A model to analyze and improve dynamic response of high light load efficiency multi-mode converters

Reduced switching frequency peak current mode is a popular method to boost up light load efficiency of isolated power converters. However, transient response decreases dramatically when low switching frequency is adopted. This paper covers small signal analysis of control scheme that peak current and switching frequency both modulated by the control signal. An interesting result is found that the control scheme can be decomposed into two parts: peak current mode part and frequency modulation mode part. At low switching frequency light load conditions the bandwidth from control to output is low. According to the proposed model, special design on voltage control oscillator is desirable to boost up the bandwidth at light load conditions. By the time domain concept the switching frequency should be designed to rise quickly during step up transient in order to overcome the drawbacks caused by low switching frequency and prevent transformer saturation. Experimental prototype Two-FET-Forward converter is build to verify the proposed idea.