A Quasi-Switched-Capacitor Resonant Converter

A quasi-switched-capacitor (QSC) resonant converter is proposed for isolated dc/dc conversion in offline power supply application. Similar to the Φ2 resonant converter, the proposed converter is operated most efficiently at fixed switching frequency and duty ratio, and it features trapezoidal voltage waveforms on all switches. Full soft switching combining ZCS, ZVS on and off is achieved within a wide load range, and, therefore, the switching loss is minimized. In addition, compared to half/full-bridge converters (e.g., the LLC resonant converter and the dual-active-bridge converter), the proposed converter reduces the voltage stress on primary-side switches to 2/3 of the input voltage, and, thus, enables more choices of low-voltage devices, which are more efficient because of better figure of merit. Furthermore, the proposed converter reduces the transformer turns ratio by 2/3, and, thus, enables less number of turns of the winding, lower winding loss, and lower transformer leakage inductance, making it suitable for high-frequency operation. A 90-W 88-V/19-V 700-kHz prototype is built with 100-V eGaN FETs. The transformer design and PCB layout are presented to minimize the transformer leakage inductance and stray inductance. The prototype achieved a power density of 172 W/in
³
, and a flat efficiency curve with a peak value of 96%.