DC BOOST CONVERTER USING FUZZY LOGIC CONTROLLER

i

OPTIMIZATION OF DC - DC BOOST CONVERTER USING FUZZY LOGIC

CONTROLLER

AIMEN KAHLIFA SOLUMAN AHTIEWSH

A thesis submitted in

Fulfillment of the requirement for the award of the

Degree of Master of Electrical Engineering

Faculty of Electrical and Electronic Engineering

University Tun Hussein Onn Malaysia

JANUARY 2013

v

ABSTRACT

DC-DC converters are electronic devices used to change DC electrical power efficiently

from one voltage level to another. Operation of the switching devices causes the

inherently nonlinear characteristic of the DC-DC converters including one known as the

Boost converter. Consequently, this converter requires a controller with a high degree of

dynamic response. Proportional-Integral- Differential (PID) controllers have been usually

applied to the converters because of their simplicity.

However, the main drawback of PID controller is unable to adapt and approach the best

performance when applied to nonlinear system. It will sufer from dynamic response,

produces overshoot, longer rise time and settling time which in turn will influenced the

output voltage regulation of the Boost converter. Therefore, the implementation of

practical Fuzzy Logic controller that will deal to the issue must be investigated.

Fuzzy logic controller using voltage output as feedback for significantly improving the

dynamic performance of boost dc-dc converter by using MATLAB@Simulink software.

The design and calculation of the components especially for the inductor has been done

to ensure the converter operates in continuous conduction mode. The evaluation of the

output has been carried out and compared by software simulation using MATLAB

software between the open loop and closed loop circuit between fuzzy logic control

(FLC) and PID control. The simulation results are shown that voltage output is able to be

control in steady state condition for DC-DC boost converter by using this methodology.

Scope of this project limited only one types that is Triangle membership function for

fuzzy logic control.

vi

ABSTRAKT

Penukar DC-DC adalah litar elektronik kuasa yang menukarkan satu aras voltan DC

kepada satu aras voltan DC yang lain. Penukar Boost digunakan untuk meningkatkan

voltan masukan untuk memenuhi syarat yang dikehendaki oleh sesuatu operasi. Operasi

peranti pensuisan yang tak linear memerlukan pengawal Proportional-IntegralDifferential

(PID).

Walau bagaimanapun, kelemahan utama pengawal PID tidak dapat menyesuaikan diri

dan mendekati prestasi terbaik apabila merujuk kepada sistem tak linear seterusnya akan

mempengaruhi peraturan voltan keluaran penukar Boost. Oleh itu, pelaksanaan pengawal

Fuzzy Logik diperkenalkan bagi meningkatkan prestasi dinamik dengan menggunakan

MATLAB @ Simulink perisian. Reka bentuk dan pengiraan komponen terutama bagi

induktor telah dilakukan untuk memastikan penukar beroperasi dalam mod konduksi

berterusan.

Penilaian output telah dijalankan dan dibandingkan dengan perisian simulasi

menggunakan perisian MATLAB antara litar gelung terbuka dan gelung tertutup.

vii

CONTENTS

TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

TABLE OF CONTENTS vii

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF SYMBOLS AND ABBREVIATIONS xiv

