Asymmetric Dead-Time Control in 6.78 MHz WPTSs for CC/CV Output With Critical ZVS Operation

Constant current and voltage (CC/CV) output is essential for wireless power transfer systems (WPTSs), which is typically achieved by the cascaded DC-DC converters or impedance matching networks. Phase-shifting modulation (PSM) allows active rectifiers to better exploit MHz flexibility. However, the resulting asymmetric dead time at MHz disrupts both output control and critical zero-voltage switching (ZVS) operation, limiting the application of the single-stage scheme. This article proposes an asymmetric dead-time control (ADTC) method for 6.78 MHz WPTSs to achieve CC/CV output with critical ZVS operation for the active rectifier. The generation and impact of the asymmetric dead times are analyzed in detail. The critical ZVS conditions are derived, and asymmetric dead times effect is impressed via the proposed ADTC method. Each control loop is individually designed based on the dynamic model. A 20 W prototype is built to verify the effectiveness and robustness of the proposed method. The ADTC method enables the system to achieve precise synchronous phase control, stable CC/CV output, and critical ZVS operation over the whole load range. The efficiency is improved over full load range and achieves 83.67% at P_o = 20.5 W. Compared to the system without ADTC, the efficiency is improved by 6.1% at rated load and 4.7% at light load.