1-kV Input 1-MHz GaN Stacked Bridge LLC Converters

In this article, a stacked bridge LLC converter with the split resonant tanks suitable for high input voltage is proposed to reduce the voltage stress of primary-side devices by half. The 650-V enhancement-mode gallium nitride high-electron mobility transistors (eGaN HEMTs) with much lower R-ds(on) and Q(g) are applied instead of 1700-V silicon carbide (SiC) MOSFETs at 1-kV input to produce significantly reduced conduction loss and switching loss at 1 MHz. With eGaN HEMTs switching speed of 2.5 ns at 1-kV input, high dv/dt of 200 kV/mu s causes large displacement current via the interwinding capacitance of the planar transformers, which distorts the resonant current seriously and causes zero voltage switching (ZVS) lost inducing additional switching loss and root mean square loss. The split resonant tanks solution is proposed to reduce the resonant current distortion to guarantee ZVS and voltage balance over the split capacitance. To reduce the interwinding capacitance, the noninterleaving type windings are proposed to minimize the displacement current from the primary side to secondary side of the matrix transformer. A practical engineering practice of a 1-MHz prototype with 1-kV input and 32-V/3-kW output is built. It achieves the peak efficiency of 96.2% at full load and power density of 107 W/in(3), a size reduction of 69% compared to the 300-kHz SiC counterpart.