Overheating protection circuit for a power transistor

Claim 1

A thermal protection circuit for a power transistor comprising a temperature-sensitive control transistor in which the base-to-emitter junction voltage decreases with increase in temperature, a thermal-conductive mounting stud adapted to be secured to a heat sink, the power transistor and control transistor each including a semiconductor element secured to the mounting stud, a bias circuit for applying a bias voltage between the base and emitter of the control transistor, the bias voltage normally being slightly less than the base-to-emitter voltage drop in the desired temperature operating range of the control transistor when the power transistor is conducting current to a load, a second transistor coupled to the output of the control transistor, the second transistor being turned on when the control transistor is turned off, a feedback resistor connecting the collector of the second transistor to the base of the control transistor for increasing the base current in the control transistor as the second transistor is turned off, and a bias control circuit for the power transistor including an oscillator, a rectifier and means coupling the alternating current signal from the output of the oscillator to the rectifier, the rectifier providing a direct current bias to the power transistor to turn the power transistor on, a transistor switch responsive to a control signal for turning the oscillator on in response to a control signal, and means coupled to the second transistor for biasing off the transistor switch when the second transistor is turned off.