Abstract: A discharging circuit for a power supply includes a rectifying circuit for coupling to an input filter capacitor coupled to an AC input voltage for providing a rectified AC voltage. A discharge resistor is coupled to the rectifying circuit, and a power switch and a discharging current source are coupled in series with the discharging resistor. A discharge control circuit is coupled to the discharge resistor for providing a discharge signal to cause the power switch to turn on for discharging the capacitor using the discharging current source. The discharging control circuit includes an AC voltage sampling circuit for providing a sampled rectified voltage, a signal-shaping circuit for providing a time-varying signal derived from the sampled rectified voltage, a comparator circuit for comparing the sampled rectified voltage and the time-varying signal, and a counter circuit for providing the discharge signal in response to an output of the comparator circuit.

Abstract: A discharging circuit for a power supply includes a rectifying circuit for coupling to an input filter capacitor coupled to an AC input voltage for providing a rectified AC voltage. A discharge resistor is coupled to the rectifying circuit, and a power switch and a discharging current source are coupled in series with the discharging resistor. A discharge control circuit is coupled to the discharge resistor for providing a discharge signal to cause the power switch to turn on for discharging the capacitor using the discharging current source. The discharging control circuit includes an AC voltage sampling circuit for providing a sampled rectified voltage, a signal-shaping circuit for providing a time-varying signal derived from the sampled rectified voltage, a comparator circuit for comparing the sampled rectified voltage and the time-varying signal, and a counter circuit for providing the discharge signal in response to an output of the comparator circuit.