4680 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 9, SEPTEMBER 2014
Current-Fed Switched Inverter
Soumya Shubhra Nag, Student Member, IEEE, and Santanu Mishra, Senior Member, IEEE
Abstract—High-boost dc-ac inverters are used in solar photovoltaic
(PV), fuel cell, wind energy, and uninterruptible power
supply systems. High step-up and step-down capabilities and
shoot-through immunity are some of the desired properties of an
inverter for a reliable, versatile, and low-distortion ac inversion.
The recently developed Z-source inverter (ZSI) possesses these
qualities. However, the realization of ZSI comes at a cost of higher
passive component count as it needs two sets of passive filters. A
switched boost inverter (SBI) has similar properties as ZSI, and
it has one L-C pair less compared to ZSI, but its gain is less than
ZSI. This paper proposes the current-fed switched inverter (CFSI)
which combines the high-gain property of ZSI and low passive
component count of SBI. The proposed inverter uses only one L-C
filter and three switches apart from the inverter structure. The
inverter topology is based on current-fed dc/dc topology. Steadystate
analysis of the inverter is presented to establish the relation
between the dc input and the ac output. A pulse width modulation
(PWM) control strategy is devised for the proposed inverter. An
experimental prototype is built to validate the proposed inverter
circuit in both buck and boostmodes of operation. A 353-V dc-link
and a 127 V (rms) ac are obtained from a 35.3-V dc input to
demonstrate the boost mode of operation. A 200-V dc-link and a
10.5-V (rms) ac are obtained from a 37.8-V dc input to verify the
buck mode of operation of CFSI.
Index Terms—Current-fed dc/dc topology, electromagnetic
interference (EMI) immunity, switched boost inverter (SBI),
Z-source inverter (ZSI).
I. INTRODUCTION
VOLTAGE SOURCE INVERTERS (VSIs) find wide application
in uninterruptible power supplies, solar photovoltaic
(PV) and fuel-cell applications, wind power systems,
hybrid electric vehicles, industrial motor drives, etc. [1], [2].
The limitation of traditional VSI is that its peak ac output
voltage is always less than the input dc-link voltage [3]. Also,
shoot-through in any of the inverter legs is not permitted as
it results in flowing of high short-circuit current. Therefore,
a dead-band is introduced between the switching signals of
complementary switches of the inverter legs, which, in turn,
causes ac output distortion. High-boost inversion is essential
in small rooftop solar PV/fuel-cell applications when it is
connected to 110–240-V ac systems. For such applications,
either a step-up transformer at the inverter output or a two-stage
boost-inverter structure is used. Inverter systems with step-up
transformers having a high turns ratio are generally bulky and
noisy. Therefore, the alternate option is to go for a transformer-
Manuscript received March 31, 2013; revised August 17, 2013; accepted
September 19, 2013. Date of publication November 8, 2013; date of current version
March 21, 2014. This work was supported by the Science and Engineering
Research Board, Government of India, under Grant SR/S3/EECE/0187/2012.
The authors are with the Department of Electrical Engineering, Indian
Institute of Technology Kanpur, Kanpur 208016, India (e-mail: soumyasn@
iitk.ac.in; santanum@iitk.ac.in).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TIE.2013.2289907
Fig. 1. Schematic of a ZSI.
less design [4]–[6]. The maximum gain of conventional boost
converter is achieved at duty ratio (D) near unity, where the
diode and the output capacitor have to sustain a high current
with very small pulsewidth. This results in severe reverse
recovery of the diode, which increases the conduction loss and
produces electromagnetic interference (EMI). This problem
is aggravated at high switching frequencies as the reverserecovery
time (trr) of the device may be larger than the time
available during (1-D) interval [7]. Moreover, a boost converter
has a maximum output to input voltage conversion ratio of 4–5
[8]. Cascaded boost converter or quadratic boost converter can
provide higher gain, but they need more passive components
and passive switches [7]. Converter with coupled inductor can
deliver high gain without extreme duty cycle operation [9], [10].
Single-switch high-gain dc/dc converters using four terminal
switched cells and switched-capacitor cells possess high gain
at reduced switch stress [11].
For high-boost dc-ac inversion, a two-stage structure is
adapted, where a VSI follows a high-gain dc-dc boost topology.
The output of the dc-dc stage is voltage-stiff. The major
problem associated with a two-stage dc-ac inversion is that the
inverter can fail due to EMI. EMI can lead to shorting of inverter
legs, resulting in flowing of short-circuit current, and damaging
of the inverter switches.
