A DC-DC converter includes a main inductor connected to an input voltage, a main switching element connected in series to the main inductor, a main diode connected to the main inductor and a load, and a main capacitor connected to the main diode and the load, the DC-DC converter changing the input voltage to output a changed input voltage as an output voltage. The DC-DC converter further includes an oscillator connected between the main inductor and the main diode, an auxiliary switching element connected to the oscillator, the auxiliary switching element changing an operation state based on an externally applied control signal to control the oscillator, and a diode unit connected to the oscillator and the auxiliary switching element and controlling current flow based on operations of the oscillator and the auxiliary switching element to change the output voltage. The main switching element and the main diode are to be zero voltage switching or zero current switching in accordance with the operations of the oscillator and the auxiliary switching element.