Single-Stage Wireless Battery Charging Circuit with Coupling Coefficient Prediction

Abstract

This paper proposes a single-stage wireless battery charging circuit with a coupling coefficient prediction method. The proposed circuit consists of only two stages: full bridge inverter with transmitter coil in the first stage and full bridge rectifier with receiver coil in the second stage. This circuit implements the constant current (CC) charging mode at the resonant frequency of two coils and the constant voltage (CV) charging mode at a specific frequency that is dependent on the coupling coefficient of two coils. The operation at a specific frequency guarantees the CV operation regardless of load condition and reduces the switching losses than the operation at the resonant frequency owing to a zero-voltage switching (ZVS) operation. In CC-CV modes, the phase-shift technique is additionally applied to improve the output voltage/current regulation. Unlike other approaches, the proposed single-stage wireless battery charging circuit does not require multiple stages of power conversion, or additional components, a pre-measured coupling coefficient or a complex control algorithm for CC-CV charging operation. The prototype proposed circuit was tested under various coil alignment conditions, and successfully implemented the CC-CV charging operation for a 36 V battery pack. The predicted coupling coefficient had an error of ≤0.62% in the coil alignment condition, and the circuit had errors of ≤0.32%, ≤0.1% in the output current and voltage regulation, respectively.