Patent ID: 7345894

Claim:
A switching power supply comprising: an input source of power; an output terminal; a feedback means providing an output responsive to the voltage at said output terminal of said power supply; a coupled inductive element having multiple windings, each with a pair of terminals, a first terminal of a first winding being coupled to said input source of power; a first transistor having a drain, a source and a gate, said drain coupled to a second terminal of said first winding of said coupled inductive element; a reservoir capacitor coupled to said gate of said first transistor; a control circuit being coupled to said reservoir capacitor as a source of power and producing a pulse modulated output in response to said feedback means, said control circuit having an undervoltage lockout function responsive to voltage on said reservoir capacitor with two hysteretic thresholds, a first higher threshold enabling operation of said control circuit when said voltage on said reservoir capacitor exceeds said first threshold, and a second lower threshold which disables said control circuit when said voltage on said reservoir capacitor falls below said second threshold; a second transistor with a source, a gate and a drain, said source being coupled to a common node of said input source of power, said drain coupled to said source of said first transistor, and said gate being driven in response to said pulse modulated output of said control circuit; a first rectifying means with an anode and a cathode, said anode connected to said drain of said second transistor and said cathode coupled to said reservoir capacitor; and an output circuit comprising a second rectifying means with a rectifying input and a rectified output and a filtering means with a filtering input and a filtered output, said second rectifying means having said rectifying input coupled to a second winding of said coupled inductive element and having said rectified output coupled to said filtering input of said filtering means, said filtering means having said filtered output coupled to said output terminal of said switching power supply.