© EVS-25 Shenzhen, China, Nov. 5-9, 2010
The 25th World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium & Exhibition

Contactless Power Transfer System for Electric Vehicle Battery Charger
Yuichi Nagatsuka1, Shingo Noguchi1, Yasuyoshi Kaneko1, Shigeru Abe1, Tomio Yasuda2, Kazuhiko Ida2,
Akira Suzuki3, and Ryoichi Yamanouchi3
1

2

Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama-shi, Saitama 338-8570, Japan
Technova Inc., 13th Fl. The Imperial Hotel Tower, 1-1 Uchisaiwai-cho 1-chome, Chiyoda-ku, Tokyo 100-0011, Japan
3
Aisin AW Co., Ltd., 10 Takane, Fujii-cho, Anjo City, Aichi 444-1192, Japan

Abstract—A contactless power transfer system is desirable for the recharging of electric vehicles (EVs). Transformers
with single-sided windings have been popular; however, transformers with double-sided windings are expected to be
more compact and lightweight. A contactless power transfer system for EVs must have high efficiency, a large air gap,
good tolerance to misalignment and be compact and lightweight. A novel transformer was developed using series andparallel capacitors with rectangular cores and double-sided windings to satisfy these criteria, and its characteristics aredescribed. An output power of 1.5 kW and efficiency of 95% was achieved in the normal position. The characteristics ofthe system when a charge control circuit and lead acid batteries are connected to the secondary winding are alsopresented Copyright Form of EVS25.
Keywords— Contactless power transfer system, Efficiency, Electric vehicle, Plug-in hybrid vehicle, Battery charger

1. Introduction
The development and commercialization of plug-in
hybrid electric vehicles (PHVs) and electric vehicles(EVs) is actively being realized, due to environmental
concerns and rising oil prices. PHVs and EVs currently
require connection to a power supply by electric cables forbattery charging. A contactless power transfer system(such as that depicted in Figure 1) would have many
advantages [1], including safety during high-power
charging and the convenience of being cordless.
z : Verticalx : Forward

direction

directiony : Lateral

direction

Contactless
Power Transfer
SystemFigure 1: Schematic diagram of a contactless power transfer
system for an electric vehicle.

The following specifications are very important for acontactless power transfer system for PHVs and EVs:
1. An efficiency of at least 95%.
2. An air gap of at least 70 mm.
3. Good tolerance to misalignment in the lateral
direction (e.g., ±125 mm).
4. Compact and lightweight.
Because transformers have a large air gap, they have
low coupling factors (0.1-0.5). Therefore, a highfrequency (10-50 kHz) inverter is used as the power
supply in order to make the secondary voltage higher andresonant capacitors are connected to the terminals in order
to compensate leakage reactance. Various resonantcapacitor configurations have been proposed [2,3], of
New Energy and Industrial Technology Development
Organization ( NEDO) of Japan. (Sponsor)

which a configuration where the primary capacitor is inseries and the secondary capacitor is in parallel has aninteresting characteristic [3]; if the capacitors are chosencorrectly and the winding resistances are ignored, theequivalent circuit of a transformer with these capacitors isthe same as an ideal transformer at the resonant frequency,
which is equal to the inverter frequency.
Transformers with circular cores and single-sided
windings have commonly been used [4,5]. However, wehave revealed that a transformer with rectangular coresand double-sided winding has many advantages for theabove specifications [4,6].
In this work, a novel 1.5 kW transformer with doublesided windings that has good tolerance to misalignmenthas been constructed for such a contactless system. The
characteristics of the contactless power transfer systemwith a charging control circuit and batteries were
investigated. The following sections describe thecharacteristics of the transformer and present various test
results.

2. Contactless Power Transfer System forEVs
2.1 Contactless Power Transfer System
Figure 2 shows a schematic diagram of the contactless
power transfer system with series and parallel resonantcapacitors. A full-bridge inverter is used as a highfrequency power supply. The cores are made of ferrite andthe windings are litz wires.

Figure 2: Contactless power transfer system.

© EVS-25 Shenzhen, China, Nov. 5-9, 2010
The 25th World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium & Exhibition

2.2 Equivalent Circuit

2.3 Resonant Capacitors

Figure 3 shows a detailed equivalent circuit, which
consists of a T-shaped equivalent circuit to which resonantcapacitors CS and CP and a resistance load RL have been
added. Primary values are converted into secondary
equivalent values using the turn ratio a = N1/N2 (primes areused to indicate converted values). As the winding
resistances and the resistance of ferrite-core loss are much
lower than the leakage and mutual reactances at theresonant frequency, the simplified equivalent circuit
shown in Figure 4(a), which ignores the winding
resistances (r'1 and r2) and the resistance of ferrite-coreloss r'0, is used.

