There is provided a power supply capable of reducing switching elements. In a power supply, an AC/DC adapter 121 supplies electric power to a load 125 and a battery charger 122. The battery charger comprises a high side FET 102 and a low side FET 103 which operate according to a synchronous rectification method, and charges batteries 108 and 109. When a commercial power supply fails, a power outage detection circuit provided in the battery charger detects the power outage, and outputs a power outage signal. The battery charger which has received the power outage signal sets the high side FET to be ON KEEP. A discharge current from the battery flows via a discharge path 119. This enables a reduction in the number of FETs in comparison with other power supplies.