ICETIR 2019

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High-Level Number Multilevel Single-Phase Current-Source
Inverter With Reduced Switching Device CountEka R Priandana1,2,4, Toshihiko Noguchi3,5Environment and Energy System, Graduate School of Science and Technology, Shizuoka
University, Hamamatsu, Japan
2
National Laboratory for Energy Conversion Technology (B2TKE) of Agency for The
Assessment and Application of Technology (BPPT), Tangerang Selatan, Indonesia.
3
Department of Engineering, Graduate School of Integrated Science and Technology, Shizuoka
University, Hamamatsu, Japan.
1

eka.rakhman.priandana.17@shizuoka.ac.jp
noguchi.toshihiko@shizuoka.ac.jp

45Abstract. This paper proposes an advanced technique in generating a pure sinusoidal output
current waveform for the high number multilevel single-phase current-source inverter (CSI)
with reduced switching device count. Previous multilevel CSI development proposed a newtechnique in generating pure sinusoidal output by compensating the staircase current with alinear current. This linear current compensation intends to reduce the output filter capacitor
size without sacrificing inverter efficiency. However, this hybrid multilevel CSI would givehigher efficiency and performance if applied in high number of levels. Consequently, thesystem needs high switching device count. Too many switching devices utilization will
decrease the performance and the power density of the CSI as well. Therefore, this technique isintroduced and only possible to be realized if the number of levels is higher than 5-level. By
applying multilayer stage method while managing the DC current module activation
sequences, the high number of levels CSI can be constructed with only using few DC current
modules. As a result, according to the computer comparison simulation, the implementation of
the proposed technique delivers relatively equal results compared to the conventional one justby utilizing fewer switching devices.

1. Introduction
A multilevel inverter is a power electronic device that is capable of providing desired AC voltage levelat the output using multiple lower-level DC voltages as input. In the voltage topology, there are mainly
three different types of the multilevel inverter: 1). Diode clamped, 2). Flying capacitor, and 3).
Cascaded H-bridge [1][2]. However, this kind of multilevel inverters will encounter a problem while
supplying heavy inductive loads such as AC motors. The multilevel voltage-source inverter (VSI) does
not have robust self-protection against the short circuit load as well as having high dv/dt and di/dt
transient behaviors.
On the other hand, the multilevel current-source inverter (CSI) is less applied due to the main
disadvantages: high conduction losses and low power density. The highest efficiency can be achieved
if the multilevel CSI supplies the heavy load with high number of levels. Nevertheless, its advantage
such as having natural protection from short circuit and low dv/dt or di/dt cannot be omitted.
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further
distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Published under licence by IOP Publishing Ltd
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Therefore, the further multilevel CSI development is still needed in to provide an alternative solution
in power electronics [3].
Iolinear

...

0~I/M

I/M

I/M

I/M

Linear Generator
Switching Modules

Polarity
Alternator

Io2

...

Current Sources

IoM-1

QB

QA

...

vo
io Load
Co

QM-1

Q1

Q2

QC

QD

Figure 1. Generalized concept of hybrid multilevel CSI.
This paper discusses new development of the multilevel CSI with multiple DC current source type as adual circuit of the multilevel VSI. Recently, authors [4–8] have reported a new concept of a hybrid
multilevel CSI by incorporating stair case current wave form generator with a linear current wave
form generator as described in figure 1. However, this linear current generator has very poor power
efficiency [9]. Therefore, to reduce the losses significantly, a very a high number of levels must be
applied, so that the required linear compensation current amplitude can be generated as small as
possible.
The problem occurs when applying a high number of levels because the switching device countlinearly conforms to the number of levels. Too many utilizing the switching device will not reduce thelosses significantly and instead it will decrease the power density. Therefore, a new technique forcontrolling and switching the devices to generate pure sinusoidal current output with high number oflevels by using fewer switching devices is proposed in this paper. Hence, the power density may be
improved without sacrificing the power quality and power efficiency of the system.
This paper is composed as follows: first, the operation principle and the circuit configuration of themultilevel CSI are introduced. Next, the control and switching strategies are proposed. In order toevaluate and compare the fundamental operation, several multilevel CSI operations are demonstrated
by computer simulations. From the simulation results, it is possible to generate pure sinusoidal current
output with a high number of levels using fewer switching devices.
2. Operation principle and circuit configuration
Firstly, the basic operation principle and the circuit configuration of the multilevel CSI are introducedin this section.
2.1. Operation principle
As described in figure 1, the hybrid multilevel CSI consists of constant current sources, a linear
current source, switching modules, and a polarity converter (or inverter) that represented by an Hbridge circuit. In order to simplify the circuit from the viewpoint of current generation and powersource configuration, a circuit diagram in figure 2 is presented. The DC current module block consists

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IOP Publishingof a buck chopper-based constant current source and a switching module. Basically, the number of
CSI levels is expressed by:(1)

𝑁𝑁𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 = 2𝑀𝑀 + 1

The M represents the number of total DC current modules and linear current generator that used in a
multilevel CSI system. Figure 3 illustrates the basic operation principle of the hybrid multilevel CSI.

