Small-Size Light-Weight Transformer with New Core
Structure for Contactless Electric Vehicle Power
Transfer System
Masato Chigira*, Yuichi Nagatsuka*, Yasuyoshi Kaneko*, Shigeru Abe*, Tomio Yasuda**, and Akira Suzuki**** Saitama University, Saitama, Japan** Technova Inc., Tokyo, Japan*** AISIN AW CO.,LTD., Anjo, Japan
E-mail : kaneko@ees.saitama-u.ac.jp
We have developed a new 1.5kW transformer using anovel H-shaped core, which is more efficient, more robust to
misalignment, and lighter than previously employedrectangular cores, and thus, satisfies above mentioned
criteria. The specification and characteristics of this newtransformer are described here. This transformer at normal
air gap 70mm has high average efficiency of 94% within
misalignment of ±150mm in the lateral direction. Thistransformer is also small size, 240×300×40mm, and lightweight of 3.9kg in-vehicle side. The results of test at wide air
gap of 100mm, temperature rise test and 3kW operation test
are also presented.

Abstract— A contactless power transfer system for electricvehicles is required to have high efficiency, a large air gap,
good tolerance to misalignment in the lateral direction, and becompact and lightweight. A new 1.5 kW transformer has beendeveloped to satisfy these criteria using novel H-shaped cores,
which is more efficient, more robust to misalignment, andlighter than previously employed rectangular cores, and its
characteristics are described. An efficiency of 95% wasachieved across a 70 mm air gap. The results of test at wide air
gap of 100mm, temperature rise test and 3kW operation test
are also presented.

I.

INTRODUCTION

The development and commercialization of plug-in
hybrid electric vehicles (PHVs) and electric vehicles (EVs)are actively being realized, due to environmental concerns
and rise in oil prices. PHVs and EVs currently requireconnection to a power supply by electric cables for battery
charging. A contactless power transfer system (cf. Fig. 1)
would have many advantages, including the convenience ofbeing cordless and safety during high-power charging. A
contactless power transfer system for electric vehicles musthave high efficiency, a large air gap, good tolerance tomisalignment in the lateral direction and be compact andlightweight.

II.

A. Contactless Power Transfer System
Fig. 2 shows a schematic diagram of the contactless
power transfer system with series and parallel resonantcapacitors [7]. A full-bridge inverter is used as a highfrequency power supply. A double-voltage rectifier is used
as a rectifier circuit on the second side to raise efficiency.
B. Equivalent Circuit
Fig. 3 shows a detailed equivalent circuit of the system,with resonant capacitors CS and CP, rectifier, and a resistanceload RL. Primary values are converted into secondary
equivalent values by the turn ratio, a = N1/N2. Here, primes
are used to indicate converted values. The winding
resistances and the ferrite-core loss are considerably lower

Transformers with circular cores and single-sided
windings have commonly been used [1, 2]; however, wehave revealed that a transformer with rectangular cores anddouble-sided winding has advantages of good tolerance tomisalignment in the lateral direction, compactness andlightweight property [3, 4]. Boys and his group, pioneeringresearchers and developers of the inductive power transfer
systems, have presented that the weak point of a transformer
with circular cores for electric vehicles is small air gap andtolerance to misalignment [2]. They have also developed atransformer with double-sided winding of Flux pipe type tosolve this weak point[5, 6]. It is expected that a transformer
with double-sided winding for electric vehicles will be amain stream in the near future.

978-1-4577-0541-0/11/$26.00 ©2011 IEEE

CONTACTLESS POWER TRANSFER SYSTEM FOR EVSz : Vertical direction
x : Forward direction
y : Lateral direction

Figure 1.

260

Contactless power transfer system for an EV.

C. Resonant Capacitors
To achieve resonance with the self-reactance of thesecondary winding ω0L2, which is equivalent to adding amutual reactance x'0 (=ω0l’0) and a leakage reactance x2(=ω0l2), the secondary parallel capacitor CP is given by:1= ω0 L2 = xp = x0′ + x2 .
ω0 C P
Figure 2.

