A power supply that provides a source of backup AC voltage and a method of operating thereof. In one embodiment, the power supply includes a transformer having main power and backup power windings and a secondary winding. The power supply also includes multiple stages, namely, (1) a primary power stage coupled to the main power winding, (2) a backup power stage coupled to the backup power winding and having a backup battery, one of the primary and backup power stages acting as a current fed source and (3) an AC power stage, coupled to the secondary winding, that provides a source of AC voltage. The power supply is operable in a normal mode of operation in which the primary power stage supplies power to the secondary winding and further supplies power to the backup power winding to allow the backup power stage to charge the backup battery. The power supply is also operable in a backup mode of operation in which the backup power stage transfers power from the backup battery to the secondary winding via the backup power winding.