Air-core transformer integration for GaN VHF converters

This paper proposes an air-core transformer integration method, which mounts the transformer straightly into the multi-layer PCB, and maintains the proper distance between the inner transformer and other components on the top layer. Compared with other 3D integration method, the air-core transformer is optimized and modeled carefully to avoid the electromagnetic interference (EMI) of the magnetic fields. The integration method reduces the PCB area significantly, ensuring higher power density and similar efficiency as the conventional planar layout because the air-core transformer magnetic field does not affect other components. Moreover, the converters with the integrated PCB transformer can be manufactured with high consistency. With the air-core transformer, the overall height is only the sum of twice the PCB thickness and components height. In addition, the proposed integration method reduces the power loop inductance by 64%. It is applied to two resonant flyback converters operating at 20 MHz with Si MOSFETs, and 30 MHz with eGaN HEMTs respectively. The full load efficiency of the 30 MHz prototype is 80.1% with 5 V input and 5 V/ 2 W output. It achieves the power density of 32 W/in³.