11) exceeds a first limit temperature (TG1), in which power-off mode the switching transistor (11) is permanently driven into its non-conductive state by the driver stage (7) in spite of the occurrence of turn-on pulses (IS), the driver state (7) further comprising a sensing transistor (21), a sensing resistor (22), a turn-on transistor (23), a turn-on resistor (24) for monitoring the operating temperature (TS) of the switching transistor (11), as well as a zener diode (29) and a turn-off stage (32) for obtaining a short switching time of the switching transistor (11), the sensing transistor (21) being arranged so as a to be substantially uninfluenced by the operating temperature (TS) of the switching transistor (11).