Extended Summary

本文は pp.808–815

New Multilevel Current-Source PWM Inverterwith Full-Bridge Inductor Cells
Toshihiko Noguchi Member (Shizuoka University, ttnogut@ipc.shizuoka.ac.jp)
Suroso Member (University of Jenderal Soedirman, suroso.te.unsoed@gmail.com)
Keywords: current-source inverter, inductor cell, multilevel
This paper proposes a new circuit configuration of a multilevel
current-source inverter (CSI). In this new topology, a three-level
CSI of which all of the power switches are connected at the identical potential level line works as a main inverter circuit, and inductor
cells are connected in parallel with the main inverter as auxiliary
circuits. The inductor cells are composed by full-bridge power converters with inductors across the bridges. The inductor cells workto generate intermediate level currents of the output to obtain a multilevel current waveform without any additional external DC powersources.
Fig. 1 shows a schematic diagram of the proposed multilevel CSI.
A five-level CSI configuration is obtained by connecting a single inductor cell, a nine-level CSI configuration is achieved by connecting
two inductor cells in parallel with the main three-level CSI, and soforth. The relation between the level number of the CSI output current waveform and the number of the inductor cells can be obtained
from the equation below:

Fig. 1. Generalized configuration of proposed multilevel CSI

M = 2(N+1) + 1, · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (1)where M is the level number of the output current and N is the number of inductor cells. For M−level CSI, if the DC current sources
are assumed to have identical current amplitude I, the current flowing through the N th inductor cell ILcN is expressed as
ILcN =

I, · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (2)2N

Fig. 2. Five-level CSI with single inductor cellIn the proposed multilevel CSIs, the DC current-sources are indispensable, which can be obtained by employing choppers withsmoothing inductors connected with the main three-level inverter.
The choppers work as DC current-sources. The choppers consist of
two controlled switches that regulate a pair of the DC currents flowing through the two smoothing inductors L1 and L2 . Fig. 2 shows
the concrete five-level CSI configuration with the choppers based
DC current-sources. It should be noted that only a single DC voltage
source is required and is connected to the CSI. Proportional integral(PI) regulators are independently applied to control two of the DC
currents flowing through the smoothing inductors L1 and L2 , which
determine the amplitude of the PWM output current waveform, andthe current flowing through the inductor cell Lc .
The operation performance of the proposed multilevel CSI is examined and is validated through some computer simulations. Furthermore, a laboratory experimental prototype of a five-level CSI
was set up to verify the proper operation of the proposed new topology. Fig. 3 shows the experimental waveforms of the five-level CSI,
i.e., showing the steady state 8-A five-level output current and theload current waveforms. The total harmonic distortion of this fivelevel output current is 3.3%.
Fig. 3. Experimental waveforms of five-level CSI– 13 –

Paper

New Multilevel Current-Source PWM Inverterwith Full-Bridge Inductor Cells
Toshihiko Noguchi∗
Suroso∗∗,∗∗∗

MemberMemberThis paper proposes a generalized circuit configuration of a new multilevel current-source inverter (CSI). In this new
topology, a three-level CSI of which all the power switches are connected at an identical potential level line works asa main inverter circuit, and inductor cells are connected in parallel with the main inverter as auxiliary circuits. The
inductor cells are composed by full-bridge power converters with inductors across the bridges. The inductor cells workto generate intermediate level currents of the output to obtain a multilevel current waveform without any additional
external DC power sources. Five-level and nine-level PWM inverter configurations, including their chopper circuits
as DC current-power sources, are verified through computer simulations. Furthermore, an experimental prototype of afive-level CSI is setup and is tested. The results show that the circuit works properly to generate the five-level output
current waveform, which proves feasibility of the proposed strategy.
Keywords: current-source inverter, inductor cell, multilevel

1.

