From the waveforms of PWM1, /PWM1, PWM2, /PWM2, the on/off states of the switching elements S1 to S4 are obtained. In addition, by narrowing which one of the operation modes shown in FIG. 3 to FIG. 6, the on/off patterns of the switching elements S1 to S4 correspond to, current components respectively flowing through the switching elements S1 to S4 are obtained.

The left side of the sheet (the left side of the alternate long and short dashed line) of FIG. 12 shows the waveforms before the edge-alignment phase shift is executed, and the right side of the sheet shows the waveforms after the edge-alignment phase shift is executed. Among the waveforms of S1 PWM to S4 PWM, the arrows indicate that a switching loss arises. The hatching indicates that an overlap loss arises because of an overlap of currents I1+I2.

Referring to the waveforms of S1 PWM to S4 PWM after the edge-alignment phase shift, the number of switching losses is ten, and the number of overlap losses due to current overlap (I1+I2) is 0. In addition, among the ten switching losses, four switching losses are caused by the state where a current (collector current) flowing through the switching element is zero at the time of on/off switching, and six switching losses are caused by the state where current flowing through the switching element is I1 or I2. Because each switching loss is proportional to a collector current, the former may be ignored as a power loss. Therefore, the number of switching losses that are counted as a power loss is six where the collector current I1 or I2 flows at the time of on/off switching.