Aspects include a hold-up capacitor charging circuit for a power supply. The hold-up capacitor charging circuit includes a voltage boosting charge pump circuit with a hold-up capacitor electrically coupled to a voltage source. The hold-up capacitor charging circuit also includes a fly-back circuit. The fly-back circuit includes a transformer with a primary winding electrically coupled to the voltage source and a secondary winding electrically coupled to a load. A switch is electrically coupled to the primary winding and the voltage boosting charge pump circuit. A controller is operable to open and close the switch to control energy transfer from the primary winding to the secondary winding and charge the hold-up capacitor responsive to voltages of the voltage source, the voltage boosting charge pump circuit, and a reflected voltage of the secondary winding at the primary winding.