CSI (5) (7) . However, the series diode might not be necessary because new IGBTs with reverse-blocking capability(reverse-blocking IGBTs) are emerging (6) (8) . Furthermore,in order to make the multilevel CSI topology more attractive and aﬀordable, epoch-making ideas to create novel CSI
topologies are required.
In this paper, new configurations of the multilevel CSI, applying full-bridge inductor cells, are presented. The inductor
cells work to generate the intermediate current-levels throughcharging and discharging operation modes of the inductors
with no additional external DC power sources. The operation performance of the proposed multilevel CSI is examined
and is validated through some computer simulations. Furthermore, a laboratory experimental prototype of a five-level
CSI was set up to verify proper operations of the proposed
new topology, using the power MOSFETs with series blocking diodes.

Introduction

The fast ever-lasting development of power devices working at high switching frequencies for medium and high power
applications such as metal-oxide-semiconductor field-eﬀect
transistors (MOSFETs) and insulated gate bipolar transistors(IGBTs) has improved power converter performance. Development of high-performance semiconductor switches also
increases the research interest in high power converters suchas multilevel inverters because they have capability to deliver
higher output power with low-voltage or low-current rating
devices, lower dv/dt and less distorted output waveforms resulting in reduction of the filter size, compared with the conventional two-level inverters (1) (3) (4) .
In general, the inverter topologies can be classified into
voltage-source inverters (VSI) and their dual circuits, i.e.,current-source inverters (CSI). The VSI has a DC voltage
power source and generates AC voltage waveforms to theload, while the CSI delivers AC current waveforms from DC
current power sources. The latter features capability of shortcircuit protection because of its high-impedance DC powersource, but requires protection against an open-circuit toguarantee continuity of the current. The VSI is more widelyused than the CSI topology due to some drawbacks such asa heavy inductor used to obtain a smooth DC current anddiscrete diodes connected in series with the power switches.
These drawbacks often degrade the overall eﬃciency of the2.

2.1 Operation Principle of Multilevel CSI
Fig. 1(a)shows a basic circuit configuration of a three-level CSI proposed in the authors’ previous research work (2) . All of thepower switches Q1, Q2, Q3 and Q4 are connected to acommon-source (common-emitter) line. Table 1 lists theswitching states of this three-level CSI for three-level output
current waveform generation of +I, 0 and −I current levels.
Furthermore, Fig. 1(b) shows a configuration of a proposed
full-bridge inductor cell composed by four controlled powerswitches Q5, Q6, Q7 and Q8, series blocking diodes and an
inductor Lc connected across the bridge.
The newly proposed configuration of the multilevel CSI
can be obtained by connecting the former three-level CSI asa main inverter circuit, and a single or more inductor cells asshown in a generalized schematic diagram of the proposed∗ Department

of Electrical and Electronic Engineering, Shizuoka
University
Hamamatsu, Shizuoka 432-8561∗∗ Department of Electrical Engineering, University of Jenderal
Soedirman
Purwokerto, Jawa Tengah 53122, Indonesia∗∗∗ Department of Energy and Environment Engineering, Nagaoka
University of Technology
Nagaoka, Niigata 940-2188
c 2010 The Institute of Electrical Engineers of Japan.
Circuit Configuration and Operation
Principle

808

Multilevel CSI with Full-Bridge Inductor Cells(a) Fundamental three-level CSI(b) Inductor cellFig. 1. Three-level CSI and inductor cellFig. 3. Five-level CSI with single inductor cell

Table 1. Switching states of three-level CSI(a) Charging mode of inductor cellFig. 2. Generalized configuration of proposed multilevel CSI(b) Discharging mode of inductor cellFig. 4. Operation modes of inductor cell

multilevel CSI in Fig. 2. The inductor cells are used to generate a multilevel output current from the basic three-level current waveform by controlling charging and discharging operation modes of the inductors.
A five-level CSI configuration is obtained by connecting
only a single inductor cell, a nine-level CSI configuration isachieved by connecting two inductor cells in parallel with themain three-level CSI, and so forth. The relation between thelevel number of the output current waveform and the numberof the inductor cells can be expressed as the equation below:

W=

and the discharging operation mode is achieved by turning on
the switches Q5 and Q8 and by turning oﬀ Q6 and Q7. The
stored energy in the inductor is discharged to the load as acurrent I/2. In case of a resistive load, the inductor value canbe found as
Lc =M = 2(N+1) + 1, · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (1)I, · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (2)2N

