GaN-Based LLC Converters With Digital Synchronous Rectifier and Copper Foil Planar Transformer in High-Output Current Applications

With a gallium nitride (GaN) switching speed of 3.7 ns, high
dv
/
dt
of 105 kV/
μ
s for 390 V poses serious challenges for synchronous rectification (SR) using extra circuit sensing high-frequency signal of drain-to-source voltage method. A digital SR algorithm based on charge conservation is proposed in this article, which deduces the SR on-time by calculating the time of current flowing through the SR
mosfet
junction capacitor. The proposed SR requires no additional detection circuit, which effectively avoids the effect of high
dv
/
dt
and minimizes the SR conduction loss to improve the efficiency. Furthermore, in high-output current applications, printed circuit board planar transformers face problems of huge winding loss and limited winding current-carrying capacity. A design is proposed for a planar transformer using secondary-parallel multiple interleaved copper foil winding. To diminish dc resistance, the secondary side of the transformer is connected in parallel with three copper foil layers. Additionally, the primary and secondary windings are interleaved to achieve lower ac resistance. A 1.5-kW GaN
LLC
prototype is built with a 650 kHz resonant frequency. The
LLC
power density is 524 W/in
³
and the efficiency is up to 98% with 28 V/53.5 A output at full load.