The present invention relates to a protective circuit for a switching transistor having a switching-off device for the driving pulses of the switching transistor, wherein the switching-off device is addressed by a comparator which compares a voltage at the switching transistor with a reference voltage and switches off the driving pulses if the switching transistor is overloaded.
Such a protective circuit is known from the journal "Elektronik", 1980, No. 25, pages 46 and 47. With such a protective circuit, the switching transistor is monitored for desaturation, i.e., it is checked to determine if its collector-emitter voltage exceeds the value occurring if the transistor is saturated. The switching transistor can thereby be protected against short circuits and overloads. In the known circuit, the collector-emitter voltage of the switching transistor is therefore compared in a comparator with a reference voltage corresponding to the saturation value and the driving pulses are switched off if the collector-emitter voltage of the switching transistor exceeds the saturation value.
The following problem, however, arises with this known circuit: the collector-emitter voltage drops relatively slowly when the switching transistor is switched on and reaches the low steady-state value only after a certain delay time. In order to prevent the protective circuit from responding during the delay time, the former must have a corresponding delay. Since this delay is effective also when switching in the presence of an existing short circuit, the short circuit current can build up unimpeded during the delay time. If the switching transistor is operating with a large base current or if a high-gain Darlington stage is involved, the short circuit current often reaches values which are permissible for at least only a limited number of short circuit switching processes during the operating time of the switching transistor.
It is therefore an object of the present invention to develop a protective circuit of the type mentioned above without or with only a very small response delay.