++=DC Current
Module-1

DC Current
Module-(M-1)

DC Source

+

QB

QA

Im/M

vo

io Load0~Im/M...

=

Linear DC
Current
Generator

Im/M

+

...

...

DC Current
Module-2

Im/M

Co
QC

QD

Figure 2. Overview circuit configuration.

Figure 3. Basic operation principle of the 9level hybrid multilevel CSI.

For this instance, the operation of 9-level CSI with 20 Amperes peak-to-peak is presented. In the caseof 9-level CSI, by using equation (1) the number of required DC current modules is four. The first
three modules are used for generating staircase current waveform, meanwhile the remaining one isrequired for generating linear current waveform. After all generated currents are summed up in a DC
link node, a H-bridge is utilized to alternate the polarity of the current whenever the zero-crossing of
the reference is detected. Due to linear current utilization as compensator, a small output capacitor isrequired for smoothing the waveform from high-frequency very-low amplitude ripples.
2.2. DC current module
A DC current source can be generated from DC voltage buck regulator. In order to compact thecircuit, the buck chopper circuit is combined with the switching module part. This merged circuit iscalled DC current module as depicted in figure 4. The operation principle of this circuit is explained intable 1 [4]. The buck chopper circuit consists of a controlled switch Q1, a smoothing inductor L, and afreewheeling diode D1. The Q1 is used for converting the voltage source to a current source byregulating the DC current flowing through the L. To reduce the smoothing inductor size L, a highswitching frequency PWM operation is applied.
The D1 is used to keep continuous current flowing through the L. The switching module part consistsof a controlled switch Q2 and two reverse-blocking diodesD2 and D3. The Q2 directs the current from3

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the L whether flowing forward to the load or circulating back to the chopper. The switching module
part works based on the comparison between the absolute value of the sinusoidal reference with thecurrent limit of each source. Therefore, this part is driven by low switching frequency operation.
IL
L

IN

D3

Q1
D1

OUT

+

VREF

+
-

-

D2

DC Source

RSENSE

Io = VREF / RSENSE
Blocking Diode

Q2

Io

Figure 4. DC current module.

RLOAD

Figure 5. Linear current generator.

Table 1.DC current module operation principle.
Q10
Switching
X

Q2
X
0
1

Output0IL02.3. Linear current generator circuit
The key feature of this multilevel CSI development is applying linear circuit as the current
compensator. The output current from this circuit is superimposed onto staircase current in order toreshape the waveform into pure sinusoidal. As shown in figure 5, the linear current generator isconstructed from class-A amplifier with emitter-follower configuration [10]. In spite of giving better
output THD results, this circuit has an extreme low power efficiency. Therefore, in order to apply thiscircuit, the generated current must be minimized as low as possible. And the only way to do isincreasing the number of levels of the multilevel CSI.
3. Proposed control and switching strategies
In order to increase the number of levels without adding more switching devices, the basic operationof figure 3 needs to be modified. As shown in figure 2, each DC current module must have equalcurrent reference IREF and limit which are determined by the main controller as shown in figure 6. On
the other hand, the proposed method enables the reference and limit of each DC current module are
not necessarily equal. The simplest way to apply this method is by making the current reference andlimit of the next DC current module is half of the previous one as shown in figure 7. The first DC
current module is set to generate a half of IREF, meanwhile the next module is set to generate a half ofthe previous one. This process is repeated until the last utilized module.
Because of this arrangement, a multilayer stage of staircase current waveform generation is formed. Itmeans there is a smaller staircase current waveform inside the larger one as described in figure 8. The
largest staircase current waveform is referred as the first layer or the outmost layer. In case of hybrid
multilevel CSI, the last layer is the linear current compensator. Except the last one, each layer may
contain one or more DC current modules that depends on the design requirements.
In case of figure 7, each layer contains one DC current module. This arrangement allows the DC
current module-1 (DCM-1) to generate the current amplitude two times compared to the conventional

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IOP Publishingmethod. DCM-2 generates an equal rating but with three times switching frequency. And DCM 3
generates the current with a half of the conventional method but with seven times switching frequency.
Figure 9shows the operation changes that occur as a result of implementing this method. This
proposed method also affects the linear compensator current. It generates a half compared to theconventional method. Due to smaller amperage, the losses that dissipated by the linear circuit should
be reduced.
DC Current
Module-1

Linear DC
Current
Generator

Staircase
Generator

IREF
+

Plant

–

DC Source

Linear
Generator

RMS

DC Current
Module-(M-1)

0~Im/2

Im/2

Im/4

...

