In the above aspect, a phase shift amount for the overlap phase shift may be set in correspondence with fluctuation predictions of the on-duty periods, and the controller may be configured to, when a total period of the on-duty periods of the first and second PWM signals based on the fluctuation predictions becomes short once every period, set the phase shift amount such that a fall of one of the first, and second PWM signals in a predetermined period coincides with a rise of the other one of the first and second PWM signals in the predetermined period.

In the above aspect, the controller may be configured to (i) carry out the fluctuation predictions of the on-duty periods from an execution period of the phase shift to the predetermined period, and (ii) when a first loss is smaller than a second loss as a result of comparison between the first loss and the second loss, execute the overlap phase shift. The first loss may include a loss that arises in the switching elements in a period from the execution period to the predetermined period when the overlap phase shift is executed. The second loss may include a loss that arises in the switching elements in the period from the execution period to the predetermined period when an edge-alignment phase shift is executed. The edge-alignment phase shift may be a phase shift by which, when a fall of the one of the PWM signals and a rise of the other one of the PWM signals do not coincide with each other, the fall and the rise are brought into coincidence with each other.

According to the above aspect of the invention, it is possible to execute a phase shift that reduces a power loss as compared to an existing phase shift.