Abstract: A duty control method for a low-voltage DC-DC converter (LDC) is provided in which a duty ratio of a high-efficiency and low-voltage DC-DC converter that has a boost converter and a full-bridge converter combined therein is variable-controlled to output low-voltage. In particular, control of a duty ratio of the LDC that includes a boost converter and a full-bridge converter or a half-bridge converter connected in series is improved and by controlling an output voltage of the full-bridge converter in a simple-equation-based variable-duty scheme to output a low voltage for high-voltage input, stable low-voltage output may be achieved over the input and output voltage range.