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Three-Level Single-Phase Current-Source AC/DC/AC Conversion Systemwith Reduced Switching Device Counts
Eka Rakhman Priandana＊, Toshihiko Noguchi (Shizuoka University)
This paper proposes a new circuit configuration of a three-level single-phase AC/DC/AC conversion system with reduced
switching device counts. The circuit incorporates a three-level single-phase half-bridge current-source PWM inverter and athree-level single-phase half-bridge current-source PWM rectifier. The inverter circuit part is constructed from duality principleof the conventional voltage-source PWM inverter, while the rectifier circuit part is built by means of mirroring and reversing
process of the inverter part. By combining them into one circuit with two reactors as the energy buffers, the converter can not
only convert the voltage and frequency, but also can reduce the total harmonic distortion with reduced reactor size, comparedwith a two-level one according to the circuit simulations.
Keywords: three-level, half-bridge, current-source PWM inverter, current-source PWM rectifier, duality principle, mirroring, reversing, total
harmonic distortion, circuit simulation

Introduction

In general, AC-to-AC conversion systems are classified to fourtypes: AC/DC/AC conversion systems (i.e., with a rectifier, a DC
link and an inverter), cyclo-converters, indirect matrix converters,
and direct matrix converters. Despite having lower efficiency due
to the two-step conversion, the AC/DC/AC conversion systems
with the DC link have the simplest arbitrarily switching control
feature and is the most applicable in industry(1).
The voltage-source AC/DC/AC conversion systems have been
widely developed in order to convert the line voltage andfrequency from one to different one. Unfortunately, this kind of
converter will encounter problem while supplying heavy inductive
loads such as AC motors. The converter does not have robust
self-protection from short circuit load and has high dv/dt or di/dt
transient behavior. Meanwhile, the current-source types are lessapplied due to two main disadvantages: high conduction losses
and low power density. Nonetheless, its advantages such as having
natural protection from short circuit and low dv/dt or di/dt cannot
be omitted(2).
The fundamental of single-phase half-bridge current-source
AC-to-AC converters as shown in Fig. 1 has been proposed by theauthors in order to give an alternative solution for powerelectronics, especially while supplying high inductive loads(3).
Technically, the converter consists of half-bridge current-source
rectifier and half-bridge current-source inverter. The output rms
voltage is controlled by adjusting the DC current. Q1 and Q2
switches are driven by pulse-width modulation (PWM) generator
which is its reference regulated by the output voltage error, the DC
current error, and the input sinusoidal waveform. Meanwhile, Q3
and Q4 are driven by independent PWM generator with fixed
modulation index. Near unity modulation index would be applied
if converting to lower voltage is desired. Otherwise, lowering themodulation index will increase the output voltage.

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PI
CONTROL+-RMS

LIMITERPI
CONTROL+-DC CURRENT

1.

LIMITERS1X
TRIANGLE
GENERATOR
3kHz

COMP

S2

INPUT SINE
GENERATOR

INPUT
PLL

OUTPUT SINE
GENERATORTRIANGLE
GENERATOR
6kHz

S3COMP

Fig. 1. Single-phase half-bridge current-sourceAC/DC/AC conversion system and its controller.

Fig. 2. Simulated waveforms of AC/DC/AC
conversion from 220V/50Hz to 100V/60Hz.

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Fig. 2 shows the simulation result when the circuit converts
220V/50Hz to 100V/60Hz. The top waveform is a DC link current,
the middle one is enlargement waveforms of the source voltage
and the source current, and the bottom one is enlargement of theload voltage and the load current. It is shown that the input current
contains relatively high harmonics that may damage the powersource although it has unity power factor. This problem can beaddressed by increasing its power quality.
The development of high-performance semiconductor devices
increases the research interest in high power converters such as
multilevel converters because they have capability to produce
higher output power with low-voltage rating devices, lower
switching losses, and higher power quality with less harmonic
distortion compared with the conventional two-level converters.
Many power semiconductor devices utilization in the multilevel
converter circuit increase circuit complexity such as control andgate drive circuits, as well as conduction losses. Therefore,
half-bridge configuration is the most efficient approach to reducethe device counts utilization to a half (4).
This paper proposes a novel circuit configuration of themultilevel half-bridge current-source AC/DC/AC converters alongwith its control system. The circuit incorporates a three-level
single-phase half-bridge current-source PWM inverter andthree-level single-phase half-bridge current-source PWM rectifier
into one circuit with two reactors as the energy buffers. Prior to
the simulation result discussion, the operation principle of theconversion system will be explained, including a duality principleof the multilevel converter configuration that gives higher power
quality contribution to this research.

