Multilevel Current-Source Inverters using Inductor-Cells and DC-Current
Modules: A Comparative Discussion
Suroso* (Nagaoka University of Technology), and Toshihiko Noguchi (Shizuoka University)

Abstract This paper presents a comparison discussion of new multilevel current-source inverters (CSIs) created from the basic H-bridge
CSI. The first topology is obtained by connecting the inductor-cells in parallel with the main H-bridge CSI. The inductor-cells with its
intermediate inductor work generating the intermediate current levels of multilevel waveform without any additional external power sources.
The second configuration is achieved by superimposing the output of the DC-current modules to the H-bridge CSI. This strategy can reduce
the number of power switching devices and simplify the gate drive circuits of the inverter. Furthermore, the superimposing technique canachieve lower inductor loss than the inductor-cell strategy. Several experimental results are presented in the paper, which demonstrates thefeatures of each topology in generating multilevel output current with low harmonics distortion and with small size of inductors.
Keywords-current-source inverter; multilevel; inductor-cell; DC-current module

1. Introduction
Generally speaking, the multilevel inverter topologies can beclassified into two types; voltage-source inverters (VSI) and its dual
circuit, i.e., current-source inverters (CSI). Multilevel inverters havecapability to deliver multilevel AC waveform with low-voltage or
low-current rating devices, less distorted output waveforms, lower
dv/dt or lower di/dt, and resulting in reduction of voltage or current
stress in the switching devices, reduction of EMI noise and reductionof the output filter size, if compared with the conventional two-level
inverters[1], [2]. In distributed power generation, multilevel CSIs
have some interesting features as power interface between the
renewable energy systems and power grid. Various international
standards, like IEEE-1547, IEEE-929 and EN-61000-3-2, impose
requirements on the inverter’s output power quality, such as harmonic
currents and total harmonics distortion (THD) of the output current [3].
These requirements can be easily achieved by using multilevel CSI.
Moreover, control of the grid connected CSI is comparatively simpler
than its counterpart, VSI. A grid connected CSI can buffer the output
from grid voltage fluctuation, generates a predetermined magnitude ofcurrent to the grid without AC current feedback loops, and can achievea high power factor operation. Moreover, it has inherent short circuit
protection [4].
Some topologies of multilevel CSIs have been proposed byresearchers and engineers. A conventional method to generate themultilevel output waveforms is by paralleling some H-Bridge CSIs as
shown in Fig. 1 [2]. However, the requirement of many isolated DC
current sources is a problem introduced by this configuration. Fig. 2
shows another topology of multilevel CSI called as the multilevel
multi-cell CSI [5]. This topology has drawback with its hugeintermediate inductors and its control complexity for balancing
control of the intermediate level currents. Some control methods havebeen proposed for balancing control of the intermediate level currents
in [6] and [7], but very large in size of the intermediate inductors(>100 mH) are still used. These bulky inductors will be costly andcause low efficiency to the inverter. Reference [2] presented theconfiguration of multilevel single-rating inductor CSI which can be
regarded as a modified configuration of multilevel paralleled Hbridge CSI with intermediate-inductors.

Fig. 1. Five-level paralleled H-Bridge CSI

Fig. 2. Five-level multi-cell CSIThis paper discusses new circuit configurations of multilevel CSIs
obtained from the basic H-bridge CSI. In the first topology, the Hbridge CSI working as a main inverter circuit is connected in parallelwith the inductor-cells. The inductor-cells work generating theintermediate level currents of the output to obtain a multilevel current
waveform without any additional external dc power sources. The
second topology is created by connecting a single or more DCcurrent modules to the H-bridge CSI. The operating performance andthe features of each multilevel CSI are analyzed and examinedthrough some computer simulations, and experimentally using
laboratory experimental prototypes.

2. Circuit Configuration and Principle Operation
2.1. Multilevel Inductor-Cell Type CSI
Fig. 3 shows the configuration of the proposed inductor-cell
circuits composed by four controlled power switches QC1, QC2, QC3
and QC4 with series discrete diodes, and an intermediate inductor Lc
connected across the bridge [8]. The newly proposed configuration of
the multilevel CSI can be obtained by connecting the three-level Hbridge CSI as the main inverter with a single or more inductor-cells
as shown in Fig. 4. A five-level CSI configuration is obtained byconnecting only a single inductor-cell, a nine-level CSI configurationis achieved by connecting two inductor-cells in parallel with the main
three-level CSI, and so forth. The relation between the level numberof the output current waveform and the number of the inductor-cells
can be expressed as the equation below:

M = 2( N +1) + 1 ,(1)where M is the level number of the output current, and N is thenumber of the inductor-cells. The inductor-cell is used to generate amultilevel output current from the basic three-level current waveformby controlling the charging, and discharging operation modes of theintermediate inductor, Lc. If the amplitude of the DC current source
in a M-level CSI is I, the current flowing through the Nth inductor-cell
ILcN is expressed asI
I LcN = N ,2

Fig.5. Five-level inductor-cell type CSI
Table 1 Switching states
Output current

1

Q4 QC1 QC2 QC3 QC4
1
0
0
0
1
0
1
1
0
0

1

0

1

0

1

0

+I/2

0
0
1

0
0
1

1
1
0

0
1
0

1

0

1

+I/2

0

1

0

1

0

1
0
1

0
1
0

0
1
1

0
0
-I/2

0

1

1

0

1

0

1

0

-I/2

0

0

1

1

-I

1

1

0

0

-I

Q1

Q2

Q3

1

0

1

1

0

1

0

1
1
0

0

(2)

0

1
1

0
0

1
1

+I
+I

Fig. 5 shows the configuration of a five-level CSI composed by the
H-bridge CSI and a single inductor-cell. Table 1 lists the switchingstates for five-level output current waveform generation. Furthermore,
Fig. 6 shows the operation modes of the inductor-cell during a
positive cycle operation of the five-level CSI.

(a) Charging mode(b) Discharging mode

Fig. 3. Proposed inductor-cell circuits(c) Circulating current mode
Fig. 6. Operation modes of inductor-cell

Fig. 4. Proposed M-level inductor-cell type CSI

In the five-level H-bridge with inductor-cell CSI, in order to generateintermediate level currents +I/2 and –I/2, a constant current I/2 is kept
constant flowing through the inductor-cell. It is achieved using thealternating charging and discharging operation modes of the inductor
as shown in Fig. 6. In the nine-level CSI configuration with two
inductor-cells, the current flowing through the second inductor-cell isregulated to be I/4, as expressed in (2).

Table 2 Switching states of five-level CSI

2.2. Multilevel DC-Current Module Type CSI
Fig. 7 shows the basic circuit configuration of a DC-current module,
and its output waveform [9]. The circuit is composed by a single DC
current source, a controlled power switch Q with a series diode, and a
blocking diode, Dc. Another new configuration of the multilevel CSI
can be obtained by connecting the three-level H-bridge CSI with asingle or some DC-current modules as shown in Fig. 8. The output of
the DC-current modules are superimposed to the three-level current
waveform of the H-bridge CSI to generate a multilevel output current
waveform. The level number of the output current waveform (M) thatcan be produced by using D-number of the DC-current modules
connected to the H-bridge CSI can be obtained from the equation
below:

M = 3 + 2D ,(3)

A five-level CSI is obtained by connecting a single DC-current
module, a seven-level CSI configuration is achieved by connecting
two DC-current modules with the H-Bridge CSI, and so forth.

Q1

Q2

Q3

Q4

Q5

Output

1
1
1
0
0

0
0
0
1
1

1
1
0
0
0

0
0
1
1
1

0
1
1
1
0

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

As can be seen in the proposed M-level CSI in Fig. 8, the powerswitches Q3, Q4, and the power switches of the DC-current modules
Q5 to Qk are connected at a common-emitter line or at the samepotential level. Two isolated gate drive circuits are required to drive
switches Q1 and Q2, and one more to drive Q3, Q4, Q5 and Qk.
Hence, no matter how many DC-current modules are connected, thewhole inverter circuit needs only three isolated gate drive circuits.
Using this circuit topology, the number of the isolated gate drive
power supplies can be simplified; hence, it can reduce the circuitcomponents and the gate drive circuits complexity, such as isolationtransformers; in case of isolated power supplies are used for eachpower switch; or the capacitors; in case of bootstrap technique isapplied. Fig. 9 shows a configuration of the proposed five-level CSI
composed by an H-bridge CSI and a single DC-current module. Allof the DC current sources are assumed to have identical current
amplitudes I/2, and the amplitude of the output current is I. Table 2
lists the operation modes for five-level output current generation.

