Modelling, Analysis and Design of Synchronous Buck

Modelling, Analysis and Design of Synchronous Buck
Modelling, Analysis and Design of Synchronous Buck
Converter Using Soft Switching Technique for PV Energy
System
Abhishek Singh (110EE0232)
Himansu Sekhar Sahoo (110EE0568)
Partha Sarathi Mahala (110EE0204
Department of Electrical Engineering
National Institute of Technology,
Rourkela
1
MODELING, ANALYSIS AND DESIGN OF
SYNCHRONOUS BUCK CONVERTER USING SOFT SWITCHING
TECHNIQUE FOR PV ENERGY SYSTEM
A Thesis submitted in partial fulfilment of the requirements for the degree of
Bachelor of Technology in Electrical Engineering
BY
Abhishek Singh (110EE0232)
Himansu Sekhar Sahoo (110EE0568)
Partha Sarathi Mahala (110EE0204)
Under the Guidance of
Prof. K.R SUBHASINI
Department of Electrical Engineering
National Institute of Technology, Rourkela –
769008
2
DEPARTMENT OF ELECTRICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA, ODISHA, INDIA - 769008
CERTIFICATE
This is to verify the fact that that the thesis entitled Modelling, Analysis and Design of
Synchronous Buck Converter Using Soft Switching Technique for PV Energy System,
submitted by Abhishek Singh (110EE0232), Himansu Sekhar Sahoo (110EE0568), Partha Sarathi
Mahala (110EE0204) in accomplishment of the demand for the honour of Bachelor of Technology
in Electrical Engg. for the duration of session 2013-2014 at National Institute of Technology,
Rourkela. A bona fide record of study exertion conducted by them underneath my guidance and
supervision.
The students have completed all the suggested experiments.
The Thesis representing student’s individual work, has not given in to anywhere else for a degree or
coursce.
In my belief, the thesis is of standard necessary for the degree of a B.Tech. in Electrical Engineering.
Place: Rourkela
Prof. K.R.Subhasini
Department of Electrical Engineering
National Institute of Technology
Rourkela-769008
3
ACKNOWLEDGEMENT
On the submission of our thesis entitled “Modelling, Analysis and Design of Synchronous
Buck Converter Using Soft switching Technique for PV Energy System”, we wanted to express our
appreciation and thankfulness to our supervisors Prof. K.R.Subhasini Asst. Professor, Department
of Electrical Engineering and Prof. Dr.B.Chitti Babu, Asst. Professor, Department of Electrical
Engineering due to their inspiration and encouragement during the time of our work in the final year.
We are very thankful for their esteemed guidance and inspiration from the genesis to the apocalypse of
the project. Their guidance and motivation at the time of crisis would be memorised always. And
from the bottom of our heart we express our gratitude to our beloved professors for being lenient,
consoling and encouraging when we were going through pressured phases during placements.
We are very thankful to our teachers Dr. B.D.Subudhi and Prof. A.K.Panda for providing
solid background for our studies, with their exemplary class room teaching and also Prof.
S.Samanta for his exquisite teaching of MATLAB and Simulink during Lab.
At last but not least, we are grateful to staff of Electrical engineering department for constant support
and providing place to work during project period. Especially I want to acknowledge the help of
Mr.Gangadhar Bag, Lab Asst., Dept. of Electrical Engineering for his continuous help during
experimental work.
Abhishek Singh (110EE0232)
Himansu Sekhar Sahoo (110EE0568)
Partha Sarathi Mahala (110EE0204)
B.Tech (Electrical Engineering)
4
ABSTRACT
If we start forecasting in the view of electrical power generation, in the upcoming decade all the fossil fuels are
going to be extinct or the worst they are going to be to a middle class man, so renewable energy power generation
systems are going to make a big deal out of that. It is extremely important to generate and convert the renewable energy
with maximum efficiency. The Photovoltaic (PV) energy system is a contemporary theory in use, which advances
status because of growing reputation to study on different causes of energy over exhaustion of the conformist fossil
fuels universal. The renewable sourced are commercialised to obtain power from the sun in efficiently and outfit
them to the obtainable loads deprived of moving their presentation.
In our work, we present the exquisite design of Synchronous Buck Converter with the application of Soft switching
Modelling to implement precise control design for the converter by the help of MATLAB/Simulink. The
Synchronous Buck Converter thus designed is used for portable appliances such as mobiles, laptops, iPod’s etc. Now,
the converter concepts cast-off, usages extra MOSFET which eliminates losses of conduction which is originate
conspicuously in the simple buck converter, thus performance of the converter is enhanced. But in this project our
main intention is to interface the PV array with the Synchronous Buck Converter we designed and we will depict that
our converter is more efficient than the conventional Buck converter in order of maintaining constant output voltage,
overall converter efficiency etc. We also studied and modelled, MPPT for the given PV energy system and the
simulations are carried out in MATLAB-Simulink environment. More, the relative study is proposed among the both
synchronous and simple Buck converter.
At last we show that the output voltage remnants fixed irrelevance of fluctuations in load and source. And
finally we see the performance of Synchronous Buck Converter, which is interfaced with PV array having the
practical variations in temperature and irradiance will also maintain a constant output voltage throughout the
