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.

I. Introduction

The converters with small profile and high efficiency are more and more required in industry applications. As the switching frequency goes up, high power density is satisfied. However, there is a limit in power density when the switching frequency reaches a high level [1]–[2]. The power density is mostly influenced by the magnetic components. The structure of inductors and transformers also has effect on the power density. The PCB windings are frequently used in high frequency power converters due to the benefit of high quality factors and low profile. Conventionally, the PCB transformers are integrated tightly to the other components horizontally [3]–[5]. The transformer occupies a large PCB area that impacts the power density. Therefore, it is important to reduce the transformer occupied area to minimize the converter volume. With the great need of the power density, the transformer core is embedded into the PCB to increase the integration density [6]. The power density of a 2 W converter with the switching frequency of 1 MHz reaches 73 W/in³. However, the transformer is cored so that the magnetic field is constrained. In VHF (30 MHz-300 MHz) range, the transformers are typically air-cored so that the transformer overall height is only the PCB thickness. Therefore, the VHF converters are suitable for the applications requiring low thickness. However, with the elimination of the magnetic core the magnetic field induced by the air-core transformer spreads widely, causing severe EMI and eddy current in the coppers. So the decoupling design of the transformer and other components is important to the converter efficiency and reliability.