Such power circuits are especially useful in switching converters or the like for the purpose of insuring that, in the case of accidental short circuiting, the chopping transistor is protected. It is well known that apparatuses of the switching type for supplying electric motors with direct current or alternating current are very critical as regards protection against internal or external short circuits or similar anomalous operating conditions.
In fact, as is well known, the current gain of the transistor tends towards 1 with an increase in the current passing through the transistor; thus, if the current I between the collector and the emitter is strongly increased and the voltage V.sub.ce between the collector and the emitter tends to increase, the possibility exists of the transistor being destroyed through the excess of power dissipated.
There are essentially two known systems which are principally used for protecting transistors, the first employing means for sensing the maximum current which flows upon short circuiting while the second employs magnetic sensing. In the first case, the increase in current which occurs with the short circuit is sensed by a precision threshold sensor or trigger whose response generates the signal which blocks the converter. This system, to be effective, must discriminate between the two working levels, that is, the level of normal operation and the level of overcurrent which occurs in the case of short circuiting. In practice, the two levels are separated by at least 20% to allow for possible errors due to the tolerances of the circuit components; this involves the disadvantage of a reduced utilization of the power elements including the transistors, in order to provide the required safety margin, or else the sensing circuit has to be very precise, rapid and stable and consequently very complicated and expensive.
In the second case the magnetic sensing of the abnormal condition causes blocking of the control circuit of the transistor, but this system has the disadvantage either of requiring the use of very expensive external magnetic sensing elements or of not being effective in the case of very slow phenomena and of suffering from possible external capacitive disturbances.