As described above, in the parallel step-up/step-down mode of the electric power converter 11, the on/off operations of the switching elements S1 to S4 are controlled in response to the logical addition of the PWM signal PWM1 for the first step-up/step-down circuit CNV1 and the PWM signal PWM2 for the second step-up/step-down circuit CNV2. When the falling edge of the PWM signal PWM1 and the rising edge of the PWM signal PWM2 are brought into coincidence with each other, the number of switching operations (the number of on/off switching operations) reduces, with the result that it is possible to reduce a switching loss.

FIG. 8 shows an example of the edge-alignment phase shift. As shown in the drawing, when the PWM signal PWM2 is shifted by a half of the on period (Duty1)/2 of the PWM signal PWM1 and a half of the on period (Duty2)/2 of the PWM signal PWM2, the rising edge (rise) of a shifted PWM signal PWM2′ coincides with the falling edge (fall) of the PWM signal PWM1. When a phase shift amount is indicated by angle, an edge-alignment phase shift amount θsft is expressed by the following mathematical expression (1).

$\begin{array}{c}(\frac{\mathrm{Duty}??1}{2}+\frac{\mathrm{Duty}}{}\end{array}$