Download International Electrical Engineering Journal (IEEJ) Vol. 5 (2014) No.7, pp. 1484-1489

International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.7, pp. 1484-1489
ISSN 2078-2365
http://www.ieejournal.com/
Performance Analysis of Diode Clamped 3
Level MOSFET Based Inverter
Dnyaneshwar D. Khairnar and V. M. Deshmukh
Department of Electronics and Telecommunication, COET Bambhori, Jalgaon, India
Email: dnyanu33@gmail.com, deshvm2006@yahoo.co.in
Abstract— At present, multilevel inverters are extensively used
in industries for high power and high voltage applications. Two
categories into which inverters can be broadly classified are two
level inverters and multilevel inverters. The multilevel began
with the three level converters. One advantage that multilevel
inverters have compared to two level inverters is minimum
harmonic distortion. This paper deals with study and analysis
of three level diode clamped MOSFET based inverters and its
applications in industries. The main purpose of the paper is to
study and implement 3 level diode clamped inverter using
MOSFET’s, the PWM signals used to switch these MOSFET’s
has been generated using PWM IC HEF4752VP. However, the
output voltage is smoother with a three level converter, these
results in smaller harmonics, but on the other hand it has more
components and it is little complex to control in hardware.
Index Terms—multilevel, diode clamped, MSFET, PWM,
harmonics.
I.
INTRODUCTION
In our previous paper, we have studied the different
topologies of multilevel inverters and compare their
advantages and disadvantages. The short summary of these
topologies has been explained in next section [7].
II.
The basic three types of multilevel topologies used are:
1. Diode clamped multilevel inverters
2. Flying capacitors multilevel inverter or capacitor
clamped multilevel inverter
3. Cascaded inverter with separate DC sources.
We have chosen the diode clamped multilevel inverter
topology to study and implement just because of its numerous
advantages over other topologies and some of them are given
as [6], [7],
a.
In recent years, industry has begun to demand higher
power equipment. Multi-level inverters have been attracting
increasing attention for power conversion in high-power
applications due to their lower harmonics, higher efficiency,
and lower voltage stress compared to two-level inverters [1],
[13].
Multilevel inverter is based on the fact that sine wave can
be approximated to a stepped waveform having large number
of steps [6]. The steps being supplied from different DC
levels supported by series connected batteries or capacitors.
The unique structure of multi- level inverter allows them to
reach high voltages and therefore lower voltage rating device
can be used. As the number of levels increases, the
synthesized output waveform has more steps, producing a
very fine stair case wave and approaching very closely to the
desired sine wave [3][9]. It can be easily understood that as
motor steps are included in the waveform the harmonic
distortion of the output wave decrease, approaching zero as
the number of levels approaches infinity. Hence Multi- level
inverters offer a better choice at the high power end because
the high volt- ampere ratings are possible with these inverters
without the problems of high dv/dt and the other associated
ones [2].
DIFFERENT TOPOLOGIES OF MULTILEVEL
INVERTERS
b.
c.
d.
e.
f.
III.
When the number of levels is high enough, the
harmonic content is Low to avoid the filters.
Inverter efficiency is high because all devices are
switching at the Fundamental frequency.
The control method in diode clamped inverter is
simple.
The capacitors can be pre-charged as a group.
Common Mode Voltage: Diode clamped produced a
common mode voltage which reduce the stress on
motor and protect it from the damage.
It can draw current with low distortions [16].
