A power supply apparatus includes an AC-DC rectifier, a transformer, a feedback inductor, a resonant unit, and a primary and auxiliary switch. The AC-DC rectifier rectifies an input voltage. The transformer includes a primary coil having a first primary coil and an isolated second primary coil, and transforms a first voltage of the primary coil into a second voltage of the secondary coil. The feedback inductor is connected to a tap between the first primary coil and second primary coil and an output of the AC-DC rectifier. The resonance unit is connected in parallel with the secondary coil and is configured to output an output voltage by rectifying the second voltage provided from the secondary coil. The primary switch and secondary switch are connected in parallel between the transformer and AC-DC rectifier.