For low power supply applications, flyback converter has been adopted widely because of its simple structure and low cost. In traditional flyback converter, feedback circuit is typically done with the assistance of expensive opto-coupler and TL431, to preserve electrical isolation. However it is not only increases cost but also adds size and power consumption. The primary side control flyback converter senses the voltage of auxiliary winding to regulate output voltage. It can void using expensive element of opto-coupler and TL431. Therefore primary side control flyback converter reduces the device cost and the size, and improve the efficiency of the power supply.