Optimizing Current Ratios for Multi-Tx WPT Systems With Non-Identical Coils

In a wireless power transfer (WPT) system, efficiency is the most important factor because a low-efficiency system leads to problems such as energy waste, heat generation problems, and reduced system performance. For optimal efficiency in a multiple-transmitter (multi-Tx) WPT system, the Tx currents must be supplied proportionately to the mutual inductance between the transmitting and receiving coils. However, this conventional current condition is derived under the assumption that all transmitting coils are identical. In a multi-Tx structure, if the specifications of all transmitters are not the same, the conventional optimum current condition does not guarantee achieving optimal efficiency. This study presents a novel current ratio condition that significantly reduces power losses and optimizes efficiency in multi-Tx WPT systems by accounting for mutual inductance and parasitic resistance. To verify the proposed current ratio condition, we implemented a multi-Tx WPT system consisting of transmitting coils made from litz wire and a COTS receiving coil. As a result of applying the conventional and the proposed Tx current conditions, we confirmed that the proposed Tx current condition could improve the power transfer efficiency with small currents over that with the conventional condition.