Uniformity of magnetic field distribution over a wireless charging surface can be improved by having high-permeability ferrite material. In this paper, 0.5mm-thick ferrite material is appropriately shaped and disposed above a power transmitting unit (PTU) resonator. As an example, a three-turn coil incorporating the ferrite is analyzed and measured. Simulation and measurement show that peak of magnetic field is significantly reduced by 55% while maintaining almost same average magnetic field strength on the wireless charging surface, compared to the same coil without ferrite.
The benefit of having uniform magnetic field is also demonstrated by measuring the induced voltage on a receiver where the ratio between minimum and maximum voltage is within 1:2. This indicates that a power receiving unit (PRU) receives more uniform power delivery regardless of position on a charging area. The distance from the charging coil to the charging surface is as low as 2mm from the PTU resonator.