平成 22年電気学会産業応用部門大会

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A New H-Bridge Multilevel Current-Source PWM Inverter with Inductor Cells
Suroso*, and Toshihiko Noguchi** (*Nagaoka University of Technology, **Shizuoka University)
Abstract-The paper proposes a new circuit configuration of a multilevel current-source inverter (CSI). In this new topology, a basic
H-bridge CSI working as a main inverter is connected in parallel with inductor cells operated as auxiliary circuits. The inductor cell iscomposed by four unidirectional power switches with an inductor across the cell circuit. The inductor cells work generating theintermediate level currents to obtain a multilevel current waveform without additional external DC power sources. A simple PIcontroller is adopted to control the intermediate level currents of the multilevel output waveform. Five-level PWM inverter
configuration, with a chopper based DC current source, is verified through computer simulations. Furthermore, an experimental
prototype of a five-level CSI is setup and is tested. The results show that the circuit works properly to generate the multilevel output
current waveform with low harmonic distortion by using small inductors, which proves feasibility of the proposed strategy.
Keywords: current-source inverter; H-bridge, inductor cell; multilevel

1. INTRODUCTION
Recent development of high-performance semiconductor powerswitches such as MOSFETs and IGBTs increases the researchinterest in high power converters such as multilevel voltage source
inverters (VSI) and their dual circuits, multilevel current source
inverters (CSI). Multilevel inverters have capability to deliver
higher output power with lower dv/dt or lower di/dt and lessdistorted output waveforms resulting in reduction of EMI noise andsize of an output filter [1].
In distributed generation application, as most renewable energy
sources, such as photovoltaic systems, deliver DC power, thegenerated power must be converted to AC power and is fed into thegrid through a grid connected inverter. 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 theoutput current [2]. Multilevel CSI is one of effective solutions totackle such problem. Control of the grid connected CSI iscomparatively simpler than its counterpart, VSI. A grid connected
CSI can buffer the output from grid voltage fluctuation, generates apredetermined magnitude of current to the grid without AC current
feedback loops, and can achieve a high power factor operation [2],[3]. Its output current is less affected by grid voltage, and it has
inherent short circuit protection.
Some topologies of multilevel CSIs have been proposed byresearchers and engineers. A conventional method to generate themultilevel output waveforms is by paralleling some three-level
H-bridge CSIs [4]-[6]. However, the requirement of many isolated
DC current sources is a problem introduced by this configuration.
Another topology of the multilevel CSI is by applying multicell
topology of CSI, which is the dual converter of flying capacitor
multilevel VSI [7]-[9]. However, this topology has drawback withits bulky intermediate inductors and its control complexity forbalancing control of the intermediate level currents. References[10] and [11] presented a novel common-emitter configuration ofmultilevel-CSI obtained by connecting two-level CSI modules
with the three-level common-emitter CSI. This configuration has agreat advantage because all of the power switches are connected ata common-emitter point. This topology needs only an isolated gate
drive circuit to drive all power switches of the inverter, hence thecomplexity of the gate drive circuits can be moderated.

Fig. 1. Proposed configuration of multilevel CSIThis 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 with inductor cells. The
inductor cells work generating the intermediate level currents of theoutput to obtain a multilevel current waveform without any
additional external DC power sources. The operating performanceof the proposed multilevel CSI is examined and tested throughsome computer simulations and experiments.
2. CIRCUIT TOPOLOGY AND OPERATION PRINCIPLE
2.1. Operation Principle of Proposed Multilevel CSI
Fig. 1 shows a schematic diagram of the proposed multilevel
CSI. The newly proposed configuration of the multilevel CSI isobtained by connecting the H-bridge CSI with a single or moreinductor cells. Fig. 2 shows a configuration of the proposed
inductor cell circuit composed by four unidirectional controlled
power switches QC1, QC2, QC3 and QC4, and an inductor Lc
connected across the cell circuit. A five-level CSI configuration isobtained by connecting a single inductor cell, a nine-level CSI
configuration is achieved by connecting two inductor cells inparallel with the main three-level H-bridge CSI, and so forth. The
relation between the level number of the output current waveform(M) and the number of the inductor-cells (N) can be formulated as
expressed by the equation below:

M = 2( N +1) + 1 .

