High Efficiency Wireless Charging
of Electric Vehicles for Safe and
Economic Future Transportation
Chris Mi, Ph.D, Fellow IEEE
Professor, Department of Electrical and Computer Engineering
Director, DOE GATE Center for Electric Drive Transportation
University of Michigan-Dearborn, (313)583-6434; mi@ieee.org
Conventional EV Charging
1
2
3
Normal charging
Fast charging
Battery swapping
AC charging using
level 1 or level 2，
voltage at 110V, 220V，
6-10 hours per charge
Mostly DC charging in
15 to 30 minutes.
Investment of battery
packs; standardization
is difficult; swapping
stations need a lot
investment, space and
manpower; safety and
reliability is of concern
Charge at home or
public space, need
large installation of
charge stations
For an EV with a 24kWh
battery pack, charging
in 15 minutes means
96kW. This is way over
the power available in
private homes.
Issues of Con. Charging
and Battery Swapping
Electric safety is of concern:
electric shock due to rain, etc.
Charge station, plug and cable
can be easily damaged, stolen
Charge/swap station takes a lot
of space and affect the views
Wireless Charging
Definition of WPT
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Wireless power transfer (WPT)
Inductive power transfer (IPT)
Contactless power system (CPS),
Wireless energy transfer
Strongly coupled magnetic resonance
High-efficiency inductive-power distribution
The essential principles are the same given the
distances over which the power is coupled is almost
always within one quarter of a wavelength and
therefore, the fundamental operation of all of these
systems can be described by simple coupled models
Grant Covic and John Boys, “Modern Trends in Inductive Power Transfer for Transportation
Applications,” IEEE journal of emerging and selected topics in power electronics, vol. 1, no. 1, march
2013
Methods of Wireless Power Transfer
Electromagnetic Induction
Radio wave
电磁感应式
无线电波
Wireless Power Transfer
Radiation
Microwave
电能的无线传输
辐射式
微波
Electromagnetic Resonance
Laser
电磁谐振式
激光
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In 1830’s, Faraday's law of induction
In 1890’s, Tesla had a dream to send energy wirelessly
GM EV1 used an Inductive charger in the 1990’s
2007, MIT demonstrated a system that can transfer 60W of
power over 2 m distance at very low efficiency
Wireless/inductive chargers are available on the market
Qualcomm, Delphi (Witricity), Plugless Power, KAIST, etc.
have developed EV wireless charger prototypes
Ultrasound
超声波
Predicted Wireless
Charging Market
$17 Billion in 2019
Problems and Difficulties

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Magnetic field is diminishing proportional to1/r3
Often the mutual inductance is less than 20% or 10% of the self
inductance
Analytical calculation of coil mutual inductance is next to impossible
Further analytical method is needed
Numerical simulation and coupled field - lumped parameter
simulation is also of paramount importance
High frequency HFSS instead of static FEM for high frequency
High cost Low efficiency
Large size Limited distance
Sensitive to vehicle alignment
Need novel designs
and methods to study
these systems
A Wireless Power Transfer System

Secondary
controlled
WPT
Covic, G.A.; Boys, J.T., "Inductive Power Transfer," Proceedings of the IEEE , vol.101,
no.6, pp.1276,1289, June 2013.
Equivalent Circuit Series-Series
L1, L2 – Self inductance
Series-Series Resonance Structure
Lm – Mutual inductance
L1= L1σ +Lm; L2= L2σ +Lm
1


R

j
(

L

)

j

L
1
1
m

 I 

C
V
 1 
1
 1

0 
1  I2 
 
 j Lm
RL  R2  j ( L2 
)

C2 

Power Transferred

Power of output side
2
2
V
(

L
)
RL
2
1
m
P2 | I 2 | RL 
2 2
| [ Z1Z 2  ( Lm ) ] |
1
0.9
0.8
0.7
0.6
0.5
Calculated
0.4
0.3

Power of the input side
efficiency
0.2
0.1
0
0
1
2
3
4
5
6
7
V12 | Z 2 |
P1 | V1 |  | I1 | cos  
cos 
2
| Z1Z 2  ( Lm ) |

Efficiency
P2
( Lm )2 RL
 
P1 | Z 2 [ Z1Z 2  ( Lm ) 2 ] | cos 
8
9
10
System Topology at UMD
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Key inventions:
-
Optimized multi-coil design for maximum coupling, with bipolar architecture
LCC topology for soft switching to further increase efficiency and frequency
Distributed circuit parameters to minimize the capacitor size and voltage rating
Bidirectional LCL Power factor correction circuit to maximize the front end efficiency
and reduce system cost
Foreign object detection and electromagnetic field emissions for human and animal
safety for the developed system.
Double-sided LCC Compensated
Wireless Power Transfer

Topology
S1
S3
iLf1
Vin
Lf1 +
Cf1
+
-
B
S2
M
+
A
S4
iLf2
C1
i1
+
L1
•
•
Lo
D3
+
+
L2
C2
+
Lf2
i2
Vout
CO
Cf2
Sending Side
•
D1
D2
Battery
Pack
D4
Receiving Side
Important Characteristic:
The output current at resonant frequency:
The output power can be expressed as:
I Lf 2  I Lf 2 _1 
Um
L

