電気学会論文誌●（●●●●●●●部門誌）
IEEJ Transactions on ●●●●●●●●●●●●●●●
Vol.●● No.● pp.●-●
DOI: ●.●●/ieejeiss.●●.●

Paper
Galvanic Isolation System with Wireless Power Transfer
for Multiple Gate Driver Supplies of a Medium-voltage Inverter
Keisuke Kusaka＊, Student member, Koji Orikawa, Member＊, Jun-ichi Itoh, Member＊a)
Isamu Hasegawa＊＊, Non-member, Kazunori Morita＊＊, Member, Takeshi Kondo＊＊, Non-member(Manuscript received Jan. 00, 20XX, revised May 00, 20XX)
In this paper, a gate driver supply, which supplies power to multiple gate drivers, is demonstrated. Robust isolation is requiredin the gate drive supplies of a medium-voltage inverter in order to drive high-voltage switching devices such as insulated-gate
bipolar transistors. The proposed isolation system achieves isolation with transmission coils mounted on printed circuit boards.
Furthermore, the isolation system transmits power from one transmitting board to six receiving boards. In the conventional
system, the number of receivers is limited to one. In contrast, multiple receivers are acceptable in the proposed system. These
characteristics help reduce the of the isolation system for the gate driver supplies.
This paper presents the fundamental characteristics of the isolation system. The equivalent circuit of the proposed system canbe derived by applying the equivalent circuit of a wireless power transfer system with a repeater coil. In addtion, a design method
for the resonance capacitors is mathematically introduced using the equivalent circuit. It is verified that an isolation system withmultiple receivers can be designed using the same resonance conditions as an isolation system with a single receiver.
Moreover, the isolation system is experimentally demonstrated. It is confirmed that the isolation system transmits power with amaximum efficiency of 46.9% at an output power of 16.6 W beyond an air gap of 50 mm with only printed circuit boards.
Keywords : Wireless power transfer, Inductive coupling, Isolation, Medium-voltage inverter, Gate driver1.

isolation. It can significantly downsize the isolation system.
However, it does not satisfy the safety standards of themedium-voltage inverter because the creepage distance is notenough.
In (11), galvanic isolation is assumed by an optical fiber. The
power for a gate drive is supplied through an optical fiber.
However, the transmitted power is limited to 100 mW per one
fiber.
Meanwhile, J. W. Kolar et al. have proposed an isolation systemusing printed circuit boards (PCBs)(12). It achieves galvanic
isolation with a coreless transformer. However, one transmittingside transmits power to only one receiving side one-by-one (1×1)
in this system. Thus, isolation systems are also required at eachgate driver supply. Therefore a number of isolation systems are
required in order to supply power. This prevents the downsizing ofisolation systems.
In this paper, the galvanic isolation system with multiple
receivers is demonstrated and analyzed based on an equivalentcircuit. The isolation system transmits power from one
transmitting board to six receiving boards (1×6) beyond an air gap
of 50 mm. The isolation system contributes to the cost reduction
and downsizing of the isolation system. Moreover, the isolation
with an air gap of 50-mm easily meets the standards for clearance
and creepage distance when the system voltage of the inverter is
6.6 kV. Moreover, the 50 mm air gap decreases the common-mode
current, which is induced by the high-dv/dt switching of a
medium-voltage inverter.
This paper first describes the system configuration of the
proposed isolation system. Secondly, the equivalent circuit of thewireless power transfer system with multiple PCBs is introduced.
The validity of the equivalent circuit model is evaluated byIntroduction

In recent years, the system voltage of a three-phasemedium-voltage inverter for industrial applications has risen to 3.3
kV or 6.6 kV(1–5). In a medium-voltage inverter, galvanic isolationis required at each gate driver supply. The standards of isolation,
which are published by the International ElectrotechnicalCommission (IEC), are well-known safety regulations. For
example, the safety standards of the IEC require a minimum
clearance of 14 mm and a creepage distance of 81 mm under thefollowing conditions: the working voltage of the inverter is 8.0 kV,
and the comparative tracking index (CTI) is 100 ≤ CTI < 400 witha pollution degree of 2(6). In general, isolation transformers withcores are commonly used for isolation. However, they cause anincrease in cost because isolation transformers are typicallycustom built. In addition, the isolation transformer is large in orderto obtain a high isolation voltage. For example, the typicaldimensions of the isolation transformer, which has an isolation
voltage of 20 kVrms for 10 s, are 200 mm × 200 mm × 200 mm at aweight of approximately 5.5 kg(7). These transformers must beplaced at each of the gate driver supplies.
In order to achieve the cost reduction and downsizing of theisolation system, a single-chip DC-isolated gate drive integrated
circuit (IC) has been proposed(8–10). It supplies power using a
microwave from the bottom layer of a sapphire substrate to the top
layer. In this method, the sapphire substrate assumes galvanica) Correspondence to: Jun-ichi Itoh. E-mail: itoh@vos.nagaokaut.ac.jp＊
Nagaoka University of Technology.
1603-1, Kamitomioka-machi, Nagaoka, Niigata, Japan 940-2188＊＊
MEIDENSHA CORPORATION.
515, Kaminakamizo, Higashimakado, Numazu, Shizuoka, Japan
410-8588© 200● The Institute of Electrical Engineers of Japan.

