Review of Novel Multilevel Current-Source Inverters with
H-Bridge and Common-Emitter Based Topologies

Toshihiko Noguchi

SurosoSenior Member, IEEE

Student Member, IEEE

Shizuoka University
3-5-1 Johoku, Naka-Ku, Hamamatsu, Shizuoka 432-8561, Japan
ttnogut@ipc.shizuoka.ac.jp

Nagaoka University of Technology
1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japansuroso@stn.nagaokaut.ac.jp
University of Jenderal Soedirman
Purwokerto, Jawa Tengah 53122, Indonesia
suroso.te.unsoed@gmail.com

Abstract -- This paper describes several approaches to create
new configurations of multilevel current-source inverters fromtwo kinds of basic main circuits, i.e., an H-bridge-based and acommon-emitter-based topologies|. The first approach is anapplication of inductor cells connected in parallel with the main
three-level current-source inverter. The inductor cells work to
generate intermediate current levels for multilevel output
waveform generation with no additional external power sources.
The other approach is based on multiple superposition of nonisolated DC current-source modules onto the main three-level
current-source inverter. Using these strategies, four of the newcircuit topologies are obtained, which have simpler
configurations and control algorithms.
Several computer
simulation and experimental results are presented in the paper
to demonstrate proper operations of the proposed new
topologies.
Index Terms--DC-AC power conversion, duality, harmonic
distortion, inverters, multilevel, switching circuits, topology

I.

INTRODUCTION

In general, renewable energy sources, energy storages anddistributed power generation devices such as photovoltaic
power systems, batteries and fuel cells basically supply DC
power; hence, the generated surplus power is usually fed into
the utility power grid through DC/AC power converters(inverters). On the other hand, various global standards likeIEEE-1547, IEEE-929 and EN-61000-3-2 impose strict
requirements on the output power quality of the gridconnected inverters, i.e., harmonics and total harmonic
distortion (THD) of the output currents [1]. Current-source
inverters incorporating multilevel techniques are likely to bean effective solution to satisfy such requirements in mediumand high-power applications. The most significant feature of
the current-source inverters is inherent simplicity of theirconfiguration and control algorithm because they can directly
inject the desired currents to the power grid without ACcurrent feedback control, and can achieve a high power factor|
This work has been supported by Grant-in-Aid for Scientific Research(C) 20560262 from Ministry of Education, Culture, Sports, Science andTechnology, Japan since 2008.

978-1-4244-5287-3/10/$26.00 ©2010 IEEE

operation with ease. Therefore, the current-source inverters
behave like buffers between the DC power sources and the
AC power grid because of their robustness against the gridvoltage fluctuation. In addition, the multilevel approach of
the inverters is an indispensable key technology to reduce theoutput waveform distortion [2][3]. However, researchers andengineers have explored few topologies of the multilevel
current-source inverters so far. One of the traditional
methods to generate the multilevel current waveform is aparallel connection of H-bridge inverters, which is a dual
circuit of a cascade multilevel voltage-source inverter [4].
This configuration requires, however, multiple isolated DC
current sources, and a large number of isolated gate drive
circuits as well as switching devices. Another approach is amulti-cell topology based multilevel current-source inverter,
which is a dual circuit of a full-bridge voltage-source inverterwith flying capacitors [5][6].
Complexity of theintermediate-current-level balancing control is a fatal
drawback of this topology. Another different multilevel
current-source inverter, which employs a single-rating
inductor-cell topology, is also discussed in the past work,
which is a dual circuit of the improved diode-clamped
multilevel voltage-source converter. However, both of themulti-cell and the single-rating inductor-cell topologies
require large inductors to obtain the stable intermediate level
currents. These intermediate inductors cause extra losses in
the multilevel current-source inverter, resulting in lower
efficiency of power conversion.
This paper describes several approaches to create new
configurations of the multilevel current-source inverters fromtwo kinds of basic main circuits, i.e., an H-bridge-based and acommon-emitter-based topologies. These configurations
feature fewer switching devices, fewer isolated gate drive
circuits and smaller inductors for generation of theintermediate current levels. The operation performances ofthe proposed strategies are examined through computer
simulations and laboratory experimental tests in the paper.

4006

Q1
1
1
0

Q2
0
0
1

Q3
1
0
0

Q4
0
1
1

Q1
0
1
1

Output+I
0-I
Fig. 3.

