Experimental Verification of Rectifiers with
SiC/GaN for Wireless Power Transfer
Using a Magnetic Resonance Coupling
Keisuke Kusaka

Jun-ichi Itoh

Nagaoka University of Technology
kusaka@stn.nagaokaut.ac.jp

Nagaoka University of Technology
itoh@vos.nagaokaut.ac.jp

Abstract-This paper demonstrates that SiC and GaN dioderectifiers are used in a magnetic resonant coupling (MRC) for
wireless power transfer system. The size of the resonance coils,
which are used in the wireless power transfer using a MRC,
depends on the transmission frequency. So, the MRC is desired
to operate at a high frequency in the Industry Science Medical(ISM) band such as 13.56 MHz. In the receiving side, a rectifier
which converts to the DC voltage from high frequency AC
voltage is necessary to supply the power to the applications
such as a battery charger for EV and home appliances. The
experimental results show the maximum efficiency from aRadio Frequency (RF) power supply to DC outputs is 75.2%
when the transmission distance is 150 mm. In addition, A
power loss separation method of the wireless power transfer
system is discussed in this paper. The experimental results
verify the reflected power of the resonance coil which
dominates the largest amount of the loss in the total loss.
Therefore, the suppression of the reflected power is important
for the wireless power transfer system using a MRC.

(ISM) band of 13.56 MHz. It is important that the power
converter can operate in high frequency with high efficiency.
Generally, the output side of the power transmission
system using a MRC requires DC voltage to supply thepower to the applications such as a battery charger. However,
it is difficult to obtain DC voltage from high frequency input
voltage such as tens of Megahertz because the fast speed
diodes are needed.
On the other hand, wide band gap semiconductors are
increasingly studied to obtain the high speed switching andhigh power density of a converter [11][12]. A Silicon
Carbide (SiC) and a Gallium Nitride (GaN) offer
performance to improve over Silicon (Si) for the power
semiconductors. The feature of the switching speeds, thewide band gap semiconductors are suitable for wirelesspower transfer system using the MRC.
The experimental verification of the rectifier using the SiC
diodes has been demonstrated in [13]. However, thebehaviors of the transmission characteristics and power lossof the wireless power transfer system is not demonstrated
well when the rectifier is connected to the MRC system.
This paper discusses the output side converter of the MRC
power transfer system. The wide band gap semiconductors,
such as the SiC and GaN, are tested to use in the rectifier. In
addition, the power loss separation method is proposed inthis paper in order to clarify the power loss of the transfer
part and the rectifiers.

I.

INTRODUCTION

In recent years, the wireless power transfer systems are
increasingly studied [1-5]. Especially, the wireless powertransfer using a magnetic resonant coupling (MRC) which isreported in 2007 has been attracted in community [6-10].
The MRC has some advantages compared to others wirelesspower transfer methods such as the electromagnetic
induction and microwave power transmission. First, theMRC enables a wireless power transfer at middle range
distance such as 1 m in high efficiency of over 90%. Second,the decline in efficiency caused by position gap is smaller.
Thus, the MRC is suitable to apply in the battery chargers
for electric vehicle (EV). The transmitting device and thereceiving device are placed at the ground of the parking
areas, and back side of the EV respectively. The batteries of
the EV are charged automatically when the EV is parked tothe parking areas which have wireless power transfer system.
In the MRC system, the wire length of the resonance coildecreases commensurately with increasing transmission
frequency. The resonance coils must be mounted on the EV.
Thus, the resonance coils are desired to design with small
size and light weight. The wireless power transfer using a
MRC respects the standard of Industry Science Medical

II.