1Isolation System for Gate Drivers of Medium-voltage Inverter (Keisuke Kusaka et al.)comparing the F-matrix of the wireless power transfer system
between the three-dimensional (3-D) electromagnetic analysis andthe equivalent circuit model. Then, the evaluation of theequivalent circuit and the design method of resonance capacitors
for efficient wireless power transfer are mathematically clarified.
Finally, the experimental results are presented.

Galvanic isolation

Receiving boards#1

Transmitting board#0

C1

Proposed isolation system

C0

Medium voltage inverter

L0#2

2.1 System configuration
Figure 1 shows the conceptof the proposed isolation system for a medium-voltage inverter.
The isolation system consists of one transmitting board and six
receiving boards. The power consumption in the gate drivers is
supplied through the isolation system from a low-voltage power
supply of 24 V in a medium-voltage inverter.
Incidentally, a wireless power transfer system with a highquality factor Q of the transmission coils has been studied inrecent years(13–17). Wireless power transfer systems use resonancein order to compensate for leakage inductance because weakmagnetic coupling causes a large leakage inductance (14). Common
techniques are used in the proposed system. In the conventional
system, the transformer with a core prevents the cost reduction
and downsizing of the isolation system. In contrast, the isolation
system is constructed using only PCBs in this system. The PCBs
can be manufactured easily and inexpensively in comparison withthe transformer. This configuration contributes to a weight savingof the isolation system. The total weight of the isolation system forsix gate drivers is approximately 890 g.
In the isolation system, a transmission frequency of 2.0 MHz isused because a high-frequency transmission is required in order todecrease the required inductance for resonance. Moreover, theoperation at a high frequency above 2 MHz limits the selection of
MOSFETs.
Figure 2 shows the positional relationship of the transmittingboard and the receiving boards. The maximum size of the system
is constrained up to 300 mm × 150 mm × 150 mm due to the space
limitations of the medium-voltage inverter. The receiving boards
are placed at the top and bottom of the transmitting board. Eachdistance between the receiving boards and the transmitting board
is kept at a minimum of 50 mm for the galvanic isolation. It
contributes a robust galvanic isolation and a low common-mode
current through parasitic capacitances. It is enough to fulfill thesafety standards of the IEC (6) when the output voltage of themedium-voltage inverter is 6.6 kV. Note that an air gap of 50 mm
contains a sufficient margin towards productization.
The number of receiving boards can be increased by expanding
the transmission board. However, the expansion of thetransmission board causes an increase of the equivalent series
resistance of the transmission coil. It will decrease transmission
efficiency. Besides, the greatest common denominator of thenumber of switches for three-phase inverters is six. Thus, six
receiving boards are placed in this paper.
Figure 3 and 4 show the shapes of the transmitting coils andreceiving coils, respectively. Transmitting board #0 consists of ahigh-frequency inverter, a series resonance capacitor, and atransmitting coil for a wireless power transfer system. The inverter
is operated by a square wave operation with an output frequencyof 2 MHz. On the other hand, receiving boards #1–6 consist ofreceiving coils, series resonance capacitors, and diode bridge
rectifiers. The thickness of the copper film for both boards is 175
m.

24V

6.6 kVGate
driver#3

fsw = 2 MHz

····

#6
Supply of powerwith magnetic coupling

Fig. 1. Concept of the proposed isolation system.
Receiving boards

90

15

0

10

25

300 mm

#1

#2

15

0

50

#3

#0

150

Transmitting board

50

90

#4
50

#5
50

#6
50
Unit: [mm]Fig. 2. Placement of the transmitting board andreceiving boards. Each board is placed across an air gap
of 50 mm.

R10

150

43.5

18.6

300
245
231

29.5

2.

Proposed galvanic isolation system

Capacitors
1.0
1.0
InverterGate drive
Unit: [mm]circuits

Control
circuitComponent side.

150

(a)

Inverter(b)

Solder side.

Fig. 3. Schematics of the transmitting board.

2

IEEJ Trans. ●●, Vol.●●, No.●, ●●●Isolation System for Gate Drivers of Medium-voltage Inverter (Keisuke Kusaka et al.)

2.2 Evaluation of parasitic capacitanceIn this section,
the parasitic capacitance between the transmitting board and thereceiving boards is evaluated. The parasitic capacitances between
the transmitting board and the receiving boards should be
suppressed because parasitic capacitances reduce the isolation
performance. The parasitic capacitances among the boards
correspond to the capacitance between a primary winding and asecondary winding in a conventional isolation system with atransformer. In particular, since the silicon carbide (SiC)
MOSFETs have begun to be used in place of conventional
Si-IGBT, suppression of the parasitic capacitance is important. A
SiC-MOSFET will improve the efficiency of medium-voltage
inverter because a SiC-MOSFET greatly improves the switching
speed due to its high electronic mobility (18–19). Thus, a largecommon-mode current caused by high dv/dt flows into the control
circuit through the parasitic capacitance between the windings.
The suppression of the parasitic capacitances is strongly required.
However, it is difficult to calculate the exact parasitic
capacitance because the shapes of the conductors on thetransmitting board and the receiving board are complicated. The
measurement of the parasitic capacitances between the boards isalso difficult since the parasitic capacitances are very small. Thus,
the parasitic capacitance is analyzed by an electromagnetic
analysis (Agilent, ADS) under the worst-case assumptions thatboth the transmitting board and the receiving board are covered
with copper. The electromagnetic analysis shows that the parasitic
capacitances between the transmitting board and the receiving
boards with an air-gap of 50 mm are less than 4.4 pF. This showsthat the proposed isolation system decreases the parasitic
capacitance in comparison with the conventional isolation systemwith a transformer(20–21).

