A power supply is provided, which includes a DC-DC converter being supplied with an external DC input voltage and a first switching control signal and outputting a duty sensing signal of which a magnitude is varied in accordance with the first switching control signal, the duty sensing signal being indicative of a duty ratio of the first switching control signal, and the DC-DC converter converting the input voltage into a DC output voltage of a predetermined magnitude based on the first switching control signal; and a feedback controlling unit comparing the duty sensing signal with a first reference signal to adjust the duty ratio of the first switching control signal.