SYSTEM CONFIGURATION

A. Wireless power transfer Part
Fig. 1 illustrates the system configuration of a wirelesspower transfer using a MRC. The system consists of afunction generator (FG), a radio frequency (RF) power
supply matched to 50 Ω, two resonance coils and a load.
Signals from the FG are amplified by a Radio Frequency(RF) power supply. The RF power supply which can control
the output travelling power is composed of the A-class linear
amplifier in the test bench. The amplified power is supplied
to the resonance coils. The resonance coils are designed
according to the specification in Table 1. The resonancecoils which placed in the transmitting side, is called astransmitting coil. Also the resonance coil, which place in thereceiving side, is called as receiving coil. Additionally, theresonance coils have a same structure of the helical antenna.
Thus, the resonance coils have a feeding point at the centerof the resonance coil. In addition, the resonance coils have aself-resonance frequency f0 due to distributed capacitance Cof winding pitch and winding inductance L. The selfresonance frequency is obtain0ed by1
f0 =..................................................... (1).
2π LC
Fig. 2 shows the equivalent circuit of the wireless powertransfer using the MRC [14]. The equivalent circuit of the
one-sided resonance coil is presented as a series connected
LCR circuit. So, the one-sided resonance coil works as a
non-radiate antenna. The MRC have a similar equivalentcircuit to the magnetic induction. However, the MRC hassame capacitances in the primary side and secondary side.
The frequency characteristics of the transmitting efficiency
have a diphasic with a high quality factor Q due to inductive
coupling k between the transmitting side and receiving side.
Equation (2) presents the transmitting efficiency, where
S21(ω) is the transmission coefficient which is obtained by(3) [15], Lm is the mutual inductance, R is the simulated
resistance of the radiation loss and copper loss, Z0 is thecharacteristic impedance of the transmission line, and ω isthe transmission angular frequency.

Fig. 1. System configuration of a wireless power transfer. The FG and
RF power supply is matched to 50 Ω. The Transmitting and Receiving
coil is designed according Table 1. The load includes the rectifier
illustrated in Fig. 3.
Table 1. Specifications of resonance coils in the wireless powertransfer part. The resonance coils is helical antenna which has feeding
point in the center of coil.

Number of turn
Radius20 [cm]Vertical Height

9.9 [cm]

C

C

L

Z0

L

Z0
R

η (ω ) = S 21 (ω ) × 100 .......................................... (2)2S 21 (ω ) =6 [turn]
Magnet wire φ2.3[mm]MaterialR

2 jLm Z 0ω
2

⎧
⎧
1 ⎞⎫
1 ⎞⎫
⎛
⎛
2
L2mω 2 − ⎨ R + ⎜ ωL −⎟⎬ + 2 jZ 0 ⎨ R + ⎜ ωL −⎟⎬ + Z 0
C
C ⎠⎭
ω
ω
⎠
⎝
⎝
⎩
⎩
⎭

............................................................................. (3)The transmission coefficient is proportion of thetransmission power to travelling power. By substituting theresonance angular frequencies ωm and ωe into the equation,(4) is obtained.
S 21 (ω m ) =

2
2− j

1
kQ

, S 21 ( ω e ) =22+ j

1 .......................... (4)kQ

where the ωm and ωe are the resonance frequencies
between the transmission coils, the transmission coefficient
S21 has maximum value. Note that the ωe is larger than ωm.
An electromagnetic field distribution between thetransmitting coil and the receiving coil for the wirelesspower transfer is difference in terms of the resonancefrequency of ωe and ωm. Equation (4) indicates that the
product of inductive coupling k and quality factor Q affect tothe transmission efficiency. The inductive coupling k
decreases with increasing transmission distance. However,
the k·Q still remains high because of high quality factor Q.
Generally, the magnetic induction is operated with low
quality factor Q, therefore the magnetic induction cannot
delivers the power in the middle range transmission distanceat high efficiency.

(a) Equivalent circuit.
C

L-Lm

L-Lm

Lm

C

Z0

Z0
R

R

(b) Equivalent circuit of type-T.
Fig. 2. Equivalent circuit of the wireless power transfer using a MRC.
The equivalent circuit of the one-sided resonance coil is presented asseries connected LCR circuit. The primary side and secondary side arecoupled due to inductive coupling.

