A GaN transistor based 90 W isolated Quasi-Switched-Capacitor DC/DC converter for AC/DC adapters

This paper presents the design of a 90 W isolated Quasi-Switched-Capacitor (QSC) DC/DC converter, serving as the DC/DC stage for AC/DC adapters. The converter employs a QSC DC/AC circuit and 2 different rectifier circuits, which are a synchronous-rectifier, current-doubler (SRCD) circuit and a synchronous-rectifier, center-tapped (SRCT) circuit. Compared to Flyback and LLC resonant converters, the QSC DC/DC converter features: 1) reduced voltage stress on the primary-side switches to 2/3 of the input voltage; 2) reduced voltage stress on the transformer to 1/3 of the input voltage and a lower transformer turns ratio; 3) a wide range for soft-switching operation and high efficiency; 4) a simple control strategy. The operation principles and simulation results are presented. A 90 W, 400 V/19 V, 500 kHz QSC converter with a SRCD circuit is built, using 600 V Transphorm GaN HEMTs and 200 V EPC eGaN FETs. The converter size is 70 mm×50 mm×16 mm, and the peak efficiency is 90.3%. Since the GaN HEMT is oversized for this application, the efficiency is not optimized. To pursue higher power density and efficiency, a 90 W, 85 V/19 V, 1 MHz QSC resonant converter with a SRCT circuit is built, using 100 V EPC eGaN FETs for all switches, to serve as the down-stream circuit of a Buck-PFC stage. This prototype can achieve: 1) a high power density of 10.5 W/cm³; 2) wide-range soft switching for all switches and an estimated efficiency of 96%.