The invention provides DC-DC converters comprising a load sensor, a variable Power MOS, and a Power MOS width controlling and driving device. The Power MOS width controlling and driving device is coupled between the load sensor and the variable Power MOS. The variable Power MOS comprises a plurality of PMOS transistors coupled in parallel and a plurality of NMOS transistors coupled in parallel. After the load sensor detects the load current of the DC-DC converter, the Power MOS width controlling and driving device conducts the PMOS and NMOS transistors according to the sensed load current to control the total size of the conduction paths that couple a transformed DC voltage output terminal to a source of an original DC voltage or ground.