B. RectifierFig. 3 illustrates a diode rectifier using the SiC or GaN or
Si. The diode bridge rectifier is employed. The ratings ofthe power devices which have voltage rating of 600 V andthe current rating of several amperes are shown in Table 2.
Note that the schottky barrier diode (SBD) is chosen for
SiC, and the fast recovery diode (FRD) is chosen for Si. The
rectifier is mounted on the printed circuit board (PCB) in
terms of the high frequency operation. The rectifiers have an
input terminal of a coaxial connector. So, the rectifier isconnected using a coaxial cable which has characteristics

impedance of 50 Ω to operate in high frequency region suchas 13.56 MHz. In addition, the laminated ceramic capacitorof 0.46 μF is used as a smoothing capacitor in purpose of
improvement of high frequency characteristics.

follows the proportion of the reflection current to travellingcurrent. Therefore, the ratio of the travelling power toreflected power is squared reflection coefficient Γ2.
Additionally, the transmitted power is obtained bysubtracting reflected power from travelling power. If thecharacteristic impedance of the transmission line equals toimpedance of the load, reflection coefficient is zero.
Therefore, the reflected power does not occur. From basic
experimental result, it is clarified that the reflected power
which is not consumed at the load has to be suppressed.
PF

Fig. 3. The diode bridge rectifier. The input terminal is connected to
the receiving coil in Fig. 1 using a coaxial cable and coaxial
connector. Additionally, all components are placed on the printed
circuit board. The SiC or GaN or Si diode is used as power devices.
The laminated ceramic capacitor is used as a smoothing capacitor to
improve the high frequency characteristics.

Si-FRD

Rated Current[A]4
6
3

III. FUNDAMENTAL CHARACTERISTICS OF THE REFLECTED
POWER
Fig. 4 presents the experimental circuit which clarifies therelationship between the travelling power and the reflected
power. In Fig. 4, PF is the travelling power, PR is thereflected power and PLoad is the transmitted power whichequals to the consumed power at the load resistance. The
resistance load is connected to RF power supply directly
through the coaxial cable which has characteristic
impedance of 50 Ω. In the high frequency region, thereflected power occurs in boundary point of the impedance
when the impedance is mismatched.
Fig. 5 shows the characteristics of the reflected power andtransmitted power when the travelling power is 100 W in thecircuit drawn in Fig. 4. The reflected and transmitted powers
depend on the load resistance when the characteristic
impedance has a constant value. The consumed power isdecreased with increasing reflected power due to impedancemismatching. The theoretical formulas of the reflected
power and transmitted power are obtained by (5) and (6)respectively.
*
PLoad= PF 1 − Γ 2 ................................................. (5)

(

*
R

2

)

P = PF Γ ............................................................ (6)where Γ is the reflection coefficient which is expressed by;
Z − Z Load
Γ= 0.................................................... (7)Z 0 + Z Load
The reflection coefficient is decided by using thecharacteristic impedance of the transmission line Z0 and
impedance of the load ZLoad. The reflection coefficient Γ
indicates the proportion of the reflection voltage to travellingvoltage. On the other hand, the reflection coefficient Γ

Fig. 4. The experimental circuit which clarifies the relationshipamong a travelling, a reflected and transmitted power. The reflected
power occurs at the boundary point of the impedance.

120
100
Power [W]

Table 2. Ratings of the diodes in Fig. 3.
Diode
Rated Voltage [V]
SiC-SBD
600
GaN diode

PLoad

PR

Transmitted (consumed) power
*
PLoad

80
60
40

Reflected power

PR*

20
0

20

40

60

80

100

120

Resistance [ ]Fig. 5. Characteristics of the reflected and transmitted power. Notethat the travelling power is 100 W. The proportion of the reflected
power and transmitted power which is equals to consumed power ofthe load resistance depends on the load resistance when thecharacteristic impedance has constant value.

IV. POWER LOSS SEPARATION METHOD
In this section, the power loss separation method is
proposed. In the wireless power transfer system, the power
loss occurs in the transfer part and the rectifier part. The
power loss of the transfer part should be separated from lossof the rectifier part to clarify the characteristics of the MRC
system. However, it is difficult to measure the power loss inthe high frequency system because of the effects of themeasuring instruments such as a probe. In addition, highfrequency system has the reflected power. The proposed
power loss separation method estimates the each efficiency
from the travelling power and reflected power which isobserved in the RF power supply and output power of therectifier. If the power losses of the coaxial cables are
assumed to be neglected, the MRC system has four elements
which cause the power loss; that is the reflected power fromcoils, the reflected power from rectifier, the transmission andrectifier losses.

