A Litz-Wire Based Passive Shield Design to limit EMF Emission from Wireless Charging System

In this paper, a litz wire based passive shield design is proposed which shows the ability to suppress the electromagnetic field (EMF) emission below the limit set by International Commission on Non-Ionizing Radiation Protection (ICNIRP) and also minimize the shield loss. Traditionally passive aluminum plate or active anti-turn has been used in EMF suppressing shield design. However, these designs show limited controllability of the shield current. Moreover, the high-frequency eddy current loss in the passive aluminum plate is approximately 0.4% and it can increase more than 2.0% at low coupling operation under misalignment. The shield current and associated shield loss are highly localized around the outer boundary of the shield in a unipolar pad. Therefore, it can be optimized effectively with passive litz wire turn at the outer boundary of the traditional aluminum shield. In this paper, a shield design is proposed with a thin aluminum plate and a passive litz-wire-loop around the outer radius of the pad. The result shows that approximately 80% of the shield current is induced in the newly added litz-wire-loop and results in a greater controllability on the shield current and EMF suppression. The litz wire mitigates the high-frequency skin effect and significantly reduces the shield resistance. In this paper, this advantage is applied in shield design for EMF suppression using a passive litz-wire loop. This method is also effective for shield loss minimization. Moreover, the proposed design provides a highly effective and efficient shielding for a high-power loosely coupled system.