The reason Why the average duty can be a value close to 100 percent in the above provided calculation example is mainly that the duty of the 10th cycle of the ramp signal generating circuit 3 is relatively a large value in addition to that the duties of the 1st through the 9th cycles are 100 percent. The reason that the duty of the 10th cycle becomes
the large value is as explained by referring to FIG. 8.