Postringing analysis of an inductive power transfer system to determine the coupling factor

Inductive Power Transfer (IPT) techniques are gaining more and more interest in wireless power systems. The knowledge of the coupling factor of transmitter (TX) and receiver (RX) coil is a crucial, but mostly during operation not available parameter for such systems. In this manuscript a novel method to determine the coupling factor by measuring the self-resonating frequency of the postringing signal in the TX is presented. The postringing signal occurs in bridge converter topologies, when the excitation is stopped. The cause of the postringing signal can be found in the free-wheeling diodes parallel to the MOSFETs that allow a current flow in the TX circuitry. The determination of the coupling factor with the postringing signal matches the results of benchmark methods, so that the properness of the novel method could be shown. Furthermore, the presented method has no additional hardware effort, since only a frequency measurement needs to be done. Thus, the measurements can be done on-the-fly during the application, so that the system can react on changes of the coupling factor.