Enhanced On-Off Keying Modulation for Wireless Power Transfer Systems to Improve Efficiency over a Wide Load Range

Maximum efficiency point tracking (MEPT) is a key concept to improve the ac-ac efficiency of the wireless power transfer (WPT) systems by controlling the equivalent load impedance with a dc-dc converter or an active rectifier. The on-off keying (OOK) modulation can reduce the switching losses of the converters to achieve MEPT but generates relatively large dc voltage ripples. In order to solve this problem, this paper proposes an enhanced on-off keying (EOOK) modulation method by adding a half-bridge mode based on OOK modulation for WPT systems. By combining the three operation modes, both the inverter and the active rectifier can operate in full-bridge-half-bridge mode or half-bridge-zero mode to adapt to different power levels and achieve MEPT simultaneously. Meanwhile, the ZVS condition and the low voltage ripple are achieved as well. The circuit model of the proposed method is analyzed to derive the equivalent load resistance, the output voltage gain, and the output power with respect to the modulation of EOOK. The reason for output voltage ripples and the power losses on both the transmitting and receiving sides are also revealed. An experimental prototype is built to verify the effectiveness of the proposed method. The experimental results show that the proposed method can maintain constant voltage output and achieve MEPT as well as ZVS operation, with high efficiency of up to 94.761% over a wide range of load conditions.