This paper proposes new active switchedcapacitor and switchedinductor Zsource inverter (ASC/SLZSI) topologies, which can provide a higher boost ability with a small shootthrough time. The proposed ASC/SLZSIs inherit all of the advantages of the classical ZSI, and have a stronger voltage boost inversion ability when compared with the classical ZSI. Thus, the output ac voltage quality is significantly improved. In addition, more cells can be cascaded in the impedance network in order to obtain a very high boost ability. The proposed topologies can be applied to photovoltaic or fuelcell generation systems with lowvoltage renewal sources due to their wide range of obtainable voltages. Both simulations and the experimental results are carried out in order to verify performance of the proposed topologies.
I. INTRODUCTION
In a traditional PWM inverter, the ac output voltage is limited to below the dc input voltage. Therefore, an additional dcdc boost converter is required to obtain a desired ac output voltage. In order to overcome the limitations of a traditional inverter, a Zsource inverter (ZSI) was introduced in
[1]
. The ZSI has a unique feature that allows it to boost the dc voltage by using the shootthrough operating mode, which is forbidden in a traditional PWM inverters. The ZSI provides a simple single stage approach for applications to any dc sources. However, because the shootthrough state is only regulated within a zero voltage state, the practical boost factor of the ZSI is seriously restricted. This disadvantage may limit further applications of the ZSI in application areas which require a high voltage gain to obtain a desired ac output voltage for lowvoltage energy sources such as photovoltaic arrays, fuelcell stacks, and batteries
[2]

[6]
.
In order to increase the boost factor, several approaches have been introduced. In
[7]

[10]
, a highvoltage was achieved by adjusting the turnratio of the transformer or coupled inductor. The switchedcapacitor (SC), switchedinductor (SL), and hybrid switchedcapacitor/switchedinductor combined with the classical ZSI topology were applied to dcdc conversion in
[11]

[14]
. They provided a highboost in cascade and transformerless structures. However, additional inductors and/or capacitors at the impedance network are required for further boosting of the voltage, and they increase both the circuit volume and cost. The ZSI topology is suggested in order to reduce the capacitor voltage stress and suppress the rush currents at startup
[15]
. A topology with an active SC and SL impedance network for reducing the number of passive components in the impedance network was proposed in
[16]
. However, the boosting ability was limited due to its onecell structure.
In this paper, three different topologies based on the active switchedcapacitor and switchedinductor Zsource inverter (ASC/SLZSI) are proposed, in order to obtain a higher boost ability. The operating principles for the three topologies are analyzed. The boost ability and voltage stress of the proposed topologies are compared with those of the classical ZSI. Both simulations and experimental results are carried out to validate the performance of the proposed topologies.
II. OPERATION ANALYSIS OF THE PROPOSED TOPOLOGIES
The operation principles for the three different topologies, which are named as ASC/SLZSI with onecell, ASC/SLZSI with twocells, and multicell ASC/SLZSI, will be described, respectively.
 A. ASC/SLZSI with OneCell
Fig. 1
shows the proposed impedance network, which consists of two inductors (
L_{1}
and
L_{2}
), one capacitor (C), five diodes (
D_{in}
,
D_{1}
,
D_{2}
,
D_{3}
, and
D_{0}
), and one active switch (
S_{7}
).
Shematic circuit of the ASC/SLZSI with onecell.
The combination of
L_{1}

L_{2}

D_{1}

D_{2}

D_{3}
acts on an SL cell, This SL cell is used to transfer and store energy from the capacitor to the dc–link bus by the switching actions of the inverter. The proposed inverter has two operation modes: a shootthrough state and a nonshootthrough state, which includes six active states and two zero states.
Shootthrough state:
In the shootthrough state, the load terminal is shorted by conducting the upper and lower switching devices of any of the phase legs, and switching device
S_{7}
is turned on. Diodes
D_{1}
and
D_{2}
are on, whereas diodes
D_{in}
,
D_{o}
, and
D_{3}
are off.