DIODE CLAMPED MULTILEVEL INVERTER
The most commonly used multilevel topology is the diode
clamped inverter, in which the diode is used as the clamping
device to clamp the dc bus voltage so as to achieve steps in
the output voltage [11]. A three-level diode clamped inverter
consists of two pairs of switches and two diodes [5]. Each
switch pairs works in complimentary mode and the diodes
used to provide access to mid-point voltage [4]. In a
three-level inverter each of the three phases of the inverter
shares a common dc bus, which has been subdivided by two
capacitors into three levels. The DC bus voltage is split into
three voltage levels by using two series connections of DC
capacitors, C1 and C2. The voltage stress across each
switching device is limited to Vdc through the clamping
diodes Dc1 and Dc2. It is assumed that the total dc link
voltage is Vdc and mid point is regulated at half of the dc link
1484
Khairnar and Deshmukh
Performance Analysis of Diode Clamped 3 Level MOSFET Based Inverter
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.7, pp. 1484-1489
ISSN 2078-2365
http://www.ieejournal.com/
voltage, the voltage across each capacitor is Vdc/2 (Vc1 =
Vc2 = Vdc/2). In a three level diode clamped inverter, there
are three different possible switching states which apply the
stair case voltage on output voltage relating to DC link
capacitor voltage rate. For a three-level inverter, a set of two
switches is on at any given time and in a five-level inverter, a
set of four switches is on at any given time and so on. Figure1
shows the circuit for a diode clamped inverter for a
three-level [14].
Figure 1. Topology of diode clamped 3 level Inverter
2.
3.
Voltage level Van= 0, turn on the switches S2 and
S1′.
Voltage level Van= - Vdc/2 turn on the switches S1′,
S2′.
IV.
PWM CONCEPT
Pulse width modulation (PWM) is the technique of using
switching devices to produce the effect of continuously
varying analog signals; this PWM conversion generally has a
very high electrical efficiency [3]. In controlling either a
three phase synchronous motor or a three phase induction
motor it is desirable to receive a perfectly sinusoidal current
waveform in motor windings, with relative phase
displacements of 120 [8]. The production of sine wave
power via a linear amplifier system would have low
efficiency, at best 64%. If instead of linear circuitry, fast
electronic switching devices are used, and then the efficiency
can be greater than 95%, depending on the characteristics of
the semiconductor power switch. The half bridge switching
circuit in Figure 3 is given as an example: the switches can be
any suitable switching semiconductors [3], [11].
Figure 2 shows the phase voltage and line voltage of
the three-level inverter in the balanced condition. The line
voltage Vab consists of a phase-leg a voltage and a phase-leg
b voltage. The resulting line voltage is a 5-level staircase
waveform for three-level inverter and 9- level staircase
waveform for a five-level inverter. This means that an
N-level diode-clamped inverter has an N-level output phase
voltage and a (2N-1)-level output line voltage [13], [14].
Figure 3. Half Bridge Switching Converter
A. Types of PWM:
Figure 2. Output voltage of three-level diode- clamped inverter
A. OPERATION OF DCMLI:
Figure 1 shows a three-level diode-clamped converter in
which the dc bus consists of two capacitors, C1, C2. For
dc-bus voltage Vdc, the voltage across each capacitor is
Vdc/2 and each device voltage stress will be limited to one
capacitor voltage level Vdc/2 through clamping diodes. To
explain how the staircase voltage is synthesized, the neutral
point n is considered as the output phase voltage reference
point. There are three switch combinations to synthesize
three-level voltages across a and n [4][12].
1. Voltage level Van= Vdc/2, turn on the switches
S1andS2.
There are various types of pulse width modulation that
can be employed. The types have been classified based on
method of generation and nature of PWM waveform.
1. Square wave PWM
This scheme is popular only for economical
inverters for voltage control, as it gives large
harmonics at low voltages. It can be implemented
online.
2. NSPWM ( Natural sampled PWM)
The harmonic spectrum of this implementation is
superior to all the PWM schemes. It gives the best
results with analogue control but is not suitable for
online implementation.
3. Regularly sampled symmetrical PWM ( RSPWM,
Symmetric)
The harmonic spectrum is inferior to NSPWM but it can
easily be implemented online as it is governed by simple
algebraic equations.
1485
Khairnar and Deshmukh
Performance Analysis of Diode Clamped 3 Level MOSFET Based Inverter
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.7, pp. 1484-1489
ISSN 2078-2365
http://www.ieejournal.com/
V.
HARDWARE IMPLEMENTATION
The hardware implementation of Diode clamped 3 level
inverter mainly consist of two sections and they are,
A. Power Section: The power section consists of a
power rectifier, filter capacitor, and three phase
diode clamped multilevel inverter.
