Non-linear capacitor based variable capacitor for self-tuning resonant converter in wireless power transfer

Abstract

In wireless power transfer (WPT) system, the resonant frequency of primary side and secondary side should be matched to maximize the power transferred and efficiency. However, in mass production, the variation of each component in the resonant circuit results in poor matching. One solution is to utilize a variable capacitor for self-tuning to compensate the variation. To implement a variable capacitor on a linear capacitor in power converters, most of the solutions chop part of the rated current by an auxiliary circuit. Therefore, the power rating of the auxiliary circuit is proportional to the percentage of capacitance variation, which generates considerable loss. On the other hand, non-linear capacitor, ceramic capacitor as an example, can change capacitance by varying DC bias. The auxiliary circuit to hold the DC bias only needs to supply the leakage current of the nonlinear capacitor, which is negligible. In this paper, an almost lossless auxiliary circuit for continuously varying DC bias is proposed. It is suitable for high power applications such as electric vehicles wireless charger. Experimental results based on a 100-W scale-down series resonant converter (SRC) with 20-cm air gap between the primary and secondary side are presented to illustrate the mechanism of this method.