The document is a project report on diode-clamped three-level inverters, detailing various control techniques, modulation strategies, and inverter topologies. It covers literature from multiple authors discussing PWM control, ripple current reduction, and the advantages of multilevel inverters in high-power applications. Additionally, it presents hardware and simulation results, culminating in an overview of inverter functionality and classifications.

1. 1
A
PROJECT REPORT
ON
DIODE CLAMPED THREE LEVEL
INVERTER
BY
VINAY SINGH
ELECTRICAL ENGINERRING

2. 2
CHAPTER 1
1.1 LITRETURE REVIEW
Dumitru Stanciu et.al.[1] described about a PWM control circuit for a three-level three-
phase inverter. The power circuit of inverter and the PWM control principles, using a mirror
triangular waveform, are explained. The output voltages of the inverter are represented by 21
Park vectors, from which 3 vectors are null. The PWM circuits with synchronization, used to
control the switches of the inverters, are inserted. A circuit that changes instantaneously two
phases each other, may be used for reversible electric drivers, is proposed. The principle of
this circuit issues from producing of two triangular waveforms in quadrature, that by a
functional transform converted into sinus and cosines; these waveforms are inputs of PWM
modulator.
Yuan Yisheng et.al.[2] Described about a new interleaved three-level inverter with reduced
ripple current of filter inductor is proposed. This inverter consists of six power switches,
including two interleaved power switches at the top location and the bottom location
separately, and two double frequency power switches at the middle locations. The inverter
theory and PWM method are explained. The ripple current expression equations of the
proposed inverter and other two half-bridge inverter are derived and compared. A two-loop
control strategy with disturbance forward suppression loop is proposed and adopted.
José Rodríguez et.al.[16] described about the most important topologies like diode-clamped
inverter (neutral-point clamped), capacitor-clamped (flying capacitor), and cascaded multi
cell with separate dc sources. Emerging topologies like asymmetric hybrid cells and soft-
switched multilevel inverters are also discussed. This paper also presents the most relevant
control and modulation methods developed for this family of converters. Multilevel
sinusoidal pulse width modulation, multilevel selective harmonic elimination, and space-
vector modulation. Special attention is dedicated to the latest and more relevant applications
of these converters such as laminators, conveyor belts, and unified power-flow controllers.
The need of an active front end at the input side for those inverters supplying regenerative
loads is also discussed, and the circuit topology options are also presented. Finally, the
peripherally developing areas such as high-voltage high-power devices and es
Mario Schweizer et.al.[3] explained about the alternative of using three-level converters for
low voltage applications is addressed. The performance and the competitiveness of the three-
level T-type converter (3LT2C) is analyzed in detail and underlined with a hardware

3. 3
prototype. The 3LT2C basically combines the positive aspects of the two-level converter
such as low conduction losses, small part count and a simple operation principle with the
advantages of the three-level converter such as low switching losses and superior output
voltage quality. It is, therefore, considered to be a real alternative to two-level converters for
certain low-voltage applications.
Y. Sato et.al.[4] describes about the multilevel inverters with larger number of levels suitable
for circuit integration are actively investigated. Diode-clamped multilevel inverters are
regarded as the promising solution. In the diode-clamped multilevel inverters whose number
of the levels exceeds three, voltage balancing circuits for the DC capacitors to maintain the
proper voltage are indispensable. The authors have been investigated the application of a
circuit topology of the voltage balancing circuits so called Resonant Switched Capacitor
Converters (RSCC). In the present paper, the utilization of the voltage boost function of
RSCC to enhance the allowable range of the input voltage of the inverter is investigated. The
voltage boost function is useful in the applications in which the DC source voltage is limited
such as batteries and fuel cells.
Prafulla J. Kale et.al.[7] described about, a method, to reduce the common mode voltage
(CMV) using diode clamped multilevel inverter (DCMLI) with Pulse Width Modulation
Technique for three phase induction motor drive is presented. Simulation model of
Conventional Two Level Inverter fed Induction Motor (IM); Three Level Diode Clamped
Inverter fed IM, Five Level Diode Clamped Inverter (FLDCI) fed IM with POD SPWM
Technique are developed under MATLAB-SIMULINK. A Miniature Model of Three Level
Diode Clamped Inverter fed Induction Motor Drive is developed. Simulation Results versus
Hardware Results are compared to examine Common Mode Voltage.
Avinash Verma et.al[9] Stated about s three phase Diode Clamped Multilevel Inverter
(DCMLI) to various modulating techniques. These pulses Width Modulation (PWM)
techniques include phase Disposition (PD), phase opposition Disposition (POD).Simulation
is performed using MATLAB – SIMULINK .It is observed that PODPWM method provide
output with relatively low harmonics distortion at the inverter output. Simulation result has
discussed. The MOSFET internal capacitance and body diodes are used for active clamping
which eliminates the need for snubber.
S. Halasz et.al[10] describe about two-phase dipolar and unipolar modulation techniques for
three-level three-phase inverters are suggested and compared with conventional three-phase
pulse width modulation (PWM) techniques. Two-phase PWMs with 60/spl deg/ (0/spl deg/
and /spl plusmn/30/spl deg/ shift) and 120/spl deg/ cycles are investigated from the point of
view of harmonic losses, motor voltage spectra, and torque pulsations. It is shown that two-
phase dipolar PWMs have no advantages in comparison with three-phase PWMs, while two-
phase unipolar PWMs-in contrast with three-phase PWMs-considerably decrease the motor
harmonic losses and torque pulsations in the whole motor voltage region. At the same time,
the inverter neutral point control requires reversing to three-phase PWM technique for the
duration of the control.