Contactless power transfer system.

(1)

The primary series capacitor CS (C'S denotes its secondary
equivalent) is determined as:

1ω0 CS′= xs′ =

x0′ x2+ x1′.
x0′ + x2

(2)This implied the power factor of the inverter output is 1, andthe efficiency of inverter becomes high with soft switching.
Figure 3.

D. Characteristics of an Ideal Transformer
The input voltage V'IN and the input current I'IN can be
expressed as follows [8]:Detailed equivalent circuit.

′ = IL b, b =VIN′ = bV2 = b VL , I INx′0.
x0′ + x2

(3)

This equation represents the equivalent circuit of atransformer with these capacitors, which is the same as an
ideal transformer with a turn ratio of b at the resonant
frequency.
E. Efficiency
Without rectifier circuit, the efficiency is approximatedby [6]:(a)η=

R L I L2=R L I + r1′I 1′ 2 + r2 I 22
2
LRL⎧⎪ ⎛ R ⎞ 2 ⎫⎪r′
R L + 12 + r2 ⎨1 + ⎜⎜ L ⎟⎟ ⎬b⎪⎩ ⎝ x P ⎠ ⎪⎭(4)

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

1 r1′
+1
b 2 r2

η max =

1
2r
1+ 2
xp

(5)

1 r1′
+1
b 2 r2

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.
F. Single and Double-sided Winding Transformers
The structure of contactless power transformers can beclassified into two groups: one is circular cores with singlesided winding [1, 2] and the other is rectangular cores withdouble-sided winding [3, 4, 5, 6]. The double-sided windingtransformers have a leakage flux at the back of the core, andconsequently they have low coupling factors k. To overcome
this problem, an aluminum sheet is attached to the back of
the core. The leakage flux is shielded by the aluminum sheet
and the coupling factor can be large [4]. The core width of
the single-sided winding must be doubled (winding width +pole width), whereas the core width of the double-sided(b)
Figure 4. Single and double-sided winding transformers. (a) Single-sided
winding transformer. (b) Double-sided winding transformer.

than the leakage and mutual reactance at the resonant
frequency. Therefore, the winding resistances (r'1 and r2) andthe ferrite-core loss r'0 are omitted in the circuit for analysis.
The rectifier is also omitted, and a secondary circuit foranalysis consists of CP and load resistance RL.

261

Furthermore, the coupling factor k of a single-sided
winding transformer becomes zero when the horizontal
misalignment is approximately about 40% of the core
diameter [2]. As shown in Fig. 4(a) of the single-sided
winding transformer, the magnetic flux of the primarywinding penetrates upward in the secondary winding at
position (A) of secondary winding. However, the magnetic
flux penetrates downward in the secondary winding at
position (B). Therefore, there is the position of secondary
winding in which the total flux penetrating secondary
winding and the coupling factor k become zero, between
positions (A) and (B).
In the case of the double-sided winding transformer (Fig.
4(b)), if the position of secondary winding is moved laterally
from (A) to (B), the total flux penetrating secondary winding
only decrease a little, and the decrease of coupling factor k issmall. Therefore, the double-sided winding transformer has
good tolerance to lateral misalignment.

45

Ferrite core

150

240

250

Winding

III.

A. Transformers with Rectangular Cores and H-shapedCores
A transformer with rectangular cores and double-sided
winding has many advantages such as good tolerance tomisalignment and compact and lightweight. We developed
some transformers with rectangular cores. Fig. 5 (a) shows a
1.5kW transformer for EVs and its characteristics arepresented in [4]. The gap length and misalignment
characteristic of this transformer depends on size of themagnetic poles and winding width between magnetic poles.
Even if the gap length and the winding width are kept thesame values, the coupling factor k does not change. To
reduce the resistance and the weight of windings, we
designed a transformer with H-shaped cores shown in Fig. 5(b).

Ferrite core(a)

The following effects can be expected from a transformer
with H-shaped cores.