Fig. 6 shows a principle of the loss separation method of
the MRC system. Note that although the figure of thewireless power transfer in Fig. 6 is simplified to thetransformer, the structure of the wireless power transfer isthe same to the Fig. 1. The relationship between thetravelling power and the transmitted power is expressed by(5). Additionally, the relationship between the travelling
power and the reflected power is also expressed by (6). The
reflected power and the transmitting power occur at eachboundary point of the impedance; in front side of theresonance coils and in front side of the rectifier. If it isassumed that the power which is reflected twice at theboundary points of the impedance, is small enough toneglect, the relationships among the travelling power andreflected power are observed in the RF power supply side
and output power of the rectifier which are expressed by;⎧ PF 1 − Γc2 ηT 1 − Γr2 η rec = Pout............................. (8)
⎨
2
2 2
2
⎩ PF 1 − Γc ηT Γr + PF Γc = PR

(
(

) (
)

)

where Pout is the output power of the rectifier, PF is thetravelling power observed in the RF power supply side using
a power meter for high frequency region with directional
coupler, PR is the reflected power observed in the power
supply side, Γc is the reflection coefficient of the resonancecoils, Γr is the reflection coefficient of the rectifier, ηT is thetransmission efficiency, and ηrec is the conversion efficiencyof the rectifier. The reflection coefficient of the rectifier andconversion efficiency of the rectifier can be measured easily
if the rectifier is connected to the RF power supply directlyin the condition which has same output voltage of therectifier. Note that the conversion efficiency is measured
when the output voltage of the rectifier has the same value
for the wireless power transfer part is connected. The
reflected powers occur at the boundary point of the
impedance. The reflected power can be assumed as a power
loss because the reflected power is not fed to the load. So,
the proportion of the transmitted power to the travelling
power (1-Γc2) can be considered to the efficiency at the input
point of the resonance coils. On the other hand, theproportion of the transmitted power to the travelling power(1-Γr2) can be considered to the efficiency at the input of therectifier. It is found that the efficiencies of the rectifier andtransfer part are obtained to solve the simultaneous equationin terms of Γc and ηT. The transmission efficiency is
obtained by
Pout
ηT =
............................... (9).
PF 1 − Γc2 1 − Γr2 η recwhere the reflection coefficient Γc is presented by;(

PF + PR ±
Γc =

)(

)

⎧

(PF + PR )2 − 4⎪⎨PF PR −
⎪⎩

2
Pout
αr

(1 − α r ) η
2

2
rec

2 PF
........................................................................... (10)

⎫⎪
⎬
⎪⎭

(

PF

(

PF 1 − Γc2)2

PR = PF Γc2 + PF 1 − Γc2 ηT Γr2

(

)

) (

PF 1 − Γc2 ηT 1 − Γr2

(

)

(

) (

)

Pout = PF 1 − Γc2 ηT 1 − Γr2 η rec)

PF 1 − Γc2 ηT Γr2

Fig. 6. Relationship of the power for each part. The reflected poweroccurs at a boundary point of the impedance. So, the reflected powerincreases with increasing the square of reflection coefficient.

V. EXPERIMENTAL VERIFICATIONS
A. Operation Waveforms
Fig. 7 indicates the operation waveforms of the rectifiers,
which uses the SiC, GaN and Si diode when the power
supply frequency is 11.18 MHz and transmission distance is
150 mm. Note that, the resonance frequency does notmatched to ISM band of 13.56 MHz in these experiments
due to the relationship between design of coils and thetransmission distance. In addition, the vertical axis isdifferent among the SiC, GaN and, Si. The resonancefrequency depends on the transmission distance. The DC
voltage with the low voltage ripple is obtained when the SiC
and GaN diode are used as shown in (a) and (b) in Fig. 7.
However, the DC voltage has large voltage ripples when the
Si diode is used as shown in (c). The transmitting coilvoltage includes a large component of second harmonics
from the transmission frequency due to standing wave.
Generally, the standing wave becomes power loss in thetransmission line. So, the standing wave should be
suppressed, because the standing wave radiates theelectromagnetic wave from transmission line. The total
efficiency ηtotal which includes the reflection loss,transmission loss and conversion loss; of the wireless powertransfer system which consists of SiC, GaN and Si diodes
are 75.2%, 69.2%, 5.2% respectively. The experimental
results verified that the power device which has wide band
gap semiconductor, the SiC and GaN, are more suitable forthe wireless power transfer system.
B. Total Efficiency characteristics
Fig. 8 shows a relationship between the load resistance
RLoad and total efficiency ηtotal of the wireless power transfer
system. The total efficiency is a product of each componentsof the power loss; the ratio of the transmitted power totravelling power (1-Γc2), the ratio of the transmitted power totravelling power (1-Γr2), transmission efficiency ηT andconversion efficiency of the rectifier ηrec. Therefore, the total
efficiency is expressed byη total = 1 − Γc2 1 − Γr2 η Tη rec ...............................(11).

