Analysis of a Self Turn-on Phenomenon on the Synchronous Rectifier in a DC-DC Converter:In the buck converter with MOS Fets, the low side switch,which must be still in cutoff state, becomes turn-on when thehigh side switch is turned-on and the current flows through thecircuit of the power supply, the high side switch and the low sideswitch. We call this phenomenon self turn-on. The powerdissipation by this current becomes higher when the switchingfrequency of the converter is high as MHz. This self turn-onoccurs easily in the converter of low output voltage and highvoltage conversion ratio.We present the principle of self turn-on phenomenon oncircuit simulation and experimental results.