EMC’14/Tokyo
15A-H6
Investigation and analysis on EMC reduction with
impedance matching technique in Wireless Power
Transfer system
Franklin Bien
Sai Kiran Oruganti
School of Electrical and Computer engineering
UNIST
Ulsan, Republic of Korea
bien@unist.ac.kr
School of Electrical and Computer engineering
UNIST
Ulsan, Republic of Korea
tesla@unist.ac.kr
Abstract—Impedance matching techniques were previously
investigated for maximum power transfer efficiency in wireless
power transfer system (WPT). In this paper, similar discipline is
investigated and analyzed for reducing magnetic EMC in wireless
power transfer system. Basic theory is reviewed, leading to
appropriate application in the WPT system while investigating
and analyzing the EMC reduction effect.
Keywords—Electromagnetic
intereference; power systems.
I.
shielding;
electromagnetic
INTRODUCTION
Electromagnetic compatibility is a major concern in almost
every electronic and electrical device dealing with time varying
magnetic and electric fields. Over the years wireless power
transfer (WPT) systems have also been under development [1][3]. Any WPT system is bound to produce EMF noise around
the coils. The international commission on non-ionizing
radiation protection (ICNIRP) has laid down strict conditions
to the amounts of radiation human beings can be exposed to
within the safe limits [4]. One possible solution was proposed
in [4]. It utilized a reactive resonant current loop to cancel out
the EMF noise. But, when impedance matching methods are
introduced into WPT systems, one is bound to witness changes
in the resonance frequencies [5]. This will affect the operation
of the reactive resonant current loops and thus the shielding
itself. Thus it makes it necessary to employ impedance
matching for the reactive shielding resonant current loops as
well. However, the methods proposed in [4], employs a ferrite
core, which adds up to the bulk of any commercial system.
This paper investigates a method to reduce the bulk of the
proposed method in [4], as well as a possible digital control for
the impedance matching of the same. The proposed method in
this paper focuses on a reactive field cancellation faraday mesh
with a digitally controlled soft switching variable capacitor.
The variable capacitor is controlled by an FPGA kit. The
FPGA kit provides the designer with the independence to
attach a feedback control loop as the design stage evolves in
the future. The proposed method can be effective in frequency
tuned wireless power transfer systems, where the operation
frequency changes, causing a shift in the levels of noise [6].
Fig.1. Reactive Field cancellation Mesh with FPGA driven Impedance tuning control unit.
Copyright 2014 IEICE
442
EMC’14/Tokyo
II.
15A-H6
REACTIVE FIELD CANCELLATION MESH
A. Operation princple
The reactive field cancellation mesh works on the principle
of utilizing the magnetic field component originating from the
EMF noise source itself. It induces an equivalent current to
generate magnetic field, 180 degree out of phase. As a result of
this equivalent current an opposing magnetic field cancels out
the EMF noise. The mesh has been depicted in the fig.1 which
Fig.3. Mesh Dimensions
Fig.2. Reactive Field cancellation Mesh details
additional feedback system can be included at a later stage at
will in the same FPGA by simply changing the HDL-code.
shows the proposed system. In the proposed case, at every
instance, when the transmitter (Tx) coils are energized with an
EMF, the currents in the Tx cause an equivalent opposing EMF
in the mesh structure, hence, the nearby circuit components
will not experience any coupling with the Tx coils as long as
they are not lying directly over the surface of the Tx or the
reactive mesh. This also can be inferred from the fact that, the
reactive mesh is like a 2D sheet-like wave guides, which keeps
the EM wave tightly coupled within their surface [7].
III.
As shown in the fig.2 the detail of prototype is basically a
combination of a mesh (top layer), a ground plate (bottom
layer) and a dielectric layer (middle layer).
B. FPGA control for the variable capacitor
An FPGA is used to digitally control the switching of the
variable capacitor which is attached to the reactive field
cancellation mesh. Thus, the variation of the capacitance gives
the system with a tuning of the reactance of the mesh. An
Fig.4. Experimental Setup
Copyright 2014 IEICE
PROTOTYPE
443
TABLE I.