CHAPTER 1 INTRODUCTION 1

1.1 Project overview 1

1.2 Problem statement 2

1.3 Project objective 3

1.4 Project scope 4

1.5 Project report layout 4

CHAPTER 2 LITERATURE REVIEW 5

2.1 Existing Models 5

2.2 DC-DC Converter 6

2.2.1 Non Isolated DC-DC Converters 6

viii

2.2.1.1 Buck converter 6

2.2.1.2 Boost Converter 7

2.2.1.3 Buck-Boost Converter 8

2.2.1.4 Cuk Converter 8

2.2.2 Isolated DC-DC Converters 9

2.2.2.1 Fly Back Converter 9

2.2.2.2 Forward converter 10

2.3 DC-DC converter switching 11

2.3.1 Advantages of DCM 14

2.3.2 Disadvantages of DCM 14

2.4 Boost Converter 15

2.4.2 Analyses for switch open (off) 18

2.5 Control methods for DC-DC converters 20

2.5.1 PID Controller 21

2.5.2 Fuzzy logic controller system 22

2.5.2.1 Fuzzification 23

2.5.2.2 Rule base 26

2.5.2.3Rules of Inference 26

2.5.2.4 Defuzzification 26

2.5.2.4 Defuzzification 27

2.5.2.5 Advantages of Fuzzy Logic Controller 28

CHAPTER 3 METHODOLOGY 34

3.1 Project Background 34

3.2 Parameter for Boost Converter 36

3.3 Generate SPWM switching using Matlab Simulink 38

3.4 PID Controller Design for Boost Converters 38

3.5 Fuzzy Logic Controller and Its Operational 42

3.5.1 Fuzzification 43

3.5.2 Rule Base 45

3.5.3 Inference Mechanism 46

ix

3.5.4 Defuzzification 46

3.6 Operational the Fuzzy logic Control for DC-DC

Boost Converters 47

CHAPTER 4 RESULT AND ANALYSIS

4.1 Introduction 50

4.2 Analysis for Boost Converter 50

4.2.1 Analysis for Open Loop Boost Converter 50

4.2.2 Analysis for Boost Converter with PID Controller 52

4.2.3 Analysis for Boost Converter with Fuzzy Logic Controller 53

4.3 Comparison Analysis 56

CHAPTER 5 CONCLUSION AND RECOMMENDATION

5.1 Conclusion 58

5.2 Recommendation 59

REFERENCES 60

x

LIST OF TABLES

2.1: Example of rule base 30

3.1: The value of all parameters can be determined as below 37

Parameters and values for boost converter

3.2: Rules for error and change of error 45

5.1: The reading on Peak Overshoot Ratio, Rise Time, Peak Time 56

And Settling Time from open loop and closed loop for PID and

FLC, circuit boost dc-dc converter

5.2 The deviations of voltage resulted from open loop and 57

Closed loop circuit for PID and FLC Boost converter

xi

LIST OF FIGURES

2.1 The Basic Circuit Configuration of the Buck Converter 9

2.2 Basic Circuit of Boost Converter 10

2.3 Basic Circuit of Buck-Boost Converter 10

2.4 Basic Circuit of Cuk Converter 11

2.5 Basic Circuit of Fly-Back Converter 12

2.6 Basic Circuit of Forward Converter 13

2.7 Switching ON and OFF of DC-DC converter 14

2.8 and pulse 15

2.9 Continuous Conduction Mode 16

2.10 Discontinuous Conduction Mode 16

2.11 and when inductor looks like short circuit 16

2.12 A Boost Converter Circuit 18

2.13 The Duty Cycle for Switching Period during Steady State 19

2.14 The Equivalent Circuit of Boost Converter When the 20

Switch S Is Closed

2.15 Analysis for Switch Closed ( ) 20

2.16 The Equivalent Circuit of Boost Converter When the Switch S Open 21

2-17 Analysis for Switch Opened (Off) 21

2.18 Control Methods for DC-DC Converters 24

2.19 Proportional-Integral-Derivative (PID) Controllers 25

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2.20 Fuzzy Logic Controller Schematic 27

2.21 FLC Overall Structure 27

2.22 The Features of A Membership Function 28

2.23(A) Triangular Membership Function Shape 29

2.23 (B) Gaussian Membership Function Shape 29

2.23 (C) Trapezoidal Membership Function Shape 29

2.23(D) Generalized Bell Membership Function Shape 29

2.23(E) Sigmoidal Membership Function Shape 30

2.24 Weight Average Method Defuzzification 33

3.1 Shows the Overview of the Methodology Flow Chart of This Project 35

3.2 Block Diagram For Propose DC-DC Boost Converter 36

3.3 Diagram of Open Loop Dc to Dc Boost Converter by Using Simulink 37

3.4 Simulink Generate SPWM Switching 38

3.5 Simulink Model of the PID Controller for the DC-DC 39

Boost Converters

3.6 Flowcharts of Digital PID and PI Controller 41

3.7 Block Diagram of Fuzzy Control System 43

3.8 The Membership Functions For e[K] , ceh[K] And Δd[K] 44

3.7 Simulink Model of the Fuzzy Logic Controller for the 48

Boost Converters

3.10 Flowcharts of Fuzzy Logic Controller for the Boost Converters 49

4.1(a): Results on output voltage for open loop circuit 51

DC to DC boost converters

4.2(b): results on output current for open loop circuit dc to dc boost converter 51

4.3(c): results on capacitor voltage for open loop circuit dc to dc boost converter 51

