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International Journal of Engineering and Techniques - Volume 2 Issue 5, Sep – Oct 2016
RESEARCH ARTICLE
OPEN ACCESS
A Buck-Boost AC-AC Converter
Topology Eliminating Commutation Problem with
Multiple Mode of Operations
Mr. Harikrishnan U1, Dr. Bos Mathew Jos2, Mr.Thomas P Rajan3
1,2,3
( Department of EEE, MA College of Engineering, M.G University, India)
Abstract:
AC - AC power conversions were traditionally done by using thyristor power controllers, phase angle control or by
integral cycle control, but had low PF and other disadvantages. Variable voltage, variable frequency high power conversions
are nowadays use DC link and Matrix converters, with higher efficiency and better regulation. But in situations where only
voltage regulation is required and the circuit need to be simple and less complicated, directed PWM AC-AC converters are
more preferred, due to reduced size and components. This project presents the design and simulation of a new type of AC-AC
converter which can operate as traditional non-inverting buck and boost converters, and inverting buck-boost converter as
well. This converter uses six unidirectional current flowing and bidirectional voltage blocking switches, implemented by six
reverse blocking IGBTs or series MOSFET-diode pairs, two input and output filter capacitors, and one inductor. It has no
shoot-through problem of voltage source (or capacitor) even when all switches are turned-on and therefore; PWM dead times
are not needed resulting in high quality waveforms, and solves the commutation problem without using bulky and lossy RC
snubbers or dedicated soft-commutation strategies. It has smaller switching losses because; only two switches out of six are
switched at high frequency during each half cycle of input voltage, and it can use power MOSFETs as body diode never
conducts, making it immune from MOSFET failure risk..
Keywords- PWM AC-AC Converters, Inverting Buck Boost Converter, Non Inverting Buck Boost
Converter, THD
I.
INTRODUCTION
Traditionally in industry, the ac-ac power conversions
are performed by using ac thyristor power controllers,
which use the phase angle or integral cycle control on
input ac voltage, to get the desired output ac voltage.
However, the obvious disadvantages of ac thyristor
controllers such as low power factor, large total
harmonic distortion in source current and lower
efficiency, have limited their use [3]. For ac-ac power
conversions with variable frequency and voltage, the
use of indirect ac-ac converters with dc-link ,and
matrix converters have been advanced because they
can obtain better power factor and efficiency, and
smaller filter requirements. However, for applications
in which only voltage regulation is needed, the direct
PWM ac-ac
converters All of these direct PWM AC - AC
converters in are obtained from their dc-dc
counterparts, where all
ISSN: 2395-1303
the unidirectional switches are replaced with
bidirectional devices .However, each topology has its
own limitations; the buck type ac-ac converter, can
only step-down the input voltage while boost type can
only
step-up the input voltage. The buck-boost and Cuk
topology can both step-up and step-down the input
voltage, however, the phase angle is reversed.
Moreover, both topologies have disadvantage of
higher voltage stress across switches, and there are
discontinuous input and output currents in case of the
buck-boost converter. The Cuk topology can
overcome the currents discontinuity but at the cost of
additional passive components; increasing the size and
cost of converter and decreasing the efficiency .All of
the direct PWM ac-ac converters have a common
commutation problem, which occurs because
compared to the ideal situation in which the
complementary.
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International Journal of Engineering and Techniques - Volume 2 Issue 5, Sep – Oct 2016
Figure 1: Schematic of
type
Non Inverting buck-boost
buck
The switches do not have any overlap or dead-time;
dead
however, practically there exists a small overlap or
dead-time
time owing to different time delays of gating
signals and limited speed of switching devices. During
the overlap time between complementary switches,
either a short-circuit
circuit of voltage source (or capacitor)
occurs, or two capacitors with different voltages
become in parallel to each other; both of which results
in current spikes which may damage the switching
devices. During the dead-time,
time, there is no current path
for the flow of inductor current [6],, or two inductors
become in series ; resulting in voltage
tage spikes which
may also damage the switching devices. To solve this
commutation problem, the PWM dead-times
dead
are
intentionally added in switching signals to avoid
overlap time, and then bulky and lossy RC snubbers
are used to protect switching.
Figure 2 : Inverting buck-boost type converter
II. BUCK BOOST AC-AC CONVERTER
Figure 3: AC-AC
AC Converter Topology
A.
