Phase Shift by Controller—Edge-Alignment Phase Shift

The CNV controller 13 executes a phase shift for coupling the on periods of the PWM signals PWM1, PWM2 in order to reduce a loss that arises in the switching elements S1 to S4. The loss that arises in the switching elements includes a switching loss and an overlap loss. The switching loss arises at the time of switching the on/off states as surrounded by the broken lines in FIG. 17A. The overlap loss arises at the time when the on states overlap with each other. Within an on loss (steady loss), the overlap loss indicates a loss that arises at the time when the on periods of the PWM signals PWM1, PWM2 overlap with each other (at the time when both currents flow overlappingly) as indicated by hatching in FIG. 17B.

The phase shift for reducing the above-described loss will be described initially from an edge-alignment phase shift that is executed generally. In the edge-alignment phase shift, when the falling edge (fall) of the on period of the PWM signal PWM1 and the rising edge (rise) of the on period of the PWM signal PWM2 do not coincide with each other, the phase shift is executed to bring these edges into coincidence (connection) with each other.