4.4(d): results on ripple output voltage for open loop circuit dc/ dc boost converter 52

xiii

4.5(a): Results on output voltage for PID controller closed loop circuit 52

DC to DC boost converter

4.6(b): Results on output current for PID controller closed loop circuit 53


4.7(c): Results on Capacitor voltage for PID controller closed loop 53

circuit DC to DC boost converter

4.8(a): Output voltage for boost converter with fuzzy logic controller 54

4.9(b): output current for FLC controller closed loop circuit 54


4.10(c): Results on Capacitor voltage for FLC controller closed loop 54


4.11(d): Results on ripple output voltage for FLC closed loop 55


4.12: 2nd Order of Step Response Reading on Overshoot Ratio, 56

Rise Time, Peak Time and Settling Time

xiv

LIST OF SYMBOLS AND ABBREVIATIONS

µe- degree of membership function of error

Δe- degree of membership function of delta of error

u- degree of membership function of voltage ouput

∨ - maximum operator

O - output of COG

˄ - minimum operator

B - Bisector of Area

C - Capacitor

CCM - Continuous Conduction Mode

che - Change of Error

COG - Centroid of Gravity

D - Duty Cycle

DC - Direct Current

DCM - Discontinuous Conduction Mode

e - Error

FLC - Fuzzy Logic Controller

Fs - Frequency Switching

Triangular - Gaussian Membership Function

KD - Derivative gain

KI - Integral gain

xv

KP - Proportional gain

L - Inductor

MF - Membership Function

MOM - Mean of Maximum

MOSFET - Metal–Oxide–Semiconductor Field-Effect Transistor

NB - Negative Big

NS - Negative Small

PB - Positive Big

PID - Proportional Integral Derivative

PS - Positive Small

PWM - Pulse Width Modulation

R - Resistor

S - Switch

VC -Voltage (Calculation)

Vo -Output Voltage

s -Kth - switching cycle Input Voltage

Vref- Reference output

ZE – Zero

1

CHAPTER 1

INTRODUCTION

1.1 Project overview

DC to DC step-up power converter, or more popularly known as the Boost converter, is

widely used in power electronics systems. Its application is widespread and wide

ranging-Boost power supply can be found in the tiniest cell phone (mill watt) to the high

power train propulsion system (hundreds of kilowatts). One of the main requirements of

the converter is the robustness of its controller.

A good controller should perform the following tasks: (1) able to regulate output voltage

when the input voltage and reference is changed (2) able to stabilize the system for any

input disturbances and load changes. The performance of the controller is normally

characterized by its response to a step input reference, i.e. transient percentage of

overshoot, settling time and steady state error.

Due to its nonlinear and time-invariant nature, the design of high performance controller

for the Boost converter presents a challenging task. Traditionally, classical methods such

as frequency response and root locus/pole placement techniques are employed.

Examples of classical controllers are the Proportional Integral Derivative (PID)[1],

Deadbeat controllers [2] and sliding mode controllers [3].

These controllers are known as “model based”, relying heavily on mathematical model

of the converter for accurate control action. An equivalent circuit-averaging model is

derived to determine the converter’s variables within a switching period. Based on the

2

averaged model, a suitable small-signal model is obtained by performing small signal

perturbation and linearization around a specific (nominal) operating point. For a Boost

converter, it is known that the poles and a right-half plane zero are dependent on the load

resistance, R [4]. Since classical controllers are designed to operate at one nominal

operating point (i.e. fixed duty cycle, D), they are unable to respond satisfactorily to a

large operating point variation (i.e. large change in D). Similarly, it could not cope with

large load disturbance (large change in R).

Moreover, classical controllers are sensitive to the changes in system parameters,

resulting in unpredictable control performance when subjected to changes in

temperature, aging, operating point etc. To alleviate the dependency on the mathematical

model, “non-model based” controllers have been proposed. Among the most popular is

the Fuzzy Logic Controller (FLC). In essence, FLC is a linguistic-based controller that

tries to solve problems by means of systematic rule inferences. It does not require

precise mathematical model, very robust and has excellent immunity to external

disturbances [5]. Although promising, FLC requires substantial computational power

due to complex decision making processes, namely fuzzification, rule base storage,

inference mechanism and defuzzification operations. To obtain optimized performance,

FLC require a much longer time because for most cases, the design is done

heuristically [6],[7].

