Abstract：Wireless power transfer (WPT) has been widely applied in various charging fields due to its non-contact and high-efficiency transmission of high-power electric energy over medium and long distances. In contrast to the traditional plug-in charging, WPT has the advantages of misalignment adaptation and easy maintenance, also can effectively avoid the plug wear, wire aging, and contact electrical sparks. However, because of its characteristics of non-contact between primary side and secondary side, foreign object could easily enter the coupling charging area. The eddy effect caused by the high power alternating magnetic field will lead to fire and ignition. As for the foreign metal objects with small size or located in the corner area relative to detection coil, the detection sensitivity of existing common detection methods is not high enough to detect them accurately. The blind detection area will cause the safety hazards. Thus, a high-order composite resonant topology to improve the detection sensitivity of foreign object detection system was proposed in this paper, which solves the problems of low detection sensitivity and blind area of traditional detection coil. Firstly, the mutual inductance coupling model between metal foreign object and detection coil was established, and the amplifying effect of resonance topology on the impedance variation of detection coil was interpreted. Secondly, the feasibility of improving the detection sensitivity of composite resonant topology under reasonable designed parameters was analyzed. Then a high-order composite resonant topology was designed according to the amplification characteristics of the resonant topology. Also, the device parameter optimization of the proposed high-order composite resonant topology is analyzed in detail. Finally, the sensitivity of the proposed topology and its realization circuit were simulated and verified by experiments. In the WPT experiment, the proposed detection coil array and FOD system did not affect the output power and efficiency due to its decoupling characteristic with magnetic coupler. Also, the parameters of detection coils were basically not affected by the offset of magnetic coupler, which can avoid the misjudgment effectively. In the FOD experiment, the aluminum cylinder with diameter 20mm is placed at the corresponding corner positions of the detection coil, the detection sensitivity of two situations above are 62.31% and 275.68%, respectively. When the aluminum cylinder was located in the center area of the detection coil, the detection sensitivity is 187.69%. As for the foreign object with small size, such as paper clips, the detection sensitivity can reach 119.23%. The experimental results above demonstrate that the proposed topology has high enough detection sensitivity to eliminate the detection blind area completely even if the metal foreign object has a small influence on the detection coil impedance. A high-order composite resonant topology to improve the detection sensitivity of foreign object detection system was proposed in this paper. Compared with the traditional series and parallel resonant topology of detection circuit, the proposed topology is more sensitive for the impedance changes of detection coils caused by metal foreign objects. The following conclusions can be drawn from the analysis and experiment results: (1) The measured detection sensitivity of the FOD system is basically consistent with the theoretical analysis. And the detection signal has a high SNR and has no influence on the output power and efficiency of the wireless charging system. (2) The proposed high-order composite resonant topology owns the high detection sensitivity. As for the paper clips and foreign objects which located at the ultimate corner of the detection coil, the detection sensitivity can be up to 62.31% and 119.23%, respectively. (3) The existence of foreign objects can be determined accurately by combining the detection results of multiple adjacent coils comprehensively. Thus, the blind detection areas and other safety risks can be eliminated completely.
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2023, Vol. 38 