 k  U1
0 L f 0 L2f
P  U 2  I Lf 2 _1 
L
 k  U1  U 2
0 L2f
11
Comparison of Coil Design
(a) Circular pads, (b) flux-pipe pads
(c) DD-DDQ bipolar pads
Trong-Duy Nguyen, Siqi Li, Weihan Li, Chunting Chris Mi, Feasibility Study on Bipolar Pads for Efficient Wireless Power
Chargers, IEEE Applied Power Electronics Conference, Fort Worth, TX, USA, March16-20, 2014
Typical Misalignment
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Door-to-door (right-left)is more difficult
Front-rear is easier to align
Z (height)
Rectangular bipolar pads
Rectangular Unipolar pads
Five Studied Cases
Case 1:
480x1000
Case 2:
600x800
Case 3:
693x693
Case 4:
800x600
Case 5:
1000x480
771.4 x 16 x
16
600 x 16 x 16
589.1 x 16 x
16
415.4 x 16 x
16
360 x 16 x 16
925.7 x 16 x
16
720 x 16 x 16
589.1 x 16 x
16
498.5 x 16 x
16
432 x 16 x 16
7
9
11
13
15
480 x 1000 x 6 600 x 800 x 6
693 x 693 x 6
600 x 800 x 6
1000 x 480 x 6
Receiver 480 x 1000 x 8 600 x 800 x 8
693 x 693 x 8
600 x 800 x 8
1000 x 480 x 8
Sender
Ferrite bar
dimension
( L x W x H *) Receiver
Number of ferrite bars
Sender
Coil
Z (height)
Coils: Similar area
Ferrites: Similar volume
X & Y Misalignment
Kx (door-to-door misallignment)
Ky (front-to-rear misallignment)
1_1_S2R2_X600xY800_noShield_dx
0.35
1_2_S2R2_X600xY800_noShield_dy
0.35
Curve Info
Curve Info
Ky_600x800
Kx_480x1000
0.30
0.30
Ky_693x693
Kx_600x800
Ky_800x600
Kx_693x693
0.25
Ky_480x1000
Kx_800x600
0.25
Ky_1000x480
Kx_1000x480
Coupling
Coupling
0.20
0.20
0.15
0.15
0.10
0.05
0.10
0.00
0.05
-0.05
0.00
0.00
100.00
200.00
300.00
400.00
500.00
x_misalligned [mm]
600.00
700.00
800.00
1. The maximum coupling coefficient
decreases with the increase of coil’s
X-length
0.00
100.00
200.00
y_misalligned [mm]
300.00
The topology with bigger Y-size has
a better Y-misalignment tolerance
Z (height)
2. X-misalignment tolerance
increases with the coil’s X-length
-0.10
400.00
Angular Misalignment
Typically, when a driver parks an EV, the worst angular misalignment can be
limited at about 30°
Coupling coefficient vs x, y, theta
Z
Finite element analysis
using Maxwell 3D
Y
X
Coupling Coefficient Profile versus Door-to-door
and Front-to-rear Misalignments
Safety issues
Range of flux density
is within 1-1.2 meters,
with chassis - perfectly aligned position
=> It is safe to install
this WPC in an
electric vehicle
chassis, typically
about 1.8meter doorto-door size
With chassis - maximum misaligned position
Exposed field to a human of 1.8-meter high
Human body is exposed to maximum about 1.6uTesla in foot area
while about 0.06uT in head area.
Field exposed to a man of 1.8met height
1.60
Curve Info
1.40
Time='13250ns'
Time='13500ns'
Time='13750ns'
1.20
Time='14000ns'
Time='14250ns'
1.00
Mag_B [uTesla]
Time='14500ns'
Time='14750ns'
0.80
Time='15000ns'
Time='15250ns'
Time='15500ns'
0.60
Time='15750ns'
Time='16000ns'
0.40
Time='16250ns'
0.20
0.00
0.00
0.25
0.50
0.75
1.00
Distance [meter]
1.25
1.50
Human’s height [in meter]
1.75
2.00
Experimental Verification
Output current
Input current
Output voltage
Input voltage
Max power: 8kW
Max Eff.: 97%
21
Experiment Results
Xmis=0mm, Gap =200mm
Xmis=300mm, Gap =200mm
Xmis=125mm, Gap =400mm
(Rectifier + PFC) + Buck + Wireless
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PFC – power factor correction >0.98
Buck for charge control
WPT: fixed frequency, auto-tuned system.
WPT
System Efficiency for Different Vbat
Dynamic In-Motion Charging
Buried tracks
Results of Foreign Object Test #1
Experiment Result: the gum wrapper was burned and there left an
imprint, which means the temperature is high.
Conclusions
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Misalignment tolerance was analyzed and discussed
Two kinds of coupling coefficient detection methods
were proposed
8 kW wireless charger prototype with 200mm gap and
300mm door-to-door misalignment tolerance had been
built and tested
Coupling coefficient maintains at 18.8%~31.1%
With a 200mm gap, 95.66% efficiency (at about 8kW)
from DC to DC was obtained at desired position
95.39% efficiency (at about 4kW) at 300mm Xmisalignment and 200mm Gap
Acknowledgement
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Department of Energy
- GATE Program
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DENSO International
US China Clean Energy Center
GATE Industrial Partners
- Chrysler, Ford, DENSO International, Mathworks, dSPACE,
ANSYS, Hp Pelzer, EDTA, PSIM, GaN Systems
IEEE Workshop and TPEL Special
Issue on Wireless Power

2015 WoW Sponsored by six Societies of IEEE

PELS, IAS, IES, VTS, MAG, PES
June 5-6 (Fri.-Sat.), 2015, Daejeon, Korea
Held just after the 2015 ECCE-Asia (June 1-4) in Seoul
General Chairs: Dr. Chun Rim, Dr. Chris Mi
TPC: Dr. John Miller
http://www.2015wow.org
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IEEE Transactions on Power Electronics (Guest-EIC)
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IEEE Journal on Emerging and Selected Topics on Power
Electronics (Guest-EIC)