(
3
)
The CNV controller 13 obtains the shift amount θsftt of the overlap phase shift on the basis of the mathematical expression (3). Alternatively, the shift amount θsftt is called up as needed by consulting a map obtained in advance as the correlation between the shift amount θsftt and the duties Duty1, Duty2, rates Rate1, Rate2 and overlap period t.
In FIG. 7 and FIG. 9, examples in which the on period is reduced one every period are shown as an example in which the on period is gradually reduced over multiple periods; however, the example in which the on period is gradually reduced over multiple periods is not limited to this example. For example, when the on periods is gradually reduced once every two periods as well, the above-described overlap phase shift may be applied.
In addition, in FIG. 7 and FIG. 9, examples in which the on period is gradually reduced over multiple periods are shown as an example of a change in duty ratio; however, the overlap phase shift is not only limited to these examples. For example, when there arises multiple times the period in which the average rate of the PWM signals PWM1, PWM2 becomes smaller than 1.0 among arbitrary multiple periods, there is an advantage of executing the overlap phase shift as compared to the edge-alignment phase shift.