response. All simulations are carried in MATLAB/Simulink software. The suggested converter is simulated
in the MATLAB Simulink software and suggested converter is implemented practically to confirm the
results of theory. Soft Switching control of synchronous Buck converter founded on PV energy system is
experimented through ICs and investigational outcomes were concluded.
5
Contents
1
INTRODUCTION
6
1.1
Inspiration. . .
7
1.2
PV Energy. . .
7
1.2.1
Photovoltaic (PV)
7
1.3
Overview of Proposed Work Done
10
1.4
Thesis Objectives
11
1.5 Organisation of Thesis
2 PV-ARRAY CHARACTERISTICS
11
12
2.1
Introduction. .
13
2.2
PV Array Modelling
13
3 SYNCHRONOUS BUCK CONVERTER AND IT’S EFFICIENCY
4
15
3.1
Synchronous Buck Converter Design
16
3.2
Synchronous Buck Converter design and efficiency
19
Maximum Power Point Tracking (MPPT)……….
21
4.1 Introduction……………….
22
4.2 P & O Method………………..
23
4.2.1 Motivation…………….
23
4.2.2 Hill climbing Techniques…………..
23
4.2.3 P & O algorithm Implementation……………
5 SOFT SWITCHING OF DC-DC SYNCHRONOUS BUCK CONVERTER
23
25
5.1
Concept of Soft Switching
26
5.2
Problem of Hard Switching
26
5.3
Types of Soft Switching Techniques
28
5.3.1 Zero Voltage Switching
28
5.3.2 Zero Current Switching
29
5.4 Soft Switching of Synchronous Buck Converter
6 SIMULATION RESULTS AND DISCUSSION
6.1 PV system
6.2 Synchronous Buck converter
6.3 Efficiency comparison
6.4 Soft switching
6.5 Experimental results
7 CONCLUSION
8 REFERENCE
9 APPENDIX
ROAD MAP TO THE PROJECT……………
6
30
31
32
35
37
38
40
43
44
45
50
List of Figures
1
Schematic Diagram of PV Based Converter System. . .
10
2
Equivalent Circuit of PV Cell…….
14
3
Block diagram of DC-DC converter incorporating MPPT control…..
23
4
Flow chart of P& O Algorithm
24
5
hard switching phenomenon
26
6
Zero voltage switching
27
7
Switch on and turn off time of Zero voltage switching
28
8
Zero current switching
28
9
Switch on turn off time of Zero current switching
29
10
Synchronous buck converter
29
11
Soft switching of synchronous buck converter
30
12
Zero voltage switching transition waveform
30
13
I-V Characteristics at fixed temp.
31
14
P-V Characteristics at Fixed temp
31
15
I-V Characteristics at fixed Radiation.
32
16
P-V Characteristics at Fixed Radiation. . .
32
17
I-V Characteristics at different diode stability factor n
33
18
I-V Characteristics at different series resistance R
33
19
Steady state response of synchronous buck
35
20
Step load change response of synchronous buck
36
21
Efficiency comparison
37
22
Soft switching of synchronous buck converter
38
23
Experimental Set-up in Laboratory
40
24
Input and output voltage of buck converter
40
25
Voltage across MOSFETof buck converter
41
26
Comparison of saw tooth and control voltage
41
27
Voltage across MOSFET1 of synchronous buck converter
42
28
Voltage across MOSFET2 of buck synchronous converter
42
29
Output voltage of buck converter
42
7
Chapter-1
INTRODUCTION
8
1
INTRODUCTION
1.1 Inspiration
As the days go by, the demand of power is increasing gradually and on the contrary the resources used
for power generation are becoming inadequate. Apart from the reason of inadequate resources, the methods used
for power generation by fossil fuels are not even environment friendly and they devote an ultimate reason for
global warming and greenhouse effect. So it is the time to initiate the usage of renewable energy resources
on very large scale.
The three main available renewable energy resources are (i) Direct Solar Energy, (ii) Hydro Energy
and (iii) Wind Energy. Hydro Energy generation and Wind Energy generation are of course two of the
main sources of renewable energies, but the main disadvantage in Hydro Energy is that, it is seasonal dependent
and in Wind energy is that it is geographical location dependent [1]. On the other hand Solar Energy is prevalent
all over the globe and all the time. The amount of irradiance and temperature may vary from place to
place and from time to time but under given conditions Solar Energy system can be implemented.
Solar Energy or PV energy system is the most most effective method to convert the solar radiated
energy into electricity based on photo-voltaic effect. Despite of high initial costs, they are already have
been implemented in many rural areas. In future the cost of the PV panel also may diminish, because of the
advancing technology and also the competition between manufacturers. And therefore, the time is not so far
that almost every middle class person can afford his own solar panel at home for at least some basic requirements.
Thus PV Energy is going to play a vital role.
1.2 PV Energy
1.2.1 Photovoltaics (PV)
Photovoltaics is mainly defined as a technique for producing electrical power by using PV cells to transfer
solar energy to electric current. The photoelectric effect causes photons of radiation to excite negative charge into
a advanced energy level, letting them to carry the electrical current. The photoelectric effect is first understood
by Alexndre-Edmound Becquerel in 1839.
9
The term photovoltaic represent working manner of a photo diode in which the driving current is completely
due to the transduced solar energy..
FIGURE 1 Schematic Diagram of PV Based Converter System. .
Solar panels yield DC power from solar light which is utilised to supply load or to refresh a batteryoperated device. The initial use of photovoltaic was to supply circling satellites and other rocket, but today
the greatest of the photovoltaic modules are implemented for network linked control generation. In this
situation it is very important to transform the DC to AC. There is very small marketplace for off
network energy for distant dwellings electrical cars,, automobiles, backup telephone booths, remote