Fig. 1. Conventional H-bridge current-source inverter

Q1
1
1
1
0/1
0

Q2
0
0
0
1/0
1

Q3
1
1/0
0
0
0

Q4
0
0/1
1
1
1

Q5
1
1
1
1/0
0

Q6
0
0
0
0/1
1

Q7
1
0/1
0
0
0

Q8
0
1/0
1
1
1

Output
+I
+I/2
0
-I/2
-I

II.

Q3
1
0
0

Q4
1
1
0

Output+I
0-I

Three-level current-source inverter with common emitter topology(fish bone structure based topology)

Q1
0
0/1
1
1
1

Fig. 2. Conventional multilevel waveform generation with parallelconnected H-bridge current-source inverters

Q2
0
0
1

Q2
0
0
0
1/0
1

Q3
1
1/0
0
0
0

Q4
1
1
1
0/1
0

Q5
0
1/0
1
1
1

Q6
0
0
0
0/1
1

Q7
1
0/1
0
0
0

Q8
1
1
1
1/0
0

Output
+I
+I/2
0
-I/2
-I

Fig. 4. Multilevel waveform generation with parallel-connected commonemitter current-source inverters

CONVENTIONAL APPROACH OF MULTILEVELWAVEFORM GENERATIONA.
Conventional H-Bridge Current-Source Inverter andMultilevel Waveform Generation TechniqueFig. 1 shows a traditional single-phase current-source
inverter, which has an H-bridge (full-bridge) configurationand can generate a three-level (+I, 0 and -I) current waveformto the load. Each switching device requires a series diode tohave a reverse-blocking capability against the lagging load
voltage. It should be noted that the zero current output isachieved by a short circuit across the DC current source.
In order to achieve higher output power and lower
waveform distortion simultaneously with lower-rated powerdevices, a multilevel approach is the most effective solution
to address such problems. A conventional method toincrease the number of the output waveform levels is a simple
parallel connection of some three-level current-source

inverters as shown in Fig. 2. A five-level output current
waveform can be obtained with this configuration as
indicated in the switching-state table.
However, thisconventional method has a drawback in the number of theswitching devices and in requirement of the isolated DC
current sources.
B.
Common-Emitter-Topology (Fish Bone Structure)
Based Current-Source Inverter and Conventional Approachof Multilevel Waveform Generation
Fig. 3 shows a new topology of a three-level currentsource inverter, which is derived from the three-level halfbridge neutral-point-clamped (NPC) voltage-source inverteron the basis of the theory of circuit duality [7][8]. It isfound from the switching-state table that the inverter is also
capable to generate a three-level current waveform. The

4007

(a) Output current level +I
Fig. 5.

Inductor cell based multilevel waveform generation applied to Hbridge current-source inverterII(b) Output current level +I/2 in energizing mode

Fig. 6.

Inductor cell based multilevel waveform generation applied tocurrent-source inverter with common-emitter topology

most significant feature of this topology is that all of theswitching devices can be driven on an identical potential
level with no isolated gate-drive power supplies. Therefore,
it is remarkably easy to operate the inverter at a considerablyhigh frequency of MHz order, which implies that a bulky
filter capacitor is no longer needed to reject undesirable
harmonic components in the load current. However, there isa drawback in this topology from the viewpoint of utility of
the DC current sources because only one of the two DC
current sources is alternately connected to the load at any
time. This topology is named “fish bone structure” by theauthors, associated with its circuit shape.
Fig. 4 shows parallel-connected configuration of thecommon-emitter current-source inverters that can deliver a
five-level current waveform to the load. The conventional
technique is applied to this configuration to increase thenumber of the output current levels; hence, similar problems
pointed out in Fig. 2 still exist in this topology.
III.

NEW APPROACH OF MULTILEVEL WAVEFORM
GENERATION WITH INDUCTOR CELLS(c) Output current level +I/2 in discharging mode(e) Output current level 0
Fig. 7.

