Abstract: An AC power supply for providing a DC current for charging a storage capacitor. A boost converter circuit raises the voltage of a rectified input current for charging a capacitor. The power factor of the power supply is maintained above a predetermined level using a power factor controller (PFC). A comparator is used to compare a signal corresponding to the instantaneous voltage of the rectified current with that on the capacitor, and outputs a control signal corresponding to the difference between those two instantaneous voltages. The PFC utilizes this control signal to increase the voltage of the stored charge on the capacitor if the instantaneous voltage of the rectified current comes closer to that on the capacitor by less than a predetermined voltage. Power factor correction instability due to a rising input voltage, is thus eliminated.