In this project, MATLAB/Simulink is used as a platform in designing the fuzzy logic

controller. MATLAB/Simulink simulation model is built to study the dynamic behavior

of dc to dc converter and performance of proposed controller.

3

1.2 Problem statement

DC-DC converter consists of power semiconductor devices which are operated as

electronic switches. Operation of the switching devices causes the inherently nonlinear

characteristic of the DC-DC converters including one known as the Boost converter.

Consequently, this converter requires a controller with a high degree of dynamic

response. Proportional-Integral- Differential (PID) controllers have been usually applied

to the converters because of their simplicity. However, the main drawback of PID

controller is unable to adapt and approach the best performance when applied to

nonlinear system. It will suffer from dynamic response, produces overshoot, longer rise

time and settling time which in turn will influenced the output voltage regulation of the

Boost converter.

In general, PID controller produces long rise time when the overshoot in output voltage

decreases.(W.M.Utomo, April 2011). Therefore, the implementation of practical Fuzzy

Logic controller that will deal to the issue must be investigated.

The Fuzzy control is a practical alternative for a variety of challenging control

applications because Fuzzy logic control is nonlinear and adaptive in nature that gives it

a robust performance under parameter variation and load disturbances. Fuzzy controllers

are more robust than PID controllers because they can cover wider range of operating

conditions than PID, and can also operate with noise and disturbance of different

natures. Developing the fuzzy controller is cheaper than developing a model based or

other controllers for the same purpose.

Fuzzy logic is suited to low-cost implementations and systems of fuzzy can be

easily upgraded by adding new rules to improve performance or add new features.

4

1.3 Project objective

The objectives of this project are;

i) To model and analysis a DC-DC Boost converter without controller (open

loop) and simulate using MATLAB Simulink.

ii) To design Proportional-Integral-Derivative (PID) Controllers to control the

switching of DC-DC Boost converter.

iii) To design fuzzy logic controller (FLC) to control the switching of DC-DC

Boost converter

iv) To analyze the voltage output for DC-DC Boost converter between open loop

and closed loop for PID controller and fuzzy logic controller.

1.4 Project scope

The scopes of this project are to simulate the proposed method of optimization of DC-

DC boost converter using Triangle fuzzy logic controller with MATLAB Simulink

software. Analyses of the converter will be done for continuous current mode (CCM)

only. The scope of proposed fuzzy logic controller is limited Triangle as a proposed

controller. The analysis only covered the output voltage based on reading on overshoot

ratio, rise time, peak time and settling time.

6

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

Since fuzzy logic controller can mimic human behavior, many researchers applied fuzzy

logic controller to control voltage output. A thorough literature overview was done on

the usage of fuzzy logic controller as applied DC-DC Boost Converter.

Ismail, N.F.N. Musirin, I. ; Baharom, R. ; Johari, D (2010) proposed a fuzzy logic

controller using voltage output as feedback for significantly improving the dynamic

performance of boost dc-dc converter by using MATLAB@Simulink software. The

simulation results are shown that voltage output with fuzzy logic controller with 0%

overshoot shows the better performance compared to the open loop circuit (without

fuzzy logic controller) whereby it has 80% overshoot.

Md. Shamim-Ul-Alam, Muhammad Quamruzzaman and K. M. Rahman (2010)

proposed design of a sliding mode controller based on fuzzy logic for a dc-dc 7 boost

converter. Sliding mode controller ensures robustness against all variations and fuzzy

logic helps to reduce chattering phenomenon introduced by sliding controller, thereby

increasing efficiency and reducing error, voltage and current ripples.

The proposed system is simulated using MATLAB/SIMULINK. This model is tested

against variation of input and reference voltages and found to perform better than

conventional sliding mode controller.

7

Ahmed Rubaai, Mohamed F. Chouikha (2004) proposed controllers for DCDC

converters Simulation results have been obtained using appropriate scaling factors

associated with the input variables of the fuzzy controller. Simulation results show the

ease of applying fuzzy control to dc/dc converters, as an interesting alternative to

conventional techniques.

Based on those related work, the researchers make a great efforts to propose the good to

overcome the DC-DC Converter problems. Their applications of each method differ,

thus the further investigation of this controller is needed.

