VII. CONCLUSION
In this article we have demonstrated that circuit theory
can provide a precise characterization of resonant inductive
coupling wireless power transfer systems and that it shows
a complete agreement with the more extended coupled mode
theory in steady-state. Design guidelines to achieve maximum
power transfer efficiency have been given in terms of source
and load impedances and distance between coils. The optimal
values for source and load coils have been found for circuit and
coupled mode theories showing complete accordance. It has
been demonstrated that these values (obtained by setting the
efficiency derivatives with respect to load and source equal to
zero) are equivalent to apply impedance matching techniques
(conjugately matching the load to the output resistance of the
system and the source to the input resistance of the system).
The effects of a variation of distance between coils have been
analyzed and, finally, a simplified formula of maximum power
transfer efficiency obtained from the previous results has been
given, enumerating the required steps to maximize it.