Experience with 1 to 3 megahertz power conversion using eGaN FETs

Earlier planning and subsequent experience are described for the development of a 50-W, 300-Vdc power converter with power density targeted at 100 W/in³ and efficiency targeted at 95%. To achieve the desired power density, 3 – 5 MHz switching frequency was attempted, necessitating the choice of commercial eGaN^® (enhancement-mode GaN FETs). Three-level isolated ZVS (Zero-Voltage-Switched) converter topologies were selected to best utilize with the 200-V ratings of the 5-MHz switches. A stacked gate driver is thus discussed for the three-level converter. For relative permeability below 50 needed by the magnetic components, magnetic powder has high core loss while gapped ferrites leads to high winding loss. Thus, a low-temperature process was developed to synthesize polymer-coated magnetic powder with permeability similar to that of commercial magnetic powder, but with loss approaching ferrites´ loss. Constant-flux magnetic concept is described in companion papers as means to improve power density. Owing to the high reverse on-voltage of eGaN^®, synchronous rectification is not a clear winner unless gate timing is precisely controlled. Experimental results at 3 MHz show that inverter´s efficiency approaches 95% and dc-dc converter´s efficiency approaches 85% at half the voltage rating of the eGaN^® FETs.