2.2 DC / DC Converters

The DC/DC converter is a device for converting the DC voltage to step-up or step-down

depending on the load voltage required. If the requirement of voltage is step-up then it is

necessary to use a boost converter. It the requirement of voltage is step-down, and then it

necessary to use a buck converter. Sometimes, both step-up and step-down is required to

cover the load, but at different times then it is necessary to use a buck-boost converter.

Therefore, different types of DC/DC converters are used for different voltage levels in

load. Generally DC/DC converters are divided into two types [9].

1- Non isolated DC-DC converter

2- Isolated DC-DC converter

8

2.2.1 Non Isolated DC-DC Converters

2.2.1.1 Buck converter

Figure 2.1 shows the basic circuit configuration used in the buck converter. There are

only four main components namely switching power MOSFET Q1, flywheel diode D1,

inductor L and output filter capacitor C1. In this circuit the transistor that is switched ON

will put voltage on one end of the inductor. This voltage causes the current of the

inductor to rise. When the transistor is switched OFF, the current continue to flow

through the inductor. At the same time, it flows through the diode. Initially it is assumed

that the current flowing through the inductor does not reach zero; thus the voltage will

only go across the conducting diode during the full OFF time. The average voltage

depends on the average ON time of the transistor on the condition that the current of the

inductor is continuous.

Figure 2.1: The Basic Circuit Configuration of the Buck Converter

9

2.2.1.2 Boost Converter

A boost converter or a step up converter is a non-isolated converter. It the most

commonly used DC/DC converter, especially used in UPS and PV. This is because

battery charge requires high DC voltage to be fully charged. Figure 2.2 shows the basic

boost converter. The theory of a boost converter is not complicated as other converters

rather. It is simple and straight forward. If the switch, S is ON, the current flows only

through the inductor, which has stored energy. When the switch, S is OFF, the energy in

the conductor is translated to a capacitor, which usually has a large capacity to store a

bigger amount of energy. Finally, this energy converts to load with a high DC voltage.

Figure 2.2: Basic Circuit of Boost Converter

2.2.1.3 Buck-Boost Converter

The main components in a buck-boost converter are the same as in the buck and boost

converter types, but they are configured in a different way. Figure 2.3 in buck-boost

converter, a step-up or step-down voltage can change the value of duty cycle.

Nevertheless, in a similar process, once the switch is ON, the inductor begins charging

and, the converter is stored with energy. However, once the switch is OFF, the circuit

changes into inductor and capacitor simultaneously hence all the stored energy in the

inductor is converted to capacitor. One thing that controls the voltage is the duty cycle. If

the duty cycle is big, voltage is high in the load. On the other hand, when the duty cycle

is small, voltage in the load is low.

10

Figure 2.3: Basic Circuit of Buck-Boost Converter

2.2.1.4 Cuk Converter

All the three converters buck, boost and buck-boost converters, transfer energy between

input and outputs through the inductor. The analysis is based on voltage balance across

the inductor. The Cuk converter in Figure 2.4 uses capacitive energy transfer. This

analysis is based on current balance of the capacitor.

Figure 2.4: Basic Circuit of Cuk Converter

However, the buck-boost converter such as the Cuk converter can step the voltage up or

down, depending on duty cycle. The main difference between the two buck-boost and

Cuk converters is that, the series of inductors at both input and output record much lower

current ripple in both circuits.

11

2.2.2 Isolated DC-DC Converters

2.2.2.1 Fly Back Converter

Actually the fly back converter shown in Figure 2.5 can be recognized as an extension of

buck-boost converter. The buck-boost converter is shown Figure 2.3. If it changes the

inductor to a transformer, then it becomes a fly-back converter. This is because the

direction of diode ON the secondary side of Fly-back converter works in both directions.

Generally, the input to the circuit is unregulated DC voltage that is obtained by

rectifying the utility AC voltage followed by a simple capacitor filter. The circuit can

offer single or multiple isolated output voltages and can operate over a wide range of

input voltage variation. In terms of energy-efficiency, fly-back power supplies are lower

than many other circuits but it’s simple topology and low cost makes it popular in low

output power range.

Figure 2.5: Basic Circuit of Fly-Back Converter

12

2.2.2.2 Forward converter

In the Fly-back converter, there are two separate phases or energy storage and delivery

to the output. As the forward converter shown in Figure 2.6, it uses the transformer in a

more traditional style where by it transfers energy directly between input and output in

just one step. However, the forward converter reverses the polarity of magnetic flux in

the core of a transformer core for each alternate half-cycle. Hence, there are fewer

tendencies to cause saturation compared to the Fly-back converter. This means the

transformer can be significantly smaller, for the same power level. This together with the

tighter and more predictable relationship between input and output voltage makes the

forward converter an attractive choice for high power applications. [10].

