Patent ID: 7764098

Claim:
A start up circuit of power converter, comprising: a first transistor having a first terminal, a second terminal and a third terminal, in which the first terminal is coupled to receive a voltage source; a third transistor having a drain terminal, a source terminal and a gate terminal, in which the drain terminal of the third transistor is connected to the second terminal of the first transistor, the source terminal of the third transistor is coupled to provide a supply voltage to a control circuit of the power converter; a diode coupled from a transformer winding of the power converter to the control circuit of the power converter for providing a further supply voltage to the control circuit; a second transistor having a drain terminal, a source terminal and a gate terminal, in which the drain terminal of the second transistor is coupled to the gate terminal of the third transistor and the third terminal of the first transistor, the gate terminal of the second transistor is coupled to receive a control signal, and the source terminal of the second transistor is coupled to a ground; and a resistive device connected from the third terminal of the first transistor to the second terminal of the first transistor; wherein the resistive device provides a bias voltage to turn on the first transistor and the third transistor when the second transistor is turned off, the control signal is coupled to turn on the second transistor for switching off the third transistor, the third transistor and the second transistor being positive-threshold voltage devices.