A duty calculating unit receives a phase, an amplitude, a command value of a voltage across a capacitor, and a command value of a DC voltage, and calculates an original discharge duty and an original rectification duty. A duty correcting unit corrects the original discharge duty and the original rectification duty to obtain a discharge duty and a rectification duty. The duty calculating unit and the duty correcting unit can collectively be regarded as a duty generating unit that generates the discharge duty and the rectification duty.