The 2010 International Power Electronics Conference

Compact Contactless Power Transfer System
for Electric Vehicles
Y. Nagatsuka*, N. Ehara*, Y. Kaneko*, S. Abe* and T. Yasuda*** Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama-shi, Saitama, Japan** Technova Inc., Imperial Hotel Tower, 13F, 1-1 Uchisaiwaicho 1-chome, Chiyoda-ku, Tokyo, Japan
Abstract--Electric vehicles (EVs) have been attracting
considerable interest recently. A contactless power transfer
system is required for EVs. Transformers can have singlesided or double-sided windings. Transformers with doublesided windings are expected to be more compact andlightweight than transformers with single-sided windings. A
contactless power transfer system for EVs needs to have ahigh efficiency, a large air gap, good tolerance tomisalignment and be compact and lightweight. In this paper,
a novel transformer using series and parallel capacitorswith rectangular cores and double-sided windings thatsatisfies these criteria has been developed, and its
characteristics are described. It has an output power of 1.5
kW and an efficiency of 95% in the normal position. To
reduce the cost of expensive ferrite cores, a transformer
with split cores is also proposed.
Index Terms--Contactless power transfer system,
Efficiency, Electric vehicle, Plug-in hybrid electric vehicle

I. INTRODUCTION
Plug-in hybrid electric vehicles (PHVs) and electricvehicles (EVs) are increasingly becoming realities
because of environmental concerns and rising oil prices.
PHVs and EVs currently need to be connected to a power
supply by electric cables to charge their batteries. A
contactless power transfer system (such as that depicted
in Fig. 1) has many advantages, including theconvenience of being cordless (so that there is no need to
unplug the cable) and safety during high-power charging.
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.
We used two technologies to satisfy these
specifications. The first technology is a resonantcapacitor configuration in which the primary capacitor isin series and the secondary capacitor is in parallel to eachwinding. The second technology is a transformer with anovel structure in which double-sided windings arewrapped around rectangular cores.
Because transformers have a large air gap, they have
low coupling factors (0.1-0.5). Hence, a high-frequency(10-50 kHz) inverter is used as the power supply andresonant capacitors are connected to the terminals.
This research is supported by New Energy and Industrial
Technology Development Organization.

978-1-4244-5395-5/10/$26.00 ©2010 IEEE807

z : Vertical direction
x : Forward direction
y : Lateral direction

Fig. 1. Contactless power transfer system for an EV

Various resonant capacitor configurations have been
proposed [1]. Among them, a configuration in which theprimary capacitor is in series and the secondary capacitor
is in parallel has an interesting characteristic [2]: if thecapacitors are chosen correctly and the winding
resistances are ignored, the equivalent circuit of atransformer with these capacitors is the same as an idealtransformer at the resonant frequency, which is equal tothe inverter frequency.
From this characteristic, the following benefits are
obtained:
1. When there is a resistance load, the power factor of
the inverter output is always 1 and soft switching is
performed.
2. The capacitances of the resonant capacitors are
independent of the output power.
3. If the primary voltage/current is constant, thesecondary voltage/current will also be constantregardless of the load change.
4. The theoretical equation for the efficiency can beeasily derived. This enables the optimum transformer
design to be determined and the transformer to be
operated at its maximum efficiency [3].
Transformers with circular cores and single-sided
windings have commonly been used. As they have two
flux loops in their cross section, the core width is large
and the coupling factor between the windings will be zero
when the horizontal misalignment is about half the core
diameter [4]. The proposed transformer has rectangularcores with double-sided windings and has a single flux
loop in its cross section. Consequently, it has a small corewidth and there is only a moderate reduction in the
coupling factor when there is lateral misalignment.
A 1.5 kW transformer with double-sided windings thatsatisfies the above specifications has been constructed.
Furthermore, to reduce the cost and the weight of ferrite

The 2010 International Power Electronics Conference

cores, a transformer with split cores has also beendeveloped. The following sections describe thecharacteristics of these transformers and present various
test results.
II. PRIMARY SERIES CAPACITOR AND SECONDARYPARALLEL CAPACITOR CONFIGURATION

Fig. 2. Contactless power transfer system for an EV

A. Contactless Power Transfer System
Fig. 2 shows a schematic diagram of a 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
and the windings are litz wires. Fig. 3 shows a detailed
equivalent circuit, which consists of a T-shaped
equivalent circuit to which resonant capacitors CS and CP
and a resistance load RL have been added. Primary values
are converted into secondary equivalent values using theturn ratio a = N1/N2 (primes are used to indicate
converted values). As the winding resistances and theferrite-core loss are much lower than the mutual andleakage reactances at the resonant frequency, thesimplified equivalent circuit shown in Fig. 4(a), which
ignores the winding resistances (r'1 and r'2) and theferrite-core loss r'0, is used.