4. 4
Sandro Calligaro et.al[14] explaned number of modulation strategies have been proposed in
particular case of three-phase three-level NPC inverter, each one focusing on the optimization
of specific aspects and performance of the converter. Nevert. Both carrier-based and space
vector modulation techniques are analyzed in this paper, highlighting specific features and
limitations, especially related to PV applications. Basic issues are considered and compared
among modulation strategies, namely: switching losses, low-frequency oscillations of the
neutral-point (NP) voltage, total harmonic distortion (THD) and weighted total harmonic
distortion (WTHD) of the phase currents and line voltages, dynamic response of the neutral-
point voltage control loop under imbalance conditions and modulation algorithm complexity.
A hybrid modulation strategy assisted by an optimal neutral-point voltage controller is then
proposed aiming at both the reduction of the switching losses with a limited deterioration of
the output voltage/current quality and fast dynamics control of the neutral point voltage.
Those features allow the development of reduced dc bus capacitance PV inverters with
optimized efficiency and quality of the output waveforms.
1.2 ORGINATION OF THISIS
Introduction of inverter, multilevel inverter, different inverter topologies and their
differences shown in chapter 2. Different type of modulation technique and type of spwm is
shown in chapter 3. Simulation modeling of three level inverter and result of simulation is
shown in chapter 4. Hardware modeling, MOSFET drive circuit, used component and result
is shown in chapter 5. Conclusion of hardware and Simulink model is shown in chapter 6.

5. 5
CHAPTER 2
INTRODUCTION
When ac loads are fed through inverters it required that the output voltage of desired
magnitude and frequency be achieved. A variable output voltage can be obtained by varying
the input dc voltage and maintaining the gain of the inverter constant. On the other hand, if
the dc input voltage is fixed and it is not controllable, a variable output voltage can be
obtained by varying the gain of the inverter, which is normally accomplished by pulse-width-
modulation (PWM) control within the inverter.
The inverters which produce an output voltage or a current with levels either 0 or +-V
are known as two level inverters. In high-power and high-voltage applications these two-level
inverters however have some limitations in operating at high frequency mainly due to
switching losses and constraints of device rating. This is where multilevel inverters are
advantageous. Increasing the number of voltage levels in the inverter without requiring
higher rating on individual devices can increase power rating. The unique structure of
multilevel voltage source inverters’ allows them to reach high voltages with low harmonics
without the use of transformers or series-connected synchronized-switching devices. The
harmonic content of the output voltage waveform decreases significantly.
2.1 INVERTER
A dc-to-ac converter whose output is of desired output voltage and frequency is called
an inverter.
Based on their operation the inverters can be broadly classified into
 Voltage Source Inverters(VSI)
 Current Source Inverters(CSI)
A voltage source inverter is one where the independently controlled ac output is a voltage
waveform.
A current source inverter is one where the independently controlled ac output is a current
waveform.
On the basis of connections of semiconductor devices, inverters are classified as
 Bridge inverters
 Series inverters

6. 6
 Parallel inverters
Some industrial applications of inverters are for adjustable- speed ac drives, induction
heating, stand by air-craft power supplies, UPS (uninterruptible power supplies) for
computers, hvdc transmission lines etc.
Fig no-2.1
2.2 MULTILEVEL INVERTERS
Numerous industrial applications have begun to require higher power apparatus in
recent years. Some medium voltage motor drives and utility applications require medium
voltage and megawatt power level. For a medium voltage grid, it is troublesome to connect
only one power semiconductor switch directly. As a result, a multilevel power converter
structure has been introduced as an alternative in high power and medium voltage situations.
A multilevel converter not only achieves high power ratings, but also enables the use of
renewable energy sources. Renewable energy sources such as photovoltaic, wind, and fuel
cells can be easily interfaced to a multilevel converter system for a high power application.
The concept of multilevel converters has been introduced since 1975. The term
multilevel began with the three-level converter. Subsequently, several multilevel converter
topologies have been developed. However, the elementary concept of a multilevel converter
to achieve higher power is to use a series of power semiconductor switches with several
lower voltage dc sources to perform the power conversion by synthesizing a staircase voltage
waveform. Capacitors, batteries, and renewable energy voltage sources can be used as the
multiple dc voltage sources. The commutation of the power switches aggregate these multiple
dc sources in order to achieve high voltage at the output; however, the rated voltage of the
power semiconductor switches depends only upon the rating of the dc voltage sources to
which they are connected.
DC-AC CONVERTER
(INVERTER)
CURRENT SOURCE
INVERTER (CSI)
VOLTAGE SOURCE
INVERTER (VSI)
PWM (CSI) LCI MULTILEVEL
INVERTER
2 LEVEL
INVERTER

7. 7
2.3 TYPES OF MULTILEVEL INVERTERS
Multilevel inverters have an arrangement of power switching devices and capacitor
voltage sources. Multilevel inverters are suitable for high-voltage applications because of
their ability to synthesize output voltage waveforms with a better harmonic spectrum and
attain higher voltages with a limited maximum device rating.
There are three main types of multilevel inverters: diode-clamped (neutral-clamped),
capacitor-clamped (flying capacitors), and cascaded H-bridge inverter.
Fig no- 2.2
2.3.1 CASCADED TYPE MULTILEVEL INVERTER
This type of converter does not need any transformer clamping diodes, or flying
capacitors; each bridge converter generates three levels of voltages (E; 0, and ÿE). For a
three-phase configuration, the cascaded converters can be connected in star or delta. It has the
following advantages:
 It uses fewer components than the other types.
 It has a simple control, since the converters present the same structure.
However, the main drawback is that it needs separate dc sources for the conversion of the
active power, which limits its use.
The cascaded H-bridge inverter has drawn tremendous interest due to the greater demand
of medium-voltage high-power inverters. The cascaded inverter uses series strings of single-
phase full-bridge inverters to construct multilevel phase legs with separate dc sources. A
single H-bridge is shown in Figure 2.5. The output of each H-bridge can have three discrete
levels, results in a staircase waveform that is nearly sinusoidal even without filtering. A

