A boost circuit is connected to a rechargeable battery and a switch is connected
between a rectification circuit and a power factor correction converter. In the
power failure state, the switch is turned on to activate the boost circuit and
energy of the rechargeable battery is supplied to a load by rasing the voltage
at two steps in the path from the boost circuit, switch, and to power factor correction
converter. Since the voltage of the battery can be lowered, the number of serially
connected cells can be made small. The volume of UPS can be reduced and a low cost
can be realized. Since the number of serially connected cells is reduced, the reliability
against failure of battery cells can be improved.