Exposure Assessment of a High-Resonance Wireless

Power Transfer System under the Misaligned Condition

SangWook Park1, ByeongWoo Kim2, BeomJin Choi1

1 EMI/EMC R&D Center, Reliability & Safety R&D Division,

Korea Automotive Technology Institute, Korea

{parksw, bjchoi}@katech.re.kr 2 Department of Electrical Engineering,

University of Ulsan, Korea

bywokim@ulsan.ac.kr

Abstract. This paper presents the dosimetry of a wireless power transfer (WPT)

system using magnetically coupled high-resonance phenomenon under the

misalignment condition. The power transfer efficiency is investigated in the

misalignment between the transmitter and receiver, as well as in the alignment.

Additionally, the power transfer efficiency at various misaligned distances is

also investigated under matching and mismatching conditions. The numerical

dosimetry for the alignment and misalignment cases under the matching

condition is conducted with the simplified cylindrical human model. The results

show that the specific absorption rates in the misalignment case are stronger

than those in the alignment case.

Keywords: dosimetry, misalignment, specific absorption rate, wireless power

transfer.

1 Introduction

The wireless power transfer (WPT) technique has been gradually receiving greater

attention since the Massachusetts Institute of Technology (MIT) research team

proposed the WPT technique using a magnetically coupled high-resonance

phenomenon [1]. This technique is expected to be useful for various applications such

as cellular phones, laptop computers, home electrical appliances, and electric vehicles.

The WPT technique will bring convenience in daily life like wireless communications.

However, the electromagnetic energies for the WPT are much stronger than those for

wireless communications. Therefore, it is necessary to investigate electromagnetic

compatibility (EMC) and safety for electromagnetic human exposure. Electric vehicle

(EV) applications are expected to need significant power, which produces strong

electric and magnetic fields. Thus, EMC and electromagnetic field (EMF) safety

problems become more critical in EV applications.

The EMF safety of the WPT system is the focus of this work, and compliance of

such a system to international EMF safety guidelines should be investigated [2]-[5].

Fundamental studies on the dosimetry of the WPT system were reported in references

[6]-[8]. However, these studies did not take into account the misalignment between

Advanced Science and Technology Letters Vol.116 (Healthcare and Nursing 2015), pp.11-15

http://dx.doi.org/10.14257/astl.2015.116.03

ISSN: 2287-1233 ASTL Copyright © 2015 SERSC

the transmitter and receiver and the matching condition. Misalignment often occurs in

charging, and thus it should be investigated from the perspective of EMF safety. This

paper reports the design of a WPT system using high-resonance phenomenon.

Numerical dosimetry with the simplified cylindrical human model was conducted for

the system under alignment and misalignment conditions between the transmitter and

the receiver. The results for the alignment case are compared to those for the

misalignment case.

2 WPT system and misalignment feature

(a) (b)

Fig. 1. WPT system: (a) alignment case and (b) misalignment case.

A WPT system using electromagnetic resonance phenomenon, shown in Fig. 1 (a),

was designed for charging electric vehicles. The WPT system consisted of two

resonant coils and two loops. Spiral-type coils having five turns and a pitch of 5 mm

were the resonators. The electromagnetic energies are efficiently transferred through

these resonant coils. The loops acted as matching circuits. Both the coil radius and the

power transfer distance of the WPT system were 150 mm. Copper wire with a radius

of 2 mm was used for the system. A capacitance of 5 pF was added to the coil to

achieve resonance at a 13.56 MHz frequency. The power transfer efficiency (|𝑆21|2)

was 98% in the alignment case. The misalignment case between transmitter and

receiver, shown in Fig. 1 (b), was also investigated because misalignment often

occurs when a car is parked for WPT charging. The power transfer efficiency in the

misalignment case generally decreases without matching. However, a practical WPT

system should be matched for power transfer efficiency. In this work, the power

transfer efficiency was investigated with various misaligned distances (m) in the

matching and mismatching conditions. The results are shown in Fig. 2. The efficiency

Advanced Science and Technology Letters Vol.116 (Healthcare and Nursing 2015)

12 Copyright © 2015 SERSC

improvements were 83.6% at m = 250 mm. In the next section, the dosimetry for the

WPT system in this misalignment case (m = 250 mm) will be conducted and

compared to the alignment case.

Fig. 2. Power transfer efficiency comparison of WPT system with matching and without

matching.

3 Dosimetry

(a) (b)

Fig. 3. Cylindrical simplified human model position with respect to WPT system: (a) alignment

and (b) misalignment.

