Homogeneous Magnetic Coupling for Free Positioning in an Inductive Wireless Power System

Abstract

Wireless power transmission suggests the freedom of placement for power transmission. However, efficiency and emitted magnetic fields limit the inductive power transfer to close to a surface. But even there, a lateral displacement of the receiver coil to the transmitter coil may lead to a change of the coupling factor and thus an unwanted variation of the power transfer. Here, an algorithm to determine the turn distribution to achieve homogeneous coupling between coils of different diameter is described. In a first step, a current distribution is calculated from the magnetic field specification. Then, a turn distribution matching the current distribution is derived. Typically, the turns are concentrated at the outer edge of the coil and less dense at the inside edge of the coil. As long as the transmitter and receiver coils overlap, the variation of the coupling factor is very low. To achieve a lateral displacement over an even larger area, an array of transmitter coils can be used. The size of the receiver coil is selected such that it always covers a complete transmitter coil. If only the covered transmitter coil is activated by a suitable detection circuit, the power transmission area can be arbitrary large with homogeneous magnetic coupling.