This paper provides a rigorous theoretical formulation to obtain an inductive resonant wireless power transfer (WPT) link with load-independent output voltage or current. This is a crucial working condition for wireless battery recharging, where there is no deterministic knowledge of load variation with respect to the battery charging level. The ideal lossless and the realistic lossy configurations are considered for both voltage- and current-excited WPT links. The link transfer matrix (ABCD matrix) is used to determine the operating frequencies where a load-independent output voltage or current can be obtained. It is shown that in the lossy cases that an almost load-independent behavior can be achieved, provided that the load resistance lies in a suitable range and analytical conditions on the load resistance value have been derived. The analytical relationships obtained by this theory are validated by means of both circuit simulation and experimental data at 13.56 MHz.

Harmonic-balance (HB) based nonlinear techniques are exploited for the design of a high-efficiency and medium-power switching mode oscillator, used as continuous power source in a wireless power transmission system. The oscillator power is transmitted through resonant coils over variable distances. To account for the resonant frequency variations, which also tune oscillation frequency, a broadband design of the system is carried out with the oscillator load including the resonant coupling and the rectifier. The oscillator efficiency, the RF-to-DC system efficiency and the DC power are then directly optimized for any coupling distance of interest. A Royer-type oscillator is designed and prototyped using the proposed technique. The oscillator exhibits 40 W at 223 kHz with 75% conversion efficiency while the WPT system efficiency, DC-to-DC, is better than 60% for coupling distances of the order of 10 cm.

Capacitive wireless power transfer can be realized by mutually coupled capacitors operating at a common resonant frequency. An optimal load exists that maximizes either the efficiency or the power transfer to the load. In this work, we utilize the frequency bifurcation effect to propose a frequency agile mode that allows for a nearly coupling-independent regime. We analytically determine the operating conditions of the coupling-independent mode based on the different system gains. In this way, we obtain a solution that achieves nearly constant efficiency and power transfer, even at varying coupling. We compare our results to inductive wireless power transfer where a perfect coupling-independent mode is achievable.

In this paper we describe a geometry design solution to minimize performance variations of a wireless power transfer system “on the move” (WPT-Mob). A sequence of switchable couples of coils, connected in series or in parallel, is adopted at the fixed transmitting link side; the geometry of the moving receiver is optimized to keep the coupling factor, and thus the power transfer, constant during the movement. First the analytical formulation of the link coupling factor is derived as a function of its circuit-equivalent parameters computed by full-wave simulation. Then selected geometry parameters of the receiver side are optimized to keep the coupling factor constant while the link is moving. A set of TX-RX arrangements are simultaneously analysed to emulate the sliding movement of the WPT-Mob. The final optimized geometry demonstrates that a constant power transfer on-the-move is enabled, even for variable TX-to-RX link distances. Design and experimental verification are carried out for a geometry suitable for medium power transfer (tens of Watts) at 6.78 MHz, but the method is formulated in such a way that the system can be scaled up and down to accomplish different application needs.

In this article, the possibility of obtaining a load-independent regime for a wireless power transfer link consisting of two magnetically coupled resonators in parallel configuration is discussed. Analytical formulas and experimental data to achieve a load-independent output voltage or current are presented. With regard to the input side, the cases of a current and a voltage generator are considered. It is shown that when the source is a current generator, either a constant output voltage or a current can be achieved by suitably choosing the operating frequency. As per the case of a voltage generator, by acting on the operating frequency, only a constant output current can be obtained.

Capacitive wireless power transfer from one transmitter to multiple receivers is discussed and analytically solved for the realization of maximum power transfer to uncoupled receivers. Simple closed-form expressions for the optimal terminating loads and the corresponding power output and transducer gain are determined by applying the maximum power transfer theorem. The results are validated by circuital simulation.