EVS26
Los Angeles, California, May 6-9, 2012

10 kW Contactless Power Transfer System
for Rapid Charger of Electric Vehicle
Tomohiro Yamanaka1, Yasuyoshi Kaneko, Shigeru Abe, Tomio Yasuda2,
1

Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama-shi, Saitama, 338-8570 Japan,
s11mm249@mail.saitama-u.ac.jp
2
Technova Inc.,13th The Imperial Hotel Tower, 1-1 Uchisaiwai-cho 1-chome, Cjiyoda, Tokyo 100-0011, Japan

Abstract
A contactless power transfer system for charging electric vehicles requires a high efficiency, a large airgap, and a good tolerance to lateral misalignment and needs to be compact and lightweight. A double-sided
winding 10 kW transformer based on a 1.5 kW H-shaped core transformer was developed for a rapid
charger. Even though the transformer capacity was increased, the dimensions of the 10 kW transformer
were almost the same as those of the 1.5 kW transformer. In this paper, the design concept for this 10 kW
transformer and the results of experimental are described. The transformer was found to exhibit more than
94% efficiency with a mechanical gap of 70mm. Even for gap change and position change, an efficiency ofover 92% was maintained.
In a contactless power transfer system with a series resonant capacitor, the primary terminal voltage
increases with increasing power. This overvoltage may lead to the problem of breakdown voltage ofcapacitor and the breakdown of winding insulation. To prevent such an overvoltage occurring, it issuggested that the primary winding and the series capacitor are split into two pieces or more respectively,
and the split windings and capacitors are alternately connected in a series. Electric vehicles using acontactless power transformer for rapid charging would also uses normal charging. Therefore, the 10 kW
transformer was designed to be compatible with the 1.5 kW transformer. Consequently, electric vehicles
equipped with the 10 kW transformer can charge even using a 1.5 kW ground transformer, without
decrease efficiency.
Keywords: wireless charging, fast charge, efficiency , charging

1

Introduction

Plug-in hybrid electric vehicles (PHEVs) andelectric vehicles (EVs) are actively being
developed and commercialized in response toenvironmental concerns[1]. PHEVs and EVs

currently need to be connected to a power supply
by electrical cables to charge their batteries. A
contactless power transfer system would have
many advantages such as having the convenienceof being cordless and enabling high-power
charging to be performed safely. Therefore,
contactless power transfer systems are being

EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium

1studied around the world.
A contactless power transfer system for EVs
must have a high efficiency, a large air gap, and agood tolerance to lateral misalignment and becompact and lightweight. We have demonstrated
these requirements can be met by using a doublesided winding.
In previous studies, we developed a 1.5–3 kW
transformer for normal charging[2]. It had anovel H-shaped core. We newly developed a 10
kW transformer for rapid charging. This paper
describes the characteristics of this transformer.
The 10 kW transformer should be compatible
with a 1.5 kW transformer for normal charging.
To design a 10 kW transformer with the same
number of turns on the primary and secondary
windings as the 1.5 kW transformer, the distance
between the poles has to be nearly the same.
Consequently, both transformers have similarparameters, including the load resistance RL at
maximum efficiency. Therefore, in the 10 kW
system, the inverter output voltage VIN and theload voltage VL are designed to be 2.6 (the squareroot of power ratio 10 kW/1.5 kW) times larger
compared to the 1.5kW system. The crosssectional area of the ferrite core and the numberof parallel windings of the 10 kW transformer are
increased relative to those of the 1.5 kW
transformer because the flux density and current
are 2.6 times greater. As a result, the 10 kW
transformer had an equivalent performance to the
1.5 kW transformer and realized an efficiency of
94.7% when the mechanical gap was 70 mm.
In addition, even when either the primary or
secondary transformer is replaced with the 1.5
kW transformer, it is still possible to transfer 1.5
kW with an efficiency of 94% or more. Thisability to interoperate between normal charging
and rapid charging is a novel characteristic ofcontactless power transfer systems with doublesided windings, which conventional chargers do
not possess.

