Self-Oscillating Wireless Battery Charging Systems with Locking Equilibrium Points

This paper presents a self-oscillating control method with locking equilibrium points for series-series compensated wireless power transfer (WPT) systems designed for battery charging. Previous studies indicate that frequency bifurcation in the WPT system reveals three zero-phase angle (ZPA) frequency points, which act as equilibrium points in self-oscillating systems. This phenomenon can cause the system to switch between different ZPA points, potentially leading to fluctuations in output power, current oscillation or instability. The analysis in this paper focuses on the frequency characteristics of the WPT system under battery/voltage loads, highlighting significant differences compared to systems with resistance loads. Notably, multiple equilibrium points exist only within a narrow range of input impedance angles. A self-oscillating control strategy is proposed to lock the WPT system at a specified equilibrium point by comparing the designed sending side current to generate PWM signals. The locked equilibrium point is positioned close to the highest ZPA frequency point, ensuring robust misalignment tolerance and near-maximum power output. Moreover, the approach enables zero-voltage switching across the entire operational range. The effectiveness of the proposed method is validated through both simulation and experimental results.