A power supply comprising: first and second power lines (L1, L2); an inverter circuit including two pairs of switching elements (TR1, TR2, TR3, TR4) connected between the first power line and the second power line, wherein the inverter circuit performs a switching operation that alternately activates and deactivates the two pairs of switching elements (TR1, TR2, TR3, TR4), and the inverter circuit converts input DC power to high frequency AC power; an auxiliary switching circuit including an auxiliary switching element and an auxiliary capacitor, wherein the auxiliary switching element is operated in cooperation with the switching operation of the inverter circuit, the auxiliary switching element includes a first terminal connected to the first power line and a second terminal that receives the DC power, and the auxiliary capacitor is connected between the first terminal of the auxiliary switching element and the second power line, wherein the power supply is configured to perform, output control, which controls the switching operation of the inverter circuit to adjust output power that is supplied to a load, and soft switching control, which deactivates the auxiliary switching element before the two pairs of switching elements (TR1, TR2, TR3, TR4) to reduce switching loss of the two pairs of switching elements (TR1, TR2, TR3, TR4); a pulse width modulation control unit that is configured to adjust, ON pulse widths of two pairs of control pulse signals respectively provided to the two pairs of switching elements (TR1, TR2, TR3, TR4) and adjusts an ON pulse width of an auxiliary control pulse signal provided to the auxiliary switching element in accordance with the two pairs of control pulse signals; characterized in that a phase shift control unit that is configured to adjust, a phase difference of the control pulse signals in each pair provided to the two pairs of switching elements (TR1, TR2, TR3, TR4) or to adjust a phase difference of the two pairs of control pulse signals and the auxiliary control pulse signal; and a control switching unit that is configured to selectively switch the pulse width modulation control unit and the phase shift control unit so that the pulse width modulation control unit operates when a relatively large output power is required and the phase shift control unit operates when a relatively small output power is required such that the two pairs of control pulse signals and the auxiliary control pulse signal are set to have ON pulse widths that are smaller than a predetermined pulse width (WmO, WsO); wherein the phase shift control unit is configured to fix the ON pulse widths of the two pairs of control pulse signals and the auxiliary pulse signal to the predetermined pulse width (Wm0, Ws0); when the relatively small output power is required. first and second power lines (L1, L2); an inverter circuit including two pairs of switching elements (TR1, TR2, TR3, TR4) connected between the first power line and the second power line, wherein the inverter circuit performs a switching operation that alternately activates and deactivates the two pairs of switching elements (TR1, TR2, TR3, TR4), and the inverter circuit converts input DC power to high frequency AC power; an auxiliary switching circuit including an auxiliary switching element and an auxiliary capacitor, wherein the auxiliary switching element is operated in cooperation with the switching operation of the inverter circuit, the auxiliary switching element includes a first terminal connected to the first power line and a second terminal that receives the DC power, and the auxiliary capacitor is connected between the first terminal of the auxiliary switching element and the second power line, wherein the power supply is configured to perform, output control, which controls the switching operation of the inverter circuit to adjust output power that is supplied to a load, and soft switching control, which deactivates the auxiliary switching element before the two pairs of switching elements (TR1, TR2, TR3, TR4) to reduce switching loss of the two pairs of switching elements (TR1, TR2, TR3, TR4); a pulse width modulation control unit that is configured to adjust, ON pulse widths of two pairs of control pulse signals respectively provided to the two pairs of switching elements (TR1, TR2, TR3, TR4) and adjusts an ON pulse width of an auxiliary control pulse signal provided to the auxiliary switching element in accordance with the two pairs of control pulse signals; a phase shift control unit that is configured to adjust, a phase difference of the control pulse signals in each pair provided to the two pairs of switching elements (TR1, TR2, TR3, TR4) or to adjust a phase difference of the two pairs of control pulse signals and the auxiliary control pulse signal; and a control switching unit that is configured to selectively switch the pulse width modulation control unit and the phase shift control unit so that the pulse width modulation control unit operates when a relatively large output power is required and the phase shift control unit operates when a relatively small output power is required such that the two pairs of control pulse signals and the auxiliary control pulse signal are set to have ON pulse widths that are smaller than a predetermined pulse width (WmO, WsO); wherein the phase shift control unit is configured to fix the ON pulse widths of the two pairs of control pulse signals and the auxiliary pulse signal to the predetermined pulse width (Wm0, Ws0); when the relatively small output power is required.