Switching modes of inductor cell based five-level current-source
inverter with common emitter topology

inductor current at a specified constant value. The relation
between the level number of the output current waveform M
and the number of the inductor cells N can be expressed as(1)
M = 2 ( N +1) + 1 .
For an M-level current-source inverter of which DC current
source amplitude is I, the current flowing through the N-th
inductor cell ILcN must be regulated at the following value:
I LcN =

A.
H-Bridge Current-Source Inverter with Inductor
Cells
Fig. 5 shows an example of the proposed configuration ofa multilevel current-source inverter. In this configuration,
one or more inductor cells are connected in parallel with themain three-level H-bridge inverter to create more current
waveform levels. Each inductor cell consists of four
controlled switches with series diodes and an inductor Lc
across the bridge. It should be noted that the inductor cellrequires no external auxiliary power sources to keep the

I
2N

.

(2)

The combination of the three-level H-bridge inverter andthe inductor cells has redundancy of the switching states togenerate intermediate levels of the output current waveform.
In order to regulate each inductor-cell current, two redundantswitching states, i.e., charging and discharging modes of thespecific inductor, are necessary. The operation principle ofmultilevel waveform generation using the switching-state
redundancy is to be described in the next section.

4008

Fig. 8. Multilevel waveform generation with multiple DC current source
modules superposed onto H-bridge inverter

Fig. 10. Actual configuration of DC current-source modules

level current waveform.
Fig. 7 shows an example of a five-level current-source
inverter using a single inductor cell and its operation modes
during a positive output current cycle. A charging mode of
the inductor Lc is executed when the switches Q6 and Q7 areturned on, while the other switches Q5 and Q8 are turned off.
A constant current Ilc = +I/2 keeps on flowing through theswitches Q6 and Q7, which energizes the inductor Lc. A

discharging mode is achieved vice versa, where the stored energy inthe inductor is released to the load, keeping the current amplitude of+I/2. The inductance value of the inductor cell is designated as(a) Output current level +I

Lc =I Lc R ,f s ΔI Lc(3)

where R is a load resistance, fs is a switching frequency of theinductor cell, and ΔILc is an acceptable inductor current ripple. The
inductor current ILc can be regulated at an almost constant value I/2
by switching over the charging and the discharging modes of theinductor, with delivering the identical output current level +I/2.

IV.
MULTILEVEL WAVEFORM GENERATION WITH
MULTIPLE SUPERPOSED DC CURRENT SOURCE MODULES(b) Output current level +I/2

I/2

I/2

(c) Output current level 0
Fig. 9.

Switching modes of five-level current-source inverter withsuperposed DC current source modules onto H-bridge

B.
Common Emitter (Fish Bone Structure) CurrentSource Inverter with Inductor Cells
Fig. 6 shows another application of the inductor cells, i.e.,a combination of the three-level current-source inverter witha common-emitter topology and the inductor cells [9]. Thisconfiguration can be obtained by connecting one or more
inductor cells in a similar way as Fig. 5. The inductor cells
are used to generate a multilevel output current by adding or
subtracting their inductor currents to or from the basic three-A.
H-Bridge Current-Source Inverter with Multiple
Superposed DC Current-Source Modules
Another different approach of the multilevel currentsource inverter is shown in Fig. 8, which employs a multiple
superposition technique of non-isolated DC current-source
modules. Each module superposed onto the main H-bridge
current-source inverter includes a single controlled switch
with a series diode and an output diode. As illustrated in the
figure, even though the DC current-source modules areextensively added, every auxiliary switching device Q5, Q6,
and Qn is connected at a common-emitter potential level withthe switches Q3 and Q4 of the main inverter, which makes thegate drive design much simpler. No matter how many DC
current-source modules are superposed onto the main inverter,
only three isolated gate drive power supplies are needed todrive all the switching devices in this topology. The relation
between the level number of the output current waveform M
and the number of the superposed DC current-source modules
N can be expressed as follows:(4)
M = 2N + 3 .
Fig. 9 illustrates operation modes of the five-level current-4009

Fig. 11. Multilevel waveform generation with multiple symmetrical DC
current source modules (extended fish bone structure based topology)
TABLE ICOMPARISON OF FIVE-LEVEL CURRENT-SOURCE INVERTERS

8

5

6

8
1

10
1

6
0

10
0

4

4

4

4

4

1

1

2

1

1

1

2

2

4

Common-emitter with DC
module (cf. Fig. 11)884

H-bridge with DC module(cf. Fig. 8)882

Common-emitter withinductor cell (cf. Fig. 6)

8

8
0

H-bridge with inductor cell(cf. Fig. 5)

