In recent years, the development of electronic technology, making the circuit working voltage is getting lower and lower, the current is getting bigger and bigger. Low voltage operation is conducive to reducing the overall power consumption of the circuit, but also to the power supply design put forward new problems. Switching power supply loss is mainly composed of three parts: power switch loss, loss of high-frequency transformer, the output end of the loss of the rectifier. In the case of low voltage and high current output, the conduction voltage drop of the rectifier diode is high, and the output of the rectifier is particularly prominent. Fast recovery diodes (FRDs) or ultrafast recovery diodes (SRDs) can be up to 1.0 to 1.2V, resulting in a voltage drop of about 0.6V even with low voltage drop Schottky diodes (SBD), resulting in increased rectification losses Large, power efficiency is reduced. For example, the current notebook computer commonly used 3.3V or even 1.8V or 1.5V supply voltage, the current consumption of up to 20A. At this point the ultra-fast recovery diode rectifier loss is close to or even more than 50% of the power output power. Even with Schottky diodes, the losses on the rectifier will reach (18% to 40%) PO, which accounts for more than 60% of the total power loss. Therefore, the traditional diode rectifier circuit has been unable to meet the low voltage, high current switching power supply efficiency and small size needs, become a DC / DC converter to improve the efficiency of the bottleneck.