Maintaining soft switching over a wide operating range is challenging in wireless power transfer (WPT) systems due to large variations in coil coupling and load conditions. A novel control method that regulates zero-voltage-switching (ZVS) angle together with power control, has been proposed recently for series-series (SS) compensated WPT systems. In that approach, ZVS angle is effectively regulated by simple voltage sampling, which ensures soft switching in the entire working range. This paper furthers the existing control method by deriving an optimal ZVS angle reference and proposing a simple adaptive algorithm to update the optimal reference in real time. The objective is to minimize reverse conduction losses of switches while maintaining ZVS. We present a detailed analysis of the ZVS criterion and its real-time calculation process. A prototype implementing the proposed adaptive ZVS angle control is built and tested. The experimental results demonstrate the feasibility of the proposed method.
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