Abstract: Method and apparatus for controlling a DC-DC converter change the switching frequency of the switching devices and the on-off ratio of the switching devices. The on-off ratio can be changed in response to the output voltage and the switching frequency can be changed in response to the input voltage supplied by the DC power supply. Alternatively, the switching frequency can be changed while the on-off ratio is fixed at a certain value, and the on-off ratio can be changed while the switching frequency is fixed at a predetermined value after the switching frequency has reached the predetermined value, thereby preventing the switching frequency from exceeding the predetermined value.