I

Fig. 3. Detailed equivalent circuitjj(a) Simplified equivalent circuit

The maximum efficiency Șmax is obtained when RL =RLmax.

RL max = xp

(1)

x0′
x0′ + x 2

(3)

C. Efficiency
The efficiency is approximated by:=

RL I L2
RL I L2 + r1′I1′ 2 + r2 I 22
RL­° § R · 2 ½°
r1′
RL + 2 + r2 ®1 + ¨¨ L ¸¸ ¾b°̄ © x P ¹ °¿

2r
1+ 2
xp

1 r1′
+1
b 2 r2(5)

Fig. 5 shows a comparison of a single-sided windingtransformer and the proposed double-sided windingtransformer. The winding width must equal or exceed thegap 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 widthof the double-sided winding need only be 1× (winding
width + 2×pole width). When a double-sided winding isused, the transformer can be made smaller than atransformer with a single-sided winding. Furthermore, thecoupling factor of a single-sided winding transformer
becomes zero when the horizontal misalignment is about
half the core diameter.
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 Fig. 5. The leakage flux is shielded by thealuminum sheet and the coupling factor becomes 50%
larger than when no aluminum sheet is present. The
reduction in the efficiency due to eddy current losses in
the aluminum sheet is small (1-2%).

Equation (3) stands for the equivalent circuit of atransformer with these capacitors is the same as an idealtransformer (Fig. 4(b)) at the resonant frequency.

η=1III. COMPARISON OF TRANSFORMER STRUCTURE(2)

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

1 r1′
+ 1 η max =b 2 r2

If these characteristics are used, it is possible to designa transformer that has a maximum efficiency when the
output power is equal to the rated power.

The primary series capacitor CS (C'S denotes its
secondary equivalent) is determined as:x0′ x2
1= xs′ =
+ x1′
x0′ + x2
ω 0 CS′(b) Ideal transformer

Fig. 4. Simplified equivalent circuit and ideal transformer

B. Resonant Capacitors
To achieve resonance with the self-reactance of thesecondary winding Ȧ0L2, which is equivalent to adding amutual reactance x'0 and a leakage reactance x2, thesecondary parallel capacitor CP is given by:1= xp = x0′ + x2
ω0 C P

j

(4)

808

The 2010 International Power Electronics Conferencez : Vertical direction

Aluminum sheet
Ferritecorex : Forward direction

Gap

Windingy : Lateral direction

Ferritecore

GapWinding
Magnetic flux loop

Cross section in x−z plane

Cross section in x−z plane

Ferrite core

Ferrite core

Winding

Winding
Pole width

Aluminum sheet

Core widthWinding widthWinding width(a) Double-sided winding transformer(b) Single-sided winding transformer

Fig. 5. Structures of transformers

IV. CHARACTERISTICS OF TRANSFORMER WITH
RECTANGULAR CORES
A. Specification
Table I lists the specifications of a 1.5 kW doublesided winding transformer and Fig. 6 shows a photographof the transformer. A gap length of 70 mm with nomisalignment is taken to be the normal position of thetransformer. Characteristics were measured for gap
lengths in the range ±20 mm, a misalignment in theforward direction of ±45 mm, and a misalignment in thelateral direction of ±125 mm.

Rated power

TABLE I
TRANSFORMER SPECIFICATIONS
1.5 kW

Gap length70±20 mm

Core

FDK 6H40

Litz wire

0.25ĳ × 24 × 16

Size
WeightWinding

B. Experimental Results
In the experiment, the power supply voltage (VAC =100 V) and the inverter frequency (f0 = 20 kHz) were kept
constant. A full-bridge rectifier and a resistance load wereconnected to the secondary winding. Fig. 7 shows aschematic of the electric circuit.
Fig. 8 shows the transformer parameters when the gap
length or position is varied. Fig. 9 shows the transformer
values when the gap length or position is altered and they
are also given in Table II. Fig. 5 depicts the misalignment
direction. Fig. 10 shows the efficiency as a function of
the resistance load and Fig. 11 shows the primary andsecondary waveforms at 1.5 kW.
As shown in Fig. 8, the coupling factor k decreased
when the gap length or misalignment was increased. Thechange in the value of the parallel capacitor CP
determined by Equation (1) was small because thesecondary self-inductance L2 was almost constant. The
values of the resonant capacitors CS and CP remained
constant when the various values were measured in Fig. 9.

