a pfc - pwm controller with a power saving means is disclosed . a built - in current synthesizer generates a bias current in response to feedback voltages sampled from the pwm circuit and the pfc circuit . the bias current modulates the oscillation frequency to further reduce the switching frequencies of the pwm signal and the pfc signal under light - load and zero - load conditions . thus , power consumption is greatly reduced . the pfc and the pwm switching signals interleave each other , so that power can be transferred more smoothly from the pfc circuit to the pwm circuit . the saturation of the switching components can be avoided by limiting the maximum on - time of the pwm signal . further , an external resistor is used to start up the pfc - pwm controller and to provide an ac template signal for pfc control .