Fig. 3 shows an example of a five-level CSI configuration.
Furthermore, Fig. 4 shows the operation modes of the inductor cell during a positive cycle operation. Charging operation
mode of the inductor Lc is achieved by making the switches
Q6 and Q7 turned on, while switches Q5 and Q8 are turnedoﬀ. A constant current ILc = I/2 flows through the powerswitches Q6 and Q7 which energizes the inductor Lc to its
maximum energy W expressed as
電学論 D，130 巻 6 号，2010 年

ILc R, · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (4)f s ΔILc

where ILc is an inductor cell current (A), R is a load resistor(Ω), f s is a switching frequency of the inductor cell circuit(Hz), and ΔILc is an acceptable current ripple of the inductor
cell current (A).
Using this new multilevel CSI topology, several advantages can be obtained as follows:( 1 ) The power switches of the main three-level inverter
are connected to an identical potential line; hence they
can be driven by using only a single non isolated gate
drive power supply.
( 2 ) The level of the output current can be increased
by connecting some inductor cell circuits to the main
three-level inverter without any additional DC powersources.
( 3 ) All of the inductor cells have the same configuration and allow a simple and modular structure, which

where M is the level number of the output current and N is thenumber of the inductor cells. For M level CSI, if the two DC
current-sources are assumed to have identical current amplitude I, the current flowing through the N th inductor cell ILcN
is expressed as
ILcN =

1 2
Lc I , · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (3)
2 Lc

809

Table 2. Switching states of five-level CSI with inductor
cell(a) Current +I(b) Current +I/2 (charging mode)leads to cost reduction in manufacturing process.
The switching state combinations required to generate afive-level current waveform are listed in Table 2 and the moredetailed operation modes of the five-level CSI are illustrated
in Fig. 5(a) to (g). Power device utility and average switching frequency between Q6Q8 and Q5Q7 in the circulating
modes of the inductor cell current are considerations to use
redundant switching states for multilevel current generation.
It is also related to the heat distribution between the switches
Q6Q8 and Q5Q7 caused by their switching and conduction
losses. The five output current levels +I, +I/2, 0, −I/2 and −I
are generated as follows:( 1 ) Current level +I
Q1, Q2, Q5 and Q7 are turned oﬀ, while Q3, Q4,
Q6 and Q8 are turned on, making the currents +I flow
to the load;( 2 ) Current level +I/2
Charging mode: Q1, Q2, Q5 and Q8 are turned oﬀ,
while Q3, Q4, Q6 and Q7 are turned on, making thecurrents of +I/2 flow to the load and the inductor cellat the same time. The inductor cell is energized in the
charging mode.
Discharging mode: Q2, Q3, Q6 and Q7 are turnedoﬀ, while Q1, Q4, Q5 and Q8 are turned on, making the stored energy in the inductor be released to theload as a current +I/2. The inductor cell is in discharging mode;( 3 ) Current level 0
Q1, Q4, Q5Q7 or Q6Q8 are turned on, while Q2,
Q3, Q6Q8 or Q5Q7 are turned oﬀ, establishing thecurrent circulation for every DC current-source and in
the inductor cell. No current flows to the load;( 4 ) Current level −I/2
Charging mode: Q3, Q4, Q6 and Q7 are turned oﬀ,
while Q1, Q2, Q5 and Q8 are turned on, making thecurrent −I/2 flow to the load and the inductor cell current is kept at the constant value of I/2. The inductor
cell works in the charging mode even though the negative cycle.
Discharging mode: Q2, Q3, Q5 and Q8 are turnedoﬀ, while Q1, Q4, Q6 and Q7 are turned on, where thestored energy in the inductor is discharged to the load
as a current −I/2. The inductor cell is in discharging
mode; and(c) Current +I/2 (discharging mode)(d) Current 0(e) Current −I/2 (charging mode)(f) Current −I/2 (discharging mode)(g) Current −I

