Analysis, design and implementation of an active clamp flyback converter

This paper presents the detailed circuit operation, mathematical analysis, and design example of the active clamp flyback converter. The auxiliary switch and clamp capacitor are used in the flyback converter to recycle the energy stored in the transformer leakage in order to minimize the spike voltage at the transformer primary side. Therefore the voltage stress of main switch can be reduced. The active clamped circuit can also help the main switch to turn on at ZVS using the switch output capacitor and transformer leakage inductance. First the circuit operation and mathematical analysis are provided. The design example of active clamp flyback converter is also presented. Finally the experimental results based on a 120 W prototype circuit are provided to verify the system performance

I. Introduction

The flyback converters are widely used in the conventional switching mode power supplies to supply low power and low output voltage. The transformer in the flyback converter is used to achieve circuit isolation and energy storage. The switch in the flyback converter isoperated at hard switching. Therefore the voltage and current stress of switch suffered from the transformer leakage inductance is very high. These high peak voltage and current will result in the low efficiency and high switching losses. The conventional passive clamp circuit can be used to reduce the energy stored in the leakage inductance using the clamp resistor. Therefore the voltage stress of switch can be reduced. However the total efficiency of converter is not greatly improved. The resonant flyback converters [1]–[3] were proposed to increase the system efficiency and reduce the switching losses at the main switch. However, the voltage stress at the switch is too high in the resonant flyback converters especially for the high input dc voltage. The active clamp technique has been proposed [4]–[6] to absorb the surge energy stored in the leakage inductance and suppress the voltage stress at the switch by using the active clamp circuit. The clamp capacitor was assumed to be a constant voltage source. The effect of the clamp capacitor on the flyback converter stability was presented in [7].