A method of controlling the charge of the bootstrap capacitor during light load or no load conditions for a non-synchronous type of DC-DC converter consists of bootstrap capacitor voltage detector, light load detector and secondary switch controller. By turning on the secondary switch during the off-time of the power transistor when the bootstrap voltage is lower than the required value under light load condition, the bootstrap capacitor voltage will be able to charge back to the required value, yet minimizing the minimum current requirement for the DC-DC converter.