Referring to the edge-alignment phase shift at the top of FIG. 11, an auxiliary pulse is inserted between PWM20 and PWM21, and the edges of PWM1 and PWM21 are aligned. However, the on period of PWM2 is gradually reduced, so the rising edge of PWM22 delays from the falling edge of PWM1. An auxiliary pulse is inserted again between PWM22 and PWM23, and the edges of PWM1 and PWM23 are aligned.
On the other hand, referring to the overlap phase shift at the bottom of FIG. 11, an auxiliary pulse is inserted between PWM20 and PWM21, and the edges are aligned such that the on periods of PWM1 and PWM21 partially overlap with each other. After that, the coupling relation between PWM1 and PWM2 is maintained until PWM24 while the overlap width becomes narrower.
In this way, by executing the overlap phase shift, it is possible to reduce the number of times an auxiliary pulse occurs as compared to the case where the edge-alignment phase shift is executed. As a result, it is possible to also reduce a switching loss resulting from an auxiliary pulse.
Selection of Phase Shift Method Based on Loss Estimation
