Provided is a power supply apparatus which includes a comparator, an output voltage adjusting unit, a controller, and an output unit. The comparator compares a sensed AC input voltage to a reference voltage and outputs one of first through third comparison result signals. The output voltage adjusting unit outputs one of first through third output voltage level adjustment signals for changing the feedback output voltage level according to one of the comparison result signals. The controller outputs a switching signal. The output unit outputs a predetermined voltage by regulating and smoothing an input voltage induced to a secondary coil of a transformer by the switching signal. By adjusting the output voltage of a boost converter according to the input voltage, efficiency is increased because there is less power loss of the switching component, and the hold-up time may be adjusted even in a case of low AC input voltage.