A DC-DC converter is used for raising input voltage, and has a substrate forming a plurality of conductive pins, and a circuit system formed on the substrate and appropriately coupled to the conductive pins. The circuit system includes a step-up circuit coupled in series to a MOSFET with threshold voltage of 20V. A Zener diode and a diode are coupled in parallel between two transistors of the step-up circuit and are coupled in series with each other for limiting working voltage of the step-up circuit no more than 20V thereby assuring MOSFET working normally and preventing damage of electronic components in the DC-DC converter.