Multi-megahertz quasi-square-wave flyback converter using eGaN FETs

As the fast development of the better figure-of-merits gallium nitride (GaN) devices, the switching frequency of isolated flyback converter could be pushed to multi-megahertz to evaluate the improvement of the power density. A constant frequency zero-voltage switching (ZVS) quasi-square-wave (QSW) mode is applied to the 30 W flyback in telecommunication application, and a charge conservation method including capacitor non-linearity is proposed to design the coupled inductors for soft switching over entire input and load range. Leakage inductance, eddy losses and parasitic capacitance were quantified and optimised in the design of high-frequency PCB windings. A 98% coupling planar-coupled inductors by utilising nickel-zinc ferrite material with EI-shaped core were designed by finite-element simulation. To validate all the analysis, a 30 W flyback converter was fabricated and measured with peak efficiency of 90.6% operating at 5 MHz. The coupled inductors consumed almost 2.5% of the output power with the area of 180 mm2 occupied 30% of the power stage.