8Single-rating inductor cell

Multi-cell

Controlledswitches
DiodesInductors
Isolatedgate drive
circuitsDC
current
sources

Paralleled H-bridge (Fig. 2)

Devicecountssource inverter incorporating the multiple superposed DC
current-source modules.
When the maximum current
amplitude +I is required, all of the outputs from the DC
current sources are added and are delivered to the load. Anintermediate level current +I/2 is generated by a short circuitof the superposed DC current source module. In general,
each DC current-source module must have the following
amplitude of the output current:
I.
(5)
I DCN =
N +1
Fig. 10 shows an actual configuration of the DC currentsource modules, where independent current-controlled buckchoppers are employed. It should be noted that only a single
DC voltage source is used to create multiple DC currentsource modules and that small inductance value is sufficient
to regulate the DC currents owing to the high-switchingfrequency operation of the choppers.
B.
Multiple Superposed Common-Emitter CurrentSource Inverters (Fish Bone Structure Based Topology)
Fig. 11 shows an extended configuration of the three-level
current-source inverter with a common-emitter topology,
which is an extended “fish bone structure.” This multilevel
current-source inverter can be obtained by connecting
multiple symmetrical DC current-source modules to the main

Fig. 12. Five-level PWM output and load current waveforms of common
emitter current-source inverter with single inductor cell

inverter. The relation between the level number of theoutput current waveform M and the number of the superposed
symmetrical DC current-source modules N is identical to (4).
The key feature of this topology is that all of the switching
devices and the DC current sources are connected at an
identical common potential level regardless of the circuit
expansion scale. Therefore, the whole circuit requires
neither isolated gate drive power supplies nor isolated DC
current sources. In addition, no potential change occurs
during the switching operations; hence, this configuration issuitable for high-switching-frequency applications, where acommon mode noise is a delicate issue.
TABLE I represents a comparison summary of the device
counts used in various five-level current-source inverters, forexample. As can be seen here, the H-bridge inverter
incorporating the superposed DC current-source module isthe most effective to reduce the controlled switch counts.
V.

EXPERIMENTAL SETUP AND TEST RESULTS

A.
Inductor Cell Based Multilevel Current-Source
InverterSome laboratory prototypes are set up and theperformances are examined through experimental tests toverify feasibility of the proposed strategies. Fig. 12 is anexperimental result of the common-emitter current-source
inverter with a single inductor cell, which shows a proper
five-level PWM output current and low distorted (THD =3.3 %) load current waveforms. Amplitudes of the two DC
current sources and the inductor cell are regulated at 8 A and
4 A, respectively. The switching frequency of the main
inverter and the inductor cell is 22 kHz, which leads to great

4010

intermediate current levels M is increased as follows:
Rc I 2 2 Rc I 2
.
(6)
=
N +1
M −1
In the above equation, N is the number of the superposed DC
current-source modules, and Rc is an equivalent total
conduction resistance of a smoothing inductor and acontrolled switch in an individual DC current-source module.
The equation obviously represents the total conduction loss isinversely proportional to the number of the output current
levels or the number of the multiple superposed DC currentsource modules, which is another fundamental property of thecurrent-source inverters.
Ploss =

VI.

Fig. 13. Seven-level PWM output and load current waveforms of H-bridge
inverter with doubly superposed DC current-source modules

CONCLUSION

This paper discussed new approaches of a multilevel
current-source inverter, which applies inductor cells andmultiple DC current-source modules to the H-bridge and thecommon-emitter (fish bone structure) circuits. The proper
operations of generating five- or seven-level current
waveforms are confirmed through the experimental tests. In
addition, the H-bridge inverter incorporating the multiple
superposed DC current-source modules achieved the highest
efficiency of 94.2 %. The proposed configurations cangenerate a multilevel current waveform with fewer switching
device counts, lower losses and better THD as the number ofthe intermediate current levels is increased.
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Fig. 14. Efficiency of H-bridge inverter with multiple superposed DC
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B.
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Superposed DC Current-Source Modules
Fig. 13 shows operation waveforms of a seven-level
current-source inverter composed by an H-bridge inverterwith doubly superposed DC current-source modules, where
the switching frequency is 22 kHz and the amplitude of every
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waveform is I, the total conduction loss Ploss in the DC
current sources can be reduced as the number of the

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