8. 8
single H-bridge is a three-level inverter. Each single-phase full-bridge inverter generates
three voltages at the output: +Vdc, 0 and –Vdc.
The four switches S1,S2 ,S3 and S4 are controlled to generate three discrete outputs Vout
with levels +Vdc,0 and -Vdc . When S1 and S2 are on, the output is +Vdc; when S3 and S4
are on, the output is -Vdc ; when either pair S1 and S3 or S2 and S4 are on, the output is 0.
Figure 2.1.1
Fig no- 2.3 cascaded type multilevel inverter
Switching table of cascaded inverter
Table shows the switching pattern of cascaded type multilevel inverter when the switch S1
and S2 are triggered then V1 voltage is obtained, when switches S1, S2, S5 and S6 are
triggered then V2 voltage level is obtained similarly when switch S3, S4 are triggered then –
V1 voltage is obtained then switch S3, S4, S7, S8 are triggered then –V2 voltage is obtained.
When S3, S4 switches are triggered –V1 voltage is obtained.
V/S S1 S2 S3 S4 S5 S6 S7 S8
0 0 0 0 0 0 0 0 0
V1 1 1 0 0 0 0 0 0
V2 1 1 0 0 1 1 0 0
V1 1 1 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
-V1 0 0 1 1 0 0 0 0
-V2 0 0 1 1 0 0 1 1
-V1 0 0 1 1 0 0 0 0
0 0 0 0 0 0 0 0 0
Table no-2.1
2.3.2 CAPACITOR-CLAMPED INVERTER
The capacitor-clamped multilevel inverter known as flying capacitor is similar to the
diode-clamped inverter was presented in Hochgrafet al (1994) and Lai et al (1996). The

9. 9
capacitor-clamped multilevel inverter topology provides more flexibility in waveform
synthesis and balancing voltage. In capacitor-clamped inverter, the diode in the diode-
clamped topology is replaced by clamping capacitors or floating capacitors to clamp the
voltages. Each phase-leg has an identical structure. The size of the voltage increment between
two capacitors determines the size of the voltage l
Fig. 2.4 flying-capacitor type inverter
Figure 2.1.2 shows the structure of a flying-capacitor type converter. We notice that
compared to NPC-type converters a high number of auxiliary capacitors are needed, for M
level (M-1) main capacitors and (M-1)*(M-2)/2 auxiliary capacitors. The main advantages of
this type of converter are:
i. For a high M level, the use of a filter is unnecessary.
ii. Control of active and reactive power flow is possible.
The drawbacks are:
i. The number of capacitors is very high.
ii. Control of the system becomes difficult with the increase of M.
Switching table
When switches S1, S2, S3 and S4 are triggered then Vdc voltage level is obtained, when S1,
S2, S3 and S4’ are triggered then 3Vdc/4 voltage level is obtained. When switch S1, S2, S3’,
S4’ are triggered then Vdc/2 voltage level is obtained and when S1, S2’, S3’, S4’ are
triggered then Vdc/4 voltage level is obtained and when S1’, S2’, S3’,S4’ are triggered then 0
voltage level is obtained.
V/S S1 S2 S3 S4 S1’ S2’ S3’ S4’
Vdc 1 1 1 1 0 0 0 0

10. 10
3Vdc/4 1 1 1 0 0 0 0 1
Vdc/2 1 1 0 0 0 0 1 1
Vdc/4 1 0 0 0 0 1 1 1
0 0 0 0 0 1 1 1 1
Table no- 2.2
2.3.3 DIODE-CLAMPED INVERTER
The diode-clamped inverter is also known as the neutral-point clamped inverter
(NPC) which was introduced by Nabae et al (1981). The diode-clamped inverter consists of
two pairs of series switches (upper and lower) in parallel with two series capacitors where the
anode of the upper diode is connected to the midpoint (neutral) of the capacitors and its
cathode to the midpoint of the upper pair of switches; the cathode of the lower diode is
connected to the midpoint of the capacitors and divides the main DC voltage into smaller
voltages, which is shown in Figure 2.1.3. The middle point of the two capacitors can be
defined as the “neutral point”. The NPC uses a single dc bus that is subdivided into a number
of voltage levels by a series string of capacitors. For a three-level diode-clamped inverter if
the point O is taken as the ground reference, the output voltage has three states 0,+1/ 2 Vdc
and -1/2 Vdc . The line-line voltages of two legs with the capacitors are: Vdc, +Vdc, -Vdc & Vdc.
Three phases are necessary to generate a three-phase voltage.
Fig.2.5 Two-Phase Diode-Clamped Multilevel Inverter
Some disadvantages of the diode-clamped multilevel inverter may be observed that
using extra diodes in series becomes impractical when the number of levels n increases,
requiring (M-1)(M-2) diodes per phase if all the diodes have equal blocking voltages. Note
that the voltages for diodes in different positions are not balanced. For example, diode Da2
must block two capacitor voltages, Da(n-2) must block (n-2) capacitor voltages. Also, the
switch duty cycle is different for some of the switches requiring different current ratings. In

