Abstract
The impedance variation of wireless power transfer (WPT) coils owing to the presence of the human body may result in mismatches, resulting in a decrease of the transmission efficiency. In addition, one of the decisive factors of the permissible transfer power in WPT systems is a compliance assessment with the guidelines/standards for human protection from electromagnetic fields. In our previous study, we reported that a shielded loop coil can potentially reduce human interaction with WPT coils. In this study, first, the rationale for this reduction is investigated with equivalent circuit models for a WPT system using a shielded loop coil operated in close proximity to the human body. We then conducted an equivalent circuit analysis considering the capacitance between the inner and outer conductors of the shielded loop coil, suggesting the stability of the impedance matching. From computational results, the mitigation capability of the shielded loop coil on impedance matching and transmission efficiency owing to the presence of the human body was verified for 6.78 MHz wireless power transfer. Additionally, the reduction of the specific absorption rate (SAR) with coils comprised of the shielded loop structure was confirmed in the presence of anatomically realistic human body models. The maximum transferable power was increased from 1.5 kW to 2.1 kW for the restrictions of the local SAR limit prescribed in the international safety guidelines/standard.
Highlights
Wireless power transfer (WPT) technology is being put to practical use in various places [1,2,3].For example, it will be deployed for the wireless charging of electric vehicles in parking areas [4,5,6,7].When new emerging technology may appear, there has been concern about electromagnetic compatibility issues, which are mainly classified into two categories: potential health effects of electromagnetic fields and unwanted field emissions
We demonstrate that the impedance characteristics and the specific absorption rate (SAR) reduction of a shielded loop coil are improved as compared to a conventional loop coil without a shield
The transmission efficiency of the shielded loop coil was improved by 49.5% compared to that of
Summary
Wireless power transfer (WPT) technology is being put to practical use in various places [1,2,3]. In the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines and IEEE standard [12,13,14] for human protection from electromagnetic fields, two metrics are mentioned: basic restrictions and reference levels The former involves the internal physical quantities which are related to adverse health effects with a certain reduction factor. We empirically presented a near-field WPT using a shielded loop coil [42] This WPT had potential features of high transmission efficiency and reduction of external electric fields, which may be expected with the presence of the human body. The permissible field strength is evaluated based on computations using an anatomically based human model
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