Analysis of Wireless Power Transfer by Coupled Mode Theory (CMT) and Practical Considerations to Increase Power Transfer Efficiency

In this chapter the analytical model of a wireless power transfer scheme is developed through the means of Coupled Mode Theory (CMT). The derivation is made under the assumption of low internal coil losses and some particular type of resonator (coil inductance and capacitance) equivalent circuit. With the equivalent circuit modeling the wireless power transfer system the direct high frequency power source connection to the source coil and the usage of external capacitance are considered. It is shown that the maximum efficiency to resonant frequency ratio could be obtained by serial to short end antenna connection of external capacitance. At MHz frequencies especially near and at the resonant frequencies, the calculation of coil parameters which are the coefficients of the model obtained by means of CMT is not a trivial task. Equivalent resistance, capacitance and inductance of antenna become frequency dependent, and those should be specially considered. Because the equivalent resistance is a critical parameter for the efficiency maximization, the skin and proximity effects are included and the verification of the calculation process is presented. Also due to frequency dependence of equivalent inductance and capacitance, the procedure to obtain the optimal resonant frequency of antennas in terms of the efficiency of the power transfer is discussed.