2

Contactless Power Transfer
System for EVs

2.1

Contactless
SystemPower

Transfer

Figure 1 shows a schematic diagram of thecontactless power transfer system with series andparallel resonant capacitors[3]. A full-bridge
inverter is used as a high-frequency power
supply. The cores are made of ferrite and thewindings are litz wires.

Fig. 1. Contactless power transfer system.

VIN' IIN'

I2 V2 Id

-jxS'

r1'

jx1'

jx2
jx0'

r2
-jxP

RL

r0'
Fig. 2. Detailed equivalent circuit.

2.2

Equivalent Circuit

Figure 2 shows a detailed equivalent circuit. It
consists of a T-shaped equivalent circuit to which
resonant capacitors CS and CP and a load resistance
RL have been added. Primary values are converted
into secondary equivalent values using the turnratio a = N1/N2. Since the winding resistances andthe ferrite-core loss are considerably lower thanthe mutual and leakage reactances at the resonant
frequency, the winding resistances (r'1 and r'2) andthe ferrite-core loss r'0, can be ignored.

2.3

Characteristics of Series andParallel Resonant Capacitor System

The secondary parallel capacitor CP and theprimary series capacitor CS can be expressed by:1= xp = x0′ + x2 ,
ω0 C P.
(1)
1x0′ x2+ x1′= xs′ =
x0′ + x2
ω0CS′
The input voltage V'IN and the input current I'IN
can be expressed by:
V IN′ = b V 2 ,′ = ID b,
I IN
.
(2)
x 0′
b=
x 0′ + x 2
Equation (2) represents that the equivalentcircuit of a transformer with these capacitors is thesame as an ideal transformer with a turn ratio of bat the resonant frequency.
Without a rectifier circuit, the efficiency can beapproximated by:

EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium

2

η=
=

RL I L2
RL I L2 + r1′I1′2 + r2 I 22
RL
2

r′
 R  
RL + 12 + r2 1 +  L  
xP  
b





(3)

The maximum efficiency ηmax is obtained when
RL = RLmax.
1 r′
RL max = xp 2 1 + 1
b r2
(4)
1
ηmax =
2r 1 r1′
1+ 2+1
xp b 2 r2

2.4

Efficient Transformer Design

The primary winding Q1 and the secondary
winding Q2 are represented by:
ωL
Q1 = 0 1 ,
r1
(5)
ω0 L2
Q2 =
r2
Here, L1 and L2 are respectively the primary andsecondary self-inductances and ω0 is the resonant
angular frequency.
From equation (4), when:
1 Q1
(6)
>> 1
k 2 Q2
ηmax and RLmax are represented by:rQ
Q1
R L max ≅ 2 2
,
k
Q2
(7)

η max ≅1
2

(8)
k Q1Q 2Equation (8) indicates that the maximum
efficiency ηmax depends solely on the quality
factor Q and the coupling factor k, if core loss
can be ignored.

2.5

1+

Double-sided Winding Transformer

Contactless power transformers can have twodifferent structures: circular cores with singlesided winding[4][5] and rectangular cores withdouble-sided
winding[2][6].
Double-sided
winding transformers have a leakage flux at theback of the core and consequently they have low
coupling factors k. To overcome this problem, an
aluminum sheet is attached to the back of thecore. Since the leakage flux is shielded by thealuminum sheet, the coupling factor k can be large.
For the same coupling factor k, the core width(winding width + pole width) of the double-sided
winding needs to be only half that of the singlesided winding. A transformer with a double-sided
winding can be made smaller than one with asingle-sided winding. Furthermore, the coupling
factor k of a single-sided winding transformer
becomes zero when the lateral misalignment isapproximately 40% of the core diameter[3]. For
the double-sided winding transformer, if thesecondary winding is moved laterally, the total
flux penetrating the secondary winding will
decrease only slightly and the reduction in the
coupling factor k will also be small. Therefore, thedouble-sided winding transformer has goodtolerance to lateral misalignment.
However, the double-sided winding transformer
has poor heat dissipation from its core because thecore is covered by the winding. In addition, thedouble-sided winding transformer has a low heat
capacity due its small volume. Moreover the 10
kW transformer will produce greater heatcompared to the 1.5 kW transformer, thermal
problems are unavoidable for large-capacitytransformers.