Fig. 5. Switching modes for five-level CSI with inductor cell
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IEEJ Trans. IA, Vol.130, No.6, 2010

Multilevel CSI with Full-Bridge Inductor Cells

Fig. 6. Five-level CSI with single inductor cellFig. 9. Multi-carrier based sinusoidal PWMof the chopper switches are generated by comparing the error
signals of the detected steady state inductor currents, and atriangular waveform after passing through the PI regulator.
These signals are used to control the duty cycles of the chopper switches to obtain the balanced and stable DC currents.
2.3 PWM Inverter and Inductor Cell Control
Inorder to obtain a low-distortion output current waveform, apulse width modulation (PWM) technique is applied to theinverter. In this paper, a level-shifted multi-carrier based sinusoidal PWM technique is employed to generate the gate
signals for the CSI power switches and to obtain the PWM
current waveforms as shown in Fig. 9. All triangular carrier
waveforms are in phase with an identical frequency. The frequency of the modulated signal (a reference sinusoidal waveform) determines the fundamental frequency of the output
current waveform, while the frequency of the triangular carrier gives the switching frequency of the CSI power switches.
An M-level output current waveform using this modulation
technique requires (M-1) triangular carriers with DC oﬀsets.
A schematic diagram including the current controller of theinductor cell for the five-level CSI is shown in Fig. 10. The
control circuit of the inductor cell functions to switch the operation modes, i.e. charging and discharging modes, of theinductor cell. The current flowing through the inductor ILc
is kept constant. It generates the intermediate level currents
based on the output waveform of the main three-level CSI. A
PI regulator is applied to zero the error between the detectedcurrent flowing through the inductor cell ILc and the reference current which is half of the main DC currents IL1 andIL2 . The output of the PI regulator is modulated by a triangular carrier to generate the control signal i[0] determining theoperation mode of the inductor cell. In case of the nine-level
CSI, the control circuit is similar to the first inductor cell controller mentioned above. The diﬀerence is only the reference
value for the second inductor cell current ILc2 , which is quarter of the main DC current-source amplitude. Therefore, if
there are N inductor cells connected in parallel with the main
inverter, the amplitude of the current flowing through the N th
inductor cell is as previously expressed in (2).
2.4 Filter Capacitor
It is necessary to add a filter
capacitor across the load because the inverter works as a current source and the load usually has an inductive component.
The filter capacitor also functions to filter the harmonic components, e.g. switching harmonic components, of the PWM
multilevel output current. The harmonic components of thePWM current should flow into the filter Cf . In general, theparasitic resistor component of the capacitor is much lower

Fig. 7. Nine-level CSI with two inductor cells

Fig. 8. Control diagram of choppers( 5 ) Current level −I
Q3, Q4, Q6Q8 or Q5Q7 are turned oﬀ, while Q1,
Q2, Q5Q7 or Q6Q8 are turned on, making the currents −I flow to the load.
2.2 DC Current Sources
In the proposed multilevel
CSIs, the DC current sources are indispensable, which canbe obtained by employing simple choppers with smoothing
inductors connected with the main three-level inverter. The
choppers work as DC current sources, which consist of twocontrolled switches that regulate a pair of the DC currents
flowing through the two smoothing inductors L1 and L2 . Two
free-wheeling diodes are used to keep continuous currents
flowing through the inductors. Fig. 6 and Fig. 7 show theconcrete five-level and nine-level CSI configurations with the
choppers based DC current sources. It should be noted that
only a single DC voltage source is required and is connectedto the CSI.
A control diagram of the choppers for the DC current
source is presented in Fig. 8. Proportional integral (PI) regulators are independently applied to control two of the DC
currents flowing through the smoothing inductors L1 and L2 ,which determine the amplitude of the PWM output current
waveform Ipwm simultaneously. Making the inductor currents
IL1 and IL2 follow their reference current Ire f is the objective of these current regulators. The switching gate signals
電学論 D，130 巻 6 号，2010 年

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Fig. 10. Control diagram of five-level CSI with inductorcell(a) Five-level and load currents

