Voltage resonant inverter, control method, and surface treatment device

A voltage resonant inverter comprising:

• a step-up transformer having an excitation winding and an output winding;
• and
• a first switching element that switches an input voltage and intermittently applying an excitation current to the excitation winding, wherein
• excitation energy stored in the step-up transformer during a period when the excitation current is applied causes a voltage resonant state in a secondary side of the step-up transformer during a period when the excitation current is shut off and generates an alternating output voltage on the output winding, and
• the voltage resonant inverter further comprises:
• a second resonant circuit that is provided in a primary side of the step-up transformer and enters a resonant state by a voltage generated at a connection point between the excitation winding and the first switching element when the excitation current is shut off, and includes a second switching element;
• a second diode that is provided between a frame ground and the second resonant circuit, and commutates a current resulting from energy returning to an input side in a region where the output voltage is negative after the second switching element is turned on so that the current is absorbed in the input voltage;
• and
• a second switching element control circuit that keeps the second switching element off while the second resonant circuit is in a resonant state, turns on the second switching element when the resonant state ends, and turns off the second switching element during a period after the first switching element is turned on until the first switching element is turned off.