Fig. 2
(a) shows an equivalent circuit in the shootthrough state for an interval of
DT_{s}
, where
D
is the shootthrough duty ratio and
T_{s}
is a switching period. The two inductor voltages
v_{L1}
and
v_{L2}
, and the dclink voltage
v_{pn}
can be expressed as, respectively,
Equivalent circuits of ASC/SLZSI with onecell : (a) shootthrough state, (b) nonshootthrough state.
Nonshootthrough state:
In the nonshootthrough state, the circuit operates under a traditional PWM inverter, and switching device
S_{7}
is turned off. Diodes
D_{in}
,
D_{o}
, and
D_{3}
are on, whereas diodes
D_{1}
and
D_{2}
are off.
Fig. 2
(b) shows an equivalent circuit in the nonshootthrough state for an interval of (1
D
)
T_{s}
. The two inductor voltages
v_{L1}
and
v_{L2}
, and the dclink voltage
v_{pn}
can be expressed as, respectively,
where
V_{in}
is the dc source voltage, and
v_{L1_NST}
and
v_{L2_NST}
are the corresponding voltages across the two inductors
L_{1}
and
L_{2}
in the nonshootthrough state, respectively.
By applying the voltsecond balance principle to inductor
L_{1}
, the corresponding voltage across
L_{1}
in the nonshootthrough state is derived as
Since the average voltage across inductor
L_{2}
is zero from (1), (3), and (4), the capacitor voltage is derived as
Similarly, by applying the amperesecond balance principle to capacitor
C
, the two inductor currents can be derived as
The peak dclink voltage across the inverter bridge is the same as
V_{C}
as in
From (9), the boost factor
B
is expressed as the shootthrough duty ratio
D
.
The shootthrough duty ratio
D
is limited to 1/3 by setting the denominator of (10) to be greater than zero.
Thus, the output peak phase voltage of the inverter is expressed by
where
M
is the modulation index.
From (11), the voltage gain
G
can be defined and is derived as follows:
 B. ASC/SLZSI with TwoCells
Fig. 3
shows the proposed ASC/SLZSI with twocells. In order to increase the boost factor, one cell comprising one inductor and three diodes is added to the ASC/SLZSI with onecell.
Shematic circuit of the ASC/SLZSI with twocells.
Fig. 4
shows equivalent circuits of the ASC/SLZSI with twocells at the shootthrough state and the nonshootthrough state, respectively. By using a method similar to that used with the ASC/SLZSI with onecell, the capacitor voltage, inductor currents, and peak dclink voltage are derived as follows:
Equivalent circuits of ASC/SLZSI with twocells: (a) shootthrough state, (b) nonshootthrough state.
From (15), the boost factor
B
is given by
where D <1/4.
 C. MultiCell ASC/SLZSI
A generalized multicell ASC/SLZSI is shown in
Fig. 5
. It can be extended to obtain a higher boost ability by cascading more cells, where the structure of onecell is shown in the upper right corner. The
n^{th}
cell includes one inductor
L_{n+1}
and three diodes
D_{3n2}
,
D_{3n1}
, and
D_{3n}
.
Shematic circuit of the multicell ASC/SLZSI.
In the shootthrough state, the inverter bridge is shorted, and switching device
S_{7}
is turned on. During the shootthrough state, diodes
D_{3n2}
and
D_{3n1}
are on, whereas diodes
D_{in}
,
D_{o}
, and
D_{3n}
are off. All of the inductors from
L_{1}
to
L_{n+1}
are connected in parallel. In the nonshortthrough state, switching device
S_{7}
is turned off. During this state, diodes
D_{in}
,
D_{o}
, and
D_{3n}
are on, whereas diodes
D_{3n2}
and
D_{3n1}
are off. All of the inductors from
L_{1}
to
L_{n+1}
are connected in series.
