Novel H-Bridge Multilevel Current-Source PWM
Inverter with Inductor-Cells
SurosoToshihiko Noguchi

Department of Electrical Engineering
University of Jenderal Soedirman
Purwokerto, Jawa Tengah 53122, Indonesia
suroso.te.unsoed@gmail.com

Department of Electrical Electronic Engineering
Shizuoka University
Hamamatsu, Shizuoka 432-8561, Japan
ttnogut@ipc.shizuoka.ac.jp

Abstract--The paper proposes a new circuit configuration ofmultilevel current-source inverter (CSI). In this new topology, abasic H-Bridge CSI working as a main inverter is connected inparallel with inductor-cells operated as auxiliary circuits. The
inductor cell is composed by four unidirectional power switcheswith an inductor across the cell circuit. The inductor-cells work
generating the intermediate level currents to obtain a multilevel
current waveform without additional external DC power sources.
A simple PI controller is applied to control the intermediate level
currents of multilevel output waveform. Five-level and nine-level
PWM inverter configurations, with chopper based DC current
source, are verified through computer simulations. Furthermore,
an experimental prototype of a five-level CSI is setup and is tested.
The results show that the proposed circuit works properly togenerate the multilevel output current waveform with low
harmonics distortion by using small inductors.
Keywords-current-source
multilevel

inverter;H-bridge,inductor-cell;

I. INTRODUCTION
Recent development of high-performance semiconductor
power switches such as MOSFETs and IGBTs increases theresearch interest in high power converters such as multilevel
voltage source inverters (VSI) and its dual circuit, multilevel
current source inverters (CSI). They have capability to deliver
higher output power with lower dv/dt or lower di/dt and lessdistorted output waveforms resulting in reduction of EMI noiseand size of output filter [1], [2].
In distributed generation application, as most renewableenergy sources, such as photovoltaic system, deliver dc power,
the generated power must be converted to ac power and is fed
into the grid through a grid connected inverter [3], [4]. Various
international standards, like IEEE-1547, IEEE-929 and EN61000-3-2, impose requirements on the inverter’s output power
quality, such as harmonic currents and total harmonics
distortion (THD) of the output current. Multilevel CSI is one ofeffective solutions to tackle such problem. Control of the gridconnected CSI is comparatively simpler than its counterpart,
VSI. A grid connected CSI can buffer the output from gridvoltage fluctuation, generates a predetermined magnitude of978-1-4244-7397-7/10/$26.00 ©2010 IEEE

current to the grid without ac current feedback loops, and canachieve a high power factor operation abilities [3]. Its output
current is less affected by grid voltage, and it has inherent shortcircuit protection. Moreover, the discrete diodes connected inseries with the power switches to obtain unidirectional powerswitches in CSI will be unnecessary because new reverseblocking IGBTs are emerging [5].
Some topologies of multilevel CSIs have been proposed byresearchers and engineers. A conventional method to generate
the multilevel output waveforms is by paralleling some threelevel H-Bridge CSIs as shown in Fig. 1 [6]-[8]. However, therequirement of many isolated DC current sources is a problem
introduced by this configuration. Another topology ofmultilevel CSI is by applying multi-cell topology of CSI,
which is the dual converter of flying capacitor multilevel VSI[9]-[11]. But this topology has drawback with its hugeintermediate inductors and its control complexity forbalancing control of the intermediate level currents.
References [12] and [13] presented the common-emitter
configuration (or “fish bone structure”) of multilevel-CSI
obtained by connecting two-level CSI modules with the threelevel common-emitter CSI. This configuration has a great
advantage because all of the power switches are connected at acommon-emitter point. This topology needs only a single
isolated gate drive circuit to drive all power switches of theinverter, hence the complexity of the gate drive circuits can bemoderated.
This paper proposes a circuit configuration of a new
multilevel CSI. In this new topology, a basic H-bridge CSI
working as a main inverter circuit is connected in parallel withinductor-cells. The inductor-cells work generating theintermediate level currents of the output to obtain a multilevel
current waveform without any additional external dc powersources. The operating performance of the proposed multilevel
CSI is examined and tested through some computer
simulations and experimentally.
II. CIRCUIT CONFIGURATION AND OPERATION
PRINCIPLE
A. Operation Principle of Proposed Multilevel CSI445

IPEC 2010

Fig. 2 shows a configuration of proposed inductor-cell
circuit composed by four unidirectional controlled powerswitches QC1, QC2, QC3 and QC4, and an inductor Lc
connected across the cell circuit. The newly proposed
configuration of the multilevel CSI can be obtained byconnecting the three-level H-Bridge CSI with a single or more
inductor-cells as shown in a schematic diagram of the
proposed multilevel CSI in Fig. 3. A five-level CSI
configuration is obtained by connecting a single inductor-cell,a nine-level CSI configuration is achieved by connecting twoinductor-cells in parallel with the main three-level H-bridge
CSI, and so forth. The relation between the level number ofthe output current waveform (M) and the number of theinductor-cells (N) can be formulated as expressed by theequation below:(1)
M = 2( N +1) + 1 .
Fig. 4 and 5 show the configurations of five-level and ninelevel CSIs using the proposed strategy, respectively.
The inductor-cells work generating the intermediate level
currents of a multilevel output waveform from the basic threelevel current of H-bridge CSI. It utilizes the charging anddischarging operation modes of the inductor. Fig. 6 shows theoperation modes of the inductor-cell during a positive cycle
operation of the five-level CSI. Charging operation mode ofthe inductor Lc is conducted when the switches QC1 and QC3Fig. 3. Proposed configuration of multilevel CSIFig. 4. Proposed five-level CSIFig. 5. Proposed nine-level CSI