CONSTRUCTION DETAILS OF PROTOTYPE
Dimensions(mm)
Top
layer
Medium
layer
Bottom
Layer
Length
Breadth
Thickness
150
150
0.013
150
150
0.013
150
150
0.013
EMC’14/Tokyo
15A-H6
The details of the top layer mesh grid are shown in fig.3.
The dielectric layer used is a carbon fiber material which is
commercially available.
ACKNOWLEDGMENTS
IV.
EXPERIMENTS AND RESULTS
To test the effectiveness of the prototype, a test coil was
placed adjacent to the proposed prototype. The S-21
parameters would be the efficiency indicator in case of Tx and
Rx. But the test coil placed in near vicinity of the WPT system
has to be measured for S-31. The experimental setup is shown
in details in fig. 4. A measure of the S-parameters received by
the test coil due to the effect of WPT system is a vital indicator
of the effectiveness of the reactive field cancellation mesh. The
test coil used in the experiment was a spiral coil, of frequency
(resonance) 8.815 GHz. Hence a peak must be observed at this
frequency in the noise measurement results. As seen in the
fig.5, the maximum noise parameter observed is at 8.815 GHz,
when there is no mesh the noise levels are -7.76 dB and with
the proposed mesh the observed noise levels are -29.18dB.
This research was financially supported by the “R&D
Infrastructure for Green Electric Vehicle (RE-EV)” through
the ministry of trade & Energy (MOTIE) and Korea Institute
for Advancement of Technology (KIAT).
REFERENCES
[1]
[2]
[3]
[4]
V.
CONCLUSIONS
The paper presents an introductory concept of impedance
matching technique for EMC reduction due to a WPT system.
The proposed reactive field cancellation mesh utilizes the
concept of generating a 180 degree out of phase magnetic field
due to the induced noise. In the study and experiments
conducted it was also observed that tuning of the capacitance
leads to a control of noise with the changes in the operation
frequency of the WPT system e.g. a dynamic frequency
controlled WPT system, where the operation frequencies are
tuned in the transmitter coil itself, As reported in[6].
[5]
[6]
[7]
Fig.5. Noise in the test coil induced
Copyright 2014 IEICE
444
S.G. Lee, H. Hoang, Y.H. Choi, and F. Bien, “ Efficiency improvement
for magnetic resonance based wireless power transfer with axialmisalignment”. Electronics letters, 48(6), pp.339-340, 2012.
S.K.Oruganti, and F. Bien. "Flexible wireless energy transfer systems by
carbon fiber as a dielectric material: Study and experiments." In
Wireless Power Transfer (WPT), 2013 IEEE, pp. 159-162. 2013.
A. Kurs, A. Karalis, R. Moffatt, J.D. Joannopoulos, P. Fisher, and M.
Soljačić, “Wireless power transfer via strongly coupled magnetic
resonances". science, 317(5834), pp.83-86, 2007.
J. Kim, S. Kong, H. Kim, I-S. Suh, N. P. Suh, D-H. Cho, and S. Ahn.
"Coil Design and Shielding Methods for a Magnetic Resonant Wireless
Power Transfer System." IEEE, Proceedings, 101(6), pp.1332-1342,
June 2013.
S. Kong, M. Kim, K. Koo, S. Ahn, B. Bae, and J. Kim. "Analytical
expressions for maximum transferred power in wireless power transfer
systems." In Electromagnetic Compatibility (EMC), 2011 IEEE
International Symposium on, pp. 379-383. IEEE, 2011.
S. Rajagopal, and F. Khan. "Multiple receiver support for magnetic
resonance based wireless charging." In Communications Workshops
(ICC), 2011 IEEE International Conference on, pp. 1-5, 2011.
A. Noda and H. Shinoda. "Selective Wireless Power Transmission
Through High- Q Flat Waveguide-Ring Resonator on 2-D Waveguide
Sheet." IEEE Microwave Theory and Techniques, 59(8), pp.2158-2167,
2011.