Z-source inverter (ZSI) was proposed [3] for single-stage dcto-
ac inversion with buck-boost capability. ZSI allows shootthrough
of the inverter switches. Fig. 1 shows the schematic of
ZSI which has a diode and an “X”-shaped impedance network
between the dc source and the inverter. The ratio of peak
inverter input voltage ( ˆ Vpn) to the input dc voltage Vg (boost
factor) for ZSI is given by
BZSI =
ˆ Vpn
Vg
=
1
1 − 2D
whereD denotes the shoot-through duty ratio (interval in which
both switches of any leg of the inverter conduct). As the input to
the inverter is a switched voltage, shoot-through is an allowed
state of operation for this inverter, which leads to better EMI
immunity.
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NAG AND MISHRA: CURRENT-FED SWITCHED INVERTER 4681
Fig. 2. Schematic and boost factor of SBI.
Switched boost inverter (SBI) was proposed by retaining the
same dynamic states of ZSI but lowering the passive component
count by replacing the “X”-shaped impedance network with an
active network [12]–[14]. Similar to ZSI, SBI also possesses
better EMI noise immunity. SBI has only one L-C pair, as
shown in Fig. 2, which makes it a compact solution. The major
drawback of SBI is that its boost factor is (1-D) times that of
ZSI. Therefore, it is not suited for very high boost inversion.
Both ZSI and SBI have maximum gain near D = 0.5.
This paper presents a current-fed dc/dc topology (CFT)
based inverter. The new inverter is named current-fed switched
inverter (CFSI). It has the following characteristics.
1) It combines the advantages of both ZSI and SBI. The gain
of the proposed circuit is the same as the ZSI. It also has
the same component count as the SBI.
2) The proposed inverter possesses good EMI noise immunity
similar to ZSI and SBI.
3) The proposed inverter draws continuous input current
from the dc source, which makes it suitable for renewable
applications.
4) It does not require dead-band for the switching signals,
and hence, output waveform distortion is avoided.
5) It does not require extreme duty ratio operation to achieve
high voltage boost.
In the next section, the basic current-fed dc/dc topology
is reviewed. In Section III, the development of CFSI and its
steady-state characteristics are described. The implementation
and pulse width modulation (PWM) control technique of CFSI
are detailed in Section IV. Section V establishes the relation
between the shoot-through duty cycle, the shoot-through constant,
and the inverter modulation index. Performance comparison
of CFSI is done in Section VI. Section VII provides
the total harmonic distortion (THD) comparison among the
aforementioned four inverters. Closed-loop control of CFSI is
described in Section VIII. Section IX provides the experimental
verifications. Concluding remarks of this paper are presented in
Section X.
X and ˆX represent the steady-state and peak values of a
signal x(t), respectively, while ˜x and Δx represents the smallsignal
variation in x(t) and ripple in x(t), respectively. Use
of superscript “∗” for a particular signal denotes its reference
value. For a digital signal Y , its complementary signal is represented
by Y . Note that in this paper, GS,GS1,GS2,GS3, and
GS4 represent the gate control signals of switches S, S1, S2, S3,
and S4, respectively, fed through a non-inverting gate driver.
Fig. 3. (a) Circuit diagram of the current-fed dc/dc topology (CFT).
(b) Conversion ratio of the CFT.
Fig. 4. (a) Circuit diagram of the complementary current-fed dc/dc topology
(CCFT). (b) Conversion ratio of the CCFT. (c) CCFT with switch at position 1.
(d) CCFT with switch at position 0.
II. REVIEW OF THE CURRENT-FED DC/DC TOPOLOGY
Current-fed dc/dc converter can provide high boost without
operating at extreme duty cycle condition [8]. In the boost
converter, the inductor charges the output capacitor only during
(1-D) interval in a switching cycle. However, the current-fed
dc/dc converter utilizes both D and (1-D) intervals to boost up
the output voltage to a high value.
The circuit diagram of the current-fed dc/dc topology (CFT)
is shown in Fig. 3(a). Under continuous conduction mode
(CCM) operation, in D interval (position 1 of the switch), the
output terminals are connected across the inductor and ground.
In D (position 0 of the switch) interval, the output terminal
connections are reversed. From the volt-second balance of the
inductor L [8], the conversion ratio of CFT can be obtained as
in the following and as shown in Fig. 3(b)
BCFT(D) =
Vc
Vg
=
1
2D − 1
.
From the transfer characteristic of Fig. 3(b), it is noted that
the converter gain is negative when the duty ratio (D) of the
converter is between 0 and 0.5, and the gain is positive when D
is beyond 0.5. Fig. 4(a) shows the complementary current-fed
topology (CCFT) structure which is obtained by int