An HID ballast includes a flyback power converter with a simplified topology that permits power factor correction while supplying constant output power to drive an inverter supplying low frequency modulated power to the HID lamp. A switch is coupled in series with a transformer in the power converter to control current through the transformer to provide constant output power. The off time of the switch contributes to controlling the power factor of the ballast circuit. The transformer is operated in critical conduction mode, with an indication of zero current in the transformer used to determine the control for the switch. An output of the flyback power converter provides a feedback signal to obtain constant power output based on switching intervals applied to the switch coupled to the transformer. This simplified topology reduces part counts while providing a robust control that can be flexibly applied to a number of HID lamps.