...

VREF

PI
Controller+–

DC Current
Module-2

Im/2

QB

QA

M-1

vo
io Load

M

...

Co
QC

QD

vo

Figure 6. Main controller.

Figure 7. Simplified proposed method.

+

[A]
Im/2

DCM-1

DCM-2

Im/4

Im/8

DCM-3

Im/16

DCM-M

=

DC CURRENT

+

0
0π

π/8

3π/8
FIRING ANGLE

π/4

π/2

[rad]

Figure 8. Multilayer stage of staircase
current.

Figure 9. Proposed staircase current generation.

Table 2. Possible combination multilevel CSI with 4 current modules (M = 4).
Combination
3–1
2–1–1
1–2–1
1–1–1–1

Number of Layers
23
3
4

Number of Levels
9
13
13
17

Regarding to figure 7 arrangement, the possible maximum number of levels can be determined as:′
𝑁𝑁𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙(𝑀𝑀𝑀𝑀𝑀𝑀)= 2𝑀𝑀 + 1

(2)

It is inferred the equation (2) enables the system by using only four current sources or module (M= 4)
can generate the multilevel current up to 17 levels.

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3.1. Multilayer stage combination
As mentioned above, the arrangement is not necessarily following the equation (2) which one layer isperformed by one DC current module. In case of high rating current application, it will be difficult tobe done because the first DC current module must employ a large smoothing inductor. Therefore, one
layer of staircase generator may be performed by more than one module. For instance, the given
system is with four DC current modules.
The possible combination to construct a multilevel CSI is shown in table 2. The first combination with
2 layers is referred as the conventional method because it follows the equation (1). The following
combinations with 3 layers are presented as alternative methods to increase the number of levels of the
CSI. The last combination with 4 layers is referred to case of figure 7 and it follows the equation (2) toconstruct a CSI with the highest number of levels.
The number of levels can be calculated from such combination by this procedure:
1. Firstly, the number of layer and the possible combination are determined.
2. The combination consists of head number that refers to outmost layer, body numbers that referto inner layers, and the tail number that refers to the last layer and this is always one.
IL1
P-I
Controller

–

+

Q1-2

–

VREF

1/2

1/8

1/4

–

+

P-I
Controller–

IL(M-1)

P-I
Controller–(2M-1)-1

+

+–

–

+

1/8

P-I
Controller

(2

RMS

Q2-2

+–

–
+–

+

M-1 -1)

M = log2 (NLEVEL-1)
(a)

Q2-1

+–

...

(b)

1/4

...

...
Q1-(M-1)

+

+

+

QD

|ABS|

IL3

–

+

–

...

Q1-3

–

1/2

+

IL2
P-I
Controller

–

+

–

QA
H-Bridge
Overlap
QB
Time
QC
Compensation

+

Q2-3

...

+

Q1-1

(c)

AC 1 Vpp–+–

Q2-(M-1)(d)Linear
Generatorvo(e)Figure 10. Complete control system for the proposed method.
3. The head number and body numbers represent the number of stages which equals to thecorresponding number plus one. For example, if the head/body number is 4, then the numberof stages is 5.
4. All the number of stages that represented by the head and body numbers are multiplied andaccumulated. The tail number is neglected.
5. Equation (1) is then used to calculate the final number of levels.
6. For instance, the given combination of 10 current modules with 4 layers is 4-3-2-1. The headis 4 that represents 5 stages. The first body is 3 that represents 4 stages. And the second bodyis 2 that represents 3 stages. Hence, the number of levels that constructed from thiscombination is [2 x (5 x 4 x 3)] + 1 = 121-level.

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3.2. Overview control system
Figure 10 describes the complete control system for the proposed method that conforms to the circuit
diagram in figure 7. Technically, this control scheme is divided into five sections, they are: (a) load
voltage controller, (b) buck chopper current controller, (c) polarity inverter, (d) staircase current
waveform generator, and (e) linear current wave form generator.
4. Simulation results
In order to verify the proper operation of the proposed method, the multilevel CSI with four DC
current modules (M = 4) and with multilayer stage combination that listed in table 2 are tested andcompared by using simulation application PSIM. Prior to the simulation, several parameters are
determined as listed in table 3.
Figure 12 is the simulation result of the conventional 9-level CSI with stage combination 3-1 in twolayers. Figure 13 is the simulation result of 13-level CSI with stage combination 2-1-1 in three layers.
Figure 14 is the simulation result of 13-level CSI with stage combination 1-2-1 in three layers. And
figure 15 is the simulation result of 17-level CSI with stage combination 1-1-1-1 in four layers.
Four figures show minor differences on the AC output. The obvious differences are the staircase andlinear currents as well as the DC current waveforms. The critical differences can only be presented inquantity measurements such as total harmonic distortion (THD) and power efficiency.
Table 3. Simulation parameters.
Parameter
DC voltage
Inductor
Filter capacitor
Load
DCM IGBT VCE/RCE ON
DCM IGBT VD/RD
Bridge IGBT VCE/RCE ON
Bridge IGBT VD/RD

