Investigation on power optimization principles for series-configured resonant coupled wireless power transfer systems

This paper is a thorough theoretical analysis of a two-coil wireless power transfer system (WPTS) configured in series with a focus on power optimization rather than maximizing transmission (link) efficiency. Different definitions of the system efficiency in the pertaining literature are distinguished and clarified. The frequency splitting phenomenon is precisely explained, and furthermore, the analytical solutions are derived based on analysis of the input impedance. The effects of this behavior on the power transfer to a load resistance are discussed. Various aspects of the power optimization problem are explored. In particular, for the case when the system is driven at the resonance frequency, (i) the explicit expression of the optimal coupling factor between the two coils for a given load, and (ii) the optimum power with respect to the load are provided. The impedance matching methods using different circuit topologies are analytically or numerically investigated, revealing that the drive frequency can be arbitrarily chosen and not necessarily equal to the resonance frequency. This provides more options for exciting the system apart from the resonance condition, without compromising the delivered power. A comparison between optimization techniques is given in terms of the coupling factor k, showing that the bi-conjugate matching with Pi - networks results in the maximum generated power and the transducer power gain (i.e., defined by the ratio between the received power and the power available from the source), which reaches 80% at k = 84.4 x 10(-3), for example. (C) 2019 Elsevier GmbH. All rights reserved.