Shutdown circuit for blocking oscillator power supply

Claim 1

In a switching power supply having a switching transistor for controlling current through the primary winding of a power transformer and a plurality of secondary windings being arranged on the core of said power transformer said secondary windings operable for supplying current to attached loads, a circuit arrangement for protecting said power supply from electrical faults comprising: shut down circuit means coupled to said switching transistor and operable for shutting down the power supply when the flow of current therethrough exceeds a predetermined limit; first circuit means coupled to said secondary windings and operable for sensing a fault condition in said secondary windings and to generate electrical signals representative of said sensed fault, said first circuit means including a positive overvoltage (O/V) detector circuitry and a negative O/V detector circuitry with the negative O/V detector circuitry including a switching means having a plurality of leads with one of said leads providing an output signal, a second of said leads being coupled to a negative voltage to be sensed, a voltage divider circuit means being coupled to a third lead of said switching means, a second circuit means for sensing an undervoltage condition, a third circuit means for sensing unacceptable output current conditions with each one of said circuit being coupled to separate secondary windings; fourth circuit means operable for combining the output from the first, second and third means and to generate a single output signal therefrom; and a fifth circuit means coupled to one of said secondary windings, said fifth circuit means being responsive to the output signal and operable for shorting said secondary windings whereby a surge of current exceeding the predetermined limit is made to flow through the switching transistor.