Patent ID: 8873255

Claim:
A startup circuit, suitable for a power supply, the startup circuit receiving alternating current input power from the power supply, comprising: a first transistor having a first end, a second end, and a control end, the first end of the first transistor receiving the alternating current input power; a bias resistor serially coupled between the second end and the control end of the first transistor; a pull-down switch, a first end of the pull-down switch being coupled to the control end of the first transistor, a second end of the pull-down switch being coupled to a reference ground voltage, the pull-down switch being turned on or turned off according to a detection result; a voltage detector coupled to the first transistor and the bias resistor, the voltage detector generating the detection result by detecting a voltage on the second end of the first transistor; and a discharging path generator coupled to the voltage detector, the first transistor, and the bias resistor, the discharging path generator providing a discharging path between the second end of the first transistor and the reference ground voltage according to the detection result.