11. 11
addition, the capacitors do not share the same discharge or charge current resulting in a
voltage imbalance of the series capacitors. The capacitor voltage imbalance can be controlled
by using a back-to-back topology, connecting resistors in parallel with capacitors, or using
redundant voltage states which were introduced by Nabae et al (1981).
The main advantages:
i. When M is very high, the distortion level is so low that the use of filters is
unnecessary.
ii. Constraints on the switches are low because the switching frequency may be lower
than 500Hz (there is a possibility of switching at the line frequency).
iii. Reactive power flow can be controlled.
The main disadvantages are:
i. The number of diodes becomes excessively high with the increase in level.
ii. It is more difficult to control the power flow of each converter.
2.4 Switch states and the output voltages for diode-clamped multilevel inverter
The switching table shows when the switches Sa1’,Sa2’, Sb1, Sb2 are triggered then –
Vdc line to line voltage is obtained. When switches Sa1’, Sa2’, Sb2’, Sb1’ are triggered then
–Vdc/2 line volatage is obtained. When switches Sa1, Sa2, Sb1, Sb2 are triggered then o
voltage is obtained. When Sa1’, Sa2’, Sb1’, Sb2’ are triggered then 0 voltage is obtained.
When switches Sa2’ Sa1’,Sb1’, Sb2’ are triggered then then positive Vdc/2 voltage is
obtained. When switches Sa1, Sa2, Sb1’, Sb2’ are triggered then Vdc line voltage is obtained.
Sa1 Sa2 Sa1’ Sa2’ Sb1 Sb2 Sb1’ Sb2’ Vao Vbo Vab
0 0 1 1 1 1 0 0 -Vdc/2 Vdc/2 -Vdc
0 0 1 1 0 1 1 0 -Vdc/2 0 -Vdc/2
1 1 0 0 1 1 0 0 Vdc/2 Vdc/2 0
0 0 1 1 0 0 1 1 -Vdc/2 -Vdc/2 0
0 1 1 0 0 0 1 1 0 -Vdc/2 Vdc/2
1 1 0 0 0 0 1 1 Vdc/2 -Vdc/2 Vdc
Table no-2.3
2.5 COMPARISON OF INVERTERS
2.5.1 COMPARISON OF CONVENTIONAL TWO LEVEL INVERTERS
AND MULTILEVEL INVERTER

12. 12
S.No. Conventional Inverter Multilevel Inverter
1 Higher THD in output voltage Low THD in output voltage
2 More switching stresses on
Devices
Reduced switching stresses on
devices
3 Not applicable for high voltage
Applications
Applicable for high voltage
applications
4 Higher voltage levels are not
Produced
Higher voltage levels are
produced
5 Since dv/dt is high, the EMI
from system is high
Since dv/dt is low, the EMI from
system is low
6 Higher switching frequency is
used hence switching losses is
high
Lower switching frequency can
be used and hence reduction in
switching losses
7 Power bus structure, control
schemes are simple
control scheme becomes
complex as number of levels
increases
8 Reliability is high Reliability can be improved,
rack swapping of levels is
possible
Table no- 2.4
2.5.2 COMPARISON OF DIFFERENT MULTILEVEL INVERTER
TOPOLOGIES
S.No. Topology Diode
Clamped
Flying
Capacitor
Cascaded
1 Power
semiconductor
switches
2(m-1) 2(m-1) 2(m-1)
2 Clamping diodes per
Phase
(m-1)(m-2) 0 0
3 DC bus capacitors (m-1) (m-1) (m-1)/2
4 Balancing capacitors
per Phase
0 (m-1)(m-2)/2 0
5 Voltage unbalancing Average High Very small

13. 13
6 Applications Motor drive
system,
STATCOM
Motor drive
system,
STATCOM
Motor drive
system, PV,
fuel cells,
battery system
Table no- 2.5
2.6 CONCLUSION
The above chapter explains how conventional inverter is differ from multilevel
inverter and describe about the switching table of cascaded type multilevel inverter, diode
clamped multilevel inverter, flying capacitor inverter.
CHAPTER 3
MODULATION TECHNIQUES
3.1 GENERAL INTRODUCTION
when the modulating signal is a sinusoidal of magnitude Am, and the magnitude of
the triangular carrier is Ac, then ratio m=Am/Ac is known as the modulation index.
Controlling the modulation index controls the amplitude of the applied output voltage. With a
high carrier frequency, the high frequency components do not carry significantly in the ac
networks (load due the presence of the inductive elements). However, a higher carrier
frequency does result in a larger no of switching per cycle and hence in an increased power
loss. The switching frequency in the 2-15 kHz range are considered adequate for power
system applications. In three phase system it is advisable to use “fc/fm=3k”, so that all three
waveforms are symmetric. There are several modulation techniques which are as shown in
figure. As a three level NPC inverter, modulation strategies can be farmed into two main
parts shown in figure given bellow-
MultilevelModulationTechniques
Fundamental switching
frequency
High Switching
Frequency PWM
spaceVector
Scheme
Selective
Harmonics
Rejection
Sinusoidal
TrapezoidalStaircase
Stepped 3rd
Harmonic
SVP
W
M

14. 14
Modulation strategies can be divided into two main parts
i. Sinusoidal Pulse width Modulation technique
ii. Space Vector Modulation technique
Both work in closed loop and open loop & named as;
The first modulation technique, broadly used in industries, is based on the comparing,
each inverter phase separately, between suitable analogous signals. Therefore the
communication time of the MOSFET are determined by the comparators outputs.
3.2 Sinusoidal Pulse-Width Modulation
Sinusoidal Pulse-Width Modulation technique (SPWM) uses for every inverter
branches, have two separate comparators, providing the driving signal. Different modulation
method can be used; as shown in figure, the commonly used scheme uses two carrier signals
and one modulating signal. A carrier signal assumed only positive values, while the other
only negative values. When the frequency of drive is to be controlled, each modulating signal
is sinusoidal and vice-versa, in FOC the modulating signals are obtained by a closed-loop
system and they have an approximate sinusoidal shape only at steady-state.
Fig. 3.2 Sinusoidal Pulse-Width Modulation
In general implementation, generation of desired output voltage is achieved by comparing the
desired reference modulating signal with a high-frequency triangular ‘carrier’ signal, either as
given in figure depending on whether the signal voltage is larger or smaller then the carrier
signal, either the positive or negative dc bus voltage is applied at the output. Note that over
the period of one triangle wave, the average voltage applied to the load is proportional to the
magnitude of the signal during this time. The resulting hopped square waveform contains a
replica of the desired signal in its low frequency components, with the higher frequency

