As described above, hoWever, the boost capacitor CB shoWn in FIG. 3 is charged When the sWitch S1 is OFF. Accordingly, if the duty D exceeds an upper limit, the boost capacitor CB cannot be fully charged, Which leads to the disability of holding the sWitch S1 ON. This problem arises because a ?nite amount of time is required for a sWitching time, and a pulse Width shorter than the sWitching cannot be generated. Accordingly, among existing DC/DC converters, a converter Which regulates the maximum value of the duty D in order to fully charge the boost capacitor CB is knoWn.