IEEE PEDS 2017, Honolulu, USA
12 – 15 December 2017

Pure Sinusoidal Output Current-Source Inverter
Using Inductor Modules
Toshihiko Noguchi, Yosuke Iwata, and Sota Yamaguchi
Shizuoka University
noguchi.toshihiko@shizuoka.ac.jp, iwata.yosuke.16@shizuoka.ac.jp

Abstract-A novel topology of a current-source inverter (CSI) is
proposed in the paper, which can deliver a pure sinusoidal
current waveform without sacrificing its efficiency and its
equipment size. The proposed circuit has a hybrid structure,and combines a switching-operation based multilevel CSI and alinear current amplifier with a low-amplitude current output.
The former multilevel CSI employs multiple inductor modules
to generate a staircase multilevel current waveform, while thelatter linear amplifier generates a compensating current toreform the staircase multilevel waveform to the pure sinusoidal
waveform. Because the most part of the output current iscreated by the switching-operation based multilevel CSI, thesystem efficiency hardly deteriorates. In addition, the systemdoes not require large LC filters because it directly delivers thepure sinusoidal current to the load. In the paper, current
drooping phenomenon of the inductor modules due to theirpower losses is discussed, and the countermeasure is proposed.

amplitude linear amplifier.
There have been several
proposals of the hybrid inverter topologies made by theauthors so far [3]. Figs. 1 and 2 show the examples of thehybrid inverter feeding the pure sinusoidal current to the load,i.e., a DC current source type and a fish-bone structure type,respectively [4] [5]. However, these circuits demand more
DC current sources, more inductors, and more isolated gate
drive circuits for the main switching devices as the number of
output levels is increased.
A new topology of the hybrid current source inverter (CSI)
is discussed in the paper, which is based on a simple H-bridge
configuration featuring inductor modules [6] [7]. Feasibilityof the new topology is confirmed through the comparison of
circuit configurations and operation principles.
CIRCUIT CONFIGURATION AND OPERATION PRINCIPLE

Q2

Q1

vo
~I/3

I/3

I/3
Load
io

Q5

Q6

Q3

Q4

Fig. 1. Hybrid CSI with DC current source modules.

~I/3

I/3

I/3
Q5

Q1

Q2

~I/3

I/3

Q6 I/3

Q3

Q4

Load
io

In general, power converters are operated on the basis ofswitching action to maximize the conversion efficiency, but
LC filters are indispensable to reduce the harmonics causedby the switching action. On the other hand, linear amplifiers
such as a class-A or a class-AB can generate a pure sinusoidal
waveform sacrificing the conversion efficiency. The ultimate
goal for the power converters is to generate the puresinusoidal waveform with low total harmonic distortion(THD) without using large LC filters or sacrificing theefficiency. One of the approaches to achieve this goal is useof high-frequency pulse width modulation (PWM) techniques.
The approach, however, has a fatal drawback to increase theswitching losses as the frequency increases although thehigher switching frequency significantly contributes
reduction of the LC filter size. Therefore, it is definitely
necessary to develop the power converter that can solve theabove problems at the same time. Another approach is
improvement of the THD by employing multilevel techniquessuch as a neutral-point clamped (NPC) topology, a flying
capacitor topology, and so forth [1].
The multileveltechniques are very effective to reduce the harmonics and
EMC noise, but have an inherent drawback of thecomplicated circuitry and control algorithm, and the
conduction loss of the main switching devices [2].
This paper proposes a new approach as a solution to theabove problems, i.e., a hybrid power converter that combines
a switching operation based power converter and a low-

A. Inductor Module Based CSI with Pure Sinusoidal Output
Fig. 3 shows the proposed inductor module based hybrid
CSI. The circuit is composed of a main multilevel inverter
and a low-amplitude variable linear current source. The
former is a combination of an H-bridge CSI and a set of an
inductor module. The inductor module is a component block
made of two transistors, two diodes, and an inductor asvo

INTRODUCTION

Fig. 2 Hybrid CSI with fish-bone structure.