By using a similar method to that used with the ASC/SLZSI with onecell, the boost factor can be derived as
The boost factor can be easily increased by cascading more cells. Because the shootthrough duty ratio is limited to 1/(
n
+2) by setting the denominator of (17) to be positive, the multicell ASC/SLZSI uses a smaller shootthrough duty ratio at the same boost factor. Therefore, a higher modulation index is available for obtaining a better output voltage waveform.
In order to compare the boost ability of the proposed ASC/SLZSIs with that of the classical ZSI,
Fig. 6
shows the boost factors of the classical ZSI and the ASC/SLZSIs when
n
is changed from 1 to 3. It can be seen that the boost factors of all the ASC/SLZSI topologies are higher than that of the ZSI. In addition, the boost factor can be easily increased by adding more cells.
Comparison of the boost ability for the multicell ASC/SLZSIs and ZSI.
III. PWM CONTROL TECHNIQUES
With the proposed topologies, the pulse width modulation (PWM) control has to be modified to effectively control the shootthrough state for boosting. The relationship between the modulation index
M
and the shootthrough duty ratio
D
depends on the PWM control method. Three PWM control methods such as the simple, maximum, and maximum constant boost control methods based on the traditional carrier based PWM technique are presented in
[1]
,
[17]
,
[18]
. In this paper, a simple boost control method is applied to the proposed topologies, because the shootthrough time per switching period is kept constant.
Fig. 7
shows the switching patterns of the simple boost control method. PWM signals are generated by comparing the threephase reference voltages
V_{a}
^{*}
,
V_{b}
^{*}
, and
V_{c}
^{*}
with a carrier signal. The shootthrough state is controlled by two shootthrough envelope signals
V_{p}
and
V_{n}
. When the carrier signal is higher than the upper shootthrough envelope
V_{p}
or lower than the lower shootthrough envelope
V_{n}
, the inverter operates in the shootthrough state. Switching devise
S_{7}
is turned on during the shootthrough state. The obtainable shootthrough duty ratio
D
is limited to (
1M
).
Switching patterns under the simple boost control method.
IV. COMPARISON WITH THE CLASSICAL ZSI
 A. Comparison of the Number of Components
Table I
shows the number of active and passive components used in the impedance networks of the ASC/SLZSI with onecell and the classical ZSI without considering the common components used in the inverter bridge and output LC filter. As shown in
Table I
, the ASC/SLZSI with onecell saves one capacitor. However, it requires more one active switch and more four diodes.
NUMBER OF COMPONENT AT IMPEDANCE NETWORK
NUMBER OF COMPONENT AT IMPEDANCE NETWORK
 B. Comparison of the Voltage Stresses
From (11), the voltage stress across the switching devices of the inverter
V_{s}
for the ASC/SLZSI is the same as the peak dclink voltage across the inverter bridge as
V_{s}
=
=
BV_{in}
. The voltage stress across the switching devices with the classical ZSI is expressed as
V_{s}
=
V_{in}
/(1 2
D
)
[1]
. In order to properly compare the voltage stresses of the two inverters, an equivalent dc voltage is introduced
[19]
. The equivalent dc voltage is defined as the minimum dc voltage to produce an output voltage
, and it can be expressed as
GV_{in}
from (12). The ratio of the voltage stress across the switching devices to the minimum dc voltage for the classical ZSI and the ASC/SLZSI with onecell can be derived as follows, respectively.
Fig. 8
shows the voltage stress ratios for the classical ZSI and ASC/SLZSI with onecell. It can be seen that the ASC/SLZSI with onecell has a lower voltage stress across the switching devices than the classical ZSI.
Voltage stress ratios.
In order to compare the capacitor voltage stresses of the two inverters, the ratios of the capacitor voltage stress to the input dc voltage for the classical ZSI and ASC/SLZSI with onecell are derived as follows, respectively.
The capacitor voltage stresses ratios for the two inverters are shown in
Fig. 9
. The capacitor voltage stress of the ASC/SLZSI with onecell is higher than that of the classical ZSI, because the capacitor voltage is the same as the dclink voltage with the ASC/SLZSIs.
