A Digital Adaptive Driving Scheme for eGaN HEMTs in VHF Converters

Very-high-frequency (VHF) converters with the conventional driving scheme suffer from serious efficiency drop over entire input voltage range due to gate drive timing mismatch of the eGaN control and synchronous rectification (SR) HEMTs. A digital adaptive driving scheme is proposed to adjust the control and SR gate drive signals efficiently over input voltage range. A state-space model is derived to predict drive timing precisely according to different input voltage and adjust the gate drive signals correspondingly. A high time resolution circuit to improve time resolution by selecting cascaded high-speed buffers is proposed. With the proposed driving scheme, high-frequency switching loss and reverse conduction time of eGaN HEMTs are reduced significantly to maintain high efficiency over entire input voltage range. A 30-MHz VHF SR flyback with the printed circuit board (PCB) fabrication air-core transformer was built. With 18-V input and 5-V/10-W output, the measured efficiency realizes 80.0%. With 24-V input and 5-V/10-W output, the proposed solution improves the efficiency from 72.9% using the conventional driver to 75.1% (an improvement of 2.2%).