(1)Fig. 3 shows the configurations of a five-level CSI using theproposed strategy. The switching states of this five-level CSI arelisted in TABLE I. A single inductor cell is connected to theH-bridge CSI. The inductor-cells work generating intermediatelevel currents of a multilevel output waveform from the basic[I - 255]平成 22年電気学会産業応用部門大会

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Fig. 2. Proposed inductor cell circuit

Fig. 5. Five-level CSI with chopper based DC current source

Fig. 3. Proposed five-level CSI

Fig. 6. Control diagram of proposed five-level CSI(a)Charging mode of inductor cell(b) Discharging mode of inductor cellFig. 4. Operation modes of inductor cell
TABLE ISWITCHING STATES OF FIVE-LEVEL CSI

three-level current of H-bridge CSI. It utilizes charging anddischarging operation modes of the inductor. Fig. 4 shows theoperation modes of the inductor cell during a positive cycle
operation of the five-level CSI. Charging operation mode of theinductor Lc is conducted when the switches QC1 and QC3 areturned on, while QC2 and QC4 are turned off. A current ILc=I/2
flows through the power switches QC1 and QC3 which energizes
the inductor Lc. Discharging operation mode is achieved byturning on the switches QC2 and QC4 and by turning off QC1 andQC3. The stored energy in the inductor is discharged to the load asa current I/2. Similar operation modes occurred for the negative
cycle of the output current waveform. For M-level CSI, if the DC
current-source of the main H-bridge CSI is assumed to have anamplitude I, the current flowing through the Nth inductor cell ILC(i)is expressed asQ1Q2Q3Q4

Qc1

Qc2

Qc3

Qc4

Output10101100+I

10100011+I10101010+I/2

100

1010

1+I/2

100

11

1000

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 output are +I, +3I/4,+I/2, +I/4, 0, -I/4, -I/2, -3I/4, and -I current levels.

1010
100001110012.2. DC Current Source

010101

1001110
100
10

-I/2
-I/2-I0101

1

100-I

In the proposed multilevel CSI, the DC current source isindispensable. In order to test the proposed circuit, the DC current
source is obtained by employing a chopper with a smoothing
inductor connected with the main three-level H-bridge inverter.

I Lc ( i ) =[I - 256]I, i=1, 2,…, N.
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The chopper consists of a controlled switch that 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 ILi. The chopper works as aregulated DC current source. Fig. 5 shows a circuit configurationof the proposed five-level CSI with the chopper based dc current
source. The power source (Vin) may be batteries, PV modules, afuel-cell or a rectifier. A simple proportional integral (PI) regulator
is applied to control the DC current flowing through the smoothing
inductor, which determines the amplitude of the PWM output
current waveform IPWM simultaneously.
2.3. PWM Technique and Inductor-Cell Control
In order to achieve a low distortion of the output current
waveform, a pulse width modulation (PWM) technique is applied.
In this paper, a level-shifted multi-carrier based sinusoidal PWM
technique is employed to generate gate signals for the CSI powerswitches and to obtain the PWM current waveforms. A schematic
control diagram including the current controller of the chopper andthe inductor cell for the five-level CSI is shown in Fig. 6.
The control circuit of the inductor cell functions to control theoperation modes, i.e. charging and discharging modes, of theinductor cell Lc. The current flowing through the inductor cell ILc
is kept constant. It generates the intermediate level currents basedon the output waveform of the H-bridge CSI. A PI regulator isapplied to zero the error between the detected current flowing
through the inductor cell and the reference current which is half ofthe main dc current and to obtain stable and balanced intermediatelevel currents. The output of the PI regulator is modulated by atriangular carrier to generate the control signal i[0] determining theoperation modes of the inductor cell. The frequency of thetriangular carrier waveform determines the switching frequency of
the inductor cell’s power switches which also regulates thecharging and the discharging modes of the inductor-cell.
In case of the nine-level CSI, the control circuit of the second
inductor cell is similar to the first inductor cell mentioned above.
The difference is only the reference value of the second inductor
cell current, which is quarter of the main DC current source
amplitude. Therefore, for M level CSI, if the DC current source ofthe main H-bridge CSI is assumed to have amplitude I, the current
flowing through the Nth inductor cell ILc is expressed as in (2).