2.

states regarding to the output current (4).
Fig. 4 shows the output current waveforms of the three-level
current-source inverter that shown in Fig. 3. The upper waveformis the current before filtering by a capacitor, meanwhile the bottom
one is the load current. It is shown that the current conforms toTable 1 that has three states: + I, 0, and – I. This circuit will beutilized as reference to construct the three-level current-source
rectifier.
2.2 Mirroring and Reversing
In the voltage topology, the rectifier and the inverter have
similar circuit configuration. In order to apply inverter as rectifier,the AC phase reference must be shifted to –180 degree, so that theTable 1.

Switching states of three-level CSI.

Q1

Q2

Q3

Q4

Output

00

11+I

100

10

1

100–I

Circuit Configuration and Operation Principle

2.1 The Three-Level Current-Source Converter Circuit
The multilevel configuration of the current-source converter isderived from the conventional voltage-source converter. Fig. 3
shows a circuit configuration of a fundamental three-level
current-source inverter (CSI) proposed by Suroso and Noguchi (5).
This circuit was derived from the single-phase half-bridge
three-level voltage-source inverter (VSI) by converting it withduality principle. The DC voltage-sources, the anti-parallel diodes
and transistors, and the inductive load are replaced with DC
current-sources, series-connected diodes and transistors, and acapacitive load (a parallel connected C-R), respectively.
Instead of two switches, three-level CSI employs four switches
where each switch is driven so that its output will generate threestates of the current waveform. Table 1 explains the switching

Fig. 3.

Fig. 4.

Output current waveforms of three-level CSI.

Fig. 5. Mirroring and reversing processes of CSI
in order to construct CSR.

Basic configuration of three-level
current-source inverter (CSI).

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The proposed control system looks to have a similar
configuration with Fig. 1. The difference is only in the referencecomparator that needs two comparators as a consequence of using
the three-level switching configuration.

Fig. 7.

Three-level current-source rectifier (CSR)
operation principle.

Table 2.

Switching states of three-level CSR.

Current
Source

Q1

Q2

Q3

Q4

L1 State

L2 State

Forward0011Charge

Hold

Forward

1001

Hold

Hold

Forward

1100

Hold

Discharge

Reverse0011

Discharge

Hold

Reverse

1001

Hold

Hold

Reverse

1100

Hold

ChargeAC SETPOINT+-RMS

PICONTROL

LIMITERPI
CONTROL+-DC CURRENT

current flows reversely from AC side to DC side (6). On the otherhand, in the current topology, only shifting the phase reference is
not sufficient, but mirroring the inverter circuit and reversing its
components are necessary.
Fig. 5 explains how to construct the three-level half-bridge
current-source rectifier (CSR) from the corresponding CSI. In themirroring process, the load is converted to the source and viceversa. Whereas in the reversing process, all devices are connectedin the opposite direction, compared with the CSI circuit. By
combining them into one circuit with the additional reactors as theenergy buffers, it will become the three-level single-phase
AC/DC/AC conversion system. Fig. 6 shows the resultant
proposed circuit.
2.3 Operation Principle of Rectifier Part
The operation principle of the proposed circuit is divided into
two parts, i.e., the rectifier side and the inverter side. Since theoperation principle of the inverter side has been explained in Table
1, the following discussion will focus on the rectifier side.
Since the source current is AC sinusoidal, the principle will beexplained in two current directions. When the source current flows
in the forward direction, Q1 and Q2 are turned off and the others
are turned on, the reactor L1 is charged meanwhile the reactor L2
holds the current. If Q1 and Q4 are turned on and the others areturned off, both reactors will hold the current. If Q1 and Q2 areturned on and the others are turned off, the reactor L1 will hold thecurrent meanwhile the reactor L2 will be discharged. Fig. 7
describes the above operation principles.
On the other hand, when the source current flows in the reverse
direction on the next half period, what happens is the opposite
operation explained above. Table 2 summaries the operation
principle of the rectifier side.
2.4 Control System and PWM GenerationFig. 8 describes the control system of the converter and how
PWM signals are generated for each switching device. Q1 – Q4
are driven by the PWM generator which receives its reference
from calculation of the output voltage error, the DC current error,
and the 180-degree-shifted input sinusoidal waveform. Meanwhile,
Q5 – Q8 are driven by the independent PWM generator with afixed modulation index. If the buck mode operation is desired,
near unity modulation index for the inverter part may be used.
Otherwise, lower modulation index than unity must be applied.