869
978-1-5090-2364-6/17/$31.00 ©2017 IEEE

Q1

ILm
=I/3

Lm

Qm1

Q2

InductormodulesQm1

Qm2

istair-P

ILm
=I/3

Q1

QP

vo
2I/3

2I/3

Load
io

Load
io

Load
io

QP

Linear current sourcewith low amplitude

QN

ilin
=~I/3

Q4

QP

QN

Q3

Q4

(a) Holding mode (io = +2I/3~+I).
ILm
=I/3

Fig. 3. Proposed hybrid CSI with inductor module.
Q1

Qm1

QN
~I/3

~I/3
Q3

Q3

Q2

Qm2

vo

vo
IDC
=2I/3

Qm1

Qm2

istair-N

ILm
=I/3

Q1

Q2

Qm1

Q4

(b) Holding mode (io = 0~+I/3).

Q2

ILm
=I/3

Q1

Qm2

Qm1

Q2

Qm2

ILm
vo

vo

2I/3

2I/3

Load
io

Qm2
QP

Fig. 4 Inductor module.

Q1
1
1
0
0
1
0
1
0

TABLE I SWITCHING STATES OF
5-LEVEL INDUCTOR MODULE BASED HYBRID CSI.
Q2 Q3 Q4 Qm1 Qm2 QP QN
io
0
0
1
1
0
1
0
+2I/3~+I
0
0
1
0
1
1
0
+I/2~+2I/3
1
0
1
1
0
1
0
+I/2~+2I/3
1
0
1
0
1
1
0
0
0
1
0
1
0
0
1
0
1
1
0
1
0
0
1
I/2~2I/3
0
1
0
0
1
0
1
I/2~2I/3
1
1
0
0
1
0
1
2I/3~I

Load
io

QN

QP

~I/3
Q3

Mode
H
C
D
H
H
C
D
H

shown in Fig. 4. The main inverter having the H-bridge CSI
and one inductor module can generate a 5-level current
waveform to the load, and its switching states are indicated in
TABLE I. The “Mode” in the table means the operation
statuses of the inductor module, where “H,” “C,” and “D”
correspond to a holding mode, a charging mode, and adischarging mode, respectively. Since the main inverter
generate the 5-level staircase current waveform, 5 kinds of
output current levels are delivered to the load, i.e., io = +I,+I/2, 0, I/2, and I. It should be noted that there is
redundancy in the switching states. By taking advantage ofthe redundant switching states, the 5-level currents can begenerated with keeping the inductor module current constantby alternating the charging and discharging modes of theinductor. On the other hand, the variable linear amplifier
outputs a low-amplitude linear current waveform tocompensate for the difference between the staircase
multilevel current waveform fed by the main inverter and thesinusoidal output current command. This strategy canefficiently reduce the power loss of the linear amplifierbecause of the low-amplitude output, compared with theclass-A linear amplifier generating a full peak-to-peaksinusoidal waveform. In addition to the advantage described

QN
~I/3

Q4

Q3

Q4

(c) Charging mode (io = +I/2).
(d) Discharging mode (io = +I/2).
Fig. 5. Operation modes of inductor module based hybrid CSI.

LDC

IDC

Df
QDC
Vin

Fig. 6 Chopper based DC current source.

above, the filter capacitor across the load can alsosignificantly be reduced owing to the sinusoidal current
waveform output that has been synthesized by superimposing
the low-amplitude linear waveform onto the staircase
multilevel waveform.
Fig. 5 shows operation modes of the proposed hybrid CSI.
The inductor current is kept constant in the current holding
mode (a) because of a short circuit across the inductor, andthe maximum level of io = +2I/3~+I is given to the load,
where the main inverter generates istair-P = +2I/3 and thevariable linear current source outputs ilin = 0~+I/3. In thesimilar way, the current holding mode (b) makes a shortcircuit across the inductor to keep the inductor current
constant. However, only the variable current source supplies
the current to the load, resulting in the minimum level output
of io = 0~+I/3. In the charging mode (c), the inductor current
slightly increases because the inductor is connected in parallelwith the load, which makes the energy transfer from the DC
current source to the inductor possible. The output current of
the main inverter is istair-P = +I/3, and the variable linear
current source outputs ilin = 0~+I/3, resulting in the