detecting, and defence of cathode in tubes.
Cells demands fortification and are typically packed inside a glass leaf tightly. When more electricity is
required than a single cell may give, cells are electrically attached together in series to build photovoltaic
array, or solar panels. A solitary unit is sufficient to supply an urgent handset, but for a home or a generating
station the cells must be attached in combination as form panels.
1.3 Proposed Work Done overview
Various prose are utilised to perform the scheme which consists of data on photovoltaic arrays,
PV energy systems, converters topology, variation in the performance of arrays with
atmospheric conditions, etc. Reference [1]-[2] gives an overview about the applications of
photovoltaic technology. Reference [3] gives us the data on the entire Indian Energy scenario
particularly regarding with renewable energy sources. It enriched us on data on the potential
10
and of solar energy use in rural applications. Reference [4] give details about technology of
battery available in the world. Reference [5] states the converter requirement for photovoltaic
applications. References [6]-[9] describe various such converters available for use. Reference
[10] made us understand the phenomenon of soft – switching and some of the techniques are
seen in reference [9].
1.4. Objectives of Thesis
At the end of the project, the objectives to be achieved are listed following:
1. To study the solar cell model and observe its characteristics.
2. To study the proposed synchronous DC-DC Buck converter and its operation.
3. To study the design of buck converter with controller with the help of Soft Switching
techniques.
4. To study the comparison between the conventional Buck converter and the synchronous DC-DC
Buck converter suggested in terms of effectiveness enhancement.
5. To study the Maximum Power Point Tracking (MPPT) algorithms of PV Array model and to
implement in Simulink Environment.
6. To validate the experimental results obtained from the laboratory set-up and to analyse the
results with the simulated results in the MATLAB-Simulink Environment.
1.5 Thesis Organisation
The thesis is divided into six sections including the section of introduction. Each section is distinct from one
another and is stated laterally by the required knowledge and concept to represent it.
Chapter No.2 deals with PV Array Characteristics and its modelling.
First, the equivalent
mathematical modelling of the solar cell is made after studying various representations and simplification is
made for our purpose. Then PV and IV characteristics curves for both fixed temperature and fixed irradiation
for the equivalent model is studied in MATLAB Simulink model using equation corresponding to that
model.
Chapter No. 3 is results and discussion section, in which all simulation results of PV Characteristics, IV
characteristics at different constant values of temperature and irradiation and also different values of
series resistance is shown.
11
Chapter-2
PV-ARRAY CHARACTERISTICS
12
PV-ARRAY CHARACTERISTICS
2.1
Introduction
Learning and analysing PV Array characteristics plays a vital role when it comes to PV energy
generation. These characteristics vary from one model to the other. But, however we in this section study the
PV array characteristics for ideal PV Cell, which includes P-V & I-V features at fixed hotness and also
PV & I-V features during fixed Irradiance. Meticulous study of these characteristics helps us to understand the
functioning of PV Cell during the variations of temperature and irradiation which are the pioneer parameters
for PV energy generation.
These characteristics obtained, not only helps us in understanding PV system, but also helps in the study of
concept Maximum Power Point Tracking (MPPT) and also to obtain that point for maximum efficient
operation of System. These topics are discussed in later chapters in detail.
2.2 PV Array Modelling
The solar panels or PV arrays are typically manufactured from minor chunks of lone solar cell
elements. They provide rated production current and voltage that can be used for a specified set
of atmospheric data. The esteemed current depends upon amount of parallel ways of solar cells
and the esteemed voltage of the panel be subject to upon the amount of series connected solar
cells in each of parallel ways. An individual solar cell is a photo diode. The corresponding circuit
model of PV cell comprises of a dependent current basis on radiation and hotness, a diode
conducting opposite overload current, series advancing resistance of the cell.
The Figure 2, is an approximated version of actual single cell equivalent circuit. The output current (Ipv)
and the output voltage (Vpv) are dependent on the solar irradiation and temperature and also the saturation
current of diode. For that single cell, Ipv and Vpv are calculated by the equations given below:
EQUATIONS:
Figure 2 Equivalent Circuit of PV Cell
13
Module Photo Current
IL= IL(T1) + K0(T-T1)
(1)
IL = ISC(T1,nom)
(2)
(
0=
)
(
(
)
)
(3)
Module Reverse Saturation Current:
0( 1) =
(
)
(
)
(4)
)
(
The module saturation Current I0 depending on cell temperature as given by:
(
=
The output current of solar cell is:
)
( 1) ∗
(5)
(6)
Due to help of above equations, subsystems are created in MATLAB/Simulink environment to obtain PV cell equivalent
subsystem and with the help of obtained subsystem PV Characteristics are obtained.
14
Chapter-3
SYNCHRONOUS BUCK CONVERTER
AND IT’S EFFICIENCY
15
SYNCHRONOUS BUCK CONVERTER AND IT’S EFFICIENCY
3.1 Synchronous Buck Converter Design
The following parameters are considered for design:


Vin = 12 V

Vout = 3 V

Fsw = 200 KHz

Iload = 1 A

Duty cycle (D) = Vin/Vout = 0.25

The switching frequency is nominated at 200 KHz.

Assume Iripple = 0.3*Iload (typically 30)
The current ripple will be restricted to 30% of supreme load
Parameters Calculations:
a) Inductance Calculation:
Inductor and capacitor plays a major role in dc-dc converters acting like a low pass filter both combined.
Inductance helps in limiting the ripple in the output current.
For an inductor,
(7)
L=37.5 μH
Assume 37.5 μH, 2 amps inductor has a resistance of 0.05Ω. The energy dissolute owing to Cu losses is:
(Iload) 2* ES2R = 0.05 watts
(b) Output Capacitor Calculation:
The voltage wave through the output capacitor is the addition of ripple voltages due to the Effective
Series resistance (ESR), the voltage drop owing to the load current that necessity be provided by the
capacitor as the inductor is settled, and the voltage ripple due to the capacitor’s Effect Series
Inductance”. The ESL requirement is usually not specified by the capacitor seller. For this instance, we
will assume that the ESL is null.
As switching frequencies rise, the ESL ratings will convert further significant. For capacitance,
16
(7)
The expression presented here displays that we are resolving an expression with many variable, ESR, C,
and ESL. A sensible method is to eliminate relations that are not important, and then brand a sensible
estimation of the greatest significant stricture that we can control, ESR. The capacitor ESR rate was
nominated from a seller’s catalogue of amps rated capacitors. Assumed the ripple current and the
objective output voltage ripple, an ESR value of 0.05Ω was designated from a list of capacitors rated for
0.3 amp ripple current.
Assume voltage of ripple to be 50mV
Given δ I=0.3 A, ESR=0.05Ω
From that, δ T=58μsec
Assume ESL=0
Then, we will compute the mandatory capacitance of the output capacitor specified the wanted output
voltage ripple is well-defined as 50 mV. Then,
(8)
The expression’s denominator (δV-(δ I*ESR)) demonstrates that the capacitor’s
ESR rating is further significant than the capacitance value. If the designated ESR is very high, the
voltage because of the ripple current will equivalent or surpass the mark output voltage ripple.
We have a divide by 0 matter, representing that an unlimited output capacitance is requisite. If a sensible
ESR is designated, formerly the real capacitance value is sensible.
Polymer Electrolytic Capacitor with 500μF and ESR of 0.05Ω is used.
Power loss in the capacitor is (Iripple)2*ESR=0.0045 watts.
c) Input Capacitor:
The poorest case ripple current happens once the duty cycle is 50% and the nastiest case ripple current on
the contribution of a buck converter is around one half of the capacity current. Similar the output
capacitor, the input capacitor assortment is chiefly verbalized by the ESR obligation desirable to meet
voltage ripple supplies. Typically, the contribution voltage ripple necessity is not as severe as the output
voltage ripple condition. Now, the extreme input voltage ripple was definite as 200 mV. The input ripple
current score for the input capacitors may be the greatest significant standards for choosing the input
capacitors. Frequently the input ripple current will surpass the output ripple current.
17
Input ripple current is assumed to be Iload/2
Input voltage ripple is accepted to 200mV
ESR value of capacitor is 0.12Ω
Compute capacitance:
Power loss at the capacitor is (Iripple)2*ESR= 0.0107 watts
d) Diode Selection:
The diode’s regular current is same to the load current product the share of time the diode is directing.
The duration the diode is on is: (1 - duty cycle)
ID = (1-D)* Iload= 0.75 amps
Max diode reverse voltage is 12 volts, for this, select schottky diode 1N5820,
20 V and 3 amps rating.
Forward voltage drop assumed at peak current is assumed to be 0.4 volts
Power dissipation in the diode is VF*Id=0.3 watts
e) MOSFET Selection:
To make simpler the gate-drive electric circuit for the MOSFET, a P-channel switch was designated. An
N-channel switch wanted a gate drive circuit that includes a technique to control the gate voltage around
the supply. The price of a equal translator and control drive will overshadow the investments of by
means of an N-channel device as opposed to a P-channel device. A 20 volt MOSFET was not designated
since the obtainable switches in the catalogue had extreme gate to source voltage ratings of individual 12
volts. With a 12 volt input voltage, the functional gate volts may surpass the MOSFET ratings A 30 volt
MOSFET was designated on the base of the 20 volt gate to source requirement.
For above design parameters for converter design, select N-channel MOSFET for comfort of lashing
gate. Select 30 V, 9.3 amps with low typically 0.02Ω.
Assume Trise=Tfall= 50nsec
Conduction loss= (Id)2*Rds(on)*D= 0.005watts
Switching loss= 0756 W
18
(9)
(Assume Coss=890pF)
Total loss= 0.005+0.0756= 80mW.
3.2 Synchronous Buck Converter Efficiency and Comparison
A) Buck Converter Efficiency:


Pout= 3 watts (3V @ 1a)

