Patent ID: 8810075

Claim:
A power off delay circuit, connected between a power supply unit and a load circuit, the power off delay circuit comprising: a first diode having an anode and a cathode, the anode of the first diode connected to the power supply unit; a boost transformer circuit, connected to the cathode of the first diode, to output power signals to the load circuit, the boost transformer circuit comprising: a primary winding having a positive input and a negative input, the positive input of the primary winding connected to the cathode of the first diode and outputting the power signals to the load circuit; a second diode, a cathode of the second diode connected to the negative input of the primary winding, and an anode grounded; a first switch, comprising a first pole connected to the negative input of the primary winding, a second pole grounded, and a control pole; a first secondary winding, wound on a same magnetic core with the primary winding, and having a high voltage terminal and a low voltage terminal, wherein a number of turns of the first secondary winding is greater than a number of turns of the primary winding; a second secondary winding, wound on the same magnetic core with the primary winding, and having a high voltage terminal and a low voltage terminal, wherein a number of turns of the second secondary winding is less than the number of turns of the primary winding, and the high voltage terminal of the second secondary winding is connected to the high voltage terminal of the first secondary winding; a capacitor, one end of the capacitor connected to the high voltage terminal of the first secondary winding; a third diode, an anode of the third diode connected to the other end of the capacitor, and a cathode of the third diode connected to the low voltage terminal of the first secondary winding; and a second switch, comprising a first pole connected to the low voltage terminal of the second secondary winding, a second pole connected to the other end of the capacitor, and a control pole; a feedback circuit, connected to the positive input of the primary winding and the high voltage terminal of the first secondary winding, to generate first feedback signals based on whether input power provided by the power supply unit is off; and a pulse width modulation (PWM) controller, to generate PWM signals according to the first feedback signals; wherein the feedback circuit further sends the PWM signals to the control pole of the first switch or the control pole of the second switch according to whether the input power is off.