2.3. DC Current Source Circuits

Fig. 7. Proposed DC-current module, and its typical output
current waveform

Fig. 8. Proposed M-level DC-current module type CSI

In this paper, the DC current sources required in the multilevel
CSIs are obtained by employing chopper circuits with small inductors.
The chopper circuit consists of a controlled switch Qc, controlling theDC currents flowing through the smoothing inductor, and assuring
the intermediate levels balance of the multilevel output waveform.
Free-wheeling diodes are used to keep continuous currents flowing
through the inductors. Fig. 10 and Fig. 11 show the configurations of
the five-level inductor-cell type and the five-level DC-current module
type CSIs with chopper circuits as the DC current supplies,
respectively. The inductors Li, L1, and L2 are the smoothing
inductors used to generate the DC currents. In the multilevel DCcurrent module type CSI, by connecting more and more DC-current
modules, the higher level number of the output current waveformwith smaller current rating of the switching devices and smaller
distortion of the output current waveform can be obtained. It should
be noted that, only a single DC power source (Vin) is required toobtain the multiple DC current sources of the CSI. In real application,
the DC power source can be PV arrays, fuel cell, battery or rectifier.

2.4. Current Control and PWM Modulation Strategy

Fig. 9. Proposed five-level DC-current module type CSI

The current control diagram of the chopper circuits and inductorcell of the five-level inductor-cell type CSI is presented in Fig. 12.
Proportional integral (PI) regulators are independently applied tocontrol the currents flowing through the smoothing inductor Li andand inductor-cell Lc, which determine the amplitude of the PWM
output current waveform (Ipwm) and the intermediate current levels,
simultaneously. Making the smoothing and the intermediate inductor
currents follow their reference current (Iref) is the objective of thesecurrent regulators. The switching gate signals of the chopper switch
are generated by comparing the error signal of the detected steadystate currents flowing through the smoothing inductor, ILi and atriangular waveform after passing through the PI regulator. Thissignal is used to control the duty cycles of the chopper switch toobtain a stable DC current. This control circuit is similar for all
chopper circuits accompanying the DC-current modules in themultilevel DC-current module type CSI.
In order to obtain a better output current waveform, a pulse widthmodulation (PWM) technique is applied, instead of a staircase
waveform operation. Staircase waveform can easily be obtained at
the fundamental switching frequency, so the switching losses can be
negligibly low. However more distortion of the output waveform isgenerated and a larger filter is needed. In this paper, a level-shifted

Fig. 13. Multi-carrier based sinusoidal PWM
multi-carrier based sinusoidal PWM technique is employed togenerate the gating signals of the inverter’s power switches to obtain
the PWM current waveforms as shown in Fig. 13. All carrier
waveforms are in phase with the identical frequency. The frequencyof the reference sinusoidal waveform determines the fundamental
frequency of the output current waveform, while the frequency of
triangular carrier waves gives the switching frequency of the powerswitches.

2.5. Conduction Loss of Inductors

Fig. 10. Experimental circuit of five-level inductor-cell type CSI

The conduction loss of huge inductors in the multilevel CSI is
one of the main loss components which cause the efficiency of the
CSI lower than the multilevel VSI topology. In the M-level CSI with
D-number of DC-current modules, if the amplitude of the M-level
output current is I, the current flowing through the smoothing
inductor IlD is expressed as
I ,(4)
I lD =( D + 1)
If the smoothing inductor has resistance Rl, the inductor’s conduction
loss (Plc) caused by this current is expressed as
2

⎛ I ⎞
(5)
Plc = ⎜⎟ Rl
⎝ D +1⎠
The total conduction loss due to the inductors in an M-level DCcurrent module type CSI (Plc-M) can be calculated as

Plc −M ==Fig. 11. Five-level DC-current module type CSI with chopper
based DC current-sources

Fig. 12. Control diagram of five-level inductor-cell type CSI

I 2 RlD +1(6)

In the five-level DC-current module type CSI, the current flowing
through the smoothing inductors is half the amplitude of the fivelevel output current. In the seven-level CSI configuration, theamplitude of the smoothing inductor currents is one third of theamplitude of seven-level current, and so forth. The higher the levelnumber of the output current leads smaller currents flowing throughthe smoothing inductors as expressed in (4). Therefore, it reduces thetotal conduction losses (Plc-M) caused by the smoothing inductors asshown in Fig. 14. In this figure, the inductor’s losses are normalized
to the inductor’s loss of the H-bridge CSI. The total inductor’s
conduction loss of the five-level CSI (Plc-5) is half the inductor’s
conduction loss of the three-level H-bridge CSI (Plc-3). The total
inductor’s conduction loss of seven-level CSI (Plc-7) is one third of
the inductor’s conduction loss of the H-bridge CSI, and so forth.
However, in the case of the multilevel inductor-cell type CSI, theinductor-cell circuits with its intermediate inductors will give theextra losses to the inverter. This condition is also the same as themultilevel multi-cell CSI configuration shown in Fig. 2.

