A DC-DC convertor comprises: a switching element; a drive circuit for turning the switching element on and off; a bootstrap capacitor electrically connected to the drive circuit; and a control circuit electrically connected to the drive circuit. The control circuit operates to charge the bootstrap capacitor during the charging period in each charging cycle that is longer than the on and off cycle for turning the switching element on and off. The DC-DC convertor is capable of efficient step-up and step-down operation.
