The optimal design problem for a wireless power transfer link based on a resonant inductive coupling is addressed in this paper. It is assumed that the magnetic coupling coefficient and the inductor quality factors are known. By employing the conjugate image impedances, the values of the inductances realizing the optimal design with respect to given values of the network input and load impedances are derived. It is demonstrated that there is just one optimal design maximizing both the power delivered to the load and the power transfer efficiency of the link. The four possible schemes corresponding to the use of a parallel or a series arrangement for the two coupled resonators (Parallel-Parallel, Series-Series, Parallel-Series, and Series-Parallel) are considered and discussed. Closed form analytical formulas are derived and validated by circuital simulations.

This paper focuses on non-radiative wireless power transfer implemented by means of a resonant magnetic coupling. The case of one transmitter and two receivers is considered and a rigorous analytical procedure is developed demonstrating that maximum power transfer or maximum efficiency can be achieved by appropriately selecting the load values. Both cases of coupled and uncoupled receivers are solved; closed formulas are derived for the optimal loads, which maximize either power or efficiency. It is shown that the resistances that realize maximum power transfer are always greater than the resistances that realize maximum efficiency. According to this observation, an optimal range of operation for the load resistances is also determined. Furthermore, it is demonstrated that in the case where the receivers are coupled the introduction of appropriate compensating reactances allows retrieving the same results corresponding to the uncoupled case both for powers and efficiency. Theoretical data are validated by comparisons with numerical results.