2.6Interoperability between Normal
Charging and Rapid Charging

When an EVs is equipped with a contactless
power transfer system for rapid charging, its
secondary transformer is desirable to beinteroperable with a primary transformer for
normal charging. If the turns of primary andsecondary windings and the operationalfrequencies are the same and the distances between
the poles are nearly the same between two
transformers, each primary transformer isinteroperable with another secondary transformer.
In this case, the transfer power is limited to thelower rated power in two transformers. Thus, thecompatibility can be realized between rapid
charging and normal charging.
Usually, a conventional rapid charging that uses a
cable connection requires a different connector andcircuit to those used for normal charging. In
contrast, a contactless power transfer system canuse the same charging circuit when a transformer
that can be used for both rapid and normal
charging is employed.

EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium

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Table 1. Specifications of transformers.

Transformer
Voltage
Litz wire
Primary
Secondary
Weight of secondary
Shielding Al sheet

Winding

1.5 kW
10 kW
1φ AC
3φ AC
100 V
200 V
0.1 mmφ×800
20 T (3p)
20 T (4p)
6 T (9p)
6 T (12p)
3.9 kg5.74 kg
600×400×1 mm (0.6 kg)(a) Without winding split.

(b) With winding split.
Fig. 3. Split winding voltage vector.

3

10 kW contactlesstransfer system

power

3.1

Countermeasure for Overvoltage
of Primary Winding(a) Photograph of 10 kW transformer.
300
300
x

280

240

y

gap

gap

In the 10 kW contactless power transfer system,
the primary current IIN increases compared to
1.5 kW system, and consequently the primaryterminal voltage V1 (i.e., the voltage after CS in
Figure 1) increases in proportion to IIN.
Increasing V1 may cause problems such as
overvoltage of the capacitor and the winding. To
prevent overvoltage, the primary winding andseries resonant capacitors are split and the split
windings and capacitors are alternately
connected in a series (circuit is depicted in
Figure 1). Figure 3 depicts the voltage vectors
when the primary winding and the seriescapacitor are split (Figure 3(b); it shows that
reducing the capacitor voltage reduces theprimary terminal overvoltage.

FerriteWindingx Forward directiony Lateral direction

unit : mm(b)1.5 kW transformer. (c)10 kW transformer.
Fig. 4. (a) Photograph and(b), (c) dimensions of transformer.

EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium

4Fig. 5. Experimental results for 10 kW transformer.

3.2

10 kW Transformer

The 10 kW contactless power transformer was
designed and made based on the 1.5 kW
transformer. It has an H-shaped ferrite core anddouble-sided winding. An aluminum sheet isattached to the back of the transformer. Table 1 lists
the transformer specifications. Figure 4 shows aphotograph and the dimensions of the transformer.
When designing the 10 kW transformer, thenumber of winding turns had to be same and the
distance between the poles had to be nearly thesame to ensure compatibility with the 1.5 kW
transformer. When there is a nearly the same
distance between the poles in the 10 kW and 1.5
kW transformers, the inductances and load
resistance that maximize the efficiency are the sameas those for the 1.5 kW transformer. Therefore, inthe 10 kW system, VIN and VL will be increased 2.6
times and the flux density and current will be about
2.6 times greater.
To avoid the increase in the flux density, thecross-sectional area of the ferrite core wasincreased. To avoid the increase in the current
density, the number of parallel windings wasincreased. In addition, the distance between thepoles was increased by 40 mm so that more litz
wire could be wound and to increase the coupling
factor k by reducing the leakage flux.

