GaN High Electron Mobility Transistors (GaN HEMT) have lower driving loss and shorter deadtime circuit benefits due to significantly reduced gate charge (Qg) and output capacitance (Coss) compared to Silicon MOSFETS. Therefore, GaN HEMTs show significant advantages over Silicon MOSFETs in high-frequency soft-switching resonant topologies such as an LLC resonant converter. With increased switching frequency (fsw), the transformer core size can be reduced. Furthermore, a 3-D PCB structure is employed to increase the power density. A 190-Watt, 400V-19V GaN Systems E-HEMT based LLC DC-DC resonant converter is carefully designed, and the transformer is optimized for high-end adapter applications operating above 600kHz. The converter shows a complete design with a power density over 63W/inch3, including the 400V bus capacitor, with a peak efficiency of 96%.