Transistor protection circuit for automotive motor control applications

In an automotive control circuit including a storage battery, and the serial combination of an electric motor and a power transistor connected across the battery, the power transistor being biasable to a conductive state in which current is supplied from the battery to the motor through the primary conduction circuit of the power transistor when a conduction producing control voltage is applied across its conduction control terminals, a protection circuit which protects the power transistor from damage due to the generation of transient inductive voltage across the motor when the conduction producing control voltage is removed from the conduction control terminals, the protection circuit comprising:

• a protection transistor having an emitter-collector circuit connected in series with a resistor across the primary conduction circuit of the power transistor, the resistor being connected across the conduction control terminals of the power transistor, and a Zener diode connected in series with the emitter-base circuit of the protection transistor across the motor, the Zener diode being poled to oppose current flow through such emitter-base circuit, such that when a transient inductive voltage is generated across the motor upon removal of the control voltage, the reverse breakdown voltage of the Zener diode is exceeded and the protection transistor is biased conductive to permit current flow in a circuit comprising the battery, the motor, the emitter-collector circuit of the protection transistor, and the resistor, thereby to generate a voltage across the resistor which biases the power transistor to a partially conductive state for reducing the rate of decrease in current through the motor and the magnitude of the resulting transient inductive voltage.