3.

45.25

29.25

50
45
30

90

R10

0.5
0.5

Capacitors

Rectifier

RectifierUnit: [mm](a)Component side.

(b)

Solder side.

Fig. 4. Schematics of the receiving board.
iinv

Lle0
C0

r0

N0:N1-6

Lm1

Lle1

r1 C1

Lle2

r2 C2

iin1 v Receiving board
in1
#1

k10
Lm2

iin2 vin2

#2

iin3

vin3

#3

iin4 v
in4

#4

vin5

#5

vin6

#6

k20

vinv

Transmitting
board
#0

Lle3

Lm3

r3 C3

k30
Lle4

Lm4

r4 C4

k40
Lm5

Lle5

r5 C5

iin5

Lle6

r6 C6

iin6

k50
Lm6
k60
Ideal transformer

Analysis of proposed isolation system

Fig. 5. Equivalent circuit of the proposed isolation
system without converters.

In this section, this paper first presents the equivalent circuit of
the multiple wireless power transfer system because the equivalentcircuit is required in order to design the resonance capacitance tocompensate for the leakage inductance. The validity of theequivalent circuit model is clarified by a comparison of theF-matrix between the equivalent circuit model and a 3-D
electromagnetic analysis. According to the equivalent circuit,resonance capacitors that compensate for the leakage inductanceare selected. It is shown that the resonance conditions are likewise
given for the isolation system with one receiver.
3.1 Derivation of equivalent circuit
The proposedsystem supplies power using a weak magnetic coupling from thetransmitting coil to the each receiving coils. For this reason, a
power factor from the view point of the output of the inverter isextremely low. In order to compensate the leakage inductanceusing resonances, series or parallel capacitors are commonly usedin the research field about a wireless power transfer(14). In thissystem, series-series compensation method is used. The resonancecapacitors (C0–6) are inserted in a series to the each coil. These
resonance capacitors should be selected in order to cancel out areactance from the view point of the output of the inverter. It
means that the output power factor of the inverter should be unity.
Figure 5 shows the equivalent circuit of the isolation system
where Lle0–6 are the leakage-inductances, Lm1-6 are the magnetizing
inductances, kij is the coupling coefficient between boards #i and#j, N0–6 are the numbers of turns, r0–6 are the equivalent series
resistances, and a is the turn ratio defined as N1–6 / N0. The methodof the derivation of the equivalent circuit is described in the next
paragraph. The magnetic coupling among the receiving coils canbe ignored because the magnetic coupling between the receiving
coils, which are placed in side by side such as the boards #1 and#2, is approximately k12 = 0.005. The magnetic coupling between
the coils #1 and #2 is three times smaller than the magnetic
coupling between the coils #0 and #1.
Figures 6(a) and (b) show a schematic and the equivalent circuitof a wireless power transfer system with repeater coil(22),respectively, where LA–C are the self-inductances of each coil. The
equivalent circuit of the proposed system shown in Fig. 5 isderived by applying the equivalent circuit of the repeater coil in(22–23). The repeater coil was invented in order to extend thetransmission distance(22–23). In the wireless power transfer systemwith a repeater coil, the power supply, which has the inner
impedance Z01, is connected to the coil A. The power is supplied
to the coil C through the coil B because the coil B is magneticallycoupled to both the coil A and coil B.
Figures 6(c) and (d) show the schematic and the equivalentcircuit of a wireless power transfer system with multiple receivers,
respectively. By interchanging the connection of the power supply
from coil A to coil B in Fig. 6(b), the equivalent circuit of thewireless power transfer system with multiple receivers is
3

IEEJ Trans. ●●, Vol.●●, No.●, ●●●Isolation System for Gate Drivers of Medium-voltage Inverter (Keisuke Kusaka et al.)introduced. Because the difference of the wireless power transfer
system with a repeating coil and the multiple receivers are only
the connection of the power supply. Thus the equivalent circuit ofa wireless power transfer system with multiple receivers is shownin Fig. 6(d). It is notable that the magnetizing inductances LAB and
LBC are connected in series to the power supply VB with the inner
impedance of Z02 in the wireless power transfer system withmultiple receivers.
Since the magnetizing inductances are connected in series, theequivalent circuit of the proposed system shown in Fig. 5 isdetermined. The coil B in Fig. 6(d) corresponds to the transmitting
coil #0. The coils A and B correspond to the receiving coils #1-6.
The power is transmitted from the transmitting coil to the six
receiving coils with the magnetizing inductances, which isconnected in series from the view point of vinv as shown in Fig. 5.
3.2 Evaluation of the equivalent circuit
In order toevaluate the validity of the equivalent circuit, the F-matrix(ABCD-matrix) of the frequency characteristics (which is definedby Eq. (1)) analyzed by 3-D electromagnetic analysis (Agilent,
Momentum) is compared with the F-matrix calculated by theequivalent circuit. The F-matrix shows the relationship of a system
between the input port i and output port j based on voltage andcurrent. The suffixes i and j indicate the number of PCBs (i, j = 0,1, ∙∙∙, 6). For simplicity, the F-parameter from one board #i toanother board #j is indicated as F(i, j).