3.3

Characteristics of 10 kW Power
Transfer Tests

When using contactless power transfer systems
for EVs, misalignment by the driver's skill and gapchange due to the car weight cannot be avoided. A
mechanical gap length of 70 mm with nomisalignment is taken to be the normal position.
The transformer characteristics were measured forgap lengths in the range ±30 mm, a misalignment in
the forward direction x of ±60 mm, and amisalignment in the lateral direction y of ±150 mm.
Misalignment in the x direction can be made small
by using a wheel stop, but a large misalignment
tolerance in the y direction is required to allow foreasy parking.
The operating frequency f0 is 30 kHz; it isFig. 6. Characteristics with change in the resistanceload.

Fig. 7. Leakage flux measurement (30 kHz).

constant during the experiments. A full-bridge
rectifier circuit and a load resistance RL are connected
to a secondary transformer. Figure 5 shows thetransformer parameters for different gap lengths andpositions. The misalignment directions (x and y) are
shown in Figure 4. The mutual inductance l0 and thecoupling factor k decreased with increasing gap
length or misalignment because the magnetic
reluctance of the main flux path became larger.
However, the secondary self-inductance L2 wasalmost constant; consequently, the capacitance of theparallel capacitor CP given by equation (1) can beconstant. Therefore, in the experiment thecapacitances of CS and CP remained constant during
the experiment and the load resistance RL was kept
constant at 15 Ω. As shown in Figure 5, the coupling
factor k decreased when the gap length or
misalignment was increased and the ideal transformer
turn ratio b also decreased and the voltage ratio

EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium5(V2/VIN) increased according to equation (2). The
output power POUT remained constant at 10 kW
when the input voltage VIN was varied. The
transformer efficiency η at the normal position was
94.7%; even when the mechanical gap was 100
mm, η was 92.6%. The transformer efficiency η
was always greater than 92% during theexperiments.

the transformer is attached to the center of the car, it
might be safe on the vehicle side. In addition,
charging must be stopped automatically when aperson goes under the car.

4

Compatibility of 1.5 kW and 10
kW Transformers

3.4

4.1

Comparison of Characteristics

Characteristics of Varying Load
ResistanceThe characteristics of the 1.5 kW and 10 kW
transformers were compared in terms ofcompatibility. Table 2 lists the transformer
parameters. Table 2 shows that the two transformers
have similar inductances and load resistance at the
maximum efficiency.
Table 3 compares the efficiencies of the 1.5 kW and
10 kW transformers for different gap lengths andmisalignments. In Table 3, the gap length of 70±30mm and misalignments of 0–40 mm in the x direction
and 0–150 mm in the y direction represent average
ranges. Tables 2 and 3 show that both transformers
have approximately the same specifications andcharacteristics.

The charging power varies during charging. As achange in the charging power can be simulated bychanging the resistance load RL, it is important toknow the characteristics when the resistance load
RL is changed. Figure 6 shows the transformer
parameters when the resistance load RL is increased
from 12 to 75 Ω at the normal position and forconstant output voltage VL. While the transformer
efficiency η decreased slightly, the voltage ratio
almost constant. This
(V2/VIN) remained
demonstrates that this transformer has thecharacteristics of the ideal transformer.

3.5

Characteristics of Leakage Flux

Table 2. Transformer parameters.