870

Q11
Q

IIL1
L1
Qm1
Q
m1

Qm2
Q
m2

Qm3
Q
m3

IIL2
L2
Qm4
Q
m4

Qm(2N-1)Q
m(2N-1)IILN
LN
Qm(2N)Qm(2N)

iistair-P
stair-P
IIDC
DC

Q22
Q

iistair-N
stair-N

vvoo

CFF
C

iioo
Load
Load

QPP
Q

Q33
Q

QNN
Q
iilin
lin

Q44
Q

TABLE II COMPARISON OF COMPONENT COUNTS.
CircuitDC-source
Proposed
Fishbonecomponents
moduleinductor module
Transistor
8
10
9
Diode
11
16
9
Inductor2
4
2
Gate drive power
554
supplyLiner current
12
1
sourceTABLE III COMPARISON OF COMPONENT COUNTS.
CircuitDC-source
Proposed
Fishbonecomponents
moduleinductor module
Transistor
M+3
2M
M+4
Diode
(3M + 7)/2
3M + 1
M+4
Inductor(M  1)/2
M1(M  1)/2
Gate drive power(M + 5)/2
M
4supplyLiner current
12
1
sourceFig. 7.
7. Generalized
Generalized multilevel
multilevel inductor
inductor module
module based
based hybrid
hybrid CSI.
CSI.
Fig.
Number of diodes

DC current module
Fish-bone
Inductor module

60
40
20
0
0

5

10
15
20
25
Level number of output current

30(a) Output levels and diode counts.
Number of inductors

25
DC current module
Fish-bone
Inductor module

20
15
10
5
0
0

5

10
15
20
25
Level number of output current

30(b) Output levels and inductor counts.
Number of gate drive power supplies

intermediate output level of io = +I/3~+2I/3. The DC current
source is shorted and the inductor releases its energy in the
discharging mode (d); thus, the same intermediate output
level of io = +I/3~+2I/3 is fed to the load with decreasing theinductor current slightly. The main inverter, i.e., the inductor
module based CSI, alternates the charging mode (c) and thedischarging mode (d) among the four modes to output theintermediate current level corresponding to the half of the DC
current source. This alternating operation of the inductor
module makes it possible to keep the inductor current
constant at I/3, which is efficient utilization of the switchingstate redundancy.
B. DC Current Source
A CSI needs a DC current source, where a large reactor isoften employed in series with a DC voltage source toconfigure the DC current source. However, this scheme
makes the power converter large and heavy, so anotherscheme has been taken in the proposed circuit as shown in
Fig. 6. The DC current source is composed of a current
regulated buck chopper with high-frequency switching
devices to reduce the smoothing inductor value. The current
control is achieved by a current feedback loop using a PI
regulator and a triangular wave based modulator.
C. Generalized Configuration of Proposed Hybrid CSI
Fig. 7 shows a generalized configuration of the proposed
hybrid CSI, which employs multiple inductor modules togenerate a staircase multilevel current waveform and to
reduce the amplitude of the variable linear current. Assumingthat the number of the inductor modules is N and that the
number of the output current levels is M, the following
equation is derived from the configuration of the CSI:
M = 2N + 3.
(1)
Let the kth inductor module current IL(k), the current can be
expressed by the following recursive relationship:I
I L k 1  I L k  
, and
(2)
N 2

80

24

DC current module
Fish-bone
Inductor module

20
16
12
8
4
5

10
15
20
25
Level number of output current

30(c) Output levels and gate drive power supply counts.
Fig. 8. Comparison of component counts.

N
I.
(3)
N 2
TABLE II and TABLE II indicate the component counts ofthe conventional circuits and the proposed circuit. It can befound in the tables that the proposed circuit requires the
fewest components to generate the M-level current waveform.
In addition, because the amplitude of the variable linear
current can be reduced as the number of the current levels of
the main inverter increases, higher conversion efficiency is
I L 1 871

Ilm_ref
=I/3

－+

IL

2I/3

－

io (A) 0

0

+
-

-5
-10

P1

+
-

P2

iref
－+

+
-

N2

－+

+
-

N1

-2I/3

-I/3

80
40

Switching mode table

I/3

－+

10
5

1

+

vo (V) 0

Q2

-40
-80

Q1
Q4

6

Q3

ilin (A) 4
2
0

Qm2
Qm1

6

istair_P 4(A) 20

Fig. 9. Control block diagram of 5-level main CSI.