Transmitting coil Repeating coil Receiving coil
Z01

A

B

Z03

C

VA
(a)

Wireless power transfer system with repeating coil.

C

LA-LAB

VA

R

LB-LAB-LBC

C

LAB

LBC

LC-LBC

C

Z01

(b)

Z03

R

R

Equivalent circuit of a repeating coil without cross coupling
between the coil A and coil C.

Transmitting coil Repeating coil Receiving coil
A

Z01

B

C

Z03

VB

Z02

(c)

Wireless power transfer system with multiple receivers.

C

LA-LAB

VA

R

Z01

LB-LAB-LBC

C

LAB

LBC
R

VB

LC-LBC

C

Z03

R

Z02

V   A Bij Vj Vi    Fi, j  j    ij  I 
 I j   Cij Dij  I j  .......................... (1)
 
 i
  

(d) Equivalent circuit of a multiple wireless power transfer system
without cross coupling between coil A and coil C.

Fig. 6. Equivalent circuit of the wireless power transfer
system with a repeater coil(22) and the wireless powertransfer system with multiple receivers.

Figure 7 shows the definitions of the F-parameters in the
proposed system. In this evaluation, only the characteristics of thecoils are evaluated. Thus, the ideal power supply with resistancesof 50 for the inner impedance is connected instead of theconverters. The inner impedance is necessary because theF-parameters are calculated from the analysis results of thescattering parameter (S-parameter) by a simulator. Moreover, theresonance capacitors are omitted because the evaluation of theequivalent circuit of the transmission coils is the main purpose ofthis section.
Figure 8 shows each of the F-parameters between thetransmitting board and receiving board #1. The solid line shows
the F-parameter analyzed from 3-D electromagnetic analysis. The
dashed-dotted line shows the F-parameter calculated from theequivalent circuit. The dotted line shows the F-parameter
calculated from the equivalent circuit with correction coefficients,
which are mentioned later. The F-parameter with the 3-D
electromagnetic analysis has a self-resonance at a frequency ofapproximately 7 MHz. However, the equivalent circuit model does
not consider self-resonance because the self-resonance frequency
is far from the operating frequency. This is one of the reasons forthe error between 3 MHz and 10 MHz.
Moreover, the F-parameter with the equivalent circuit hasoffsets of approximately 10 dB between 100 kHz and 3 MHz. The
reason for the error is the difference between the numbers of theactual windings and effective windings. This means that a portionof the windings is not effective to induce the voltage. This is a
specific problem when a transmitting coil is placed on PCBs. The
difference can be corrected using a correction coefficient . The
corrected turn-ratio a´ of the transformer is represented as Eq. (2)using the correction coefficient .

Receiving board
F(0,1)

Coil

Transmitting board F(1,0)#0F(1,1)50

#1
F(1,2)

F(2,1)
F(2,0)
F(0,0)

Coil50

F(0,2)#2

F(0,3)#3

F(3,0)

F(4,0)
F(0,4)
F(5,0)#4#5

F(0,5)
F(6,0)
F(0,6)#6

Fig. 7. Definitions of the F-parameter in the isolation
system.

a  

N1....................................................................... (2)N2The dotted line shows the F-parameter with the equivalentcircuit considering the correction coefficient in Fig. 8. The
correction coefficients, which are derived by trial and error, are1–6 = 2.26, 2.28, 2.27, 2.26, 2.28, and 2.27, respectively, in the
prototype. The role of the correction coefficients  is tocompensate the difference between the actual number of turns andthe effective number of turns. Induced voltages at coils are notproportional to the actual number of turns because the radii of the
4

IEEJ Trans. ●●, Vol.●●, No.●, ●●●Isolation System for Gate Drivers of Medium-voltage Inverter (Keisuke Kusaka et al.)




V 
1 
1  1 r2  R2   j Lle 2  Lm 2 r6  R6   j Lle 6  Lm6 Iinv  inv r1  R1   j Lle 1  Lm1 




 
C1 
C2  
C6 




1 

r0  j  Lle 0  Lm1  Lm 2    Lm 6 
C0 
 jLm1

C1 

 jLm1

r1  R1  j Lle 1  Lm1  1 

 jLm 20 jLm 60 jLm 2 jLm 60

0

r2  R2   j Lle 2  Lm 2  1  

0

1



1

 2 Lle 16  Lm16   2 L16







(4)





r6  R6   j Lle 6  Lm6  1 

0



120

3-D model (Agilent momentum)
Equivalent circuit without correction coefficient 
Equivalent circuit with correction coefficient 

80

B10

40

A10

0

C6 



D10
C10

-40
Self resonance110Frequency f [MHz]Fig. 8. F-matrix between the transmitting board #0 andreceiving board #1.
provided by Eq. (3), can be simplified as
6


V 
1 
 .............. (9),Iinv  inv r1  R1   j Lle 1  Lm1  C1 
where  is




1 
1 




  r0  j  Lle 0  6Lm1 r1  R   j  Lle 1  Lm1 C 0 
C1 






1 


6 2 L2m r1  R   j  Lle 1  Lm1 
C1 





6

5

.............................................................................................. (10).
Figure 10 shows the vector diagram under the resonanceconditions. For simplicity, all of the parameters on the receivingsides (including the load resistance) are assumed to be the sameamong the receiving boards. Moreover, the fundamental
components are only considered here. The output voltages Vin1–6 isexpressed by Eq. (11) when all of the parameters on the receiving
boards and coupling coefficients are the same. Note that it iseffective only when the AC resistance loads are connected to thetransmitting coil directly, not the rectifiers.

