Modeling and Design of Passive Shield to Limit EMF Emission and to Minimize Shield Loss in Unipolar Wireless Charging System for EV

In this paper, a detailed lumped model of the shield is developed, and an optimization method is proposed to minimize the shield loss while suppressing the emission within the standard limit. The electromagnetic emission in a wireless charging system (WCS) is a serious concern for health and safety, especially for medium- and high-power applications, where strong magnetic field propagates through a large air gap. Usually, a flat aluminum plate is used as a shield to suppress the emission outside the charging area. This field emission increases proportionally with the coil ampere turn; therefore, a much higher shield current is required to suppress the additional emission under the limit set by the International Commission on Non-Ionizing Radiation Protection. In this paper, a novel hybrid shield structure is proposed to limit the electromagnetic field (EMF) emission, as well as to minimize the shield loss. The proposed shielding is evaluated on a laboratory prototype of 7-kW WCS designed for electric vehicle application. The experimental result shows that, while suppressing the EMF under the limit, the shield loss has been reduced by 21% compared to the commonly used aluminum shield.

I. Introduction

Wireless charging systems (WCSs) are being investigated for many different applications for the past few decades [1]–[4]. In a typical vehicular WCS, there is one transmitter (Tx) and one receiver (Rx) pad separated by about 100–200 mm air gap. [4]–[6]. Each of these pads is usually made with one or multiple coils, a ferrite core, and an aluminum (Al) shield, as shown in Fig. 1. High frequency ac current through the transmitter coil generates a high frequency magnetic field which propagates through a large air gap to reach the receiver. Therefore, there is a high possibility of magnetic field emission outside the charging region. This emission is quite small for low-power wireless charging applications. However, medium-to-high power applications with a large air-gap generate a considerable level of emission outside the charging area. One of the most promising applications of such power range is automotive charging, where the power can range from several hundred watts to as high as 50 kW. Usually, vehicular charging systems are installed in a commonly accessible area, such as home garage or automobile parking areas. Therefore, when regarding health and safety, it is crucial to limit the electromagnetic field (EMF) emission for this application [7], [8]. In this paper, the EMF emission characteristics and its suppression methods are investigated for a medium-to-high power WCS in automotive applications. Fig. 1.

Typical position and components of the transmitter and receiver pads in a WCS for EV.