A power supply is provided. The power supply includes: a switching module including switching elements and is configured to receive rectified power; a transformer configured to transform first power received from the switching module; an outputter including first and second switching elements, and is configured to receive the transformed first power from the transformer and output an output voltage that follows a preset reference voltage; and a controller configured to control the switching module to operate in a full bridge mode or a half bridge module based on a peak voltage of the rectified power, adjust a switching frequency of the switching module based on the output voltage, control switching of the first and second switching elements based on the output voltage, and adjust a duty ratio of each of the first and second switching elements based on the rectified power.