Effect of Coil Shape on the Critical Load Resistance of Frequency Splitting Phenomenon in Magnetic Resonance Wireless Power Transfer

The frequency splitting phenomenon (FSP) implies a rapid decrease in output power at the resonant frequency, with maximum output happening at the other two frequencies. This occurs when the transmission distance is shorter than the conventional distance or when the load resistance is below a certain threshold. Variation in the load resistance, such as those found in batteries, is more frequent than the variation in the transmission distance during the charging process. In addition, the load resistance depends on the battery capacity of the load type. Therefore, the critical load resistance at which FSP starts should be as low as possible to ensure high output power over a wide range of load resistances, so that FSP does not occur. However, the lower the critical load resistance, the lower the efficiency; therefore, even if FSP occurs, the critical load resistance can be tuned to improve efficiency as long as the target output power is met. Therefore, the critical load resistance at which the FSP is started is an important factor to consider when designing a resonant coil. In this article, the effect of resonant coil shape on the critical load resistance that induces FSP was analyzed by the 2-D finite element method (FEM) and verified by experiment.