A control device has a first control part and a second control part. The control device is applied to a DC-DC converter. The control device selects a first switch turning-on instruction or a first switch turning-off instruction generated by the first control part during a voltage boost control period, and transmits the selected first switch turning-on instruction or the selected first switch turning-off instruction to a first switch in the DC-DC converter. The control device selects a second switch turning-on instruction or a second switch turning-off instruction generated by the second control part during a voltage step-down control period, and transmits the selected second switch turning-on instruction or the selected second switch turning-off instruction to a second switch in the DC-DC converter.