As the contactless power transformer has smallcoupling, the leakage flux is distributed around thetransformer. Figure 7 shows the leakage flux
densities of the 1.5 kW and 10 kW transformers at
the normal position. The leakage flux density
decreases according to the 2.7 power of the distancefrom the center of the transformer. Thus, when the
distance is doubled, the leakage flux density
decreases to 15%. As shown in Figure 7, the output
power affects the leakage flux, but the reduction
ratio is due to only the transformer structure.
In practical applications, the effect of leakage flux
poses serious problem to human health. Figure 7
also shows criteria for exposure to electromagnetic
lines; the 6.25 and 27μT lines represent reference
levels for exposure to the general public given in
ICNIRP1998 and ICNIRP2010[7], respectively. If

Transformer

1.5 kW10 kW

r1 [mΩ]106

107

r2 [mΩ]

9.30

12.7

l0 [μH]

55.4

57.1

l1 [μH]115

111

l2 [μH]

9.70

10.18

k

0.333

0.338

b

0.340

0.335

RLmax [Ω]

8.69

8.03

ηmax [%]

97.9

97.6

Table 3. Transformer efficiencies.

Transformer
Normal position
Gap lengthη[%]xTolerance tomisalignmenty

Average(30–100 mm)
100 mmAverage(0–60 mm)
60 mm
Average(0–150 mm)
150 mm

1.5 kW
94.9

10 kW
94.7

94.5

94.5

93.1

92.6

94.0

94.2

92.6

93.2

94.0

93.9

93.0

92.4

EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium

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VDC IDC
Regulated DC
Power Supply＋－

VIN IIN V

1

I2 V2 ID

CS

VL IL
＋
－

CP

RL

2kW HF Inverter

P1

P2

Pout

Fig. 8. Contactless power transfer system for case A(primary: 1.5 kW; secondary: 10 kW).

Fig. 9. Contactless power transfer system for case B(primary: 10 kW; secondary: 1.5 kW).

Fig. 10. Experimental results for case A (primary: 1.5 kW; secondary: 10 kW).

Fig. 11. Experimental results for case B (primary: 10 kW; secondary: 1.5 kW).

4.2

Characteristics
ofCompatibility Tests

1.5

kW

Two 1.5 kW tests were carried out in thecompatibility test: case A in which an EVs isequipped with the rapid type(10 kW) and ground is
the normal type(1.5kW), and case B in which an
EVs is equipped with normal type and ground is the
rapid type. The normal position is taken to have amechanical gap length of 70 mm with nomisalignment. The transformer characteristics were
measured for gap lengths in the range ±30 mm, a
misalignment in the forward (x) direction of ±40mm, and a lateral (y) direction of ±150 mm.
The operating frequency f0 was constant at 30
kHz throughout the tests. Figure 8 shows aschematic diagram of case A; a full-bridge rectifier
circuit and load resistance RL=15 Ω are connectedto the secondary transformer. Figure 9 shows aschematic diagram of case B; a double-voltage
rectifier circuit and a load resistance RL=80 Ω areconnected to the secondary transformer.
Figure 10 and 11 show the parameters andcharacteristics for cases A and B, respectively.
Similar to the results for 10 kW operation, the mutual
inductance l0 and the coupling factor k decreased withincreasing gap length or misalignment. The
experimental results of case A and case B show bothtransformer
hasapproximatelythesamecharacteristics.
Table 4 lists the transformer parameters fordifferent gap lengths and positions. In case A, thetransformer efficiency η at the normal position is
94.8%; even when the mechanical gap is 100mm η is
93.0%. In the misalignment experiment, thetransformer efficiency η is always more than 93.4%.
In case B, η at the normal position is 94.7%; evenwhen the mechanical gap is 100mm η was 93.0%. In
the misalignment experiment, the transformer
efficiency η is always more than 93.9%. In contrast
with the results in Table 3, even if the 1.5 kW
transformer only replaces the primary or secondaryside of the 10 kW transformer, the transformer
efficiency η remains approximately the same.

EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium

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Table 4. Experimental results for compatibility tests.