LIMITERS2X
UPPER
TRIANGLE
GENERATOR
3kHz

COMP

LOWER
TRIANGLE
GENERATOR
3kHz

COMP

UPPER
TRIANGLE
GENERATOR
3kHz

COMP

LOWER
TRIANGLE
GENERATOR
3kHz

COMP

S4

INPUT SINE
GENERATOR

S1
INPUT
PLL

OUTPUT SINE
GENERATOR

S3

S6
S8

S5Fig. 8. Proposed converter control system and PWM
generation.

Fig. 6. Proposed three-level single-phasecurrent-source AC/DC/AC conversion system.

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3.

Simulation Results

Table 4.

Some simulations were conducted in order to verify theperformance of the proposed converter. Prior to the simulations,several circuit parameters Fig. 8 were determined as listed in Table
3. Fig. 9 shows the result of simulation waveforms that consist of
the DC link current, the enlargement of the input switched current,
the enlargement of the source voltage and current, the enlargementof the output switched current, and the enlargement of the loadvoltage and current.
It is shown in Fig. 9 that the source current has better waveformthan the source current in Fig. 2 and still demonstrates the unity
power factor. Table 4 reports the comparison between thetwo-level and the three-level configurations, where the input
THD-I was successfully reduced to 13.7 %. These results provedthat the multilevel configuration effectively improves the power
quality of the converter without sacrificing the power factor.
The merit of the proposed converter is not only increasing theconverter power quality, but also reducing the utilized reactors to aquarter. Previous research reported that the employed reactor size
was 100 mH for the two-level configuration, but the investigatedconverter in this study uses only 25 mH each (3). Due to smaller
reactor size, the consequent output THD-V and THD-I are slightly
increased but are not significant. The converter efficiency alsoslightly decreased due to higher semiconductor counts utilization,
which causes higher conduction losses.

Input

Output

Input

220 V

95.1 V

220 V

95.1 V

THD-V

0.09 %

3.39 %

0.09 %

4.09 %

Current

44.8 A

84.0 A

43.3 A

84.0 A

THD-I

34.8 %

1.27 %

13.7 %

Output1.75 %

Reactor size

100 mH

25 mH

DC-link current

249.8 A

249.8 A

Efficiency

79.8 %

78.9 %

4. Conclusion
The three-level single-phase current-source AC/DC/AC
conversion system with reduced switching device counts has beenpresented with some simulation results. The multilevel
configuration application does not only improve the converter
power quality and the power factor, but also reduces the reactor
size utilization. In order to verify the simulation results, theproposed circuit prototype is currently under development forexperimental tests.
References(1)

Simulation parameters.

Parameters

Three-levelVoltage(2)

Table 3.

Recorded simulation data.
Two-level

Measuredquantities

Values(3)

Source voltage

220 V / 50 Hz

Set point

100 V / 60 Hz

Loads

1.07 Ω / 0.98 mH

Modulation index

0.6

Carrier frequency

3000 Hz
Input: 100 uF / ESR 1 mΩ
Output: 680 uF / ESR 1 mΩ

(5)Filter capacitors
Series inductor

1 mH / ESR 1 mΩ

(6)

Reactors

25 mH / ESR 1 mΩ

(4)

J. W. Kolar et. al., “The essence of three-phase AC/AC converter systems”,
Proceedings of the 13th Power Electronics and Motion Control Conference(EPE-PEMC'08), Poznan, Poland, pp. 27 – 42, Sept. 1 - 3, 2008.
L. A. Costa et. al., "Single-phase AC-DC-AC current source converter with
reduced DC-link oscillation," 2017 IEEE Applied Power Electronics
Conference and Exposition (APEC), Tampa, FL, 2017, pp. 480-487.
E. R. Priandana and T. Noguchi, “Single-phase current-source AC/DC/AC
conversion system with reduced switching device counts”, The 2018
Annual Meeting of The IEEJ.
Suroso and T. Noguchi, "New generalized multilevel current-source PWM
inverter with no-isolated switching devices," 2009 International Conference
on Power Electronics and Drive Systems (PEDS), Taipei, 2009, pp.
314-319.
Suroso and T. Noguchi, ‘Three-level current-source PWM inverter with noisolated switching devices for photovoltaic conditioner’, IEE J Trans. Ind.
Appl., 2009, 129, pp. 505–510.
M. P. Akter, et. al., "Modified model predictive control of a bidirectional
AC–DC converter based on lyapunov function for energy storage systems,"
in IEEE Transactions on Industrial Electronics, vol. 63, no. 2, pp. 704-715,
Feb. 2016.

Fig. 9. 220V/50Hz to 100V/60Hz conversion
simulation result.

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