In-Hwan Oh
Fairchild Semiconductor

In a typical Flyback converter, the major switching losses are caused when the power switch is turned off. Particularly, the tune on switching loss is small or almost zero because the power device is turned on with zero starting current. There must be also a snubber circuit to limit the voltage spike caused by stray and leakage inductance of the main transformer. Typically, a Resistor-Capacitor-Diode (RCD) snubber is widely used for this purpose. However the resistor in the RCD snubber dissipates power. In addition, there must be an auxiliary winding at the main transformer to provide a low operating voltage for the control IC. This paper proposes a method of utilizing this useless energy for a low operating voltage based on a lossless snubber circuit and compares its performance with the RCD snubber. The experimental results are provided by using a 60W AC-to-DC Flyback converter. This proposed utilization of useless energy based on a lossless snubber circuit, effectively eliminates the need for auxiliary winding. This method also improves system efficiency. Therefore, a simple, low cost, higherefficiency power converter can be realized.