A helical air-core transformer with even current distribution for VHF converters

Very High Frequency (VHF) converters allow the use of the air-core transformers owing to small required inductance. The traditional air-core transformers suffer from uneven current distribution caused by the proximity effect, which results in serious AC conduction loss. A new helical air-core transformer structure with even current distribution is proposed. In the proposed structure, all layers of windings with the same size are completely overlapped to reduce eddy current loss. The layer internal distance of overlapping PCB windings is adjusted to realize the desired transformer parameters. A 30 MHz GaN flyback converter with the PCB fabrication air-core transformer was built. With 15 V input and 15 V/ 21 W output, the proposed transformer structure improves the efficiency from 75.9% using the conventional transformer to 82.7% (an improvement of 6.8%). With 15 V input and 15 V/ 24 W output, the efficiency of the proposed transformer realizes 82.7%.