B. Control Section: The control circuit using PWM
technique with VCO and Opto-isolator circuitry,
signal amplifier to generate 12 gating pulses to drive
MOSFET based power inverter circuit [14].
phase induction motor can be generated by a use of a
Pulse Width Modulated (PWM) inverter. The
system consists of a rectified single phase A.C.
supply, which is usually smoothed to provide the
D.C. supply rails for the main switching devices
[10].
The picture of the hardware will be clearer after observing
the image of the actual hardware given as,
An AC input voltage is fed to a three phase diode bridge
rectifier, in order to produce dc output voltage across a
capacitor filter. A capacitor filter, removes the ripple contents
present in the dc output voltage. The pure dc voltage is
applied to the three phase multilevel inverter through
capacitor filter. The multilevel inverter has 12 MOSFET
switches that are controlled in order to generate an ac output
voltage from the dc input voltage.
The various sections of the hardware are explained below.
1.
2.
3.
4.
5.
6.
7.
PWM IC: The CMOS based PWM Pulse Generator
IC HEF4752VP is the heart of control circuit. This
IC generates three sets of 120° out of phase signals
that are Pulse Width Modulated to behave as
sinusoidal signals. It is provide with six PWM
output signals.
VCO: Voltage controller IC NE566 is used to
generate the clock frequency to provide external
clock to PWM IC.
Regulated Supply Sections: The +5V and +12V
regulated supply has been achieved using IC 7805
and 7812 to provide supply to the PWM IC and
VCO IC respectually.
Opto-Darlington: Opto-isolators are used to
electrically isolate the control circuit from the power
circuit in order to protect the control circuit from
potentially fatal power surge from the power circuit.
Next, we use Darlington pair transistors after each
isolator to boost current to level that is required to
trigger the Power MOSFET without loading on the
opto-isolators. The outputs of the Darlington pairs
are suitably connected to the Power MOSFET
switches for correct operation of the inverter.
Four winding Transformers: It is a specially design
transformers consist of four secondary windings
used to provide 12 separate, +12V supply to the
section of 12 opto-Darlington pairs.
D.C. Supply to Inverter: The D.C. supply to the
inverter is derived from single phase 230V A.C.
Mains supply given to bridge rectifier along with
100 microfarad, 6A capacitor which is capable to
provide 350V D.C. supply to the inverter.
PWM Voltage Source Inverter: A variable voltage,
variable frequency three phase supply for three
Khairnar and Deshmukh
Figure 4. Hardware Setup
The implementation of the hardware includes the
procedure of layout planning for PCB, Artwork, Painting,
Etching, Drilling, Mounting of various components like
Resistors, Capacitors, RF coils, Transistors, MOSFET’s and
IC’s, after mounting finally we solder all the components.
The HEF4752V is a circuit for A.C. motor speed control
utilizing LOCMOS technology. The circuit synthesizes
Three 120° out of phase signals, of which the average voltage
varies sinusoidally with time in the frequency range 0 to 200
Hz. The method employed is based upon the pulse width
modulation principle, in order to achieve a sufficient
accuracy of the output voltages over the whole frequency
range. A pure digital waveform generation is used. Input
group consist of the following signals.
FCT = frequency clock
VCT = voltage clock
RCT = reference clock
OCT = output delay clock
All outputs are of the push-pull type. Inputs and outputs
are protected against electrostatic effects in a wide variety of
device-handling situations. However, to be totally safe, it is
desirable to take handling precautions into account.
VI.
RESULTS
The reading (Phase voltages and Line voltages) of the
inverter is obtained over the oscilloscope and digital
multimeter, but before that the test waveforms of external
clock signals (FCT, RCT, OCT and VCT) has been measured
1486
Performance Analysis of Diode Clamped 3 Level MOSFET Based Inverter
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.7, pp. 1484-1489
ISSN 2078-2365
http://www.ieejournal.com/
over oscilloscope to evaluate their amplitudes and
frequencies. The oscilloscope snaps of all external clock
signals and phase voltages and line voltages are given with
their amplitudes and frequencies.