Capacitor voltage stresses.
V. SIMULATION AND EXPERIMENTAL RESULTS
 A. Simulation Results
The circuit parameters used for the simulation and the experiment are shown in
Table II
.
Fig. 10
shows the simulation results for the ASC/SLZSI with onecell under the simple boost control when
D
= 0.295 and
M
= 0.705. From
Fig. 10
(a) it can be seen that the capacity voltage is boosted to 260 V from 40 V input dc voltage, and that a filtered peak linetoline output voltage of 156 V can be produced.
Fig. 10
(b) shows the steadystate waveforms of the inductor and input currents, and the gating signal of switching device
S_{7}
. In the shootthrough state, the inductor current increases and the input current is zero. In the nonshootthrough state, the inductor current decreases and the input current is identical to the inductor current.
CIRCUIT PARAMETERS
Simulation results of ASC/SLZSI with onecell. (a) Input and capacitor voltages, output voltage and current, and dclink voltage. (b) Inductor and input currents and S_{7} signal.
Fig. 11
shows the simulation results for the ASC/SLZSI with twocells, when
D
= 0.22 and
M
= 0.78. The capacitor voltage is boosted to 260 V and the peak linetoline output voltage is 175 V. Compared with the simulation results shown in
Fig. 10
(a), the ASC/SLZSI with twocells produces a higher output voltage when the capacitor voltage is identical, because a higher modulation index is available.
Simulation results of ASC/SLZSI with twocells.
Fig. 12
shows the simulation results for the classical ZSI when
D
= 0.295 and
M
= 0.705. The capacity voltage is only boosted to 70 V from 40 V input dc voltage, and the filtered peak output linetoline voltage is 69 V.
Simulation results of classical ZSI.
 B. Experimental Results
A prototype of the ASC/SLZSI has been built in the laboratory for experiments.
Fig. 13
shows a photograph of the experimental setup, which consists of an impedance network, a threephase inverter, an LC filter, and a control board.
Photograph of experimental setup.
The control system is implemented by a 32bit DSPtype TMS320F28335 operating at a clock frequency of 150MHz. The sampling period for the ASC/SCZSI control at the DSP is 100 msec. The switching frequency of the inverter is determined to be 5 kHz, because the shootthrough duty ratio is controlled twice during one switching period
T_{s}
as shown in
Fig. 7
. The cutoff frequency of the LC filter is650 Hz, which is between the inverter frequency of 60 Hz and the switching frequency of 5 kHz. The parameters of the LC filter are described in Table II.
The experiment was performed with the same parameters and operating conditions as the simulation.
Fig. 14
shows the experimental results of the ASC/SLZSI with onecell at
M
= 0.705,
D
= 0.295, and
V_{in}
= 40V, which are the same operating conditions used to obtain the simulation result shown in
Fig. 10
.
Fig. 14
(a) shows the experimental waveforms of the linetoline output voltage, the linetoline output voltage filtered by an LC filter, the capacitor voltage, and the input dc voltage. The capacitor voltage is boosted to 250 V, which is slightly lower than the capacitor voltage in the simulation results due to both the forward voltage drop on the diodes and the parasitic resistance in the inductors. The peak linetoline output voltage is 147 V, which is also slightly lower than the simulation results.
Fig. 14
(b) shows the experimental waveforms of the inductor and input currents, the dclink voltage, and the gating signal of switching device
S_{7}
.
Experiments results of the ASC/SLZSI with onecell at M = 0.705, D = 0.295, V_{in} = 40V: (a) output voltage, capacitor and input dc voltages, (b) inductor and input currents, dclink voltage, and gating signal of S_{7}.
Fig. 15
shows the experimental waveforms of the filtered linetoline output voltage, the output current, the capacitor voltage, and the input dc voltage when
M
= 0.78,
D
= 0.22, and
V_{in}
= 40V. Compared with the experimental results shown in
Fig. 14
(a), the capacitor voltage can be boosted to 250 V with a lower shootthrough duty ratio. In addition, the peak linetoline output voltage is 164 V, which is higher than that of the ASC/SLZSI with onecell. The aphase output current lags to the linetoline output voltage by 30° at the resistive load condition. Since a higher modulation index is available with the ASC/SLZSI with twocells, a higher output voltage can be produced.
