Analysis And Design Of Active Clamp Flyback Converter Based On Secondary Resonance

With the continuous improvement of the performance of portable electronic devices,small and medium power converters are being developed towards high efficiency and high power density.The commonly used traditional active clamp flyback converter relies on the resonance between the transformer leakage inductance and the primary clamp capacitor to achieve soft switching,but this method increases the conduction loss and the current oscillation of the output rectifier.Therefore,comparing and analyzing the influence of different resonance modes on the working state of the circuit,and exploring the design method of the active clamp flyback converter under the optimal working state,have important guiding significance for improving the efficiency and power density of the flyback converter.In order to explore the optimal operating mode of the active clamp flyback converter,the converter is divided into four operating modes according to whether the excitation inductor current and the secondary current are continuous or not.Through in-depth analysis of the working characteristics of different modes,it is found that the secondary current zero-crossing DCM is the best working mode of the converter,the voltage and current stress of the switching device in this mode are deduced,and it is concluded that the primary side main switch tube and clamp tube realize soft switching conditions of.According to the power supply situation of the load,it is found that the converter works in CCM mode,and there are two energy transmission modes;when it works in DCM mode,there is one energy transmission mode.The critical inductance and output ripple voltage analytical formulas in each mode are deduced,and it is pointed out that the conversion When the transformer works in DCM mode,the output ripple voltage decreases with the increase of the transformer inductance and clamping capacitor;when it works in CCM mode,the output ripple voltage decreases with the increase of the clamping capacitor,regardless of the transformer inductance.Therefore,the critical inductance between CCM and DCM is the magnetizing inductance that minimizes the output ripple voltage.In order to solve the problems of large conduction loss and current oscillation of the output rectifier in the traditional active clamp flyback converter,an equivalent circuit model under different resonance modes was established,and the influence of different resonance modes on the working state of the converter was deeply analyzed.The research found that the secondary side The resonance mode significantly reduces the effective value of the primary current,reduces the conduction loss of the converter,and eliminates the current peak of the output rectifier,which is beneficial to the implementation of synchronous rectification and improves the efficiency of the converter.According to the design index,make an experimental prototype of 45W active clamp flyback converter,and build a test platform.The experimental results show that the designed converter not only realizes the soft switching of power devices,but also reduces the effective value of the primary current and improves the current oscillation phenomenon of the synchronous rectifier;in the full input voltage and load range,the converter efficiency is greater than 90%,the maximum efficiency reached 94.06%,and the power density and load regulation were 1.16W/cm3 and 0.35%,respectively.Compared with traditional flyback circuits,both efficiency and power density are significantly improved.