809

Core

240 × 250 × 5 mm

Winding width

150 mmPrimary

4.4 kgSecondary

4.6 kgPrimary

1p × 18 turns

Secondary

2p × 9 turns

Aluminum sheet

400 × 600 × 1 mm

Fig. 6. Photograph of 1.5 kW transformer

The secondary voltage V2 (determined by Equation(3)) increased because the coupling factor k and the idealtransformer turn ratio b decreased when the gap length
was increased. The resistance load RL was adjusted to the
output power POUT = 1.5 kW when the transformer values
were measured as a function of the gap length.

The 2010 International Power Electronics Conference

VDC
VAC

+

VIN IIN

I2

V1

V2 IL

VOUT IOUT

CS

+

+

RL

CP

η

P1

P2

Fig. 7. Experimental circuit

0.3
k

l0

0.2

L2

20
50

60

0.2

0

30

x [mm]

0.4

0.2

20

L2

0

45

0.3

k

l0

40

0.1

15

0.5

l1

60

0.3

L2

20

90

k

l0

40

0.1
70
80
Gap [mm]

0.4

60

k

L [ǴH]

k

80

0.5

l1

k

0.4

60
40

80

L [ǴH]

0.5

l1

L [ǴH]

80

0.1
50
y [mm]

100

Fig. 8. Transformer parameters with change in gap length or position

B2
5050

60

70
80
Gap [mm]

V2

150

100

POUT 1
VIN

0.5

B2
500

90

ǯ

15

x [mm]

30

45

200

1.5
V2

150

100

POUT 1

VIN

ǯ

0.5

B2
500

50
y [mm]

POUT [kW], B2 [T]

0.5

ǯ

1.5

Voltage [V], ǯ [%]VIN

Voltage [V], ǯ [%]1

POUT [kW], B2 [T]Voltage [V], ǯ [%]

V2

150

100

200

1.5

POUT

POUT [kW], B2 [T]

200

100

Fig. 9. Transformer values with change in gap length or position
TABLE II
TRANSFORMER SPECIFICATIONSFrequency [kHz]2070x [mm]

045

y [mm]

0125

RL [ȍ]

23.1

50.0

VIN [V]*

112

110

V2 [V]*

139

200

VOUT [V]186

281

POUT [kW]

1.49

1.57

Ș [%]

95.3

90.2

B2 [T]

0.14

0.20

CS [ȝF]

0.696

CP [ȝF]

2.30

Efficiency [%]Gap length [mm]

100
90
80

Curve calculated using equation (4)Experimental result

70
60
0

10

20
30
RL [ǡ]

40

50

Fig. 10. Efficiency as a function of resistance load

misalignment increased.
In the misalignment test in the directions of x and y,
the power supply voltage VAC was 100 V, the resistanceload RL was 50 ȍ, and the gap length was 70 mm; theseparameters were kept constant. As shown in Fig. 9, theefficiency Ș was about 90% or higher. As Table 2 shows,
it is possible to transfer 1.5 kW and maintain thetransformer efficiency at over 90% even at the highest
misalignment (x = 45 mm, y = 125 mm).

* rms value

The input voltage VIN and the secondary voltage V2
almost satisfy Equation (3), even when the gap length
was varied. The efficiency Ș was 93.4% when the gap
length was 90 mm.
The voltage ratio (VIN/V2) changed when the positionor the gap length was varied. When the input voltage VIN
and the resistance load RL were constant, the secondary
voltage V2 and the output power POUT increased when the810

The 2010 International Power Electronics Conference

1 0 0V

V IN

V IN

V IN

10 A

I IN

I IN

I IN

1 0 0V

V2

V2

V2

2 0A

IL

IL

IL

0

0 .0 50.1[m s ]

00.0 5(a) Rectangular cores

0. 1[ ms ]

0(b) Split cores #1

0 .0 50 .1(c) Split cores #2

[m
m]
[ms]

