A 30-W flyback converter operating at 5 MHz

Motivated by the recent commercialization of gallium-nitride (GaN) switches, an effort was initiated to determine whether it was feasible to switch the flyback converter at 5 MHz in order to improve the power density of this versatile isolated topology. Leakage inductance, harmonic losses in the coupled windings, core loss, layout impedances, gate drivers, and other issues problematic at high frequency were quantified by analysis and simulation for a bus converter stepping 36-72 V down to 12 V at 0-30 W. The layout of the coupled windings was optimized by finite-element simulation to achieve 30 nH of leakage inductance with respect to 0.96 μH of magnetizing inductance in order to limit voltage stress. Even with optimal layout, the windings consumed almost 3% of the output power, and dominated the total loss. After breaking down the copper losses in frequency domain, harmonic analysis revealed that 38% of the winding losses were consumed by the harmonics. In order to validate all analysis, a quasi-square-wave (QSW) flyback converter with zero-voltage switching (ZVS) was measured with peak efficiency of 87%. The coupled inductors were composed of a commercial planar ER core fabricated from nickel-zinc ferrite; the size was measured as 18×3.2×10 mm³ and occupied 30% volume of the power stage. In addition, the high dv/dt of 16 V/ns and high di/dt of 10 A/ns caused by the smaller output capacitance of GaN FETs necessitated negative 2 V between gate and source to prevent false turn-on of the switches.