Losses in output capacitor= 4.5mW

Diode loss= 300mW

Total losses= 445mW

Inductor loss= 50mW

Losses in input capacitor= 10.8mW

MOSFET loss=80mW
Converter efficiency = (Pout?(Pout+total losses)*100= 85.6%
Here diode forward voltage drip (0.4 V) causes the overall losses of 60%. The converter effectiveness
could be elevated if the diode’s forward voltage drip resolve be depressed.
B) Synchronous Buck Converter Efficiency:
This portion displays a Synchronous Buck converter. It is like to the preceding conventional buck
converter, excluding the diode is connected in parallel with a new transistor. It is named a synchronous
buck converter since MOSFET M2 is switched on then off synchronously with the process of the main
switch M1. The knowledge of a synchronous buck converter is to custom a MOSFET as a rectifier that has
very short forward voltage drop as related to a normal rectifier. By dropping the diode’s voltage drop, the
over-all productivity for the buck converter can be enhanced. The synchronous rectifier (MOSFET M2)
needs an other PWM signal that is the counterpart of the main
PWM signal. M2 is on once M1 is off and reverse is true. This pwm arrangement is called
Complementary PWM.
Pout = 3W (3 V @ 1 A)
Choose N-channel MOSFET with Rds(on) = 0.0044Ω, Use similar method for loss calculation as stated
above.
19
Transmission loss= (Id)2*Rds(on)*(1-D) = 15mW
Primary MOSFET (S1) loss= 10mW
Resonant capacitance(Cr) loss = 10mW
Resonant Inductance(Lo) loss= 50mW
Losses of Output capacitor (Co) =4.5mW
MOSFET(S2)= 75mW
Diode(D) loss= 5
Inductor(Lr) loss= 20mW
Whole Loss = 190mW
Converter efficiency= (3/3+0.190)*100= 94.8%
NOTE: The relative graph of efficiency among Buck Converter and Synchronous Buck converter is
exposed in RESULTS AND DISCUSSION section.
20
Chapter 4
Maximum Power Point Tracking
21
4. MAXIMUM POWER POINT TRACKING (MPPT)
4.1 Introduction
Maximum power point tracking (MPPT) is a practise that is used in solar battery-operated stallions and
identical procedures use to get the extreme conceivable power from one or more photovoltaic equipments,
classically solar panels, however optical power broadcast systems can profit from comparable technology.
Solar cells need a compound relationship among solar radiation, temperature and total resistance that
products a non-linear production efficiency which can be examined founded on the I-V curve. It is the
determination of the MPPT scheme to model the output of the cells and put on the good resistance (load)
to get extreme power for somewhat given ecological circumstances. MPPT procedures are characteristically
combined into an electrical control converter scheme that pro-vides voltage or current adaptation, sifting,
and directive for heavy various loads, with power networks, sequences, or motors.
The Maximum Power Point Tracker (MPPT) is desirable to enhance the quantity of power gotten from the
solar panel towards the power source. The production of a solar panel is considered by a characteristic
curve of voltage versus current, named the I-V curve. The supreme power point of a solar panel is the point
laterally the I-V curve that agrees to the extreme output power conceivable for the panel. This magnitude
can be calculated by estimating the extreme area underneath the I-V curve. MPPT’s are utilised to precise
for the differences in the I-V features of the solar panels The I-V curve will change and distort dependent
upon such things as high temperature and lighting..
Meanwhile the extreme power point rapidly transfers as illumination conditions and weather change, a
equipment is desirable that treasures the supreme power point and adapts that voltage to a voltage equivalent
to the system voltage. Price is a chief issue when determining to use solar power as a source. A buyer wants
to extract the extreme energy per rupee consumed on a panel. Solar panel do contemporary an stimulating
difficulties in the transmission of power to a load, though. Aimed at additional obvious clarification, we
can state that solar array have a nonlinear IV characteristic, with a different maximum power point (MPP),
which be contingent on the ecological factors, such as temperature and radiation. In order to uninterruptedly
produce extreme power from the solar plates, they have to function at MPP in spite of the unavoidable
variations in the atmosphere. Therefore the supervisors of all solar electrical converters employment some
technique for maximum power point tracking (MPPT). Over the previous year’s several MPPT methods
have been available. The three procedures that anywhere found most appropriate for large and medium size
photovoltaic (PV) claims are perturb and observe (P and O), incremental conductance (InCond) and fuzzy
logic control (FLC). Here in this scheme we suggest P and O method, which overwhelmed the poor
presentation when the radiation vagaries constantly. This model was validated with simulations.
22
Figure 3: Block diagram of DC-DC converter incorporating MPPT control
4.2 P and O Method
4.2.1 Motivation
MPPT algorithms are essential in PV submissions since the MPP of a solar array differs with the radiation
and hotness, so the usage of MPPT schemes is mandatory in directive to get the extreme power from a PV
panel. Since many years lots of methods are used to treasure the MPP have been technologically advanced
and printed. These practises vary in many facets such as obligatory devices, intricacy, cost, range of
efficiency, conjunction speed, precise tracking when radiation and/or hotness change, hardware required
for the application or admiration, among others. Among these methods, the P & O and the InCond
procedures are the greatest shared. These methods have the benefit of an calm carrying out but they
similarly have disadvantages. Other methods founded on different ideologies are fuzzy logic control, neural
grid, fractional open circuit voltage or short circuit current, current sweep, etc. Greatest of these technique
harvest a local extreme and about, like the fractional open circuit voltage or short circuit current, stretch an
approached MPP, not the precise one. In standard circumstances the V-P curve has only one extreme, so it
is not a problematic. Though, if the solar panel is partly dappled, there are many maxima in these arcs. In
directive to dismiss this difficulty, some procedures have been applied as in.
4.2.2 Hill Climbing Techniques
Individually P and O and InCond procedures are founded on the “hill-climbing” standard, which contains
of touching the action point of the PV panel in the path in which power gains. Hill climbing procedures are
the maximum general MPPT approaches due to their comfort of application and decent presentation when
the radiation is fixed. The benefits of two approaches are the effortlessness and little computational. The
inadequacies are also renowned: fluctuations about the MPP and they can get misplaced and track the MPP
in the wrong way in quickly altering impressive conditions.
4.2.3 P and O Algorithm Implementation
The P and O procedure is also named “hill-climbing”, nonetheless both terms mention to the similar
procedure liable on how it is applied. Hill-climbing includes a worry on the duty cycle of the buck converter
and P and O a agitation in the working voltage of the DC connection among the PV panel and the buck
converter. In the situation of the Hill-climbing, disturbing the duty cycle of the buck converter suggests
adjusting the voltage of the DC connection among the PV panel and the buck converter, so both terms refer
to the identical practise.
23
Figure 4: Flow Chart of P and O Algorithm.
In this technique, the sign of the previous agitation and the sign of the previous increase in the power are
cast-off to choose what the following agitation should be. As described in Fig. 4, on the left-hand of the
MPP increasing the voltage upsurges the power while on the right-side decreasing the voltage growths the
power. If there is an increase in the power, the agitation would be kept in the similar way and if the power
reductions, then the following agitation would be in the conflicting way. Founded on these truths, the
procedure is applied. The procedure is recurrent pending the MPP is touched. Then the working point
fluctuates about the MPP. This difficulty is shared also to the InCond technique, as was referenced later. A
arrangement of the procedure is exposed in Fig. 4.
In P and O and InCond structures, how quickly the MPP is touched be contingent on the scope of the
increase of the orientation voltage. The disadvantages of these practises are chiefly two. Firstly the main
one is that they can effortlessly misplace track of the MPP if the radiation varies quickly. In situation of
step variations they touch the MPP very healthy, since the variation is rapid and the arc does not keep on
varying. Though, when the radiation changes subsequent a slope, the curve in which the processes are
founded vagaries incessantly with the radiation, so the variations in the voltage and current are not alone
due to the agitation of the voltage.
24
CHAPTER 5
Soft-Switching of DC-DC buck Synchronous
Converters
25
Soft-Switching in DC-DC buck Converters
5.1 Concept of soft switching:
Conventional PWM converters operate on hard switching phenomenon where voltage and current
pulses, during their transition from higher to lower values or lower to higher values interact with each
other and cause power losses called switching losses and generate a substantial quantity of
electromagnetic intrusion. Switching losses rise due to output capacitance of transistor, capacitance of
diode, reverse recovery diode. It is experiential that switching losses are relative to switching frequency.
So, developed switching losses cause to the restraint of switching frequency. Due to extensive variety
of harmonics existing in PWM waveform, a great Electro Magnetic Interference (EMI) happens. EMI
also fallouts from extraordinary current spikes produced by diode recovery.
Figure 5: hard switching phenomenon
Switching losses and EMI can be reduced by using soft switching techniques at the expense of stress on
the device. If the semiconductor device is made to turn off or turn on when current or voltage is zero,
then the product of voltage and current during transition is zero which leads to no loss in power. Thus
switching losses are removed and the MOSFET can be made to function at high switching frequencies.
Magnitude and heaviness of the MOSFET also reduces because of no requirement of heat sink.
5.2 Difficulties of Hard-Switching