To achieve resonance with the self-reactance of thesecondary winding ω0L2, which is equivalent to adding amutual reactance x'0 and a leakage reactance x2, thesecondary parallel capacitor CP is given by:1= ω0 L2 = xp = x0′ + x2 .
ω0 C P(1)

The primary series capacitor CS (C'S denotes its
secondary equivalent) is determined as:
1
x′ x
(2)
= xs′ = 0 2 + x1′.
x0′ + x2
ω0CS′
2.4 Characteristics of an Ideal Transformer

I

V'IN and I'IN can be expressed as:′ = IL b, b =VIN′ = bV2 = b VL , I IN

x0′
. (3)
x0′ + x2

Equation (3) represents the equivalent circuit of atransformer with these capacitors, which is the same as an
ideal transformer with a turn ratio of b (Figure. 4(b)) at theresonant frequency.

Figure 3: Detailed equivalent circuit.

2.5 Efficiency
The efficiency is approximated by:η=(a) Simplified equivalent circuit.

=(b) Ideal transformer.

Figure 4: Simplified equivalent circuit and ideal transformer.

Aluminum sheet

R L I L2R L I L2 + r1′I 1′ 2 + r2 I 22.
RL⎧⎪ ⎛ R ⎞ 2 ⎫⎪r1′
R L + 2 + r2 ⎨1 + ⎜⎜ L ⎟⎟ ⎬b⎪⎩ ⎝ x P ⎠ ⎪⎭(4)

z: Vertical direction

Ferritecore

Ferritex: Forward direction

core
Gap

WindingGapy: Lateral direction

Winding
Magnetic flux loop

Cross section in the x−z plane

Cross section in the x−z plane

Ferrite core

Ferrite core

Winding

Winding

Pole widthCore widthWinding width(a) Double-sided winding transformer
Figure 5: Structures of single and double-sided winding transformers.

Winding width(b) Single-sided winding transformer© EVS-25 Shenzhen, China, Nov. 5-9, 2010
The 25th World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium & Exhibition

The maximum efficiency ηmax is obtained when RL =RLmax:
RL max = xp

1 r1′
+ 1 η max =b 2 r2

. (5)

1
1+

3. Characteristics of Double-Sided Winding
Transformer
3.1 Specifications of the 1.5kW Transformer

1 r1′
+1
b 2 r22r2
xp

A rectangular core and double-sided winding was used
to provide the transformer with compactness and goodtolerance to misalignment. Table 1 lists the specificationsof a 1.5 kW double-sided winding transformer and Figure
6 shows a photograph of the transformer. The cores are
made of ferrite and the windings are litz wires. A gap
length of 70 mm with no misalignment is taken to be thenormal position.

If these characteristics are used, it is possible to design atransformer that has a maximum efficiency when the
output power is equal to the rated power.
2.6 Comparison of Transformer Structure
Figure 5 shows a comparison of single- and doublesided winding transformer structures. The winding width
must equal or exceed the gap length for the coupling factor
to be greater than 0.2. The core width of the single-sided
winding must be 2×(winding width + 2×pole width),
whereas the core width of the double-sided winding need
only be 1×(winding width + 2×pole width). Therefore,
double-sided winding results in a transformer that can bemade smaller than a single-sided winding transformer.
Furthermore, the coupling factor of a single-sided windingtransformer becomes zero when the horizontal
misalignment is approximately half the core diameter [5].
However, double-sided winding transformers have aleakage flux at the back of the core, and consequently they
have low coupling factors. To overcome this problem, an
aluminum sheet is attached to the back of the core, as
shown in Figure 5(a). The leakage flux is shielded by thealuminum sheet and the coupling factor becomes 50%
larger than that without the aluminum sheet. The reductionin efficiency due to eddy current losses in the aluminum
sheet is small (1-2%).

Table 1: Specifications of the 1.5 kW transformer.
Rated power
1.5 kW
Gap length70±20 mmTolerance to
Forward direction x±45 mmMisalignment Lateral direction y±125 mmSize
240×300×40 mm
Weight of the secondary transformer
4.6 kgFigure 6: Photograph of the 1.5 kW transformer.