(a)

Five-level and load current waveforms

3. SIMULATION RESULTS
In order to test the proper operation of the proposed multilevel
CSI topology, a five-level CSI configuration with a chopper based
dc current source as shown in Fig. 5 was tested by using computer
simulation with a PSIM software. The test parameters are listed inTABLE II. Fig. 7(a) shows the computer simulation result of theproposed five-level CSI, where the five-level PWM current andthe load current waveforms are presented. Fig. 7(b) shows thesmoothing inductor current and the inductor cell current
waveforms. As can be seen in the result, the amplitude of theinductor cell current is properly controlled at 50 % of the 8-A
smoothing inductor current.
TABLE IITEST PARAMETERS
Smoothing inductor and inductor
1 mH and 5 mHcellPower source voltage
160 V
Inverter switching frequency

22 kHzFilter capacitor Cf

5 μFLoadOutput current frequency

R = 6.2 Ω , L =1.2 mH
60 Hz(b) Smoothing inductor current ILi and inductor cell current ILc waveforms
Fig. 7. Simulation result of the proposed five-level CSI

4. 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
control circuits were designed with mixed-signal electronics using
analog Op-Amps and EPROMs. The opto-isolator based gate drive[I - 257]

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flowing through the smoothing inductor and inductor cell. The
inverter worked properly generating a five-level 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 the experimental waveforms agree with those of
the computer simulation results. The measured THD value of the
five-level PWM current is 2.93%.
5. CONCLUSION
In this paper a new configuration of a multilevel CSI, which
employs inductor cells as auxiliary circuits, has been proposed. The
inductor cells are connected in parallel with the main H-bridge CSI
to generate multilevel output current waveforms without additional
external DC power sources. A chopper based dc current source isalso introduced in order to reduce the size of the smoothing
inductor. The simulation and experimental results show that using
the proposed multilevel CSI a low distortion of output current canbe obtained by using small inductors.
REFERENCES[1](a)

J. Rodiguez, J. S. Lai, and F. Z. Peng, “Multilevel inverter: a surveyof topologies, controls, and application,” IEEE Transaction on
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[3] P. G. Barbosa, H. A. C. Braga, M. C. Barbosa, and E. C. Teixeria,“Boost current multilevel inverter and its application on single phasegrid connected photovoltaic system,” IEEE Transaction on PowerElectronic, vol. 21, p.p. 1116-1124, July 2006.
[4] Z H. Bai, Z. C. Zhang, “Conformation of multilevel current source
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[9] B. P. McGrath, and D. G. Holmes, “Natural current balancing ofmulticell current source inverter,” IEEE Transaction on PowerElectronic, vol. 23, p.p. 1239-1246, May 2008.
[10] Suroso and T. Noguchi, “New generalized multilevel current-source
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2009, p.p. 1968-1975.

Five-level and load current waveforms

ILi (A)

1284ILc (A)0128

40-0.02

0.00

0.02

Time (s)(b) Smoothing inductor current ILi and inductor cell current ILc waveforms
Fig. 8. Experimental test result of five-level CSI

circuits are used in the prototype. The implemented circuitspecifications are identical with the computer simulation
parameters in TABLE II. Fig. 8(a) shows the experimental
waveforms of the five-level CSI, i.e., an 8-A, 60-Hz five-level
PWM output current and a load current waveforms withmodulation index 0.9. Fig. 8(b) shows the current waveforms[I - 258]