Disclosed is an uninterrupted power supply apparatus, comprising a rectifying unit outputs a positive half-cycle of voltage potential and a negative half-cycle of voltage potential; a backup battery outputs a backup voltage; a first boost unit receives the positive half-cycle of voltage potential or the backup voltage and outputs a first voltage; a second boost unit receives the negative half-cycle of voltage potential or the backup voltage and outputs a second voltage; a current shunt unit receives the first voltage and the second voltage and outputs an AC output voltage; a charge circuit, coupled to the rectifying unit, the backup battery, and the first and/or the second boost unit, for being capable of controlling the first and/or the second boost unit to either store a charging power or charge the backup battery by applying the stored charging power alternatively and successively.
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