.................................. (6)The output current of the inverter is developed with theresonance conditions as Eq. (7) and Eq. (8), which clearly showthat the unity input power factor of the load is achieved by theresonance.

Vin16   re  Vinvr1  R1 r2  R2 r6  R6  .... (7)Iinv   re    re 

5jLm1 R1r1  RV
6
5 inv (11)r0 r1  R1   6 2 L2m1 r1  R1 

4. Experimental verifications   re   r0 r1  R1 r2  R2  r6  R6   2 L2m1 r2  R2 r3  R3  r6  R6   2 L2m 2 r1  R1 r3  R3  r6  R6 

C2 

-80
0.1

1
1
C0  2 2 ................... (5) Lle 0  Lm1  Lm 2    Lm6   L0
C16 

F-parameters [dB]receiving coils are not constant when a spiral coil is used. Thus amagnetic flux partially interlinks across the receiving coils. It is
specific characteristic when a spiral coil without magnetic core isused. For this reason, the correction coefficients are necessary in atime-domain analysis in order to analyze the circuit performance.
The F-parameter of the equivalent circuit considering thecorrection coefficient shows agreement with one of theelectromagnetic analyses between 100 kHz and 3 MHz.
3.3
Design of resonance capacitors
Figure 9 showsthe equivalent circuit of the proposed system with the converters.
In the following calculation, the rectifiers are assumed as load
resistances R1–6 for simplicity. The resonance capacitors aredesigned to cancel out the reactance owing to the leakage
inductance. From the equivalent circuit, the output current of theinverter is expressed as Eq. (3), where  is Eq. (4). The prime
parameters are the parameters referring to the primary side.
In order to correct the load power factor, the reactance of the Eq.
(3) has to be zero. Thus, the resonance conditions are derived as
Eq. (5) and (6), where  = 2f is the output angular frequency ofthe power supply, and L0-6 are the self-inductances of the coils.
Equations (5) and (6) indicate that the resonance capacitors should
be selected to resonate with each self-inductance because the sumof the resonance leakage inductance and the magnetizing
inductances is equal to each self-inductance. This means that theresonance conditions of the isolation system with multiple
receivers are the same as those of the isolation system with one
receiver.

(3)

4.1 System setup of the prototype
Figure 11 shows aphotograph of the isolation system prototype. The transmittingboard, which is shown in Fig. 3, and the receiving boards, which
are shown in Fig. 4, are placed as shown in Fig. 2. The chassis is
made from acrylic except for clinchers because eddy current
losses should be suppressed.
Table I shows the specifications of the prototype. The laminated.................. (8)  2 L2m 6 r1  R1 r2  R2  r4  R4 r5  R5 

If all of the parameters on the receiving boards and the coupling
coefficients are the same, the inverter output current, which is
5

IEEJ Trans. ●●, Vol.●●, No.●, ●●●Isolation System for Gate Drivers of Medium-voltage Inverter (Keisuke Kusaka et al.)ceramic capacitors for resonance are selected to satisfy theresonance conditions shown as Eq. (5) and (6). Note that the
coupling coefficients between the boards are simulated values
because it is difficult to measure the coupling coefficientaccurately.
4.2 Fundamental characteristics
Figure 12 represents
the operation waveforms of the proposed isolation system. Notethat the resistances of 38  are connected at the outputs of thesystem instead of the gate drivers for simplicity. The inverter isoperated at a frequency of 2.0 MHz. The DC voltages are obtained
as the outputs of receiving board #1. The other boards obtained
DC voltages, similarly. In theory, the input voltage of vin1-6 is 90
degrees phase lead to the inverter output voltage vinv according toeq. (11). However the phase difference between these voltages issmaller than theoretical calculation. It is caused by the error
between the resonance frequency of each board and thetransmission frequency because the ceramic capacitors have large
tolerances on capacitances. In order to overcome this problem,
high-accuracy capacitors with low temperature coefficient should
be used.
Figures 13 and 14 show the output voltage and output power
characteristic of the proposed isolation system. Each output
voltage increases gradually with an increase in load resistance.
The output voltages of the receiving boards #2 and #5 arehigher than the others because the coupling coefficients areapproximately twice the values of the others. The different
coupling coefficients are caused by the positional relationship of
the receiving boards. For this reason, the output power is also
different with each board. Because the output voltage depends on
the load, voltage regulators are necessary in the gate drive circuits.
At light load, the output voltage is too low to drive the voltage

regulators. Thus, it is necessary to consume power above a certain
level in the gate drive circuit.
Figure 15 represents the measured total efficiency of the
proposed system. The total efficiency is defined by Eq. (12),where Pin is the input DC power of the isolation system. The total
efficiency is the ratio of the input power and the sum of the output
power for all of the receiving boards. Note that the same resistanceloads are connected to all of the receiving boards as a load.
..................................................... (12) Poutn n 1, 2 ,, 6