0.3

L2
60

20

0.2
70

80

90

80

0.4
k(JMAG)

60

0.3

k(LCR)

0.2
L2

20

0

0

50
100
[mm]
yx [mm]k(JMAG) 0.4
0.3

k(LCR)

40

0.1

gap[mm]
Gap
[mm]

0.5
l1

k

k(LCR)

40

0.5

k

k

L[μH]

50

60

k(JMAG) 0.440l1

L [μH]

0.5

60

20

80

0.6

l1

L [μH]

80

0.2
L2

0.1

15
30
yx [mm][mm]

45

Figure 7: Transformer parameters.

B2
50
50

60

70
80
gap[mm]Gap[mm]

Figure 8: Transformer values.

90

100

POUT 1
VIN

η
B2

50
0

50
[mm]
yx[mm]

100

0.5

150
100
50
0

1.5
V2
POUT 1VIN

1530yx [mm]
[mm]

η 0.5
B2

45

POUT[kW], B2[T]

η 0.5

V2

Voltage[V], η[%]

VIN
100

150

200

1.5
POUT[kW], B2[T]1

Voltage[V], η[%]

V2

150

200

1.5

POUT

POUT[kW], B2[T]Voltage[V], η[%]200

© EVS-25 Shenzhen, China, Nov. 5-9, 2010
The 25th World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium & Exhibition

Table 2: Experimental results.
Frequency [kHz]Gap length [mm]
70
k
0.376
b
0.369
RL [Ω]
19.8
VIN [V]107
V2 [V]
128
η [%]
95.3
B1 [T]
0.11
B2 [T]
0.13
CS [μF]
0.696
CP [μF]
2.30

20
140
0.158
0.154
40.3
69.2
179
89.5
0.18
0.18
0.654
2.44

y [mm]
Figure 9: Characteristics of the 1.5 kW transformer withmisalignment in the y direction.

ID

IC

IOUT VOUT

Figure 10: Contactless power transfer with charging system.

3.2 Experimental Results
3.2.1 Fundamental Characteristics
To investigate the fundamental characteristics of the 1.5
kW transformer, the power supply voltage (VAC = 100 V)
and the inverter frequency (f0 = 20 kHz) were kept
constant. The values of the resonant capacitors CS and CP
for the normal position were kept constant. A full-bridge
rectifier and resistance load were connected to thesecondary winding.
Figure 7 shows the transformer parameters when the
gap length or position is varied. In Figure 7, k (JMAG)
shows the coupling factor calculated with JMAG, anelectromagnetic field analysis software, and k (LCR)
shows that calculated from inductances measured using an
LCR meter. Figure 8 shows the transformer values forvarious gap lengths or positions. Figures 1 and 5 depict themisalignment directions.
From Figure 7, the coupling factor k decreased when the
gap length or misalignment was increased, because theleakage flux became larger. The change in the value of theparallel capacitor CP determined by Equation (1) was
small, because the secondary self-inductance L2 wasalmost constant. The coupling factor k and idealtransformer turn ratio b decreased when the gap length
became larger, as shown in Figure 8; therefore, thesecondary voltage V2 and the output power POUT increased,
as indicated by equation (3). The efficiency η was 95.3%
at the normal position and 93.4% even at the highest gap
length of 90 mm.
The voltage ratio (VIN/V2) changed when the position
was varied and also when the gap length was changed.
When the input voltage VIN and the resistance load RL

were constant, the secondary voltage V2 and the output
power POUT increased when the misalignment increased.
The efficiency was always greater than 91%, as shown in
Figure 8. The results demonstrate that the rectangular
double-sided winding transformer has good tolerance tomisalignment.
3.2.2 Characteristics with a Wide Gap
The minimum ground clearance for PHVs and EVs isapproximately 140 mm. The characteristics of thetransformer with a wide gap are of particular interest withrespect to practical application of the transformer forcontactless power transfer. Table 2 shows the experimental
results for the 1.5 kW transformer with gap lengths of 70
and 140 mm. Although the efficiency η decreased to89.5%, it is possible to transfer 1.5 kW even at a gap
length of 140 mm.
3.2.3 Characteristics with a Large Misalignment
One of the important features of the double-sided
winding transformer is its characteristics with large
misalignment of the lateral direction. The misalignment of
the forward direction is easily limited by the use of wheel
chocks, which help drivers to position the forward
direction of the PHV/EV for charging.
The experimental results for a large misalignment of thelateral direction are shown in Figure 9. The efficiency was
greater than 85% and the output power was 1.5 kW, evenfor a misalignment of 250 mm, which is equal to the core
length. During the experiment, the input voltage VIN was
adjusted to maintain a constant secondary voltage V2 andthe values of the resonant capacitors Cs and Cp remained
constant.