6

istair_N 4
2

-1

+

istair_N(A) 0
Switching mode table

+

iref

istair_P
-

4
3

ilin
QN
QP

ILm (A) 2
1
0

Time10(ms/div)

Fig. 11. Simulation waveforms of proposed 5-level hybrid CSI.

Q1
Qm1

+
-

Fig. 10. Control block diagram of variable current source.

CF

Load
QP

QN
ilin
=~Im/4Q3

Q4

Fig. 12. Inductor module based 7-level hybrid CSI.
IL2-ref
=I/3

1

+
－

0

IL2
IL1-ref
=2I/3

1

+
－
IL1

0

+
-

P1

－+

P2

－+

+
-

P3

－+

+
-

+3I/4
+2I/4
+I/4

Q1

Q4

iref
-I/4

+
-

N1

-2I/4

+
-

N2

+
-

N3

－+

-3I/4

Q2

Switching mode table

A. Simulation Results of 5-Level Hybrid CSI
Fig. 9 shows a control block diagram of the main 5-level
CSI. The switching signals are created by comparing asinusoidal current command iref with four different DC
threshold references. The current of the inductor module isdetected with a current sensor, and is controlled by a relay
regulator with a hysteresis band. The control error between
the inductor current command ILm_ref and the feedback current
ILm is quantized by the relay regulator. Either a charging
mode or a discharging mode is selected, according to thequantized signal “1” or “0,” respectively. The main inverter
can provide the load with istair-P = +I/3 by keeping the inductor
current around its command with the charging and thedischarging modes. Fig. 10 shows a control block diagram ofthe variable linear current source. It delivers the linear
current waveform corresponding to the difference between
the sinusoidal current command iref and the staircase
multilevel current waveform istair-P and istair-N generated by the
5-level CSI. Either a switch QP or QN is turned on in apositive or a negative half cycle of the output current.

vo
io

－+

VERIFICATION OF OPERATION BY SIMULATIONS

Qm4

Qm3
IDC
=3Im/4

Q2

IL2
=Im/4

L2

－+

expected. One of the remarkable features of the proposed
circuit is constant counts of the gate drive circuit power
supplies regardless of the increase of the number of the output
current levels; hence, the proposed circuit has an advantage
when more current levels are demanded. Some relationships
between the number of the output current levels and thecomponent counts are shown in Figs. 8 (a)-(c). The
component counts increase with the output current levels, but
the proposed circuit has a gradual increase tendency of thecomponent counts, compared with the other circuits.

IL1
=Im/2
Qm2

L1

Q3

Qm2
Qm1

Qm4
Qm3
QP
QN

Stair operation signal

Fig. 13. Control block diagram of 7-level main CSI.

Assuming that the DC current source is 6 A, the output
current command is 9 Apeak and 50 Hz, some simulation tests
have been conducted using a power electronics simulator
PSIM. The inductor current is controlled to follow its

872

Q1
1
0
1
1
0
0
1
0
1
0
1
1
0
0
1
0

Q2
0
1
0
0
1
1
0
1
0
1
0
0
1
1
0
1

TABLE IV. SWITCHING STATES OF
7-LEVEL INDUCTOR MODULE BASED HYBRID CSI.
Q3 Q4 Qm1 Qm2 Qm3 Qm4
io
0
1
1
0
1
0
+3I/4~ +I
0
1
1
0
1
0
+I/2~ +3I/4
0
1
0
1
1
0
+I/2~ +3I/4
0
1
1
0
0
1
+I/2~ +3I/4
0
1
0
1
1
0
+I/4~ +I/2
0
1
1
0
0
1
+I/4~ +I/2
0
1
0
1
0
1
+I/4~ +I/2
0
1
0
1
0
1
0~+I/410
10
100~－I/410
1
0
1
0 －I/4~ －2I/4
1
0
0
1
1
0
－I/4~ －I/2
1
0
1
0
0
1
－I/4~ －I/2
1
0
0
1
1
0 －I/2~ －3I/4
1
0
1
0
0
1 －I/2~ －3I/4
1
0
0
1
0
1 －I/2~ －3I/4
1
0
0
1
0
1
－3I/4~ －I

10
5
0
-5
-10
240
120
vo (V) 0-120-240

io (A)

L1
H
D
C
H
H
D
C
H
H
C
D
H
H
C
D
H

L2
H
H
D
C
D
C
H
H
H
H
C
D
C
D
H
H

2
0
6

istair_P 4
(A) 2
0
6

istair_N 4(A) 20

IL1 (A), 6
4

IL2(A) 20Time 10(ms/div)

Fig. 14. Simulation waveforms of proposed 7-level hybrid CSI.