(

)(

)

The total efficiency using the SiC diode in the rectifier ishigher than using the GaN at total range of the load
resistance.
C. Power Loss Separation
Fig. 9 shows the stacked bar chart which indicates the loss
separation results of the wireless power transfer system
including the diode rectifier using the SiC and GaN diodes.
The SiC and GaN follow the similar characteristics. In theregion of the high load resistance, the conversion loss of therectifier using the SiC or GaN decrease with increasing load
resistance. The voltage drops of the diodes are decided ineach material. Therefore, the conduction loss of the diodes
decreases because the input current is reduced when the
input travelling power has configured with constant. On theother hand, the sum of the reflection loss of the resonancecoils and reflection loss of the rectifier is lowest value at theload resistance of 50 Ω. Since the characteristic impedanceof the power supply and the coaxial cable is matched in 50 Ω,
the characteristic of the sum of the reflection loss seems to
be caused by the impedance matching. The loss of thereflected power dominates a partial amount of the power loss
in a wireless power transmission system. The reflection loss
can be suppressed by a matching circuit. In the theoreticalconsideration, the reflection loss can be suppressed to zero.
So, the total loss can offer the improvement of the total
efficiency from 75.2% to 92.2% when the load resistance of
100 Ω and the switching devices are SiC diodes.

(a) SiC-SBD(b) GaN-diodeVI. CONCLUSION
This paper clarifies the characteristics of the rectifier
which is composed by the SiC and GaN diodes in a wirelesspower transfer. The MRC is desired to operate in highfrequency such as 13.56 MHz. So, the rectifier whichconverts to the DC voltage from high frequency AC voltage
is needed to supply the power to the applications such as abattery charger for the EV and home applications. Therefore,
the rectifier using the SiC and GaN power devices isrequired to obtain DC voltage at high frequency such as over
10 MHz. The experimental results confirmed the maximum
efficiency from a RF power supply to DC output is 75.2%
using SiC-SBD when the transmission distance is 150 mm.
In addition, the power loss separation method of thewireless power transfer system including the rectifier is
proposed to clarify the power loss. The loss separation
method is considered to focus attention on the reflection
coefficient. The experimental result demonstrated thereflected power of the rectifier and resonance coils dominate
the largest amount of the total loss. The reflected power canbe suppressed the impedance matching.
In the future work, the wireless power transfer systemusing the MRC, which present a higher efficiency due to the
suppression of reflection power, will be shown.

(c) Si-FRD
Fig. 7. Experimental waveforms when the rectifier is connected to thewireless power transfer using a MRC as a load. The travelling powerat the RF power supply is 100 W and the transmission distance is 150
mm. The rectifier using the SiC and GaN diodes can be obtain the DC
output voltage from high frequency AC voltage of over 10 MHz. The
rectifier using the Si diodes cannot operate in the high frequency
region.

80

SiC

60

GaN

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[2]

40
20

[3]

0

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100

[4]

Load resistance [ ]Fig. 8. The relationship between the load resistance and total
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[6]

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[7]

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[8]
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Transmission[9]20
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[11]
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[14]
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loss of the rectifier using GaN is larger than SiC. Therefore, Total
efficiency of GaN is lower than SiC.

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