240

Fig. 11. Wave forms

x ㅴⴕ: Forward direction
y
ᣇะ
x Ꮐฝ
ᣇะ
y : Lateral direction

unit: mm

250

250

250

(a) Rectangular cores(b) Split cores #1(c) Split cores #2

Fig. 12. Transformers with various corestransformer with split cores was developed by splitting
the ferrite core into separate sections, as shown in Fig.
12(b) and (c). The same coils for the rectangular cores
were used. Split core #1 is 40% (0.6 kg) and split core #2
is 60% (0.9 kg) lighter than the rectangular cores; thecross-sectional areas of these split cores were respectively
40% and 60% smaller than that of the rectangular core.
The core flux densities of split cores #1 and #2 are
expected to be 170% and 250% higher than that of therectangular cores, respectively.

The above results demonstrate that the rectangular
double-sided winding transformer has good tolerance tomisalignment. Fig. 10 reveals that there is good
agreement between the measured efficiency and that
calculated using Equation (4). As shown in Fig. 11, VIN
and V2 and IIN and IL were coherent. This demonstrates
that the transformer has the characteristics of an idealtransformer.
V. CHARACTERISTICS OF TRANSFORMER WITHSPLIT CORES

B. Experimental Results
Fig. 13 shows the variation in the parameters of thetransformers with split cores when the gap length orposition is changed. The coupling factor k was lower thanthat for the rectangular cores because the main flux path
had a higher magnetic reluctance. The change in thevalue of the parallel capacitor CP was small because thesecondary self-inductance L2 was almost constant evenwhen the gap length or position was changed.
The same experiment was performed as that for thetransformer with rectangular cores. When the transformer
characteristics were measured as a function of the gap
length, the power supply voltage VAC was 100 V and theresistance load RL was adjusted to give an output power
POUT of 1.5 kW. In the misalignment test in the directions

To reduce the weight and cost of the transformer withrectangular cores, we developed a transformer with split
cores. In the case of the transformer with rectangularcores, the flux density of the secondary core (B2) is higher
than that of the primary core and B2 is much lower than
the saturation flux density BS (= 0.53 T), as shown in Fig.
9. Thus, it is possible to reduce the amount of ferrite core
without reducing the power transfer performance.
A. Specifications
It is important when reducing the ferrite core to ensurethat the transformer performance does not decrease when
the gap length or position is varied. The external
dimensions of the transformer must not be alteredbecause the transformer performance depends on them. A

811

The 2010 International Power Electronics Conference

l1 0.4

60

20

L2

0.3
0.2

0.4

20

0.1

0.3

k

40

0.2
L2

0.5
l1

60

k

k

k

40

80

0.1

40

k

20

L2
Split cores #1

0.4
0.3
k

60

0.5

l1

L [ǴH]

80

L [ǴH]

0.5

L [ǴH]

80

0.2
0.1

Split cores #2

7080
Gap [mm]

30

0

45

50
y [mm]

ǯ

0.5

B2
70
80
Gap [mm]

1.5

200

V2
POUT

150

VIN

100

0.5

ǯ
B2

500

90

1

15

x [mm]

30

200

V2
POUT

150

ǯ

ǯ
5050

60

70
80
Gap [mm]

90

VIN
B2

100

0.5

ǯ
50

0

15

x [mm]

30

POUT [kW], B2 [T]

0.5

POUT 1150

50
y [mm]

100

200

V2

Voltage [V], ǯ [%]B2

POUT[kW], B2[T]

VIN

1

1.5

V2

200
Voltage [V], ǯ [%]Voltage[V], ǯ[%]

V2

150
100

1.5

45

0.5

B2
500

Fig. 14. Experimental results for transformer with split cores #1

POUT

1

VIN

100

45

1.5

1.5

POUT 1

150

VIN
B2

100

ǯ
50

0

POUT [kW], B2 [T]

VIN

200

100

50
y [mm]

POUT [kW], B2 [T]

POUT [kW], B2 [T]

1

150

60

x [mm]

POUT [kW], B2 [T]Voltage [V], ǯ [%]

V2

100

15

Fig. 13. Parameters of transformers with split cores

POUT 1.5

200

5050

0

90

Voltage [V], ǯ [%]60

Voltage [V], ǯ [%]