Lm

Q1

Q2

ILm

Qm1

Qm2

istair-P

istair-N

vo
io

IDC

Bidirectionalswitches

Load
QN

QP

Q3

Q4

ilin

Fig. 15. Inductor module based hybrid CSI with bidirectional switches
ILm_ref
=I/3

1

+
IL

－

0

+
-

P1

vc1

vc2

+
-

P2

+
-

N2

+
-

N1

iref
vc3

vc4

Switching mode tablecommand ILm_ref = 3 A within the hysteresis band of 0.4 A.
The load consists of a 6- resistor and a 0.6-mH inductor
with a parallel filter capacitor of 20 F. The variable linear
current source is regarded to be ideal, and superimposes thelinear current waveform to compensate for the staircase 5level current waveform generated by the main inverter. Fig.
11 shows the operation waveforms obtained by the simulation.
As seen in the figure, the output current io to the load is asinusoidal wave with low distortion, which is created bysuperimposition of the variable linear current ilin onto thestaircase currents istair-P and istair-N generated by the 5-level CSI.
The inductor module current ILm is kept at 3 A by alternating
the charging mode and the discharging mode.
The proposed circuit can improve the total conversion
efficiency as the number of the output current level isincreased because the amplitude of the variable linear current
is decreased to the contrary.
B. Simulation Results of 7-Level Hybrid CSI
The 7-level hybrid CSI is depicted in Fig. 12. Two sets of
the inductor modules are used in the H-bridge CSI to generate
the 7-level current waveform. The switching states are shownin TABLE IV. It is required to control independently boththe inductor currents at constant values at the same time.
When the main inverter outputs istair = ±I/2, ±I/4, the two
inductors can be operated in the charging mode, the holding
mode, and the discharging mode; hence, both the inductor
currents can be kept at constant values by changing the modes
appropriately among the three. Fig. 13 shows a control block
diagram of the 7-level hybrid CSI. Each switching signal iscreated by comparing the sinusoidal current command with
one of the six different DC threshold references. In the caseof the 7-level inverter, if one inductor is controlled to charge
or to discharge, the other inductor continues the charging or
the discharging mode. Therefore, one of the inductors is
controlled with a priority, and the control is switched to theother inductor at the moment of polarity change of the otherinductor current. The inductor currents are controlled to be6ilin (A) 4

Q2

Q1
Q4
Q3
Qm2
Qm1

Fig. 16. Proposed control method.

constants of IL1 = I/2 and IL2 = I/4, according to the recursive
relationship (2) and (3) described previously.
The simulation has been conducted under the condition of
the inductors L1 = L2 = 0.7 mH, and the hysteresis bands for
the IL1 control and the IL2 control are set at 0.2 A. The load iscomposed of a resistance of 25  and a 1.5-F filter capacitor.
Fig. 14 shows the simulation result, where the pure sinusoidal
output current waveform can be confirmed. In addition, thestaircase 7-level current waveforms istair-P and istair-N are
properly generated, and the variable linear current
compensates for the staircase 7-level current to synthesize the873pure sinusoidal current waveform. The inductor module
currents are also controlled to follow their commands, andthey are kept at constants of 4 A and 2 A, respectively.
C. Compensation Technique for Drooping Current of
Inductor Module
It is important to reduce the power loss in the inductor
module in the proposed multilevel CSI because the inductor
has to keep its current at a constant value at any time.
However, the power loss of the inductor such as a copper loss
and an iron loss causes the current drooping, especially in theholding mode. Therefore, a low-frequency PWM technique
is introduced into the system to compensate for the drooping
current of the inductor module, which may sacrifice theconversion efficiency due to the switching losses but theswitching frequency can be limited to several kHz at mostowing to the long time constant of the drooping phenomenon.
Figs. 15 and 16 show the main circuit configuration and thecontrol block diagram. In order to achieve the low-frequency
PWM, the constant threshold level signals are replaced withmultilevel triangular waveforms, and the sinusoidal current
command is compared with the triangle waveforms. By
using this approach, the charging mode often interrupts theholding mode even in the zero-current level output and the
maximum-current level output situations, resulting in thealmost constant inductor current.
The proposed compensation technique for the drooping
current has been evaluated by the simulation.
Thefrequencies of the output current command and the PWM
carrier are 50 Hz and 3 kHz, respectively. The inductanceand the winding resistance of the inductor in the inductor
module are 1 mH and 0.2 , respectively, and the hysteresis
band of the inductor current control is set at 0.5 A. The load
is composed of a 50- resistor and 1.5-F capacitor. Figs. 17(a) and (b) show the operation waveforms of the proposed
methods without and with the PWM. As can be seen in the
inductor module currents of the waveforms, the current
droops during the holding mode of the inductor module when
the PWM is not applied, but even a low-frequency PWM issignificantly effective to keep the inductor current at constantvalue around 1.5 A within the hysteresis band.