Fig. 11. Schematic diagram of CSI, filter Cf and load
Table 3. Test parametersthan the load resistance; hence the losses and heat causedby the harmonic components flowing into the filter capacitor are much smaller. Moreover, using the higher switchingfrequency with its constraints, and with the higher level number of the output current, a smaller size of the filter capacitor
is applicable. A proper choice of the capacitor is importantto minimize the heat in the filter, e.g. capacitor with smallequivalent series resistance (ESR).
Fig. 11 shows a circuit diagram of a CSI connected withthe filter capacitor (Cf ) with its internal resistor (RCf ), andthe load (R and L). The equivalent impedance of this circuit
is expressed as1(R + jωL) Rc f − jωC f . · · · · · · · · · · · · · · · · (5)Zeq =1(R + Rc f ) + j ωL −ωC f(b) Waveforms of DC current sources IL1 , IL2 and inductor cell current ILc

Fig. 12. Simulation result of five-level CSI

such resonance in the circuit. In addition, as in dual property with the voltage source inverter, since the inverter behaves as a current source, it should be connected with a capacitive load (including the filter capacitor). Thus, the total
impedance connected to the CSI as expressed in (5) should
be a capacitive. It is another key in choosing the value of the

For this circuit, the resonance frequency (ωo ) can be found as⎤1⎡
1 ⎢⎢⎢⎢ R2C f − L ⎥⎥⎥⎥ 2
ω0 = 
⎥⎦ . · · · · · · · · · · · · · · · · · · · · · (6)⎢⎣LC f R2c f C f − LThis capacitor value should be avoided in order to prevent
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IEEJ Trans. IA, Vol.130, No.6, 2010

Multilevel CSI with Full-Bridge Inductor Cells(a) Nine-level and load currents

Fig. 13. Test result of load step change in five-level CSI(from 4 Ω to 8 Ω)

filter capacitor.
3.

Computer Simulation Results

In order to test the operation of the proposed multilevel
CSI topology, computer simulations are conducted by using
a PSIM software. This section presents the computer simulation results of the proposed five-level and nine-level CSI
configurations. The simulation parameters are listed in Table 3. The five-level and nine-level configurations shown in
Fig. 6 and Fig. 7 are tested with a single DC power source.
Fig. 12(a) shows a computer simulation result of the fivelevel CSI, where the five-level and the load current waveforms are presented. Fig. 12(b) shows the current waveforms
flowing through inductors L1 , L2 and Lc . The amplitude of
the DC current sources are well balanced for both smoothing
inductors IL1 , IL2 , and the current amplitude of the inductor
cell ILc corresponds to half of the peak level of the five-level
output current waveform. Fig. 13 shows the test result of aload step change from 4 Ω to 8 Ω showing the five-level current, inductor cell current and load voltage waveforms at constant output current and constant gains of PI controller.
Furthermore, Fig. 14(a) shows another computer simulation result of the nine-level CSI. Fig. 14(b) shows the DC
current sources IL1 and IL2 , and the first and the second inductor cell current waveforms ILc1 and ILc2 of the nine-level
CSI. The amplitude of the DC current sources are also wellbalanced for both smoothing inductors, and the amplitudes of
inductor cell current waveforms ILc1 and ILc2 are correspondto half and quarter of the peak level of the nine-level output
current waveform, respectively. It is confirmed that the fivelevel and the nine-level CSIs can generate proper multilevel
電学論 D，130 巻 6 号，2010 年(b) Waveforms of DC current sources IL1 , IL2 and inductor cell currents
ILc1 , ILc2

Fig. 14. Simulation result of nine-level CSI

current waveforms.
4.

