Design approach for high efficiency NFC systems with magnetic shielding materials

摘要

The performance of a Near Field Communication (NFC) antenna may be diminished when it is placed close to any conductive surface such as a metallic case or a battery. This degradation is caused due to the stray magnetic field created by the eddy currents induced on the surface, which is opposite to the intended field generated by the NFC antenna. One of the first solutions that come to mind to designers when facing this problem is the use of high permeability magnetic shielding based on sintered ferrite sheets. This is a good approach but something that is not generally taken into account is that these materials introduce an additional inductance to the NFC antenna. If the permeability of the material is too high (respecting the necessary value for solving the problem), this additional inductance results in shifting the resonance frequency to lower values than the desired (13.56 MHz). Thereby this contribution focuses on the analysis of a ferrite-polymer composite magnetic shielding that provides lower relative permeability (µr = 25) at the communication frequency. This approach is more effective against the presence of a metallic element when there is a gap of some millimeters between the conductive surface and the NFC antenna. Therefore, different thicknesses of the same ferrite-polymer material are evaluated and the effect of introducing this kind of shielding between the conductive surface and the NFC antenna is analyzed from the standpoint of the loop antenna equivalent circuit. The results presented are based on the Smith Chart measurement as well as a simulation model that corroborates the results obtained experimentally.