300

150

240

40

150

•

Weight reduction by decreasing the length ofwindings and volume of ferrite cores.

•

Copper loss reduction by decreasing the length ofwindings.

•

Tolerance enhancement to misalignment in thelateral direction by increasing the length of magnetic
poles in lateral direction.

Ferrite corexWindingyFerrite core

COMPARISON OF CHARACTERISTICS OF
TRANSFORMERSThe power supply frequency is increased at the same time toimprove the transformer efficiency.

unit : mm(b)

Table I lists the design goal of a transformer with Hshaped cores. The misalignment of the forward direction iseasily limited by the use of wheel stopper, which help drivers
to position the forward direction of the PHV/EV for charging.

Figure 5. Transformer’s photographs and their dimensions.
(a) with rectangular cores. (b) with H-shaped cores.

winding needs only half of it. When a double-sided windingis used, the transformer can be made smaller than atransformer with a single-sided winding.

262

TABLE I.

DESIGN GOAL

Rated power
WeightGap length
Tolerance to
Misalignmentxy

TABLE II.

Table II lists the specifications of both transformers with
H-shaped cores and rectangular cores. Fig. 5 shows thephotographs and dimensions of these transformers. The
weight of a secondary transformer with H-shaped cores is
3.9kg and lighter than that with rectangular cores of 4.6kg.
The weight of winding part in a transformer with H-shapedcores is 2.0kg and lighter than that with rectangular cores of
2.9kg.

1.5kW
4.0kg
70±30mm±45mm±150mmSPECIFICATIONType

RectangularCore

FDK 6H20

Litz wire
Weight of thesecondary
Size
Primary
Winding
Secondary
Aluminum sheet

0.25mmφ×24×16

H-shaped core
FDK 6H20
TDK PC40
0.1mmφ×800

4.6kg

3.9kg

TABLE III.

In the contactless power transfer system for EVs, themisalignment of the transformer by the drivers’ skill and thechange of the gap length by the car weight can not be
avoided. A mechanical gap length of 70 mm with nomisalignment is taken to be the normal position. The
characteristics of transformers were measured for gap
lengths in the range ±30 mm, a misalignment in the forward
direction x of ±45 mm, and a misalignment in the lateral
direction y of ±150 mm.

240×250×40mm
240×300×40mm
1p×18T
3p×20T
2p×9T
9p×6T
400×600×1mm

The transformer parameters and the experimental resultsof the transformer with H-shaped cores compared with thatwith rectangular cores were shown in Table III and Table IV.
The resistances R and the inductances L when either primaryor secondary terminals of the transformer are opened or
shorted were measured with LCR meters, and thetransformer parameters were calculated from these values.
The vales of the resonance capacitors CS and CP for thetransformer with H-shaped cores and rectangular cores were
determined by (1) and (2) respectively. An aluminum sheetwith 1mm in thickness (400mm×600mm) was attached to theback of the transformer to shield the leakage flux.

PARAMETERS OF NORMAL POSITION(gap=70mm)Type
f0 [kHz]r0 [mΩ]
r1 [mΩ]
r2 [mΩ]
l0 [μH]l1 [μH]l2 [μH]L1 [μH]L2 [μH]kb
RLmax [Ω]ηmax [%]TABLE IV.

Rectangular
20
0.45
82.5
20.9
40.2
64.3
17.2
104
27.3
0.38
0.37
9.85
96.6

H-shaped core
30
0
106
9.30
55.4
115
9.70
170
14.7
0.33
0.34
8.69
97.9

B. Characteristics of Normal Position
Table III lists the transformer parameters of normal
position (mechanical gap length of 70mm). The maximum
efficiency ηmax of the transformer with H-shaped corescalculated by (5) is higher than that with rectangular cores
though the coupling factor k of the transformer with Hshaped cores is lower than that with rectangular cores. Thisis because the secondary winding resistance r2 of thetransformer with H-shaped cores is smaller than that withrectangular cores and the exciting reactance x0 (=2πf0l0) of
the H-shaped cores is large by increasing the number ofprimary windings and the frequency f0.

