A charging circuit in a back-up power system is provided. The charging circuit includes an output terminal electrically connected to a main power for providing an AC output voltage, a transformer having a secondary electrically connected to the output terminal, an energy storage and supply device providing a DC voltage, an inverter having an output electrically connected to a primary of the transformer and an input electrically connected to the energy storage and supply device and including four gate control switch devices to form a bridge switching device, a bridge rectifier having an input electrically connected to the output of the inverter in parallel, and a charging switch device electrically connected to an output of the bridge rectifier in parallel so as to charge the electrical energy storage and supply device through one of a conduction state and a cut-off state of said charging switch device.