Switching losses
Device strain, thermal organisation
EMI because of more di/ dt and dv/ dt
Power loss in lost L and C
26
5.2.1 Probable Solutions




Snubbers to decrease di/dt and dv/dt
Circuit layout to reduce lost inductances
Gate drive
 circuit layout
 switch on / off rapidity
Soft switching to attain ZVS or ZCS typically no variation in losses
5.2.2 Advantages –



Less losses
Less EMI
Permits high frequency process
5.3 Types of soft switching techniques
The soft switching techniques are widely categorized into two types namely


Zero Voltage Switching(ZVS)
Zero Current Switching(ZCS)
5.3.1 Zero Voltage Switching(ZVS):
The technique in which the MOSFET or any other semiconductor turns on at zero voltage is called ZVS.
ZVS is used during turn on of the device. Initially the primary MOSFET S is off and the secondary
MOSFET S1 is on. Hence current through primary switch is zero whereas voltage will not be zero.
During switch on voltage is ended zero across the switch and current is assumed some time delay such
that current begins to rise after the voltage becomes zero. This is called ZVS.
FIGURE 6: ZERO VOLTAGE SWITCHING (ZVS)
27
Turn on
 Switch voltage made to zero before gate voltage is supplied
 Ideal, no loss changeover
Turn off
 Less loss changeover
 Parallel capacitor like a less loss snubber
Favoured arrangement for very higher frequency operations consuming MOSFETs
FIGURE 7: switch on and switch off of Zero Voltage Switching(ZVS)
5.3.2 Zero Current Switching(ZCS):
This technique in which MOSFET or any other semiconductor device turns off at zero current is called
ZCS. ZCS is used during turn off of the device. Originally the device is conducting. Hence current
passing over the device is non zero and the voltage crosswise the device is 0. In the ZCS method, current
is brought zero and the voltage is increased only after the current is zero. Thus there is no power loss
during turn off of the device.
FIGURE 8: ZERO CURRENT SWITCHING(ZCS)
28
Turn off
 Switch current transported to zero earlier gate voltage is detached
 Zero loss changeover. Lossless process
Turn ON
 Low loss change
 Series inductor by way of a less loss snubber
 Power in junction capacitance is misplaced
Finest suitable for converters through IGBTs due to end current at turn-off
FIGURE 9: ZERO CURRENT SWITCHIN(ZCS)
5.4 Soft switching of Synchronous Buck Converter
FIGURE 10: SYNCHRONOUS BUCK CONVERTER
In this converter two MOSFETS are used which are synchronized. The second MOSFET is used in place
of diode so that conduction loss is minimised. But in this converter, no auxiliary circuit is present for
reducing the switching losses. Thus this converter can be used only for low switching frequency
applications.
29
FIGURE 11: SOFT SWITCHING OF SYNCHRONOUS BUCK CONVERTER
FIGURE 12: ZERO VOLTAGE SWITCHING TRANSITION WAVEFORM IN SYNCHRONOUS
BUCK CONVERTER
30
Chapter-6
SIMULATION OBSERVATIONS RESULTS AND
DISCUSSION
6.1 PV System
In direction to verify the proposed model of small PV system of 19.2 W is considered. This segment reveals
the simulation outcomes of solar panel using the equations depicted in last section in MATLAB/Simulink
environment. In this section we will explore the characteristics of PV array with the change in
irradiance and temperature and we will observe the changes in output power and current
Fig. 13 depicts the variation of Module current with Module Voltage with the variation of irradiance (G=1, 1.25.
1.6 and 2 sun) on the module at the constant temperature i.e. of 250C
Fig. 14 depicts the variation of Module power with Module Voltage with the variation of irradiance
(G=1, 1.25. 1.6 and 2 sun) on the module at the constant temperature i.e. of 250C.
31
Fig. 15 depicts the variation of Module current with Module Voltage with the variation of temperature
(T=0, 25, 50, 750C) on the module at the constant irradiance i.e. of 1000W /m2
Fig. 16 depicts the variation of Module Power with Module Voltage with the variation of temperature (T=0,
25, 50, 750C) on the module at the constant irradiance i.e. of 1000W /m2.
32
Fig. 17 depicts the variation of Module current with Module Voltage with the variation of diode stability factor n
(n=1.00, 1.25, 1.50, 1.75 and 2) on the module at the constant irradiance and temperature
Fig. 18 depicts the variation of Module current with Module Voltage with the variation of series resistance R
(R=0, 0.05, 0.2) on the module at the constant irradiance and temperature
33
6.2 Synchronous Buck Converter
To verify the suggested study of small rated PV system of 19.2 W with dc-dc synchronous buck converter
unit of is demonstrated and examined in MATLAB/Simulink software. The parameters taken for simulation
model study are assumed in the appendix. The performance of synchronous buck converter is analysed
under different operating conditions and the corresponding results are presented here.
6.2.1 during Steady State Conditions
Fig. 19 depicts the steady state response of Synchronous Buck Converter for constant load. From Fig. 19(a)
and fig 19(c), one can be see that, the output voltage and current of the converter settles in less than 6ms
with the aid of above designed PI controller. The corresponding output voltage and current ripple are shown
in Fig. 19(b) and Fig. 19(d) respectively and the voltage and current ripple of output voltage which is
maintained very low with the help of the designed output capacitor which limits the output voltage and
current ripple. Voltage stress across MOSFET ‘M1’ and MOSFET ‘M2’are illustrated Fig. 19(e) and Fig.
19(f) with limited values according to desired value. Fig. 19(g) shows the response of input voltage from
