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In this study, the analysis of a three-coil wireless power transfer (WPT) system, which can be divided into source, communication and load circuits, is discussed in details. Among the three-coil WPT systems features, it is demonstrated, for instance, that maximum efficiency (η MAX) and maximum power transferred to the load (P 3MAX) do not depend on the load resistance, neither on the mutual inductance between communication and load coils. In fact, it is shown that η MAX and P 3MAX depend only on source and communication circuits parameters. Practical results are also presented, showing good agreement with the developed theory and validating the proposed analysis.

In wireless power transfer (WPT) systems with more than two coils, the intermediary or relay circuits are used to extend the link distance. Thus, to achieve this extension efficiently in terms of power transfer, these relay circuits must have low losses. However, there are several instances in which there are restrictions in reducing the ohmic losses in all the relay circuits of the system. This is the case of biomedical applications where commonly there are size and access restrictions since one of the circuits can be implanted and also in applications using high-temperature superconductor (HTS) coils due to the difficulty in implementing the necessary cooling system for all the coils of the system. Therefore, in these situations, the designer need to choose which relay circuit will be optimized. In this work, it is presented an analysis on the impact that losses in individual relay circuits have on efficiency, and power transfer, of typical four-coil wireless power transfer systems consisting of circuit 1 (transmitter), relay circuits 2 and 3, and circuit 4 (load). It is shown that the losses on relay circuit 2 have greater impact on efficiency, while the losses of relay circuit 3 have a greater impact on power transfer for a given condition. Practical experiments confirm the developed analysis.

In this work we present the electrical characterization of closed-and openbifilar solenoid coils by comparing their self-capacitance, self-inductance, and self-resonant frequency (SRF). The analysis is done by relating these parameters with those of a conventional solenoid coil with same dimensions and number of turns. It is shown that bifilar coils have a greater self-capacitance and lower SRF when compared with the respective conventional coil and that this relation can be expressed only in terms of the number of turns of the coils. Therefore, these components have a potential use in applications that need to avoid the use of a real capacitor and need to operate in a smaller resonant frequency, such as in wireless power transfer systems, for example. In a similar way, the self-inductances of closed-and open-bifilar coils are also compared with those of conventional coils with the same dimensions. It is also shown that the SRF of closed-and open-bifilar coils are approximately the same for practical values of the quality factor and that their impedance behaves as series and parallel RLC circuits. Practical experiments are conducted validating the developed theory.