© EVS-25 Shenzhen, China, Nov. 5-9, 2010
The 25th World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium & Exhibition

4. Charging System
4.1 Configuration of the Contactless Charging System
Figure 10 shows a schematic diagram of a contactlesscharging system. A charge control circuit, which consistsof a full-bridge rectifier and chopper, was connected to thesecondary winding. Constant current control is employedat the start of charging. If the charging voltage VOUT
reaches the transfer voltage, then the control is changed toconstant voltage control.
4.2 Experimental Results
Six automotive lead acid batteries (12 V, 32 Ah)connected in series were used. The charging current was 4
A and the transfer voltage was 87 V for a charge time of 7
h. The batteries were discharged at 20 Ah from full charge.
The experiment was started when the open circuit voltageof the lead acid batteries was 72 V.

90

4

800

80

3

70

2

60

1

50
0

1

2

3
4
5
時間t[hour]t [h ]

6

7

600
400
200

充電電力P
[W]
OUT[W]
POUT

1000
充電電流IOUT[A]

5

VV
[V]
OUT
充電電圧
OUT[V]

100

0Figure 11: Characteristics of the charging system.

As shown in Figure 11, full charge of the batteries was
successfully achieved and the estimated amount of charge
was 18 Ah.
Figure 12 shows the waveforms of charging system atstages (i)-(iv) shown in Figure 11. The inverter output
voltage VIN and secondary voltage V2 were constant, even
if output power was varied. In addition, the inverter output
voltage VIN and current IIN, and the secondary voltage V2
were almost coherent. This demonstrates that thetransformer with the charging system has thecharacteristics of an ideal transformer.

5. Conclusion
A contactless power transfer system is proposed that issuitable for application as an EV battery charger. A
configuration with in series capacitors in the primarywinding and in parallel capacitors in the secondary
winding has an interesting characteristic, in that its
equivalent circuit is the same as an ideal transformer.
Consequently, the transformer has simple efficiency
equations. A transformer consisting of rectangular coreswith double-sided windings is compact and insensitive to
misalignment in the lateral direction. A 1.5 kW
transformer was constructed with dimensions of
240×300×40 mm, a gap length of 70±20 mm, a
misalignment tolerance in the lateral direction of ±125 mm,
a secondary winding and core mass of 4.6 kg, andefficiency of 95% in the normal position. Successful test
results for a wide gap of 140 mm, for a large lateral
misalignment of 250 mm and for lead acid battery
charging were presented. In the future, we intend to impro-

100V

VIN

100V

VIN

100V

VIN

100V

VIN

5A

IIN

5A

IIN

5A

IIN

5A

IIN

100V

V2

100V

V2

100V

V2

100V

V2

7A

IID2

7A

IID2

7A

IID2

7A

IID2

0

0.05

0.1

0

[ms]

VC

120V

0

0.05

0.1
[ms]

0.05

Figure 12: Waveforms of the charging system.

VC

0

VOUT

IC

VOUT

40V

IOUT

0.05

0.1
[ms]

IOUT

2A

0

0.05

0.1

(ⅲ)

[ms]

0.1

VC

2A

2A

(ⅱ)

0.05

[ms]

120V

IC

40V

IOUT

0

0.1
[ms]

2A

VOUT

2A

(ⅰ)

0

120V

IC

40V

IOUT

2A

VC

2A

VOUT

40V

0.1
[ms]

120V

IC

2A

0.05

0

0.05

0.1

(ⅳ)

[ms]

© EVS-25 Shenzhen, China, Nov. 5-9, 2010
The 25th World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium & Exhibitionve the design of the cores and windings to develop atransformer that is more lightweight and has higher
efficiency.
This research was sponsored by the New Energy andIndustrial Technology Development Organization (NEDO)
of Japan.