PV system which maintains constant at 12V.
6.2.2 During Step Load variation
Fig. 20 portrays the dynamic response of Synchronous Buck Converter during step changes in the load.
From Fig. 20(a) an, we could observe that, the output voltage settles in more time than 10ms and maintained
constant irrespective of the load variation from 1A to 1.5A as illustrated in Fig. 20(d) During load
variations, the transients in output voltage persist and it settles within 20ms from the evidence of Fig.20(b).
Voltage stress across MOSFET ‘M1’ and MOSFET ‘M2’are illustrated Fig. 14(e) and Fig. 14(f) with
limited values according to desired value. Fig. 14(g) shows the response of input voltage from PV system
which maintains constant at 12V.
34
Figure 19: Response of Steady state of synchronous buck converter (a) output voltage (b) output voltage ripple (c)
output current (d) output current ripple (e) voltage stress across MOSFET M1 (f) voltage stress across MOSFET
M2 (g) input voltage
35
Figure 20: Response of step load variation of synchronous buck converter (a) output voltage (b) output voltage
ripple (c) output current (d) output current ripple (e) voltage stress across MOSFET M1 (f) voltage stress across
MOSFET M2 (g) input voltage.
36
6.3 Efficiency Comparison
Fig. 21 represents the efficiency comparison between two basic buck converter topologies. Since, voltage
drop against MOSFET M2 is lower than the voltage drop across diode in buck converter topology. So,
synchronous buck converter has low or less power dissipations and higher efficiency is obtained. From the
figure it’s evident that, Synchronous Buck Converter has improved productivity than Conventional Buck
Converter. The performance of synchronous buck converter at low load is higher than nonsynchronous
buck converter. However, under higher load level, the efficiency also depends on duty cycle. However the
trade-off for better efficiency in Synchronous Buck Converter is the price of additional MOSFET used.
And also MOSFET saves space but complexity of control is increased because both switches should not
conduct simultaneously ( Any simultaneous conduction could cause to overload and damage the system
called as ”shoot through ”.To get rid of this a suitable delay called ”dead-time” must be incorporated.)
Figure 21: Efficiency Comparison between Synchronous Buck Converter and Conventional Buck
Converter
6.4 Soft switching of synchronous buck converter
Soft switching technique is implemented on synchronous buck converter through matlab Simulink model
and through driver circuit in hardware implementation. In this technique, two MOSFETS are used which
are synchronized. The second MOSFET is used in place of diode so that conduction loss is minimised. But
in this converter, no auxiliary circuit is present for reducing the switching losses. Figure 22 displays the
respective waveforms of output current and voltage graphs. We can conclude from fig 22(a) and 22 (c) that
there are little ripples in the voltage and graphs. Fig 22 (b) and 22 (d) shows the respective ripple voltage
and current waveforms. Fig 22(e) and 22 (f) shows the delay in turn on and turn off time for MOSFET 1
and MOSFET 2. Fig 22(g) shows the input voltage waveforms.
37
Figure 22: Soft switching of synchronous buck converter (a) output voltage (b) output voltage ripple (c)
output current (d) output current ripple (e) voltage stress across MOSFET M1 (f) voltage stress across
MOSFET M2 (g) input voltage.
38
6.5 Experimental Results
As discussed in previous chapters, various components of Synchronous Buck Converter are designed
and bought through stores. The catalogue of items are given below:
 Ready-made Inductor of value around 40μH.
 Input Capacitor of value 100μF
 Output Capacitor of value 500μ
 Two N-Channel MOSFETS i.e. SiHG20N50C
 Two resistors of 1.5Ω each.
 Higher Voltage and High Speed power MOSFET or IGBT driver IR2213
As shown in the figure Fig. 18, experimental set up in laboratory is going to require
Voltage Source, CRO, Bread Boards, Connecting probes, Function generator etc., to carry out the
experimental work intended.
We operate at 170 kHz and we use a duty cycle of 40 % for the flexible operation of the two MOSFETs.
39
Figure 23: Experimental Set-up in Laboratory
6.5.1 Conventional Buck Converter
Input voltage shown in figure Fig. 19 is given through voltage source for conventional buck converter
set-up. With the help of CRO we can observe the obtained output voltage which is shown in the figure
Fig. 20, which concurs with the theoretical calculations of Buck Converter.
Figure 24 (a) Input voltage (b) output voltage of conventional buck converter.
40
Figure 25 Voltage across MOSFET of buck converter
6.5.2 Synchronous Buck Converter
Input voltage same as given to Buck Converter as shown in figure Fig. 24 is given through voltage source
for Synchronous buck converter set-up. With the help of CRO we can observe the obtained output
voltage which is shown in the figure Fig. 2, which concurs with the theoretical calculations of Buck
Converter. In figures Fig. 27 and Fig. 28 we can observe the voltage stress across the main MOSFET
and Synchronous MOSFET respectively.
From the figures, Fig. 29 and Fig. 20 it is evident that the output voltages for both
Conventional Buck Converter and Synchronous Buck Converter are identical for a given duty cycle.
However, as studied theoretically there will be a great deal of difference in the efficiencies in the
comparison of both converters, in which Synchronous Buck Converter is more effective and efficient
than Conventional Buck Converter as shown in the figure Fig. 21.
