Patent ID: 7274575

Claim:
A switching power supply, comprising: a transformer including a primary winding having a first terminal receiving an input voltage, a first secondary winding providing an output voltage of the switching power supply, and a second secondary winding having a center tap connected to a ground terminal; a switching transistor having a conduction path coupled to a second terminal of the primary winding; a pulse width modulated control circuit having a control input coupled for receiving the output voltage of the switching power supply and an output coupled to a control input of the switching transistor; and a bias circuit having an output for supplying an operating voltage to the pulse width modulated control circuit, the bias circuit consisting of: (a) a first capacitor having a first terminal coupled to a first terminal of the second secondary winding, (b) a second capacitor having a first terminal coupled to a second terminal of the second secondary winding, the first and second terminals of the second secondary winding being on opposite ends of the winding with the center tap between the first and second terminals of the second secondary winding, (c) a first diode having a cathode coupled to a second terminal of the first capacitor at a first node and an anode coupled to a second terminal of the second capacitor at a second node, (d) a second diode having an anode coupled to the first node and a cathode coupled to the output of the bias circuit, (e) a third diode having a cathode coupled to the second node and an anode coupled to the ground terminal, and (f) a third capacitor having a first terminal coupled to the output of the bias circuit and a second terminal coupled to the ground terminal.