 100
Pin

Receiving board #1

Lm1
C0

r0

Lle0

#4
Transmitting coil

#6

High-frequency Inverter

Receiving coil

Input voltage (DC)

VDC

24

V

Switching frequency

fsw

2.00

MHz

Transmitting coil

L0

20.3

H

Self-inductances
Receiving coils

L1-6

14.8

H

EquivalentTransmitting coil

R0

790

m

series resistances

Receiving coils

R1-6

690

m

Transmitting board #0

C0

310

pF

Receiving boards #1-6

C1-6

430

pF

capacitances

VDC1

Receiving board #1

k01

0.017

Coupling coefficients

Receiving board #2

k02

0.025

#2

(between the receiving

Receiving board #3

k03

0.018

∙∙∙

iinv

vin1

#5

Table 1. Specifications of the prototype.

R
C1

vms1
a´=1 N0
N1

#3

Fig. 11. Prototype of the isolation system.

vle1 vr1 vc1 iin1
Lle1 r1

vm1#2

Transmitting board #0#1
vc0 vr0 vle0

RectifierGate drivers for high-frequency inverter

coils and transmitting

Receiving board #4

k04

0.017

#6

coil)

Receiving board #5

k05

0.025

Receiving board #6

k06

0.017

vinv
Lm2

∙∙∙
Lm6

Fig. 9. Equivalent circuit with the power converters.
ImVVms1  m1
a

qm
Vle0

Vm1

Vm2

Vm4
Vm 3

Vm5

Vm6

Inverter output voltage vinv [50V/div]

Vr 0

0
Input voltage of rectifier #1 vin1 [10V/div]

Vle1

 Vr10Vc 0

Vin1

Vc1

Input voltage of rectifier #2 vin2 [10V/div]

0

r R

q m  tan 1 1
Lm

İinv

Vinv

Output voltage of receiving board #1 VDC1 [5V/div]

Re

O

0

Fig. 10. Vector diagram of the proposed isolation system
under the resonance conditions.

200[ns/div]

Fig. 12. Operation waveforms of the proposed systemwith resistance loads of 38 .

6

IEEJ Trans. ●●, Vol.●●, No.●, ●●●Isolation System for Gate Drivers of Medium-voltage Inverter (Keisuke Kusaka et al.)The maximum efficiency is 46.9% at an output power of 16.6 W.
In the isolation system for a medium-voltage inverter, lowefficiency can be acceptable because the power loss in theisolation system is considerably smaller than the rated power of a
medium-voltage inverter (i.e., 1 MW). Thus, the isolation system
is designed to give priority to the isolation performance over theefficiency.
4.3
Operation with gate drivers
In this subsection,
the prototype with the gate drivers is tested.
Figure 16 shows the experimental setup for the experiments ofthe proposed isolation system with the gate drivers. The gate
drivers (which are operated at a switching frequency of 10 kHz atgate-emitter voltages of ±15V) and the IGBTs (Cge = 1800 pF) areconnected as a load. Note that DC/DC converters are connected asvoltage regulators at the front end of the gate drivers. The DC/DC
converters output voltages of ±15V. Moreover, photocouplers areused in order to drive the IGBTs despite the deficient isolation
distance because the isolation of the PWM signal is not a main
topic of this paper.
Figure 17 shows the operation waveforms with the IGBTs. Even
if the gate drivers and IGBTs are connected as a load, theoperation system achieves wireless power transfer beyond an air
gap of 50 mm.
From the experimental results, it is confirmed that the proposedsystem can be used as an isolation system for a medium-voltage
inverter.

Output voltage VDC(n) [V](n = 1, 2, ∙∙∙, 6)

20
Receiving board #5

15
10

:#4
:#1

#6

#2

#3

5
0

10

20

30

40

50

Load resistances R []Output power Pout(n) [W](n = 1, 2, ∙∙∙, 6)

Fig. 13. Output voltage characteristic of the proposed
isolation system. The output voltage increases with anincrease in load resistance.

6
5

Receiving board #5

4

#2

3
2

#6

#4
#1

#3

1
0

10

20

30

40

50

Load resistances R []5. Conclusion

Fig. 14. Output power characteristic of the proposed
isolation system.

A conventional galvanic isolation system for a gate driver
supply with PCBs has been developed. However, the cost
reduction of the conventional system is limited because thetransmitting side transmits power to only one receiver.
In this paper, the equivalent circuit of the multiple gate driver
supplies using PCBs and resonant conditions were discussed forwhen the number of receivers increases from one to multiple. The
isolation system transmits power from one transmitting board to
six receiving boards. Due to this characteristic, the cost of the gate
driver supplies can be decreased.
Firstly, an equivalent circuit model was derived and evaluatedbased on the equivalent circuit of the repeating coil system. It was
confirmed that the F-parameters of the equivalent circuit, which
has six magnetizing inductance in series in the transmitting side,shows good agreement with the analysis results of 3-D
electromagnetic analysis. The resonance conditions are derived
based on the introduced equivalent circuit. Then, the isolation
system was experimentally demonstrated. Finally, theexperimental results showed that the prototype transmits power
beyond an air gap of 50 mm using resonance. Moreover, theisolation system achieves a maximum efficiency from an input DC
to output DC of 46.9% at a summed output power of 16.6 W.