Experiments results of the ASC/SLZSI with twocells at M = 0.78, D = 0.22, V_{in} = 40V.
Fig. 16
shows the experimental results of the classical ZSI under the same operating conditions as those used to obtain the experimental results shown in
Fig. 14
. Compared with experiments results of the ASC/SLZSI with onecell, both the boost factor and the ac voltage gain of the classical ZSI are 72% and 33% lower, respectively.
Experiments results of the classical ZSI at M = 0.705, D = 0.295, V_{in} = 40V.
VI. CONCLUSIONS
This paper proposed three novel topologies with active switchedcapacitor and switchedinductor impedance networks, which provide a high boost ability with a small shootthrough time. In comparison with the classical ZSI, the proposed ASC/SLZSI with onecell provides a higher boost factor over the whole range of the shootthrough duty ratio. The voltage stress across the switching devices of the main inverter is lower at the same input dc voltage and output ac voltage. In addition, the proposed topology can be easily extended by cascading more cells in order to obtain a higher boost ability. However, the capacitor voltage stress is higher, because the capacitor voltage is identical to the peak dclink voltage.
Both simulation studies and experimental results for the proposed ASC/SLZSIs and the classical ZSI are carried out, respectively, in order to verify the theoretical analysis. Through the experimental results, the capacitor voltage can be boosted to 250 V under 40 V input dc voltage. In addition, the output voltage of the ASC/SLZSI with twocells is higher than that of the ASC/SLZSI with onecell at the same capacitor voltage condition. Both the boost factor and the ac voltage gain of the ASC/SLZSI with onecell are 350% and 150% higher than those of the classical ZSI, respectively, at the same modulation index, shootthrough duty ratio, and input dc voltage.
Acknowledgements
This work was supported by the Power Generation & Electricity Delivery of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Ministry of Trade, Industry, and, Energy (No. 20111020400260) .
BIO
AnhVu Ho received his B.S. and M.S. degrees in Electrical Engineering from the Ho Chi Minh City University of Technical Education, Ho Chi Minh City, Vietnam, in 2005 and 2009, respectively. He is currently working towards his Ph.D. degree in Electrical Engineering from University of Ulsan, Ulsan, Korea. His current research interests include power converter/inverter, power quality, and renewable energy systems.
TaeWon Chun was born in Korea in 1959. He received his B.S. degree in Electrical Engineering from Pusan National University, Busan, Korea, in 1981, and his M.S. and Ph.D. degrees in Electrical Engineering from Seoul National University, Seoul, Korea, in 1983 and 1987, respectively. Since 1986, he has been a member of the faculty in the Department of Electrical Engineering, University of Ulsan, Ulsan, Korea, where he is currently a full Professor. From 1996 to 1997, he was a Visiting Scholar with the Department of Electrical and Computer Engineering, University of Tennessee, Knoxville, USA. From 2005 to 2006, he was a Visiting Scholar with the Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, USA. His current research interests include gridconnected inverter systems, Zsource inverters, and ac motor controls.
HeungGeun Kim was born in Korea in 1956. He received his B.S., M.S., and Ph.D. degrees in Electrical Engineering from Seoul National University, Seoul, Korea, in 1980, 1982 and 1988, respectively. Since 1984, he has been a member of the faculty in the Department of Electrical Engineering, Kyungpook University, Daegu, Korea, where he is currently a full Professor. From 1990 to 1991, he was a Visiting Scholar with the Department of Electrical and Computer Engineering, University of WisconsinMadison, Madison, USA. From 2006 to 2007, he was a Visiting Scholar with the Department of Electrical Engineering at Michigan State University, East Lansing, USA. His current research interests include the control of ac machines and PV power generation.
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