Analysis and Design of High Frequency Asymmetrical Half Bridge Flyback Converter

The asymmetrical half bridge flyback converter is gaining popularity because it can achieve ZVS operation. However, the relationship between voltage transfer ratio and duty cycle is nonlinear in the condition of high switching frequency and small blocking capacitor. It is improper to use the linear model to guide the design of asymmetrical half bridge flyback converter. This paper deduces a steady-state voltage transfer function of the converter at high frequency, and the effect of load is also taken into consideration. According to the steady-state voltage transfer function, the characteristics of voltage transfer ratio and maximum duty cycle can be determined. Finally, a 19V/6A, 250kHz prototype is designed and built to verify the analytical results. The experimental results indicate that, compared to the linear model, the asymmetrical half bridge flyback converter designed according to the voltage transfer function can be controlled more accurately and has significant merits at high frequency.