Effect of frequency and load on efficiency of inductive wireless power transfer systems

Wireless charging of smartphones using inductive wireless power transfer (IWPT) has become very popular. However, the efficiency of IWPT systems can decrease substantially when the transmitting and receiving coils are not closely coupled. One popular approach to mitigate the efficiency degradation is to utilize higher frequency. However, the higher efficiency is only achieved at a specific range of load resistance, which may not be feasible in real applications. In this work, we analyze the effect of frequency and load variations on the power transfer efficiency of a typical two-coil IWPT setup, based on a numerical model. The results show that lower frequency systems are favored for low load resistances, and vice versa for high load resistances. For example, when the coils' inductance is 20μH and the load resistance is below 3.4 ohms, then operation at lower frequencies is recommended even when the coupling factor is as low as 0.1.