AC – AC Converter
This section Figure 3.1 shows the circuit topology of
the AC-AC
AC converter consisting of six unidirectional
current flowing bidirectional voltage blocking
switches S1 - S6 , one inductor L , and two input and
output filter capacitors Cin and Co . The six
unidirectional current switches can be realized by
series combination of power MOSFETs with externa
lfast recovery diodes, as shown in Figure 3.1. In this
figure, body diodes of MOSFETs are not shown as
they never conduct, and thus, their poor reverse
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recovery problem is eliminated. For high power
applications, it can either use six reverse blocking
IGBTs(RB-IGBTs)
IGBTs) , or six IGBTs (without body
diode) with external fast recovery diodes in series.
This converter can operate as traditional non-inverting
non
buck and boost converters with voltage gain of D and
1/(1-D)) , respectively, and also as inverting buckbuck
boost converter with voltage gain of ( D / (1 - D ) . By
using only six switches, it can combine the
functionality of eight switches non--inverting buckboost converter shown in Fig. 1.1 and four switches
inverting buck-boost
boost converter shown in Fig 1.2.
Therefore ,it can be used as non-inverting
inverting buck
buck-boost
converter to replace the traditional inverting buckbuck
boost converter in various ac--ac conversion
applications For its as DVR, the non--inverting buckboost mode can be used to compensate voltage
sags(which occurs more often), and inverting buckbuck
boost mode for voltage swells (which occurs less
often). The current and voltage stresses of the
components in the converter are calculated and In this
section, the design parameters of inductor and
switches will be determined based on maximum
values of their stresses for its operation
B. Non Inverting Buck Mode
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VL = -VO
Figure 4: Non Inverting Buck mode
[1].
.
Operating principle
The PWM switching sequence of the
converter during non-inverting buck mode
operation and key waveforms are shown in
Figure 4.The modes of operations include
a. Postive Cycle
Figure 5 shows operation circuit Positive half of input
ac voltage (vin > 0 ), switches S1 S3 S6 are always
turn on and S4 S5, are always turn off, while switch
S2 is switched at high frequency. Fig.3.3 shows the
equivalent circuits of the proposed converter for vin >
0.
Figure 6. Operation during negative half of input ac
voltage (a)During (D) (b) During (1-D)
For Vin < 0 , switches S2,S4,S5 are always turn on
while switches S3 S6 are always turn off, and S1
becomes high frequency switch. The operation for vin
0 is same as explained for vin < 0 , with only
difference is that now the switch S1 performs same
asS2 (for vin > 0 ), and vice-versa. The equivalent
circuits during this negative half-cycle are shown in
Fig. 6 (a) and (b). By applying volt-sec balance
condition on inductor L from above equations, the
gain in this buck mode is given by
VO/Vi= D
It can be concluded that the voltage gain of the
proposed ac-ac converter in this operation mode is
same as that of non-inverting buck ac-ac converter.
C. Non Inverting Boost Mode
The switching sequence of the converter during noninverting boost mode operation and key waveforms
are shown in below figure 7.
Figure 5. Operation during positive half of input ac
voltage (a)During (D) (b) During (1-D)
The circuit shown in Fig.5 (a) is during DT interval in
which switch S2 is turned on and the input energy is
stored in inductor L . Even though the switch S1 is
also turned on, however, its external series diode
becomes reverse biased because of inverse input
voltage vin across it. Therefore, no current flows
through switch S1 during this interval, as shown in
Fig.6 (a). Applying KVL, we get
VL = Vin - Vo.
b. Negative Cycle
During (1 - D )T interval as shown in Fig.6(b), switch
S2 is turned off whileS1 conducts in this interval as its
series diode becomes forward biased due to
freewheeling action of inductor L current. Energy
stored in inductor is released to load in this interval.
Applying KVL yields,
ISSN: 2395-1303
Figure 7: Non Inverting Boost mode
a. Positive Half Cycle
For vin > 0 , switches S2,S3,S6 are always turn on and
S1 S4 are always turn off, while switch S5 is switched
at high frequency. Fig.8 shows the equivalent circuits
of the proposed converter for vin 0 . The circuit shown
in Fig. 8(a) is during DT interval in which switch S5 is
turned on and the input energy is stored in inductor L .
The switchS6 is also turned on, however, its external
series diode becomes reverse biased because of
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International Journal of Engineering and Techniques - Volume 2 Issue 5, Sep – Oct 2016
inverse output voltage vo across it. Therefore, no
current flows through switch S6during this interval, as
shown in Fig. 8a).
a). Applying KVL, we get
VL = Vin
During (1-D)T
D)T interval as shown in Fig. 3.6(b), switch
S5 is turned off while S6conducts in this interval as its
series diode becomes forward biased due to free
freewheeling action of inductor L current. Energy stored
in inductor is released to load in this interval.