FIGURE 26: COMPARISON OF SAW TOOTH AND CONTROL VOLTAGE
The fig. 26 shows the pulse formation occurrence. The reference signal is matched with the sawtooth
input and gate pulse is produced.
The gate pulse S1 is generated by relating the reference signal with clock signal, to produce the ON for
a time t02, which can be observed in Fig 27
As it can be observed from the fig.28, the gate pulse of MOSFET S2 is get by relating the clock signal
with the reference signal, to produce the pulse for time duration t7-t8.
41
FIGURE 27: GATE PULSE FOR MOSFET S1
FIGURE 28: GATE PULSE FOR MOSFET S2
FIGURE 29: OUTPUT VOLTAGE OF SYNCHRONOUS BUCK CONVERTER
The response output of the synchronous buck converter that output a voltage of 3V when given an
input of 8V, which is observed in the oscilloscope as in fig 29
42
Chapter-7 CONCLUSION AND FUTURE WORK
CONCLUSION
The no load P-V, P-I, I-V curves we got from the simulation of the solar panel designed in MATLAB
simulink model describes in fact its constraint on the radiation levels and degree of hotness. The whole
energy transformation scheme have been considered in MATLB-SIMULINK software. The many values
of the voltage and current found have been conspired in the non-circuited I-V curves of the PV panel at
isolation stages of 100 mW/cm2 and 200 mW/cm2. The voltage and current magnitudes occur on the
characteristics screening that the link of the PV panel with the Buck converter is appropriate. Though
the enactment of the photovoltaic device be contingent on the spectral circulation of the solar emission.
As solar panel is cast-off as a foundation of energy, it is essential to usage a extreme power point tracker
to guarantee negligible energy loss. The maximum power point tracker is applied to find the extreme
power point in our synchronous buck converter design. The core idea of paper is to use Soft Switching
technique for modelling of converter which decides precise values for PI controller used in control
circuit. Synchronous buck converter with closed loop PI controller precisely improved the dynamic
response of the system during load as well as source variation with reduced voltage and current ripple.
Furthermore, the circuit construction is modest and more economic relative to other control practices
where great amount of apparatuses is required. Further, the converter design and its efficiency also
determined. As results, the effectiveness of synchronous buck converter is advanced than conventional
dc-dc buck converter for same power rating.
FUTURE WORK:
The converter designed in this project operates at 200 KHz. However, for faster response at higher
frequencies with easily customisable control, FPGA implementation can be made and can be integrated
with micro controller control for more stability in output at various conditions. Such low cost systems
with less error due to digital operation can be used to operate low power high current devices and also
the isolated house power can be managed with these microcontroller based systems which has an added
advantage of flexibility and ability to interact with other devices. Thus the freedom to get electricity
anywhere and the adaptability of micro controllers to suit many conditions easily can be exploited to
make such portable systems in an effective and user-friendly manner.
43
7. References
[1] S. Rahmam, M. A. Khallat, and B. H. Chowdhury, “A discussion on the diversity in the applications of
photovoltaic system,” IEEE Trans., Energy Conversion, vol. 3, pp. 738–746, Dec. 1988.
[2] J.P. Benner and L. Kazmerski, “Photovoltaics gaining greater visibility,” IEEE Spectrum., vol. 29, no. 9,
pp. 34–42, Sep. 1999.
[3] F.Blaabjerg, Z. Chen, and S. B. Kjaer, “Power electronics as efficien interface in dispersed power
generation systems,” IEEE Trans., Power Electron., vol. 19, no. 5, pp. 1184–1194, Sep. 2004.
[4] M.Nagao and K. Harada, “Power flow of photovoltaic system using buck- oost PWM power
inverter,”Proc. PEDS’97, vol. 1, pp. 144–149,1997
[5] J.P.Lee, B.D. Min, T.J. Kim, D.W.. Yoo, and J.Y.Yoo,”Design and control of novel topology for
photovoltaic dc/dc converter with high efficiency under wide load ranges.” Journal of Power Electronics.
vol.9. no.2, pp.300-307, Mar,2009.
[6] E.Achille, T. Martiré, C. Glaize, and C. Joubert, “Optimized DC-AC Buck converters for modular
photovoltaic grid-connected generators,” in Proc. IEEE ISIE’04, pp. 1005–1010,2004,
[7] H.Bodur and A.Faruk Bakan,”A new ZCT-ZVT-PWM DC-DC converter,” IEEE Trans., Power Electron.,
vol. 9, no.3,pp 676-684.
[8] I.H.Altas, A. M. Sharaf, “A photovoltaic array simulation model for Matlab-Simulink GUI environment,”
Proc. ofInternational Conf. on Clean Electrical Power, 21-23, 2007, ICCEP, May 2007.
[9] Panda, A.K.; Aroul, K.; "A Novel Technique to Reduce the Switching Losses in a Synchronous Buck
Converter," Proc. of International Conference of Power Electronics, Drives and Energy Systems. pp.15,PEDES '2006.
[10] B.Chitti Babu, R.Vigneshwaran, Sudarshan Karthik, Nayan Ku. Dalei, Rabi Narayan Das, “A Novel
Technique for Maximum Power Point Tracking of PV Energy Conversion System’, Proc. of
InternationalConf. on Computer Applications in Electrical Engineering, IIT Roorkee. pp.276279,CERA 2010.
44
APPENDIX
45
MATLAB SIMULINK MODELS
1. PV ARRAY MODEL
PHOTOVOLTAIC SIMULATION MODEL
46
PV MODEL BLOCK
SATURATION CURRENT BLOCK
47
REVERSE SATURATION CURRENT BLOCK
2. SUNCHRONOUS BUCK CONVERTER MODEL
SIMULINK MODEL OF SYNCHRONOUS BUCK CONVERTER
48
SOFT SWITCHING SIMULINK MODEL OF SYNCHRONOUS BUCK CONVERTER
49
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