Diagnostic circuit for units providing current control and protection against excessive heat dissipation for semiconductor power devices

A diagnostic circuit for units providing current control and protection against excessive heat dissipation for semiconductor power devices, comprising a comparator having a first input connected to the power device so as to be supplied with a voltage proportional to a current passing through the device and a second input connected to a reference voltage generator, a signal generator connected to the output of said comparator so as to generate a diagnostic signal whenever the output from the comparator indicates a growth of said voltage proportional to the current beyond a reference voltage generated by said reference voltage generator, and means sensitive to the temperature of the power device which act on said reference voltage generator so as to reduce said reference voltage in response to an increase in temperature beyond a preset limit, wherein said comparator includes a differential stage formed by two transistors having a common base supplied with a constant current through a current mirror circuit, one of said transistors being polarized through a resistance connected in series to said power device and the other of said transistors being polarized through a further resistance defining said reference voltage, said means sensitive to temperature including a voltage divider used for the polarization of a transistor used to commutate to conduction when the temperature goes beyond a preset limit defined by said voltage divider, said transistor acting on said current mirror circuit so as to reduce said constant current supplying the transistors of the differential stage of comparator and thus vary said reference voltage.