A switching power supply control-circuit according to the present invention uses
bleeding resistors to charge a start-up capacitor and discharge an EMI filter.
No extra discharge device is needed to accelerate the discharge of the input capacitor,
since the start-up capacitor is charged up by the AC input source. A latch circuit
of the power supply can be quickly reset after the AC input source is shut off.
After the control-circuit begins to operate, the auxiliary winding of the transformer
will power the control-circuit. To further reduce power consumption, the auxiliary
winding generates a bias voltage to enable line-voltage detection. This allows
the power supply to perform line-voltage detection and startup, without having
to connect resistors or transistors to the input capacitor.