15. 15
components being at frequencies of and approximate to the carrier frequency. Note that the
root mean square value of the ac voltage waveform is still equal to the dc bus voltage, and
hence the total harmonic distortion is not influenced by the PWM process. The harmonics
due to inductances in the ac system. The duty cycle, ton, is calculated as;
Fig. 3.3 Sinusoidal Pulse-Width Modulation input & output signal
The Multilevel SPWM method is the extension of bi-level SPWM. One reference
signal is used to compare the carriers which is shown in Figure. If the reference signal is
higher than the carrier, the corresponding inverter cell outputs positive voltage, otherwise, the

16. 16
corresponding inverter cell outputs negative voltage. The output voltage of the inverter is
shown in Figure.
3.3 VARIOUS TYPES OF SPWM TECHNIQUES
a. Phase Disposition PWM (PD)
b. Phase Opposition Disposition PWM (POD)
c. Alternative Phase Opposition Disposition PWM
d. Phase Shift PWM
e. Carrier Overlapping PWM
f. Multi Carrier Sinusoidal Pulse Width Modulation with Variable Frequency PWM
A. Phase Disposition PWM (PD)
In this method all the carrier signals of same frequency, amplitude and phase, but
having different DC offset to occupy different levels, are compared with a single sine
modulating signal. The intersection points of the modulating signal with the respective
triangular signals are the points, where the gating signals for the switches of respective levels
are generated. Since all carrier waves are selected with the same phase, the method is called
as PD. The method is illustrated in the Fig.1. in which the number of triangular signals
required is (n-1) where n = number of levels.
Fig. 3.4 Phase Disposition PWM
B. Phase Opposition Disposition PWM (POD)
This method also contains carrier signals with same frequency, amplitude but differ in
phase, the carrier signals above reference zero voltage are in 180o out of phase with the
carrier signals bellow the zero reference voltage, this method is illustrated in the Fig.3.3(b).
Fig.3.5 Phase Opposition Disposition PWM

17. 17
C. Alternative Phase Opposition Disposition PWM
This method also contains carrier signals with same frequency and amplitude but each
carrier wave is 180o out of phase with the adjacent one, this method is illustrated in the Fig.
3.3(c).
Fig. 3.6 Alternative Phase Opposition Disposition PWM
D. Phase Shift PWM
In this method all carrier signals have same amplitude, frequency and DC offset but
they are phase shifted to each other by 90o, this method is illustrated in the Fig.3.3(d).
Fig. 3.7 Phase Shift PWM
E. Carrier Overlapping PWM
In this method for an m-level inverter, m-1 carrier signals are used which have the
same frequency and same peak to peak amplitude. The carrier signals are disposed such that
the band they occupy overlap each other and overlap each other till half of its amplitude, and
the reference signal is centred in the middle of the carrier signals, this method is illustrated in
Fig. 3.3(e).
Fig.3.8 Carrier Overlapping PWM

18. 18
F. Multi Carrier SPWM with Variable Frequency PWM
This method is used for multilevel inverters where the switching frequency of the
upper and lower switches is more than the intermediate switches, this method is used to
equalize the member of switching's, this method is illustrated in Fig. 3.3(f).
Fig. 3.9 Multi Carrier SPWM with Variable Frequency PWM
3.4Advantages of PWM
 The output voltage control is easier with PWM than other schemes and can be
achieved without any additional components.
 The lower order harmonics are either minimized or eliminated altogether.
 The filtering requirements are minimized as lower order harmonics are eliminated
and higher order harmonics are filtered easily.
 It has very low power consumption.
 The entire control circuit can be digitized which reduces the susceptibility of the
circuit to interference.
3.5 CONCLUSION
In this chapter we study about the various type of SPWM technique and we study
about modulating signal and carrier signal, by changing the frequency and phase of carrier
signal we can change the width of output modulated pulse.

19. 19
CHAPTER 4
SIMULATION DESCRIPTION & RESULT
4.1 SIMULINK MODEL OF THREE LEVEL INVERTER
The figure 4.1 shows the Simulink model of three level inverter in this model for one
leg 4 MOSFET drive circuit is used. Two diodes are clamped parallel in each leg. The
inverter is works in the principle of pulse width modulation, so for PWM two carrier signal
and three modulating signal is used, one carrier signal takes positive value and other carrier
signal takers negative signal. Both the carrier and modulating signals are compared with
logical operators and resultant square pulse is obtained. This pulse is given to the gate of the
MOSFET drive circuit.
Fig. 4.1 Simulink model of three level inverter

20. 20
4.2 SPWM GENERATION FOR 3-LEVEL INVERTER
The figure 4.2 shows the Simulink model for generating pulse width modulating
signal. Here repeating sequence i.e. triangular wave whose frequency is very high as compare
to modulating signal. In this model carrier signal and modulating signal is compared together
by using NOT operator the resulting signal is the square pulse which is used for triggering the
MOSFET drive circuit.
Fig 4.2 SPWM for gate pulse
4.2.1 INPUT OF PWM GENERATOR
In the PWM generator we use two type of signal first one is the sine wave and other is
triangular wave we can describe this one is modulating signal and other is carrier signal.
In the three phase three level inverter use three sine wave signal that are apart of 120 degree
and its frequency is 50 Hz. The modulating signal is triangular wave that have high
frequency. Figure shows the input of pwm generator

21. 21
Fig no- 4.3 input of PWM generator
4.3 RESULTS
4.3.1 GATE PULSE
Figure shows the PWM output and the pulse is giving to the MOSFET switch. The
Simulink model is produce six pulse we use that pulse at switches Sa1, Sa2, Sb1, Sb2, Sc1,
Sc3 and next six pulse is that produce by using logical operator as not gate and that are given
in switches Sa3 , Sa4, Sb3, Sb4, Sc3 and Sc4.
Fig. 4.4 gate pulse