Experimental Test Results

In order to verify and to prove feasibility of the proposed multilevel CSI configuration experimentally, a laboratory prototype of the five-level CSI was constructed with813

Fig. 15. Experimental result of five-level and load currents

Fig. 16. Experimental result of DC current sources IL1 ,IL2 and inductor cell current ILc

power MOSFETs (FK30SM-5) and fast recovery diodes(HFA16PB120). The implemented circuit specifications are
identical as the computer simulation parameters in Table 3.
Fig. 15 shows experimental waveforms of the five-level
CSI, i.e., a steady state 8-A five-level output current and aload current waveforms. Fig. 16 shows the current waveforms flowing through inductors L1 , L2 and Lc . The inverter
works properly generating a five-level output current waveform. As can be seen in the figure, a low distorted sinusoidal
load current waveform is also obtained after filtering by asmall 5-μF filter capacitor. All of the experimental waveforms agree with those of the computer simulation results.
The ripple diﬀerence between the DC input current IL1 andIL2 is caused by the diﬀerence in the delivering periods ofthe current to the load. In one cycle, a current source supplies for only half cycle of the current to the load. The restof the cycle is a current circulation mode. In addition, thediodes and power MOSFETs switches used in the experimental setup have relatively high forward voltage drop andhigh on-resistance, which introduce a problem in controllingthe inductor cell current, and cause more ripple in the inductor cell current. Using higher-performance diodes and power
MOSFETs, the ripple of the inductor cell current can be reduced. Fig. 17 shows a frequency analysis result of the fivelevel PWM output current waveform including the switchingharmonic component and its sidebands, with total harmonic
distortion (THD) of 3.3%. Fig. 18 shows a frequency spectrum of load current presenting elimination and reduction of
the harmonic components with a small 5-μF filter capacitor.
Fig. 19 shows an eﬃciency characteristic of the prototype
five-level CSI. As can be seen, the eﬃciency is low in a lightload condition which is caused by conduction losses of the inductors and switching devices becoming relatively dominantFig. 17. FFT of five-level output current

Fig. 18. FFT of load current

against the load power. In general, this is a substantial drawback of current-source power converters. However, the eﬃciency increases as the load becomes heavier, and the maximum eﬃciency of the prototype was confirmed to be 87.8%
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IEEJ Trans. IA, Vol.130, No.6, 2010

Multilevel CSI with Full-Bridge Inductor Cells

(4)

(5)

(6)

(7)

(8)

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Fig. 19. Eﬃciency characteristic of five-level CSI
Toshihiko Noguchi (Member) was born in 1959. He received theB.Eng. degree in electrical engineering from Nagoya
Institute of Technology, Nagoya, Japan, and theM.Eng. and D.Eng. degrees in electrical and electronics systems engineering from Nagaoka University ofTechnology, Nagaoka, Japan, in 1982, 1986, 1996,
respectively. In 1982, he joined Toshiba Corporation,
Tokyo, Japan. He was a Lecturer at Gifu National
College of Technology, Gifu, Japan, from 1991 to
1993 and a Research Associate in electrical and electronics systems engineering at Nagaoka University of Technology from 1994
to 1995. He was an Associate Professor at Nagaoka University of Technology from 1996 to 2009. He has been a Professor at Shizuoka University
since 2009. His research interests are static power converters and motor
drives. Dr. Noguchi is a Senior Member of the IEEE.

at the load of 0.8 kW.
5.

Conclusion

In this paper, a new generalized configuration of a multilevel CSI, which employs auxiliary full-bridge inductor cells
has been proposed. A multilevel current waveform is obtained by controlling the charging and the discharging operation modes of the inductor cells connected in parallel withthe main three-level CSI. The validity of the proposed topology has been verified through computer simulations and in
an experimental setup.
(Manuscript received Nov. 3, 2009,
revised Feb. 17, 2010)

Suroso (Member) was born in 1978. He received the B.Eng. degree inelectrical engineering, from Gadjah Mada University,
Indonesia in 2001 and the M.Eng. degree in electricalengineering from Nagaoka University of Technology,
Japan in 2008. He is a Lecturer at electrical engineering department, University of Jenderal Soedirman,
Purwokerto, Jawa Tengah, Indonesia. He is currently
a doctor student at energy and environment engineering department, Nagaoka University of Technology,
Japan. His research interest includes power electronics and its application in renewable energy technology. He is a student member of the IEE-Japan and the IEEE.

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