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| Critical Zero Voltage Switching Control Method for a 6.78 MHz Wireless Power Transfer System Based on LCC-S Networks |
| Zhao Chenxu, Xiao Guochun, Gao Zetu, Mohamad Abou Houran, Wang Laili |
| School of Electrical Engineering Xi’an Jiaotong University Xi’an 710049 China |
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Abstract The 6.78 MHz wireless power transfer (WPT) system is an attractive solution for consumer electronics, offering excellent spatial freedom, lightweight coils, and high power density. However, the full-bridge converters in MHz scenarios suffer from high switching losses, reverse conduction losses, and challenges in synchronous phase control. This paper proposes a bilateral adaptive dead-time control strategy that adaptively adjusts dead time and driving phase angle across the entire load range, ensuring continuous operation under critical zero-voltage switching (ZVS) conditions.
Firstly, a frequency-domain analysis model is established based on the equivalent circuit. The fundamental and higher harmonic components of the resonant current are determined. Considering the trade-off between computational complexity and accuracy, harmonic components of the 9th order and above are reasonably neglected.
Secondly, the critical ZVS conditions for the inverter and rectifier are derived from time-domain analysis. Both the inverter and rectifier need to reduce the hard-switching and reverse-conduction losses in GaN devices during critical ZVS operation. When the charge provided by the current (Qzvs) exceeds the charge required by the device (Qoss), the converter operates in the post-ZVS condition, leading to significant reverse-conduction losses due to the high reverse-conduction voltage of GaN devices. When Qzvs is less than Qoss, the converter operates in partial ZVS, resulting in hard-switching losses due to turn-on ringing. When Qzvs equals Qoss, the converter operates in critical ZVS mode, eliminating both hard-switching ringing and reverse-conduction losses. By integrating the higher-harmonic components calculated from numerical solutions for the critical ZVS operation, the converter operation is derived.
Then, a bilateral adaptive dead-time control strategy is proposed. The converter continuously monitors the reverse conduction signal and the zero-crossing point of the secondary resonant current. Through phase angle and dead-time control, the system adaptively achieves critical ZVS operation for the bilateral converters under varying load conditions. Specifically, the primary side employs a reverse-conduction detection (RCD) circuit to monitor the drain-source voltage of the lower switch in the inverter, thereby determining the ZVS current status. The resonant current zero-crossing point is detected and processed by a digital PI controller to achieve precise regulation of the driving phase angle on the secondary side. It is integrated with the RCD circuit signal to adjust the dead time and the reference value of phase angle, enabling critical ZVS operation across the entire load range to optimize efficiency.
Finally, two 40 W prototypes, one based on GaN rectifiers and the other on passive rectifiers, are constructed to validate the proposed approach. Experimental results demonstrate that the system maintains high efficiency across varying loads, with a peak overall efficiency of 86.82% at 40.2 W output using GaN-based rectifiers. An improvement of 2.65% at rated load (RL=20 Ω) and 5.49% under light load (RL=50 Ω) conditions is achieved compared to the system with passive rectifiers. The constant voltage output characteristic of the system with the proposed method under varying load conditions has been effectively validated. Moreover, a detailed comparison with existing methods is presented in a table. This research provides a solution for future WPT system requirements.
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Received: 15 September 2025
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2026, Vol. 41 