22. 22
4.3.2 SIMULATION RESULT
Figure 4.5 shows the Simulink result of three level inverter the scope1 shows the
voltage between phase A and B (Vab). The second scope shows the voltage between phase B
and C (Vbc) and 3rd scope shows the voltage between the phase A and C (Vca).
The result shows that we obtained line voltage in three level that is +Vab, +Vab/2, 0, -
Vab/2, and –Vab. Similarly we get the result for three phases.
Fig. 4.5 simulation result
4.4 THD RESULT OF THREE LEVEL INVERTER
Simulation studies have been performed on three level diode clamped three-phase
inverters. The output voltage waveform and its frequency spectrum for a three-level inverter
at a switching frequency of 1 kHz are gives the THD and switching losses in each phase
voltage at different switching frequencies. As the switching frequency is increased THD is
reduced.
The total harmonics distortion in diode clamp three level inverter is calculated by FFT
analysis of Simulink model and total 36.23% harmonics is obtained at fundamental frequency
50 Hz.

23. 23
Fig no- 4.6
4.5 SIMULINK MODEL OF TWO LEVEL INVERTER
fig no- 4.7

24. 24
4.6 THD ANALYSIS RESULT OF TWO LEVEL INVERTER
In the figure 4.7 we observed that the harmonics obtained by FFT analysis of two
level inverter is 91.14% at fundamental frequency 50 Hz.
Fig no- 4.8
4.7 CONCLUSION
In chapter 4 we analyse the Simulink model and result of three level diode clamped
inverter and we also perform FFT analysis for obtaining total harmonic distortion and also
analyse Simulink model of two level inverter and perform FFT analysis for THD.

25. 25
CHAPTER 5
HARDWARE IMPLEMENTATION
5.1 MOSFET DRIVE CIRCUIT
Gate Driver is a power amplifier that accepts a low-power input from a controller IC
and produces the appropriate high-current gate drive for a power MOSFET. The gate driver
must source and sink current to establish required Vgs. A gate driver is used when a pulse
width- modulation (PWM) controller cannot provide the output current required to drive the
gate capacitance of the MOSFET. Gate drivers may be implemented as dedicated ICs,
discrete transistors, or transformers. They can also be integrated within a controller IC.
Partitioning the gate-drive function off the PWM controller allows the controller to run cooler
and be more stable by eliminating the high peak currents and heat dissipation needed to drive
a power MOSFET at very high frequencies.
Fig. 5.1 MOSFET Drive Circuit
5.2 COMPONENTS USED
1. MOSFET (IRF 840)
2. TRANSISTER (BD 139)
3. OPTOCOUPLER (PC 817)
4. VARISTOR (MOV-20D751K)
5. ZENER DIODE
6. RESISTANCE
7. CAPACITOR
8. DIODE

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5.2.1 MOSFET
MOSFET is an acronym for metal oxide semiconductor field effect transistor and it is
the key component in high frequency, high efficiency switching device. The focus of the
topic is the gate drive requirements of the power MOSFET in various switch mode power
conversion application.
The bipolar and the MOSFET transistor exploit the same operating principle
fundamentally both type of transistors are charge control device which means that their
output current is proportional to charge established in the semiconductor by the control
electrode before. When these device are used as a switch, both must be drive from a low
impedance source capable of sourcing and sinking sufficient current to provide for fast
insertion and extraction of the controlling charge from this point of view, the MOSFET have
to drive just as hard during turn on and turn off as a bipolar transistor to achieve comparable
switch speed . theoretically the switching speed of the bipolar MOSFET devices are closed to
identical, determine by the time required for the charge carrier to travel across the
semiconductor region typical value in power devices are approximately 20 to 200
picoseconds depending on the size of the device.
The popularity and profliration of MOSFET technology for digital and power
application is drive by two of their major advantage over the bipolar transistor. One of these
benefits is ease of. USE of the MOSFET devices in high switching frequency application.
The MOSFET transistor are simpler to drive because their control electrode is isolated from
the current conducting silicon, therefore a continuous on current is not required. Once the
MOSFET transistor are turn on their drive current is practically zero. Also the controlling
charge and accordingly the storage time in the MOSFET transistors is greatly reduced.
MOSFET MODELS
There are numerous model available to illustrate how the MOSFET works
nevertheless finding the right representation might be difficult. Most of the MOSFET
manufactures provide spice and /or saver models for theirs devices, but these models say very
little about the application drive designer have to face in practice. They provide even favour
clues how to solve the most common design challenge.
A relay useful MOSFET model which would describe all the important properties of
the device from an application point of view would be very complicated. On the other hand,
very simple and meaningful models can be drive of the MOSFET transistor if we limit the
applicability of the model to the certain problem areas.

27. 27
Fig-5.2 MOSFET (IRF 840)
5.2.2 OPTOCOUPLER
An Optocoupler, also known as an Opto-isolator or Photo-coupler, is an electronic
components that interconnects two separate electrical circuits by means of a light sensitive
optical interface.
The basic design of an optocoupler consists of an LED that produces infra-red light
and a semiconductor photo-sensitive device that is used to detect the emitted infra-red beam.
Both the LED and photo-sensitive device are enclosed in a light-tight body or package with
metal legs for the electrical connections as shown.
An optocoupler or opto-isolator consists of a light emitter, the LED and a light
sensitive receiver which can be a single photo-diode, photo-transistor, photo-resistor, photo-
SCR, or a photo-TRIAC with the basic operation of an optocoupler being very simple to
understand.
Fig-5.3 Optocoupler
5.2.3 TRANSISTER
All types of transistor amplifiers operate using AC signal inputs which alternate
between a positive value and a negative value so some way of “presetting” the amplifier
circuit to operate between these two maximum or peak values is required. This is achieved
using a process known as Biasing. Biasing is very important in amplifier design as it
establishes the correct operating point of the transistor amplifier ready to receive signals,
thereby reducing any distortion to the output signal.