50

0.5

100

Fig. 15. Experimental results for transformer with split cores #2of x and y, the power supply power voltage VAC was 100
V, the resistance load RL was 59.8 ȍ (split cores #1) or
81.2 ȍ (split cores #2), and the gap length was 70 mm;
these parameters were kept constant. An experiment withthe transformer with split cores #2 at y =125 mm was not
performed since the required power exceeded the power
supply capacity. Figs. 14 and 15 respectively show theparameters of the transformers with split cores #1 and #2
when the gap length or position was altered; theparameters are also given in Table III. Fig. 10 shows theprimary and secondary waveforms at 1.5 kW.
Fig. 11 shows that the waveforms of the transformers
with split cores almost were the same as those of atransformer with rectangular cores. The flux densities ofthe secondary core B2 for split cores #1 and #2 werehigher than that of the rectangular cores, as shown in Figs.
14 and 15. The flux density B2 of split cores #1 was lower
than the saturation flux density BS, whereas the flux
density B2 of split cores #2 was close to BS. This indicatesthat that split cores #2 may have saturated.
C. Comparison of Performances of Transformers
The secondary voltage V2 of transformers with split
cores was higher than that of the transformer withrectangular cores because the coupling factor k was
smaller. The efficiency Ș decreased since the copper loss

812

TABLE III
EXPERIMENTAL RESULTS
Core type

Rectangular

Split #1

Frequency [kHz]20

Gap length [mm]70x [mm]

0y [mm]

Split #2

0

k

0.38

0.34

0.28

RL [ȍ]

23.1

26.5

40.3

VIN [V]*

112

111

106

V2 [V]*

139

147

178

VOUT [V]

186

195

245

POUT [kW]

1.49

1.54

1.47

Ș [%]
B2 [T]

95.3
0.14

94.1
0.24

0.45

CS [ȝF]

0.696

0.751

0.897

CP [ȝF]

2.30

2.46

3.04
* rms value

89.2

at the secondary winding became larger. As Table 3
shows, the reduction in the efficiency Ș of the transformer
with split cores #1 was small, whereas that of thetransformer with split cores #2 decreased by 89% due tosaturation of the secondary cores.

The 2010 International Power Electronics Conference
The flux density of the secondary core B2 was 170%
(split cores #1) or 320% (split cores #2) higher than thatof the rectangular cores. The reason why B2 for split cores#2 is much higher than expected must be the highsecondary voltage V2 due to low coupling factor k.
These results demonstrate that split cores have equal
performances as rectangular cores even when the gap
length or position is changed, provided the cores are notsaturated.

This research is sponsored by NEDO, New Energy andIndustrial Technology Development Organization in
Japan.

VI. CONCLUSIONSREFERENCES

length or the position is changed. In the future, we intend
to improve the design of cores and windings to develop atransformer that is more lightweight and has a higher
efficiency.
ACKNOWLEDGMENT

A contactless power transfer system suited for PHVs
and EVs is proposed. The configuration with capacitors
in the primary winding being in series and those in thesecondary winding being in parallel has an interesting
characteristic: its equivalent circuit is the same as an idealtransformer. Consequently, it 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 and tested. Its dimensions
are 240 mm×250 mm, its gap length is 70±20 mm, its
misalignment tolerance in the lateral direction is ±125mm, the mass of the secondary winding and core is 4.6
kg, and its efficiency is 95% in the normal position.
Using split cores reduces the size, weight, and costwithout reducing the performance even when the gap[1] A.W. Green and J.T. Boys: “10 kHz Inductively Coupled
Power Transfer – Concept and Control”, IEE PowerElectronics and Variable Speed Drives Conference, PEVD,
No.399, pp.694-699 (1994)[2] T. Fujita, Y. Kaneko, S. Abe: “Contactless Power Transfer
Systems using Series and Parallel Resonant Capacitors”,
IEEJ Trans. IA, Vol.127, No.2, pp.174-180 (2007) (inJapanese)[3] Y. Kaneko, S. Matsushita, Y. Oikawa, S. Abe: “Moving
Pick-Up Type Contactless Power Transfer systems andtheir Efficiency Using Series and Parallel Resonant
Capacitors”, IEEJ Trans. IA, Vol.128, No.7, pp.919-925(2008) (in Japanese)[4] M. Budhia, G.A. Covic, J.T. Boys: “Design and
Optimisation of Magnetic Structures for Lumped Inductive
Power Transfer Systems”, IEEE Energy Conversion
Congress & EXPO, pp.2081-2088 (2009)813