EXPERIMENTAL RESULTS(gap=70mm)Type
f0 [kHz]gap [mm]
x [mm]y [mm]RL [Ω]
VIN [V]V2 [V]VL [V]**
POUT [W]η [%]B2 [T]CS [μF]CP [μF]

Rectangular
20
70*
0
45
0
0
0
150
23
50
23
112
117
86.6
139
178
137
186
241
186
1489
1166
1528
95.3
91.9
93.1
0.14
0.18
0.14
0.696
2.30

0
0
168
128
346
1507
94.9
0.21

H-shaped core
30
70
45
0
0
150
80
143
128
129
129
346
346
1503
1506
93.7
93.0
0.21
0.22
0.189
1.91

Table IV lists the experimental results at the normal
position and Fig. 6 shows the primary and secondary
waveforms of the transformer with H-shaped cores. The
efficiency η (=P2/P1) of the transformer with rectangularcores was higher than that with H-shaped cores in Table IV.
The efficiency of the transformer with rectangular coresbecame 94.6% when the transformer covers was attached,
and the efficiency of the transformer with H-shaped cores of
94.9% was higher than that with rectangular cores. As shownin Fig.6, VIN and IIN and V2 were coherent. This demonstrates
that this transformer has the characteristics of an idealtransformer.

0 ***
0
226
182
491
3060
94.7
0.29

* If the transformer covers are put up to rectangular cores, the gap length becomes 60mm.
** VL is the output voltage value of either full-wave rectifier for Rectangular core or doublevoltage rectifier for H-shaped core.
*** Values when 3kW feeding power.

263

1

150

IIN

V2

100

200V

V2

50A

0.05
Time [ms]η(normal
0.5
100mm)(standardgap
gap100mm)
k

L2

(a)

0.1

250
Voltage [V], L2 [μH]

Figure 6.

50

040 50 60 70 80 90100110120130
gap [mm]ID

0

η(normal
70mm)(standardgap
gap70mm)

VIN

Wave forms of transformer with H-shaped core.

C. Characteristics with Change in Gap Length
Fig. 7 shows the transformer values of the transformer
with H-shaped cores when the gap length or position ischanged. The misalignment directions are shown in Fig. 5.
The mutual inductance l0 and the coupling factor k weredecreased when the gap length or misalignment wasincreased because magnetic reluctance of main flux path
became larger. In Fig. 7 (a), (b), and (c), the secondary selfinductance L2 was almost constant. As the value of theparallel capacitor CP determined by (1), the values of theresonant capacitors CS and CP remained constant during theexperiment.

200

POUT

1.5

η

VIN

1

150
V2

100

0.5

50

k

L2
0

POUT [kW], k, η

15A

200

1.5

POUT

POUT [kW], k, η

VIN

15

30

45

x [mm](b)

250
Voltage [V], L2 [μH]

The coupling factor k and ideal transformer turn ratio b
decreased when the gap length became larger, therefore, thevoltage ratio (V2 / VIN) increased, as indicated by (3). The
input voltage VIN was adjusted to give an output power POUT
of 1.5 kW. VIN and V2 roughly satisfied the relation of (3)though the gap length changed. The efficiency η decreased
from 95.2% to 93.1% when the gap length increased from
40mm to 100mm. The average value of the efficiency was
94.5%.

200

100

1.5

η

VIN

1

150
V2
k

50

0.5

L2
0

D. Characteristics with Change in Positions
Fig. 7 (b) and (c) show the transformer values when theposition (misalignment) is changed. The efficiency η of thetransformer with rectangular cores and H-shaped cores were
93.1% and 93.0% respectively at maximum misalignment in
the lateral direction y. The reduced efficiency of thetransformer with H-shaped cores compared with efficiency
of normal position was 1.9%, and smaller than that withrectangular cores of 2.2%. The transformer with H-shapedcores has good tolerance to misalignment in the lateral
direction by increasing the length of magnetic poles.

