Disclosed are power supply systems and methods of operating the same. An exemplary power supply system includes a primary rectifier configured to rectify an AC voltage to produce a bus voltage on a DC bus and a voltage monitor configured to monitor the AC voltage. A capacitor is switchably coupled to the DC bus via a switch and a charger is configured to charge the capacitor with power from the DC bus. A switch controller is configured to close, in response to the voltage monitor indicating a sag in at least one phase of the AC voltage, the switch to enable the capacitor to discharge to the DC bus.