Fig. 1. Parallel H-Bridge five-level CSI(a) Charging mode of inductor-cell(b) Discharging mode of inductor-cell
Fig. 6. Operation modes of inductor-cell

Fig. 2. Proposed inductor cell circuit

446

TABLE I. SWITCHING STATES OF FIVE LEVEL CSI

Q1

Q2

Q3

Q4

Qc1

Qc2

Qc3

Qc4

Output1010

1

100+I0

1

1

10

100

10

10

10

10+I+I/2

100

10

10

1+I/2

100111000101010000111001101

1000100
1011101-I/2-I/2-I

01011100-I

current is kept at the constant value of I/2. The inductorcell works in the charging mode even though the negative
cycle of the output.
Discharging mode: Q2, Q3, QC1 and QC3 are turned on,
while Q1, Q4, QC2 and QC4 are turned off, causing thestored energy in the inductor is discharged to the load as acurrent -I/2. The inductor cell is in the discharging mode;and5) Current level –I
Q2, Q4, QC3 and QC4 are turned on, while Q1, Q3, QC1and QC2 are turned off, making the current -I flow to theload.

0

are turned on, while QC2 and QC4 are turned off. A current
ILc=I/2 flows through the power switches QC1 and QC3 whichenergizes the inductor Lc. Discharging operation mode isachieved by turning on the switches QC2 and QC4 and byturning off QC1 and QC3. The stored energy in the inductor isdischarged to the load as a current I/2. Similar operationmodes occurred for the negative cycle of output current
waveform. For M-level CSI, if the DC current-source of themain H-bridge CSI is assumed to have an amplitude I, thecurrent flowing through the Nth inductor cell ILC(i) is expressedasI Lc (i ) =I, i=1, 2,… N.
2iFig. 7. Five-level CSI with chopper based DC current source(2)

The output current levels of the five-level CSI are +I, +I/2, 0, I/2, and -I current levels. For nine-level CSI, the outputs are +I,+3I/4, +I/2, +I/4, 0, -I/4, -I/2, -3I/4, and -I current levels.
TABLE I lists the switching states of the proposed five-level
CSI.
The five output current levels +I, +I/2, 0, -I/2 and -I of fivelevel CSI are generated as follows:
1) Current level +I
Q1, Q3, QC1 and QC2 are turned on, while Q2, Q4, QC3
and QC4 are turned off, making the current +I flow to theload;2) Current level +I/2
Charging mode: Q1, Q3, QC1 and QC3 are turned on,
while Q2, Q4, QC2 and QC4 are turned off, making thecurrents of +I/2 flow to the load and to the inductor-cell at
the same time. The inductor-cell is energized in thecharging mode.
Discharging mode: Q1, Q4, QC2 and QC4 are turned on,while Q2, Q3, QC1 and QC3 are turned off, making thestored energy in the inductor be released to the load as acurrent +I/2. The inductor-cell is in the discharging mode;3) Null current level
Q1, Q4, QC1 and QC2 are turned on, while Q2, Q3 QC3and QC4 are turned off, making the current loop for input
dc current source and inductor-cell current. No currentflows to the load;4) Current level –I/2
Charging mode: Q2, Q4, QC2 and QC4 are turned on,
while Q1, Q3, QC1 and QC3 are turned off, making thecurrent of –I/2 flow to the load, and the inductor-cell447

Fig. 8. Multi-carrier based sinusoidal PWM

Fig. 9. Control diagram of proposed five-level CSI

discharging modes of the inductor-cell. In case of the ninelevel CSI, the control circuit of the second inductor-cell issimilar to the first inductor-cell mentioned above. The
difference is only the reference value of the second inductorcell current ILc2, which is quarter of the main dc currentsource amplitude. Therefore, for M level CSI, if the dc current
source of the main H-bridge CSI is assumed to have amplitude
I, the current flowing through the Nth inductor-cell ILc isexpressed as in (2).