Value
200 V
560 uH/0.09 Ω
2.2 uF/1 mΩ
5 Ω/1 mH
1.4 V/20 mΩ
1.0 V/20 mΩ
1.8 V/20 mΩ
1.2 V/20 mΩ

Parameter
DCM blocking diodeBridge blocking diodeBJT hFE gain
BJT bias/VCE sat
Sense resistor
Operational amplifier
Sawtooth frequency
Output voltage

Figure 12. Conventional 9-level CSI

Value
1.0 V/20 mΩ
1.0 V/12.5 mΩ
1000
5.0 V/1.0 V
1ΩIdeal
30 kHz100 V/60 Hz

Figure 13. 13-level CSI with 2-1-1 combination

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Figure 14.13-level CSI with 1-2-1 combination
Parameter
LayersStage combination
Load current
Load voltage
DC current
THD-I
THD-V
THD-I before filtering
Efficiency

Parameter
LayersStage combination
Switch count
Diode countInductor countLoad current
Load voltage
DC current
THD-I
THD-V
THD-I before filtering
Efficiency

Figure 15.17-level CSI

Table 4. Recorded data of simulation
9-level13-level
13-level
2
3
3
3-1
2-1-1
1-2-1
20.0 A
20.0 A
20.0 A
100.5 V
100.7 V
100.6 V
13.0 A
12.5 A
12.5 A
0.76 %
0.85 %
0.92 %
1.91 %
1.95 %
2.04 %
2.04 %
2.04 %
2.02 %
77.0 %
80.5 %
80.3 %

17-level
41-1-1-1
20.0 A
100.2 V12.1 A
0.90 %
2.05 %
2.02 %
81.8 %

Table 5. Conventional vs proposed
13-level
13-level
17-levelconventional
proposedconventional
2
3
2
5-1
2-1-1
7-1
10
6
14
15
9
21
5
3
7
20.2 A
20.0 A
20.2 A101.2 V
100.7 V
101.5 V12.6 A
12.5 A
12.3 A
1.22 %
0.85 %
1.69 %
2.44 %
1.95 %
3.28 %
3.20 %
2.04 %
4.59 %
80.9 %
80.5 %
82.9 %

17-levelproposed4
1-1-1-1
6
9
3
20.0 A
100.2 V12.1 A
0.90 %
2.05 %
2.02 %
81.8 %

Table 4 shows the comparison among CSI with 3 DC current modules and a linear current
compensator, but they have different stage combination configurations. The worst efficiency waspresented by 9-level CSI with 3-1 stage combination or 9-level conventional with 77.0 % power
efficiency. The best performance was presented by 17-level CSI with 1-1-1-1 stage combination.
Despite having highest THD-V with 2.05 %, the 17-level CSI resulted the highest efficiency with 81.8%.

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While table 5 shows the comparison of the corresponding level between conventional method and the
proposed one. It is shown the proposed method CSI didn’t give better efficiency, but it gave better
THD with fewer devices and inductors. With slightly different in efficiency, the proposed method canbe considered as a solution for constructing a multilevel CSI with high number of levels.
5. Conclusions
A new technique in generating a pure sinusoidal output current for high number of levels of themultilevel CSI with using fewer switching devices has been presented with simulation results. By
arranging the activation of the current modules, the needs of switching devices of the multilevel CSI
can be reduced. However, due to linear circuit utilization, the efficiency of multilevel CSI that uses
some current modules never reaches 90%, although it gives very good THD results. It is better toapply this method in a very high number of levels with many current modules, so that the linear
compensating current is extremely decreased. Moreover, in case of high current rating application, thecombination of the conventional and the proposed methods may give a solution to reduce the inductor
size for the current modules that applied in the outmost layer. Therefore, the proper selection of thestage combination will determine the final result. In order to verify the simulations, the multilevel CSI
that controlled by using the proposed method is currently being investigated in several hardware
experimental setups.
Acknowledgment
Eka Rakhman Priandana was supported by Noguchi Laboratory of Shizuoka University and by
Research and Innovation in Science and Technology Project (RISET-PRO), Ministry of Research,
Technology, and Higher Education of Indonesia. Any opinions, findings, and conclusions expressed inthis material are those of the authors, and do not necessarily reflect the views of the funding agencies.
Authors also would like to gratitude anonymous reviewers for their very helpful and constructive
comments, which improved this manuscript from the original.
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