Figure 8. Voltage Clock Signal
After these clock signals given to the PWM digital
controller IC, the controller produces a output set of six
signals.
Figure 5. Frequency Clock Signal
Figure 5. shows the FCT signal having Amplitude of 6V
with Variable frequency. And figure 6, 7 and 8 shows the
RCT, OCT and VCT signals with Amplitude 4V with
approximately equal to 127 KHz frequency.
Figure 9. PWM signals
Figure 6. Reference Clock Signal
The figure 9 shows the two of six signals produces by
PWM IC, these signals cannot be assign directly to the power
MOSFET’s in order to protect the control circuit from
potentially fatal power surge from the power circuit. To
provide the isolation optoisolators are used along with
Darlington transistor pair to amplify the current of PWM
signals.
Figure 7. Output delay Clock Signal
Figure 10. PWM signals after isolation
The little bit amplification can be observed in figure 10 as
compared to the PWM signals before isolation. So these
signals are applied to the power MOSFET’s of inverter for
switching depending upon which inverter generates output.
1487
Khairnar and Deshmukh
Performance Analysis of Diode Clamped 3 Level MOSFET Based Inverter
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.7, pp. 1484-1489
ISSN 2078-2365
http://www.ieejournal.com/
capacitor charge and discharge can be balanced. Such a
converter, when serving for reactive power compensation is
called Static Var Generator [16].
The multi-level structure allows the entire converter to
be directly connected to a high voltage distribution or
transmission system without the need of a step down
transformer. All the three Multi – level inverters can be used
in reactive power compensation without having voltage
unbalanced problem [15].
2.
Figure 11. Output voltage RN
Figure 11 shows the 3 levels phase voltage produced by
inverter with 400V amplitude and 43Hz frequency. The other
phase voltages YN and BN have same amplitude and
frequency as of RN.
Inter connection of two Multi-level inverter with a DC
link in between is called as a Back to back intertie. In this
type of circuit the left hand side converter servers as rectifier,
while the right hand side serves as the inverter. The purpose
of the back to back intertie is to connect to synchronous
systems of different frequencies. It can be treated as a)
frequency connector b) phase shifter c) a power flow
controller [15], [16].
3.
Figure 12. Output voltage RY
TABLE I. Line VOLTAGES and Phase VOLTAGES ON DMM
Phase Voltages
VRY : 232 V
VRN : 144 V
VRB : 232 V
VYN : 143 V
VYB : 231 V
VBN : 144 V
Utility compatible adjustable speed drives
An ideal utility compatible adjustable speed drives
requires unity power factor, negligible harmonics and high
efficiency. By extended the back to back intertie, the
multi-level inverter can be used for a utility compatible
adjustable speed drive with the input as constant frequency
AC source and the output has the variable frequency AC
source.
The major differences when using as a utility compatible
adjustable speed drives and for back to back intertie, is the
control design and size of capacitor [14], [16].
Figure 12 shows 3 level Line voltage of inverter with 600V
amplitude and 40Hz frequency. The other line voltages YB
and BR have same amplitude and frequency as that of RY.
The readings of line voltages and phase voltages also
measured on digital multimeter
Line Voltages
Back to Back intertie
VIII.
CONCLUSION
This paper provides a comprehensive analysis on the
three-level diode clamped inverter, which also known as
neutral-point clamped (NPC) inverter. The diode-clamped
inverter provides multiple voltage levels through connection
of the phases to a series of capacitors. Because of industrial
developments over the past several years, the three level
inverter is now used extensively in industrial application. The
performance of the three-phase three-level twelve switch
inverter has been explained and improved by employing
PWM control scheme. The use of three-level inverters
reduces the harmonic components of the output voltage
compared with the two-level inverter at the same switching
frequency. It needs no additional reactors or transformers to
reduce the harmonic components. Then, it is suitable for high
voltage and high power systems.
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VII.
1.
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Khairnar and Deshmukh
Performance Analysis of Diode Clamped 3 Level MOSFET Based Inverter
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.7, pp. 1484-1489
ISSN 2078-2365
http://www.ieejournal.com/
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