28. 28
The aim of any small signal amplifier is to amplify all of the input signal with the minimum
amount of distortion possible to the output signal, in other words, the output signal must be
an exact reproduction of the input signal but only bigger (amplified).Collector to emitter
voltage (Vce) is 80V. Collector current (Ic) is 1.5A,Power dissipation (Pd) is
12.5W,Collector to emitter saturation voltage of 500mV at 0.5A collector current,DC current
gain (hFE) of 25 at 0.5A collector current,Operating junction temperature range from 150°C.
Fig-5.4 Transistor
5.2.4 ZENER DIODE
A Zener diode is a particular type of diode that, unlike a normal one, allows current to
flow not only from its anode to its cathode, but also in the reverse direction, when the so-
called "Zener voltage" is reached. Zener diodes have a highly doped p-n junction. Normal
diodes will also break down with a reverse voltage but the voltage and sharpness of the knee
are not as well defined as for a Zener diode. Also normal diodes are not designed to operate
in the breakdown region, but Zener diodes can reliably operate in this region. Zener diodes
are widely used in electronic equipment of all kinds and are one of the basic building blocks
of electronic circuits. They are used to generate low power stabilized supply rails from a
higher voltage and to provide reference voltages for circuits, especially stabilized power
supplies. They are also used to protect circuits from over-voltage, especially electrostatic
discharge.
Fig-5.5 Zener diode

29. 29
5.2.5 DIODE (IN5408)
A diode is a two-terminal electronic component that conducts primarily in one
direction (asymmetric conductance); it has low (ideally zero) resistance to the current in one
direction, and high (ideally infinite) resistance in the other. A semiconductor diode, the most
common type today, is a crystalline piece of semiconductor material with a p–n junction
connected to two electrical terminals. A vacuum tube diode has two electrodes, a plate
(anode) and a heated cathode. Semiconductor diodes were the first semiconductor electronic
devices Reverse Voltage 1000V, Average Rectified Current 3.0A,Max. Reverse Current
0.5Ma, Max. Forward Voltage Drop1.2.
Fig-5.6 Diode
5.2.6 Varistor (MOV-20D751K)
A varistor is an electronic component with an electrical resistance that varies with the
applied voltage. Also known as a voltage-dependent resistor (VDR), it has a nonlinear, non-
ohmic current–voltage characteristic that is similar to that of a diode. In contrast to a diode
however, it has the same characteristic for both directions of traversing current. At low
voltage it has a high electrical resistance which decreases as the voltage is raised. Varistors
are used as control or compensation elements in circuits either to provide optimal operating
conditions or to protect against excessive transient voltages. When used as protection devices,
they shunt the current created by the excessive voltage away from sensitive components
when triggered.
Fig-5.7 Varistor

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5.2.7 CAPACITOR
A capacitor is a passive two-terminal electrical component that stores electrical
energy in an electric field. The effect of a capacitor is known as capacitance. While
capacitance exists between any two electrical conductors of a circuit in sufficiently close
proximity, a capacitor is specifically designed to provide and enhance this effect for a variety
of practical applications by consideration of size, shape, and positioning of closely spaced
conductors. Capacitors are widely used in electronic circuits for blocking direct current while
allowing alternating current to pass. In analog filter networks, they smooth the output of
power supplies. In resonant circuits they tune radios to particular frequencies. In electric
power transmission systems, they stabilize voltage and power flow. The property of energy
storage in capacitors was exploited as dynamic memory in early digital computers. In the
drive circuit the capacitors are used 1000mf for 50v, 100mf for 25v, 0.1mf for 300v.
Fig-5.8 Capacitor
5.2.8 VOLTAGE REGULATOR
7812 is a famous IC which is being widely used in 12V voltage regulator circuits.it is
a complete standalone voltage regulator. We only need to use two capacitors, one on the
input and second one on the output of 7812 in order to achieve clean voltage output and even
these capacitors are optional to use. To achieve 12V 1A current, 7812 should be mounted on
a good heatsink plate. Thanks to the transistor like shape of 7812 which makes it easy to
mount on a heatsink plate. 7812 has built in over heat and short circuit protection which
makes it a good choice for making power supplies. the maximum safe current you can get
from one 7812 IC is 1A. More than one 7812 can be used in parallel in order to achieve more
than 1A current but output voltage of each 7812 can slightly vary resulting in unbalanced
load on all of them. This can result in load balancing issues and can damage the IC carrying
most current. However there is a way to overcome this problem. I have given bellow a
schematic diagram in which two 7812 ICs are attached together and both of them are carrying
almost equal load.

31. 31
Fig-5.9 Voltage Regulator
5.2.9 BRIDGE RECTIFIER
A rectifier is an electrical device that converts alternating current (AC), which
periodically reverses direction, to direct current (DC), which flows in only one direction. The
process is known as rectification. Physically, rectifiers take a number of forms, including
vacuum tube diodes, mercury-arc valves, copper and selenium oxide rectifiers,
semiconductor diodes, silicon-controlled rectifiers and other silicon-based semiconductor
switches. Historically, even synchronous electromechanical switches and motors have been
used. A full-wave bridge rectifier converts the whole of the input waveform to one of
constant polarity (positive or negative) at its output. Full-wave rectification converts both
polarities of the input waveform to pulsating DC (direct current), and yields a higher average
output voltage. Two diodes and a centre tapped transformer, or four diodes in a bridge
configuration and any AC source (including a transformer without centre tap), are needed.[3]
Single semiconductor diodes, double diodes with common cathode or common anode, and
four-diode bridges, are manufactured as single components. The average and RMS no-load
output voltages of an ideal single-phase full-wave rectifier are.
Fig- 5.10 Bridge Rectifier