η [%]

5

Primary
Transformer
SecondaryNormal position
Average(30–100 mm)
Gap length
100 mmAverage(0–40 mm)x
40 mm
Tolerance tomisalignment
Average(0–150 mm)
y
150 mmConclusion

A novel 10 kW transformer that can be used for
rapid charging was developed and its characteristics
were investigated. It had a high efficiency and wascompatible with a 1.5 kW transformer. To keep thecompatibility, the 10 kW transformer was designed
to have the same turns and the same distance
between poles. To avoid high primary terminal
voltage, the primary winding and the seriescapacitor are split into two severally, and split
windings and capacitors are alternately connectedin the series.
The interoperability tests between the 10 kW
rapid charging transformer and the 1.5 kW normal
charging transformer are carried out successfully.

Acknowledgments
This research was sponsored by the New Energy
andIndustrial
Technology
Development
Organization (NEDO) of Japan.

References[1]

G.A.Covic, J.T.Boys, M.Budhia andC.Y,Huang ： “Electric Vehicles -Personaltransportation for the future”, The World
Battery, Hybrid and Fuel Cell Electric
Vehicle Symposium and Exhibition (EVS25), Shenzhen China, pp.1-10(2010)[2]

M. Chigira, Y. Nagatsuka, Y. Kaneko, S.
Abe, T. Yasuda, and A.Suzuki "Small-Size
Light-Weight Transformer with New Core
Structure for Contactless Electric Vehicle
Power Transfer System" ECCE2011PHOENIX pp260-266(2011)[3]

Y. Nagatsuka, N. Ehara, Y. Kaneko, S. Abe
and T. Yasuda, “Compact contactless powertransfer system for electric vehicles”, IPEC
Sapporo, pp. 807-813 (2010)

Case A
1.5 kW
10 kW
94.8

Case B
10 kW
1.5 kW
94.7

94.8

94.3

93.0

93.0

94.6

94.3

94.1

93.9

94.4

94.3

93.4

94.1

[4]

M. Budhia, G.A. Covic and J.T. Boys ：“Design and Optimisation of Magnetic
Structures for Lumped Inductive Power
Transfer Systems”,IEEE ECCE, pp.20812088 (2009)[5]

C.-S.Wang, O.H.Stielau, and G.A.Covic ：“Design consideration for a contactlesselectric vehicle battery charger”, IEEE Trans.
Ind. Electronics,Vol.52, No.5, pp.1308-1314(2005)[6]

M.Budhia, G.A.Covic, and J.T.Boys ： “A
New Magnetic Coupler for Inductive Power
Transfer Electric Vehicle Charging Systems”,
IEEE IECON 2010, pp. 2481-2486 (2010)[7]

International Commission on Non-Ionizing
Radiation Protection (ICNIRP), "Guidelines
for limiting exposure to time varying electric,
magnetic, and electromagnetic fields"(2010)

Authors
Tomohiro Yamanaka
Saitama University
255
Shimo-Okubo,
Sakura-ku,
Saitama-shi, Saitama 338-8570, Japan
Tel:+81-48-858-3472
Fax:+81-48858-3475
Email:
s11mm249@mail.saitama-u.ac.jp
URL: http://akt.ees.saitama-u.ac.jp/
Yasuyoshi Kaneko
Saitama University
255
Shimo-Okubo,
Sakura-ku,
Saitama-shi, Saitama 338-8570, Japan
Tel:+81-48-858-3475
Fax:+81-48858-3475
Email: kaneko@ees.saitama-u.ac.jp
URL: http://akt.ees.saitama-u.ac.jp/

EVS26 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium

8Shigeru Abe
Saitama University
255
Shimo-Okubo,
Sakura-ku,
Saitama-shi, Saitama 338-8570, Japan
Tel:+81-48-858-9207
Fax:+81-48858-9207
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, 11 Uchisaiwai-cho 1-chome, Chiyodaku, Tokyo 100-0011, Japan
Tel:+81-3-3508-2280
Fax:+81-33508-7578
Email: yasuda@technova.co.jp
URL: http://www.technova.co.jp

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