Author’s
Photo

6. References[1] Y. Kamiya, Y. Daisho and H. Matsuki: “Inductive Power
Supply System for Electric -driven Vehicles”, IEEJ Journal, Vol.
128, No. 12, pp. 804-807 (2008) (in Japanese)[2] A.W. Green and J.T. Boys: “10 kHz Inductively Coupled
Power Transfer – Concept and Control”, IEE Power Electronics
and Variable Speed Drives Conference, PEVD, No. 399, pp. 694699 (1994)[3] T. Fujita, Y. Kaneko and S. Abe: “Contactless Power
Transfer Systems using Series and Parallel Resonant Capacitors”,
IEEJ Trans. IA, Vol. 127, No. 2, pp. 174-180 (2007) (in Japanese)[4] Y. Kaneko, N. Ehara, T. Iwata, S. Abe, T. Yasuda and K. Ida:
“Comparison of Transformer Winding Methods for Contactless
Power Transfer Systems of Electric Vehicle”, IEEJ Trans. IA,
Vol. 130, No. 6, pp. 734-741 (2010) (in Japanese)[5] M. Budhia, G.A. Covic and J.T. Boys: “Design and
Optimisation of Magnetic Structures for Lumped Inductive
Power Transfer Systems”, IEEE ECCE, pp. 2081-2088 (2009)[6] Y. Nagatsuka, N. Ehara, Y. Kaneko, S. Abe and T. Yasuda:“Compact Contactless Power Transfer System for Electric
Vehicles”, IPEC2010-Sapporo, pp. 807-813 (2010)

Author’s
Photo

Author’s
Photo

Author’s
Photo

7. Authors
Author’s
Photo

Yuichi Nagatsuka
Saitama University
255 Shimo-Okubo, Sakura-ku, Saitama-shi,
Saitama 338-8570, Japan
Tel:+81-48-858-3472 Fax:+81-48-858-0940
Email: s09mm222@mail.saitama-u.ac.jp
URL: http://akt.ees.saitama-u.ac.jp/

Author’s
Photo

Shingo Noguchi
Saitama University
255 Shimo-Okubo, Sakura-ku, Saitama-shi,
Saitama 338-8570, Japan
Tel:+81-48-858-3472 Fax:+81-48-858-0940
Email: s09mm226@mail.saitama-u.ac.jp
URL: http://akt.ees.saitama-u.ac.jp/

Author’s
Photo

Yasuyoshi Kaneko
Saitama University
255 Shimo-Okubo, Sakura-ku, Saitama-shi,
Saitama 338-8570, Japan
Tel:+81-48-858-3475 Fax:+81-48-858-3475
Email: kaneko@ees.saitama-u.ac.jp
URL: http://akt.ees.saitama-u.ac.jp/
Shigeru Abe
Saitama University
255 Shimo-Okubo, Sakura-ku, Saitama-shi,
Saitama 338-8570, Japan
Tel:+81-48-858-9207 Fax:+81-48-858-9207
Email: abe@ees.saitama-u.ac.jp
URL: http://akt.ees.saitama-u.ac.jp/
Tomio Yasuda
Technova Inc.
13th Fl. The Imperial Hotel Tower, 1-1
Uchisaiwai-cho 1-chome, Chiyoda-ku,
Tokyo 100-0011, Japan
Tel:+81-3-3508-2280 Fax:+81-3-3508-7578
Email: yasuda@technova.co.jp
URL: http://www.technova.co.jp
Kazuhiko Ida
Technova Inc.
13th Fl. The Imperial Hotel Tower, 1-1
Uchisaiwai-cho 1-chome, Chiyoda-ku,
Tokyo 100-0011, Japan
Tel:+81-3-3508-2280 Fax:+81-3-3508-7578
Email: ida@technova.co.jp
URL: http://www.technova.co.jp
Akira Suzuki
AISIN AW CO., LTD. Product development
planning department
10 Takane, Fujii-cho, Anjo City, Aichi 4441192, Japan
Tel:+81-566-73-1616 Fax:+81-566-99-1433
Email: i12902_Suzuki@aisin-aw.co.jp
URL: http://www.aisin-aw.co.jp/
Ryoichi Yamanouchi
AISIN AW CO., LTD. Product development
planning department
10 Takane, Fujii-cho, Anjo City, Aichi 4441192, Japan
Tel:+81-566-73-1820 Fax:+81-566-99-1433
Email: i22269_Yamanouchi@aisin-aw.co.jp
URL: http://www.aisin-aw.co.jp/