Adaptive Impedance Matching for Magnetically Coupled Resonators

For high quality factor magnetically coupled resonator wireless power systems, constant wireless power transfer efficiency can be achieved over a greater range and higher efficiency than an inductively coupled system. However, as the distance varies between two near-field resonators, the input and output impedances to the resonators change. Therefore, in order to achieve constant efficiency, the resonator input impedance must be adaptively matched to the source impedance of the transmit resonator and the load impedance of the output device. Previous techniques have demonstrated adaptive frequency tuning control algorithms to track the resonant peaks. However, government regulations strictly limit the radiating electric field strength of wireless applications outside specific bandwidths which will be exceeded by these frequency tuning algorithms. Adaptive impedance matching (AIM) networks automatically change the input and output impedances in order to maintain maximum wireless power transfer efficiency. In this paper, the input impedance is characterized for a four-element resonant wireless power system. Two algorithms are presented to determine the component values for any adaptive impedance matching network topology to achieve constant wireless power transfer efficiency at a single frequency. These techniques are demonstrated and verified with a π-match network.