Efficiency  [%]50
40

R = 45 

30
20
10

R=0

0

5

10

15

20

Poutn [W]
Total output powern1
, 2,, 625

Fig. 15. Measured efficiency characteristic of the
proposed system. The same resistance loads areconnected to all of the receiving boards as loads.

7

IEEJ Trans. ●●, Vol.●●, No.●, ●●●Isolation System for Gate Drivers of Medium-voltage Inverter (Keisuke Kusaka et al.)Receiving boards#1
vDC1
Transmitting board#0(3)

K. Sano and T. Masahiro : “A Transformerless D-STATCOM Based on a
Multivoltage Cascade converter Requiring No DC Sources,” IEEE
Transactions on Power Electronics, Vol. 27, No. 6, pp. 2783-2795 (2012)

vge1

DC G1
-15V
PWM signal#2

S4+15V Photocoupler
DC

C1
vin1

Vol. 24, No. 3, pp. 796-803 , March 2009

Gate drive unite 1

(4)

G1

I. Shigenori and H. Akagi : “A Bidirectional Isolated DC-DC Converter as aCore Circuit of the Next-Generation Medium-Voltage Power Conversion
System,” IEEE Transactions on Power Electronics, Vol. 22, No. 2, pp.

GDU 2535-803 (2009)24V

fsw = 2 MHz

#3

(5)

GDU 3

····

····

#6

GDU 6

(6)

S. Dieckerhoff, S. Bernet and D. Krug : “Power Loss-Oriented Evaluationof High Voltage IGBTs and Multilevel Converters in Transformerless
Traction Applications,” IEEE Transactions on Power Electronics, Vol. 20,
No. 6, pp. 1328-1336 (2005)
International Electrotechnical Commission (IEC) : “Adjustable speed
electrical power drive systems – Part 5-1: safety requirements – Electrical,

Fig. 16. Experimental setup with gate drivers.

thermal and energy,” IEC 61800-5 (2007)(7)

Inverter output voltage vinv [50V/div]

Christoph Marxgut, jurgen Biela, Johann W. Kolar, Reto Steiner and Peter
K. Steimer : “DC-DC Converter for Gate Power Supplies with an Optimal
Air Transformer,” in Proc. Applied Power Electronics Conference and0

Exposition 2010, pp. 1865-1870 (2010)(8)

Input voltage of rectifier #1 vin1 [25V/div]

S. Nagai, N. Negoro, T. Fukuda, N. Otsuka, H. Sakai, T. Ueda, T. Tanaka
and D. Ueda : “A DC-isolated gate drive IC with drive-by-microwave

0

technology for power switching devices,” in Proc. International Solid-State
Circuits Conference 2012, pp. 404-406 (2012)

Output voltage (#1) VDC1 [10V/div](9)

S. Nagai, T. Fukuda, N. Otsuka, D. Ueda, N. Negoro, H. Sakai, T. Ueda and
T. Tanaka : “A one-chip isolated gate driver with an electromagnetic0Gate-emitter voltage (#1) vge1 [25V/div]

resonant coupler using a SPDT switch,” in Proc. 24th IEEE International

0

Symposium on Power Semiconductor Devices and ICs 2012, pp. 73-76(2012)

100[s/div](10) S. Nagai, N. Negoro, T. Fukuda, H. Sakai, T. Ueda, T. Tanaka, N. Otsuka
and D. Ueda : “Drive-by-Microwave technologies for isolated direct gate(a) Gate-emitter voltage with proposed isolation system.

drivers,” in Proc. IEEE Microwave Workshop Series on Innovative WirelessInverter output voltage vinv [50V/div]

Power Transmission: Technologies, Systems, and Applications 2012, pp.
267-270 (2012)0(11) M. Ishigaki, H. Fujita : “A Resonant Gate-Drive Circuit for Fast
High-Voltage Power Semiconductor Devices with Optical Isolation of Both

Input voltage of rectifier #1 vin1 [25V/div]

Control Signal and Power Supply,” IEEJ Trans. On Industry Applications,0Vol. 129, No. 3, pp. 303-310 (2009) (in Japanese)(12) R. Steiner, P. K. Steimer, F. Krismer and J. W. Kolar : “Contactless Energy

Output voltage (#1) VDC1 [10V/div]transmission for an Isolated 100W Gate Driver Supply of a Medium Voltage
Converter,” in Proc. Annual Conference of the IEEE Industrial Electronics

0Gate-emitter voltage (#1) vge1 [25V/div]

Society 2009, pp. 302-307 (2009)(13) A. Karalis, J. D. Joannopoulos and M. Soljacic : “Efficient Wireless0

non-radiative mid-range energy transfer,” Annals of Physics, Vol. 323, No.

200[ns/div]

1, pp. 34-48 (2007)(14) S. Y. R. Hui, Y. X. Zhong and C. K. Lee : “A Critical Review of Recent(b) Expanded operation waveforms of (a).