32. 32
5.3 MOSFET DRIVE CIRCUIT TESTING
For the testing of MOSFET switch we need some equipment’s and arrangements that
is shown in figure 5.4.1 for the testing we connect the load and a power supply between
MOSFET drain and source. The gate pulse is given by a pulse generator (1KHz) and the
supply voltage is 5V.
Fig no- 5.11
Apparatus used in testing
1. PULSE GENERATOR
2. DSO
3. POWER SUPPLY
4. LOAD
5.3.1 PULSE GENERATOR
A pulse generator is either an electronic circuit or a piece of electronic test equipment
used to generate rectangular pulses. Pulse generators are used primarily for working with
digital circuits, related function generators are used primarily for analog circuits.
Pulses can then be injected into a device that is under test and used as a stimulus or
clock signal or analyzed as they progress through the device, confirming the proper operation
of the device or pinpointing a fault in the device. Pulse generators are also used to drive
devices such as switches, lasers and optical components, modulators, intensifiers as well as
resistive loads. The output of a pulse generator may also be used as the modulation signal for
a signal generator.

33. 33
5.3.2 DSO
A digital storage oscilloscope (often abbreviated DSO) is an oscilloscope which stores
and analyses the signal digitally rather than using analog techniques. It is now the most
common type of oscilloscope in use because of the advanced trigger, storage, display and
measurement features which it typically provides.
5.3.3 POWER SUPPLY
There are two power supply is used in testing of MOSFET switch
i. 5V power supply for transistor and optocoupler
ii. 12V power supply for MOSFET and drive circuit
iii. A power supply need at the load point for testing
5.3.4 LOAD
In general we use a high value resistive load between the drain and source. we
measure the testing result across the load.
5.4 MOSFET DRIVE CIRCUIT TESTING RESULT
From the pulse generator we gives the square pulse signal to the MOSFET drive
circuit and apply the signal with the help of transistor at the gate of MOSFET and MOSFET
is triggered and gives the squre pulse signal which is shown in figure 5.4.
Fig. 5.12 MOSFET drive circuit testing

34. 34
5.5 DEAD BAND CIRCUIT
A dead band (sometimes called a neutral zone or dead zone) is an interval of a signal
domain or band where no action occurs (the system is 'dead' - i.e. the output is zero). Dead
band regions can be used in voltage regulators and other controllers to prevent oscillation
repeated activation-deactivation cycles (called 'hunting' in proportional control systems).
Fig 5.13 dead band circuit
Since long dead times lead to longer body diode conduction and a consequent loss of
efficiency, it is always desirable to provide an optimally minimized dead time without
running into shoot-through conditions. This requires a detailed understanding of the transition
process and calculation of different intervals based on MOSFET and circuit parameters.
While optimum delays can be, and quite often are, determined empirically, analysis is
necessary to account for variations and to choose the right device for the highest efficiency.
Unlike hard-switched converters, ZVS designs like IBC or phase-shifted bridges must work
under strict dead time limits during switching transitions. Insufficient dead time during turn
off can result in the loss of ZVS, poor efficiency, and in the worst case,failure of the device
due to shoot-through. The minimum dead time required varies from one device technology to
another, even when they all come from the same manufacturer, but can be easily calculated
from the published device parameter
5.5.1 TESTING RESULT OF DEAD BAND CIRCUIT
Figure 5.14 shows the hardware testing result of the dead band circuit result shows the
pulse that is in input pure square wave and at the output some harmonics are available that
change the wave shape at upper and lower side.

35. 35
Fig. 5.14 dead band circuit input and output
5.6ARDUINO
Arduino Uno is a microcontroller board based on the ATmega328P (datasheet). It has
14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16
MHz quartz crystal, a USB connection, a power jack, an ICSP header and a reset button. It
contains everything needed to support the microcontroller; simply connect it to a computer
with a USB cable or power it with a AC-to-DC adapter or battery to get started "Uno" means
one in Italian and was chosen to mark the release of Arduino Software (IDE) 1.0.
The Uno board and version 1.0 of Arduino Software (IDE) were the reference
versions of Arduino, now evolved to newer releases. The Uno board is the first in a series of
USB Arduino boards, and the reference model for the Arduino platform; for an extensive list
of current, past or outdated boards see the Arduino index of boards.
Fig. 5.15 arduino

36. 36
5.7 HARDWARE RESULT
Figure shows the three level single phase result. In which three level voltage output is
obtained. In our hardware model we gives 14 volt DC supply then from the figure 5.7 we
obtained 1st voltage level at 8 volt and 2nd voltage level at 14 volt.
Fig no- 5.16
5.8 CONCLUSION
In this chapter we analysis of the hardware model of three level inverter and we also
discussed various components which are used in hardware and we also obtained various
testing result of components and three level hardware model.

37. 37
CHAPTER 6
CONCLUSION
The multilevel inverters with 3-level output is designed and developed. The
performances of commonly used carrier based modulation techniques are compared.
In the field of high performance applications, the three-level inverter is the most
promising alternative. In this work a simplified space SPWM method for three-level inverter
is proposed and described in detail.
The hardware modeling and Simulink modeling study and compare that result and
also compare with many inverter topologies and study about its advantages over two level
inverter. THD analysis is also done in Simulink model of three level inverter and two level
inverter.
From the simulation results and analysis taken for the three-level three-phase inverter,
it is observed that with the increase in the number of levels, the system performance is
improved in terms of the THD and switching losses. The voltage impressed across the
terminals of the switches is reduced from 200 to 100 volts as compared to the two-level
inverter. However, it is also observed that an unequal device rating would be necessary for
the three-level inverter.
THD obtained in two level inverter 91.14%
THD obtained in three level inverter 36.23%

38. 38
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