Fig. 14. Conduction loss characteristics of the inductors
Table 3 Comparison of component number
Multilevel CSI
Types

Paralleled HBridge CSI
Topology

DC-Current Inductor-Cell
ModuleTopology
Topology

Power switches
DiodesInductors
Isolated gate drive
circuits
DC current source
DC power source

10
10
2
6

7
8
2
5

9
9
2
5

2
2

2
1

1
1

In addition, Table 3 lists a comparison of the component number
required to implement the five-level paralleled H-bridge CSI, fivelevel inductor-cell topology, and five-level with DC-current modules
CSI. In this table it is assumed that all of the topologies use chopper
circuits as DC current generation. The number of the components
includes the devices required in the chopper circuits. As can be seenin the table, the five-level CSI with DC current modules requires theminimum number of the power switches and diodes even the chopper
circuit components are included. The inductor-cell type CSI needs asingle DC current source only, if ideal DC current sources are used.

inductor-cell current has been driven to the balanced condition of
50% of the 8-A DC current source. As can be seen in the figure, alow distorted sinusoidal load current waveform is also obtained after
filtering by a small 5-μF filter capacitor. Moreover, Fig. 16 shows theexperimental waveforms of the five-level DC-current module type
CSI presenting the five-level current, load current and the smoothing
inductor currents IL1 and IL2. Fig. 17 and Fig. 18 show the efficiency
characteristics of the five-level CSIs with inductor-cell and DCcurrent module at a constant output current, respectively. The
maximum efficiency of the five-level inductor-cell type CSI
prototype is 90.8%, and 93.4% for the five-level DC-current module
type CSI, which is higher than the inductor-cell type as predicted in
the previous sections. Fig. 19 and Fig. 20 present the harmonics
spectra of the five-level PWM current generated by the five-level CSI
using inductor-cell and DC-current module topologies with THD
values 2.93% and 2.6%, respectively

4. Conclusions
In this paper new configurations of multilevel CSI using inductorcells and DC-current modules have been presented, and the validity
has been verified experimentally. The proposed multilevel inductorcell type CSI topology can reduce the number of the isolated DC
current sources as required in the multilevel paralleled H-bridge CSI
configuration. The configuration of the multilevel DC-current module
type CSI can achieve lower inductor’s conduction loss than themultilevel inductor-cell type CSI. The inductor-cells added to the main
three-level CSI will give extra losses to the inverter. Chopper circuits
working as DC current sources are also discussed which are able toreduce the inductor size down, and to assure stable DC current sources
required in the multilevel CSI with simple PI regulators.
References
[1]

[2]

3. Experimental Test Results
In order to verify and to prove feasibility of the proposed
multilevel CSI configuration experimentally, laboratory prototypes ofthe five-level inductor-cell and the five-level DC-current module CSI
topologies were constructed with IXFK90N30 power MOSFETs inseries with DSEI120-06A fast recovery diodes. The five-level
inverter circuit configurations shown in Fig. 10 and Fig. 11 were
tested with a single 160 V DC power source. The switching and thefundamental output current frequencies are 22 kHz and 60 Hz,
respectively. The inverter was connected to an inductive load, i.e. R =
6 Ω , L = 1.2 mH with a 5-μF filter capacitor. The smoothing
inductors and the inductor-cell are 1 mH and 5 mH, respectively. Fig.
15 shows the experimental waveforms of the five-level inductor-cell
type CSI showing the five-level PWM output current, the load
current, smoothing inductor current and the inductor-cell current
waveforms. This figure also shows the start-up transient waveformsof the inverter. No additional DC power source was connected to the
inductor-cell circuits during the startup transient test. The inverter
works properly generating a five-level output current waveform. The

[3]

[4]

[5]

[6]

[7]

[8]

[9]

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Fig. 17. Efficiency characteristics of five-level inductor-cell
type CSI

Fig. 15. Experimental waveforms of startup transient for fivelevel inductor-cell type CSI

Fig. 18. Efficiency characteristics of five-level DC-current
module type CSI

Fig. 19. FFT analysis result of five-level PWM current generated
by five-level inductor-cell type CSI

Fig. 16. Experimental waveforms of five-level DC-current
module type CSI

Fig. 20. FFT analysis result of five-level PWM current generated
by five-level DC-current module type CSI