Abstract: A power supply device for transforming a DC input voltage by switching charging/discharging of an inductor, and obtaining a DC output voltage by smoothing the transformed voltage by a capacitor, includes: a transistor for synchronous rectification coupled between the inductor and the capacitor; a current determination circuit for determining whether a current flowing through the transistor is less than a lower-limit current; and a control circuit for operating the transistor in a constant current operation while the current flowing through the transistor is greater than the lower-limit current and operating the transistor in a rectification operation when the current is less than the lower-limit current, based on a determination result of the current determination circuit.