POUT

POUT [kW], k, η

200V

Voltage [V], L2 [μH]

250

50

100
y [mm]

150(c)
Figure 7. Experimental results for transformer with H-shaped core.
(a)with change in gap length. (b)with change in position x.
(c)with change in position y.

load changed. The output voltage VL was kept constantduring the test. In Fig. 8, dotted line shows the efficiency
calculated with (4) by using the resistance load RL correctedby voltage ratio (VL / V2) of the double-voltage rectifier [6].
There is good agreement between the measured efficiency
and the calculated efficiency. The changes in the input
voltage VIN and the secondary voltage V2 were small and theE. Characteristics with Resistance-Load Change
Fig. 8 shows the transformer values of the transformer
with H-shaped cores at normal position when the resistance-

264

250
POUT

1.5
VIN

150
100
50
0

Figure 8.

1V2
η(experimental value)η(calculated value)

50

0.5

POUT [kW], η

Voltage [V]200

increased from 70mm, 100mm to 130mm. Although theaverage value of the efficiency decreased to 91.9%, it ispossible to transfer 1.5 kW even at a gap length of 100 mm±30 mm.

2

B. Temperature Rise Test
The full charging time of 1.5kW with AC 100V isassumed about 4 hours and 10 hours or more for PHVs andEVs, respectively. Therefore, the temperature rise of the core
and the winding of the transformer with H-shaped cores
become more severe because the size can be smaller thanthat of the circular cores with single-sided windings
transformer. Fig. 9 shows the result of temperature rise test
for the transformer with H-shaped cores. The temperature of
the secondary core and winding became highest but weresaturated 3 hours later because of the thermal equilibrium.
The efficiency η has improved by about 1.0% compared tothe beginning of experiment. This is because the loss of theferrite has decreased with the temperature rise.

100 150 200 250
RL [Ω]

Characteristics with resistance-load change.

96ηC. Characteristics of 3kW Charge
The 3kW charging can decrease the charging time about
half compared to 1.5kW charging. As the current density of
the winding and the flux density in the core are not high, the3kW transfer test was performed. The efficiency η at normal
position was 94.7% as shown in Table IV.

95

80
Secondary core

Secondary winding

94

60
Primary core40

Primary windingη [%]Temperature [℃]

100

93

V.

Secondary aluminum sheet

20
0

The novel H-shaped cores transformer suited for PHVs
and EVs has been developed. A contactless power transfer
system using this transformer can exhibit good performancesuch as high efficiency, a large air gap, good tolerance tomisalignment in the lateral direction, and be compact andlightweight. A 1.5kW H-shaped cores transformer at normal
air gap 70mm has high average efficiency of 94% within
misalignment of ±150mm in the lateral direction. The
transformer is small size (240×300×40mm) and light weight(3.9kg). The test results at wide air gap of 100mm and 3kW
operation are also presented.

92
60 120 180 240 300 360 420 480
Time [min]Figure 9.

Temperature rise test.

voltage ratio (V2/VIN) was almost constant. This demonstrates
that this transformer has the characteristics of an idealtransformer.
IV.

CONCLUSIONACKNOWLEDGMENT

THE OTHER CHARACTERISTICS OF TRANSFORMERWITH H-SHAPED CORES

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

A. Characteristics in a Wide Gap
The characteristics of the transformer in a wide gap areof particular interest with respect to practical application of
the transformer for contactless power transfer. The minimum
ground clearance for PHVs and EVs is approximately 140
mm. The gap length was 100mm when the primary sidetransformer of 40mm in thickness was put directly on the
ground in a parking lot. The characteristics of a transformer
with normal gap length of 100mm were measured. The
values of the resonant capacitors CS and CP decided by theparameters of the transformer with normal gap length of
100mm again. The input voltage VIN was adjusted to give an
output power POUT of 1.5 kW. The coupling factor k and the
input voltage VIN were decreased when the gap length
became lager as shown in Fig. 7 (a). The efficiency ηdecreased from 93.5%, 92.1% to 89.7% when the gap length

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