Optimal operation and burst-mode control for improving the efficiency of the quasi-switched-capacitor resonant converter

The Quasi-Switched-Capacitor (QSC) resonant converter has been proposed to serve as the isolated dc/dc stage of ac/dc power adapters for portable electronics. Compared to Flyback and LLC resonant converters, its features include: the voltage stress on the primary-side switches is reduced to 2/3 of the input voltage, which is friendlier to low-voltage switches whose figure of merits is generally better; and the transformer turns ratio is reduced by 2/3, which enables less number of turns, lower winding loss and lower leakage inductance of the transformer. In this paper: Two types of resonance which lead to ZCS and ZVS respectively are identified to achieve the optimal normal-mode operation. The optimal duty ratio and switching frequency are implemented to achieve full soft switching and reduce the switching loss; and burst-mode control is implemented to improve the light-load efficiency. Simulation results and experimental results from a 90-W, 88-V/19-V prototype based on eGaN FETs validate the proposed techniques. The converter achieves a flat efficiency curve with a peak value of 96% at 700 kHz. Compared to earlier results without optimization, the efficiency is improved by 2~20% within the full load range.