Progress in Mid-Range Wireless Power Transfer,” IEEE Trans. On Power

Fig. 17. Operation waveforms of the proposed isolation
system with the gate drivers.

Electronics, Vol. 29, No. 9, pp. 4500-4511 (2012)(15) M. Budhia, G. A. Covic and J. T. Boys : “Design and Optimization of
Circular Magnetic Structures for Lumped Inductive Power Transfer
Systems,” IEEE Trans. On Power Electronics, Vol. 26, No. 11, pp.
3096-3108 (2011)(16) T. Imura and Y. Hori : “Maximizing Air Gap and Efficiency of Magnetic
Resonant Coupling for Wireless Power Transfer Using Equivalent Circuit

References(1)

and Neumann Formula,” IEEE Trans. On Industrial Electronics, Vol. 58, No.
10, pp. 4746-4752 (2011)

N. Hatti, Y. Kondo and H. Akagi : “Five-Level Diode-Clamped PWM(17) S. Lee and R. D. Lorenz : “Development and Validation of Model for

Converters Connected Back-to-Back for Motor Drives,” IEEE Trans. On

95%-Efficiency 200-W Wireless Power Transfer Over a 30-cm Air-gap,”

Industry Applications, Vol. 44, No. 4, pp. 1268-1276 (2008)(2)

IEEE Trans. On Industry Applications, Vol. 47, No. 6, pp. 2495-2504 (2011)

N. Hatti, K. Hasegawa and H. Akagi : “A 6.6-kV Transformerless Motor(18) T. Nakanishi and J. Itoh : “Evaluation of Control Methods for Isolated

Drive Using a Five-Level Diode-Clamped PWM Inverter for Energy

Three-phase AC-DC converter using Modular Multilevel Converter

Savings of Pumps and Blowers,” IEEE Transactions on Power Electronics,

Topology,” in Proc. International Energy Conversion Congress and

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IEEJ Trans. ●●, Vol.●●, No.●, ●●●Isolation System for Gate Drivers of Medium-voltage Inverter (Keisuke Kusaka et al.)

Exhibition Asia 2013, pp. 52-58 (2013)(19) Z. Tiefu, W. Jun, A. Q. Jun and A. Agarwal : “Comparisons of SiC
MOSFET and Si IGBT Based Motor Drive Systems,” in Proc. 42nd IEEE
Industry Applications Society Annual Meeting 2007, pp. 331-335 (2007)

Isamu Hasegawa(Non-member)
He is currently a PowerElectronics R&D Engineer with the MEIDENSHACORPORATION.

Kazunori Morita(Member) He is currently a Power Electronics
R&D
Engineer
withthe
MEIDENSHACORPORATION.

Takeshi Kondo(Non-member)
He is currently a PowerElectronics R&D Engineer with the MEIDENSHACORPORATION.

(20) Infineon, “Datasheet of EiceDRIVER 2ED300C17-S” (2013)(21) CT-CONCEPTTechnologie

GmbH,“Datasheetof

EiceMELTER

2SD300C17,” (2013)(22) T. Imura : “Equivalent Circuit for Repeater Antenna for Wireless Power
Transfer via Magnetic Resonant Coupling Considering Signed Coupling”,
in Proc. 6th IEEE Conf. On Industrial Electronics and Applications 2011,
pp. 1501-1506 (2011)(23) D. Ahn and S. Hong : “A Study on Magnetic Field Repeater in WirelessPower Transfer,” IEEE Trans. On Industrial Electronics, Vol. 60, No. 1, pp.
360-371 (2013)

Keisuke Kusaka(Student member) received his B.S. and M.S.
degrees in electrical, electronics and informationengineering from Nagaoka University ofTechnology, Niigata, Japan in 2011 and 2013,
respectively. He is currently a Ph.D. candidate at
Nagaoka University of Technology, Niigata, Japan.
His research interests include an inductive powertransfer and a high-frequency power conversion.
He is a member of the Institute of ElectricalEngineers of Japan, the Institute of Electrical and Electronics Engineers
and the Society of Automotive Engineers of Japan.
Koji Orikawa(Member) received his M.S. and Ph.D. degrees inelectrical, electronics and information engineeringfrom Nagaoka University of Technology, Niigata,Japan in 2010 and 2013, respectively. Since 2013,
he has been working at Nagaoka University ofTechnology as a postdoctoral fellowship. He is amember of IEEE and IEEJ. His research interests
include power conversion system especially DC-DC
converters and high frequency techniques for power

converters.
Jun-ichi Itoh

(Member) received his M.S. and Ph.D. degrees inelectrical and electronic systems engineering from
Nagaoka University of Technology, Niigata, Japan in
1996 and 2000, respectively. From 1996 to 2004, he
was with Fuji Electric corporate Research andDevelopment Ltd., Tokyo, Japan. Since 2004, He
has been with Nagaoka University of Technology as
an associate professor. He received the IEEJ
academic promotion award (IEEJ Technical Development Award) in 2007
and the Isao Takahashi power electronics award in 2010. His researchinterests include matrix converters, DC/DC converters, power factor
correction techniques and motor drives. He is a member of the Institute ofElectrical Engineers of Japan and the Society of Automotive Engineers ofJapan.

9

IEEJ Trans. ●●, Vol.●●, No.●, ●●●