6
3

io (A) 0

-3
-6
300
150
vo (V) 0
-150
-300
3.0
ilin (A) 1.5
0.0
-1.5
3.0

istair_P (A) 1.5

0.0
-1.5
3.0
1.5

istair_N (A)0.0
-1.5
3.0

ILm (A) 1.5

0.0
-1.5
Current of inductor module

63

io (A) 0

-3
-6
300
150
vo (V) 0
-150
-300
3.0
ilin (A) 1.5
0.0
-1.5
3.0

istair_P 1.5(A) 0.0
-1.5
3.0
1.5

istair_N (A)0.0
-1.5
3.0

ILm (A) 1.5

0.0
-1.5
Current of inductor module

Time10(ms/div)(b) Waveforms with inductor current drooping compensation.
Fig. 17. Simulation results of inductor current drooping compensation.

REFERENCES
[1]

[2]

CONCLUSION
In the paper, a novel topology of a hybrid CSI has been
proposed, which combines a switching operation and a linear
operation. The main inverter generates a staircase multilevel
current waveform, and the linear amplifier compensates forthe staircase multilevel waveform to the sinusoidal one. It
has been demonstrated that the pure sinusoidal current
waveform can be fed to the load without a large LC filter,
high-frequency PWM, nor a full range linear amplification
like a class-A amplifier. In addition, a compensation
technique has been proposed to avoid drooping effect of theinductor module current caused by the power loss of theinductor, which makes it possible to keep the inductor current
constant.

Time10(ms/div)(a) Waveforms without inductor current drooping compensation.

[3]

[4]

[5]

[6]

[7]

J. Lai and F. Z. Peng, “Multilevel converter-a new breed of power
converters,” IEEE Transactions on Industry Applications, vol. 32, no. 5,
1996, pp. 509-517.
McGrath B. P. and Holmes D. G., “Natural Current Balancing ofMulticell Current Source Converters,” IEEE Transactions on PowerElectronics, vol. 23, no. 3, 2008, pp. 1239-1246.
S. Yamaguchi and T. Noguchi, “Hybrid Current-Source Inverter with
High-Efficiency Characteristic and Low-Distortion Output,” IEEJ
Proceedings of Annual Conference, 2013, C4-2.
T. Noguchi and Suroso, “Review of Novel Multilevel Current-Source
Inverters with H-Bridge and Common-Emitter Based Topologies,”
IEEE Energy Conversion Congress and Exposition, vol. 5, 2010, pp.
4006-4011.
Suroso and T. Noguchi, “New H-Bridge Multilevel Current-Source
PWM Inverter with Reduced Switching Device Count,” Proceeding of
IEEJ International Power Electronics Conference (IPEC-Sapporo),
2010, pp. 1228-1235.
A. Ikegami and T. Noguchi, “Proposal of Inductor Module CurrentSource Inverter,” IEEJ Proceedings of Annual Conference, vol. 1, 2013,
pp. 61-62.
Y. Iwata, T. Noguchi, and Tran Thi Lam Quyen, “Novel Topology ofMultilevel Current-Source Inverter Using Inductor Modules,” IEEJ
Proceedings of Annual Conference, vol. 1, 2016, pp. 293-296.

874