Effects of Number of Relays on Achievable Efficiency of Magnetic Resonant Wireless Power Transfer

In magnetic resonant wireless power transfer (WPT), relays are placed between a transmitter and a receiver as a means of increasing both the operating distance and the power transfer efficiency, but this makes the analysis of the WPT system more complicated. In this letter, we derive a mathematical expression for the power transfer efficiency with an optimal load resistance based on an equivalent circuit model, and analyze the effects of varying the number of relays on the power transfer efficiency that can be achieved. By means of circuit-level simulations and experiments under a variety of scenarios, we verify the accuracy of our analysis, and we also confirm that there is an optimal number of relays for maximizing the achievable power transfer efficiency for a given end-to-end distance.