Applying KVL yields,
VL = Vin - VO
configuration requirement.The PWM switching
frequency is chosen to be 25 Khzfor
for PWM Switched
Switches and 50hz for other 4 switches.
Table 2:: Simulation Parameters
Table 3:: Prototype Parameters
Figure 8:: Operation during positive half of input ac
voltage (a)During (D) (b) During (1-D)
b. Negative Half Cycle
For Vin < 0 , switches S1,S4,S6 are always turn
on while switches S2 S3 , are always turn off, and
S6 becomes high frequency switch. The operation
for vin < 0 is same as explained for vin > 0 , with
only difference is that now the operation of switch
S6 is same as that of S5 (for vin > 0 ), and vice
viceversa. The equivalent circuits during this halfcycle are shown in Fig. 3.7(a) and (b). By
applying volt-sec
sec balance condition on inductor L
from above Equations,, the voltage gain in this
boost mode is given by
1
1
It can be concluded that the voltage gain of the
proposed ac-ac converter in this operation mode
is same as that of non-inverting
inverting boost ac
ac-ac
converter.
III.
SIMULATION RESULTS
The simulation was done in MATLAB/SIMULINK
2014a. The simulation model andresult for the two
stage configuration is shown. A constant output
voltage
age of 110 V ismaintained for input voltages in
each configuration. Simulation is done for an
inputvoltage of 70 V for boost operation and 152V for
Buck and Buck-Boost
Boost operations,withswitching
frequency of converter,fs = 50 Khz and PWM
frequency chosen is 25 Khz. Generation of switching
signals involves switching pulses for converter
switches S1to S6. For each configuration, the
switching pulses for switches changes according tothe
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Figure 8: Gate signal to converter switches (a) S1 S3
(b) S2 S4 (c) S5 (d) S6
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International Journal of Engineering and Techniques - Volume 2 Issue 5, Sep – Oct 2016
verification. Experimentally obtained value is 12.18 V
which is nearly equals to the calculated value.
Figure 9: a) Voltage and Current stress across switch
S6 (b) Voltage and Current stress across switch S4
Figure 10: Output voltage and current
Voltage and current of Buck Mode
& Input
Figure 11 : Output voltage and current & Input
voltage and current of Boost mode
IV.
Figure 12:Hardware Prototype
Figure 13: PWM Gating Signal
Figure 14: Gating Pulse at 50Hz
EXPERIMENTAL RESULTS
Prototype with reduced voltage had been developed.
For Boost converter mode an input voltage of 12 V is
given with a duty ratio of 42% and with a switching
frequency of 25 KHz for S5 & S6 and other switches
S1- S4 at 50hz and for buck converter mode an input
voltage of 12 V is given with a duty ratio of 72% and
with a switching frequency of 25 KHz for S1 & S2 and
other switches S3- S6 at 50hz
The switching pulses were developed using PIC
16F877A and driver IC used is TLP250. An output
voltage of about 18 V and 9 V was obtained. Table 3
shows the prototype parameters used for experimental
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Figure 15: The output voltage of the converter (Boost
mode)
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International Journal of Engineering and Techniques - Volume 2 Issue 5, Sep – Oct 2016
other converters, this topology has less high frequency
active switches. Six switch converter circuit with
reduced stress, less component count and high gain is
proposed for multiple mode operations. For an input
voltage of 12 V, output voltages for boost, buck
modes and buck boost mode can be obtained. A closed
loop control of converter session can be done to
maintain constant voltage at the output of the
converter.. A prototype converter section is tested and
results are experimentally verified. Future scope
involves increasing the modes included in converter
with less complexity in control.
VI.
ACKNOWLEDGMENT
Figure 16: The output voltage of the converter (Buck
mode)
V.
CONCLUSION
This paper is based on the development and analysis
of the multiple mode operations in a AC-AC
Converter topology. Compared with popularly used
It is a great pleasure to acknowledge all those who
have assisted and supported me for successfully
completing the paper. I express my deep sense of
gratitude to Dr. Bos Mathew Jos,, for the valuable
guidance as well as timely advice which helped me a
lot in completing the paper successfully. I am deeply
indebted to assistant Prof. Thomas P Rajan, for his
guidance, patience and encouragement will be a
guiding spirit in all endeavours of my future.
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