TABLE II. TEST PARAMETERS
Smoothing inductor and inductor
1 mH and 5 mHcell
Power source voltage
160 V
Inverter switching frequency

22 kHzFilter capacitor Cf5 μF

LoadOutput current frequency

R = 6.2 Ω , L =1.2 mH
60 Hz

B. DC Current SourceIII. SIMULATION RESULTS

In the proposed multilevel CSI, the dc current source isindispensable. In order to test the proposed multilevel CSI, thedc current source is obtained by employing a chopper with asmoothing inductor connected with the main three-level Hbridge inverter. The chopper consists of a controlled switchthat regulates the dc current flowing through the smoothing
inductor Li. A free-wheeling diode is used to keep continuous
current flowing through the smoothing inductor. The chopper
works as a regulated dc current source. Fig. 7 shows the fivelevel CSI configuration with the chopper based dc current
source. The power source (Vin) may be batteries, PV modules,
a fuel-cell or a rectifier.
A simple proportional integral (PI) regulator is applied tocontrol the dc current flowing through the smoothing inductor,
which determines the amplitude of the PWM output current
waveform Ipwm simultaneously. Making the inductor current
follow the reference current is an objective of this current
regulator. The switching gate signals of the chopper switch(Qc) is generated by comparing the error signal of the detected
steady state inductor current, and a triangular waveform after
passing through the PI regulator. This signal is used to control
the duty cycles of the chopper switch to obtain a stable dc
current waveform even a small smoothing inductor is used.

In order to test the proper operation of the proposed
multilevel CSI topology, a five-level and a nine-level CSI
configurations as shown in Fig. 4 and 5 with chopper based dc
current source were tested by using computer simulation witha PSIM software. The test parameters are listed in TABLE II.
Fig. 10(a) shows the computer simulation result of the
proposed five-level CSI, where the five-level and the load
current waveforms are presented. Fig. 10(b) shows the dc
input current and the inductor-cell current waveforms. Fig.
11(a) shows another computer simulation result of proposednine-level CSI showing the nine-level and the load current
waveforms. Fig. 11(b) shows the dc input current, the first(ILc1) and the second inductor-cell (ILc2) current waveforms ofthe nine-level CSI. As can be seen in the result, the amplitudesof the first and the second inductor-cell currents are properly
driven to be 50% and 25% of the 8-A dc input current,
respectively.
IV. EXPERIMENTAL TEST RESULTS
In order to verify and to prove feasibility of the proposed
multilevel CSI configuration, a laboratory prototype of thefive-level CSI was constructed with IXFK90N30 power
MOSFETs in series with DSEI120-06A fast recovery diodes.
The implemented circuit specifications are identical with thecomputer simulation parameters in TABLE II.
Fig. 12(a) shows the experimental waveforms of the fivelevel CSI, i.e., an 8-A, 60-Hz five-level PWM output current
and a load current waveforms with modulation index 0.9. Fig.
12(b) shows the current waveforms flowing through thesmoothing inductor and inductor-cell. The amplitude of theinductor-cell current is confirmed to be half of the 8-A dc
input current. The inverter worked properly generating a fivelevel output current waveform. In addition, a low distorted
sinusoidal load current waveform is also obtained after
filtering by a small 5-μF filter capacitor. All of theexperimental waveforms agree with those of the computer
simulation results. The measured THD value of the five-level
PWM current is 2.93%.

C. PWM Technique and Inductor-Cell Control
In order to achieve a low distortion of the output current
waveform, a pulse width modulation (PWM) technique isapplied. In this paper, a level-shifted multi-carrier basedsinusoidal PWM technique is employed to generate gate
signals for the CSI power switches and to obtain the PWM
current waveforms as shown in Fig. 8. A schematic control
diagram including the current controller of the chopper and theinductor-cell for the five-level CSI is shown in Fig. 9.
The control circuit of the inductor-cell functions to control
the operation modes, i.e. charging and discharging modes, ofthe inductor-cell (Lc). The current flowing through theinductor-cell ILc is kept constant. It generates the intermediatelevel currents based on the output waveform of the H-Bridge
CSI. A PI regulator is applied to zero the error between thedetected current flowing through the inductor-cell and thereference current which is half of the main dc current to obtainstable and balanced intermediate level currents. The output of
the PI regulator is modulated by a triangular carrier to generate
the control signal i[0] determining the operation modes of theinductor-cell. The frequency of the triangular carrier waveform
determines the switching frequency of the inductor-cell’s
power switches which also regulates the charging and theV. CONCLUSIONSIn this paper a new configuration of multilevel CSI, which
employs inductor-cells as auxiliary circuits has been proposed.
The inductor-cells are connected in parallel with the main Hbridge CSI to generate multilevel output current waveforms
without additional external dc power sources. A chopper based
dc current source is also introduced in order to reduce the size448

of the smoothing inductor. The validity of the proposed
topology has been verified through computer simulations andexperimentally. The simulation and experimental results showthat using the proposed multilevel CSI a low distortion of
output current can be obtained by using small inductors.
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(a) Five-level and load current waveforms(b) Dc input current ILi and inductor-cell current ILcFig. 10. Simulation result of the proposed five-level CSI449

(a) Nine-level and load current waveforms(a) Five-level and load current waveforms(b) Dc input current and inductor-cell current waveforms
Fig. 11. Simulation result of nine-level CSI(b) Dc input current and inductor-cell current waveforms
Fig. 12. Experimental test result of five-level CSI450