A power supply apparatus includes a converter to convert AC power into DC power, an SMPS to convert the DC power into DC powers desired by loads, a capacitor to interconnect the converter and the SMPS, a PTC element connected to the converter, a first switch connected in parallel with the PTC element, and a second switch connected in series with the first switch. The method includes turning on the second switch to start charging of the capacitor, turning on the first switch to charge the capacitor to a target voltage level, and turning off both the first switch and second switch if a voltage across the capacitor rises over the target voltage level, to discharge the voltage across the capacitor so as to lower the voltage across the capacitor to the target voltage level or lower.