A Hybrid Modulation Scheme for Efficiency Optimization and Ripple Reduction in Secondary Side Controlled Wireless Power Transfer Systems

This paper presents a hybrid modulation (HM) scheme to facilitate secondary-side control in wireless power transfer (WPT) systems. To achieve broad power regulation in WPT systems, the conventional pulse width modulation (PWM) exhibits a significant efficiency drop under light loads, while the existing pulse density modulations (PDMs) lead to considerable current and voltage ripples. To address this issue, an optimal discrete PDM (D-PDM) is proposed for active rectifier modulation. By symmetrically and uniformly distributing pulses, the proposed optimal D-PDM eliminates even-subharmonics in rectifier input voltage, thereby reducing the current distortions and output voltage ripple while removing the capacitor DC blocking voltages. Moreover, the pulse width regulation is incorporated into the optimal D-PDM, enabling continuous output tuning and further minimizing subharmonics in the voltage. Based on a WPT prototype, the proposed HM is benchmarked with the existing PWM and PDMs. Experimental results show that the proposed HM significantly mitigates current and voltage ripples while facilitating continuous tuning when compared with the existing PDMs. Additionally, when compared to the PWM, the proposed HM demonstrates notable efficiency improvements within the 10%-60% power range, achieving a maximum efficiency enhancement of up to 5.5%.