Fig. 3 shows the cylindrical model position with respect to the WPT system. The

SARs were calculated in the five different cases. The cylinder was 1700 mm high and

had a diameter of 280 mm, similar to the size of an average human. The dielectric

properties of the cylindrical model were set to be two-thirds that of muscle tissue,

which represents the average dielectric properties of the human body. The electrical

properties of muscle tissue were taken from Gabriel’s Cole–Cole models [9]. The

localized SAR (SAR10g) and whole-body SAR (SARwb) for the five cases are shown

in Fig. 4. The results show that the SARs of the misalignment cases are larger than the

Advanced Science and Technology Letters Vol.116 (Healthcare and Nursing 2015)

Copyright © 2015 SERSC 13

SAR of the alignment case. This is because the electric and magnetic fields are

strongly distributed over a large area in the misalignment case to achieve the best

power transfer efficiency. The maximum allowable powers (MAPs) to comply with

the ICNIRP guideline are shown in Fig. 5 for the five cases. The results show that the

MAPs for the misalignment cases are lower than the MAP for the alignment case.

These results suggest that we should consider the misalignment condition of the WPT

system when conducting exposure assessment.

Fig. 4. Localized SAR and whole-body SAR for alignment (case1) and misalignment cases

(case2-5).

Fig. 5. Maximum allowable powers to comply with the ICNIRP guideline for five different

cases.

Advanced Science and Technology Letters Vol.116 (Healthcare and Nursing 2015)

14 Copyright © 2015 SERSC

4 Conclusion

Dosimetry was conducted for WPT under conditions of alignment and misalignment

between transmitter and receiver. The SARs in the misalignment condition were

higher than that in the alignment condition. The dosimetric results of the WPT system

indicated that the worst case for exposure generally occurred in the misalignment case.

Therefore, dosimetry for the WPT system should be conducted in the misalignment

case, as well as the alignment case. In future works, dosimetry will be conducted with

a precise whole-body voxel human model based on anatomical structures.

Acknowledgments. This work was supported by a grant “Development of

induction/magnetic resonance type 6.6kW, 90% EV Wireless Charger (No.

10052912)” from the Ministry of Trade, Industry and Energy.

References

1. A. Kurs, A. Karalis, R. Moffatt, J. D. Joannpoulos, P. Fisher, and M. Soljacic.: Wireless

power transfer via strongly coupled magnetic resonances. Science, 317, 83–86 (2007).

2. ICNIRP.: Guidelines for limiting exposure to time-varying electric, magnetic, and

electromagnetic fields (up to 300 GHz). Health Phys., 74, 494–522 (1998).

3. ICNIRP.: Guidelines for limiting exposure to time-varying electric and magnetic fields (1

Hz to 100 kHz). Health Phys., 99, 818–836 (2010).

4. IEEE Standard for Safety Levels With Respect to Human Exposure to Electromagnetic

Fields, 0–3 kHz, IEEE Standard C95.6 (2002).

5. IEEE Standard for Safety Levels With Respect to Human Exposure to Radiofrequency

Electromagnetic Fields, 3 kHz to 300 GHz, IEEE Standard C95.1 (2005).

6. I. Laakso, S. Tsuchida, A. Hirata, and Y. Kamimura.: Evaluation of SAR in a human body

model due to wireless power transmission in the 10 MHz band. Phys. Med. Biol., 57, 4991–

5002 (2011).

7. A. Christ, M. G. Douglas, J. M. Roman, E. B. Cooper, A. P. Sample, B. H. Waters, J. R.

Smith, and N. Kuster.: Evaluation of wireless resonant power transfer systems with human

electromagnetic exposure limits. IEEE Trans. Electromagn. Compat., 55(2), 265–274 (2013).

8. S. W. Park, K. Wake, and S. Watanabe.: Incident electric field effect and numerical

dosimetry for a wireless power transfer system using magnetically coupled resonances.

IEEE Trans. MTT. 61(9), 3461–3469 (2013).

9. C. Gabriel and S. Gabriel.: Compilation of the dielectric properties of body tissues at RF and

microwave frequencies. Brooks AFB, San Antonio, TX, USA (2006) [Online] Available:

http://www.brooks.af.mil/AFRL/HED/hedr/reports/dielectric/home.html.

Advanced Science and Technology Letters Vol.116 (Healthcare and Nursing 2015)

Copyright © 2015 SERSC 15