Figure 2.6: Basic Circuit of Forward Converter

13

2.3 DC-DC converter switching

The MOSFET and IGBT are the most popularly used electronic switch devices in

DC/DC converters. MOSFET can be modeled as an ideal switch with a series resistance,

and a capacitor connected in parallel Figure 2.7 Every time it is turned ON, the parallel

capacitor is discharged through the resistance and (0.5 × Cout × ) units are lost. A

MOSFET in a Zero Current Switch will have to turn ON with a high drain source

voltage, and there will be capacitive switching losses. Additionally, the reactive

overcurrent in the switch is very high, and as the MOSFET does not perform well in

overcurrent conditions, the conduction losses will be very high. Therefore the MOSFET

is not very suitable as a Zero Current Switch (ZCS) [11].

There are two switching condition that need to be applied, that is when ON and OFF

as shown in figure 2.7.

When ON,

Output voltage is the same as the input voltage and the voltage across the switch is 0V.

When OFF,

Output voltage = 0V and current through the switch = 0A. In ideal condition, power

loss = 0W since output power equal to input power.

Figure 2.7: Switching ON and OFF of DC-DC converter.

14

ON and OFF resulting in pulse as shown in Figure 2.8 where switching period, , is a one

full cycle (360°) of a waveform ranging from to pulse T

Figure 2.8: and pulse.

Thus, duty cycle, D, which depends on and range of duty cycle, is

0 D 1. If switching frequency, , is given,

(2-1)

Average DC output voltage,

∫

( )

∫

(2-2)

There are two modes of operation in DC-DC converters based on inductor current, ,

i) Continuous Conduction Mode (CCM), when

ii) Discontinuous Conduction Mode (DCM) when goes to 0 and stays at 0 for

some time.

15

Figure 2.9: Continuous Conduction Mode.

Figure 2.10: Discontinuous Conduction Mode.

In steady state and periodic operation, inductor charges and discharges with DC

voltage across inductor in one period = 0. Thus, inductor looks like a short circuit as

shown in Figure 2.11.

Figure 2.11: and when inductor looks like short circuit.

16

2.3.1 Advantages of DCM

i. The CCM boost, buck-boost, and fly back topologies have a right half planes zero

(RHPZ) in their control to output transfer function. The right half plane zero is

nearly impossible to compensate for in the company loop. As a result, the control

loop in these CCM converters is typically made to cross over at a frequency much

lower than the RHPZ frequency resulting in lower transient response bandwidths.

The DCM version of the boost, buck-boost, and fly back converters do not have a

right half plan zero and can have higher loop crossover frequency allowing higher

transient response bandwidths.

ii. In the buck, boost, buck-boost, and all topologies derived from these, the input to

output and control to output transfer functions contain single pole responses while

operating in DCM. Converters with only single pole transfer functions are easier to

compensate than converters having a double pole response.

2.3.2 Disadvantages of DCM

i. In DCM, the inductor current reaches zero while in non-synchronous operation, the

end of the inductor connected to the switch (also called the freewheel end), must

immediately transition to the voltage at the other end of the inductor. However,

there will always be inductive and capacitive parasitic elements which will cause

severe ringing if damping is not implemented. This ringing can produce

undesirable noise at the output of the power supply

ii Boost, buck-boost, and derived topologies are commonly operated only in DCM to

avoid the adverse effects of the RHPZ described earlier. However, to achieve the

same power in DCM as in CCM, the peak and RMS currents are substantially

higher resulting in greater losses in the conduction paths and greater ringing

because the energy store inductances is proportional to the square of the current.

Energy stored that is not delivered to the output causes ringing and losses.

17

iii In DCM, the inductance must be much smaller in value to allow the current to fall to

zero before the start of the next cycle. Small inductance results in higher RMS and

peak inductor currents. Because the RMS and peak currents are greater in DCM

than in CCM, the transformers must be sized larger to accommodate greater flux

swings and copper and core losses. DCM has a physically larger transformers and

inductors required for same power output as CCM.

2.4 Boost Converter

A boost converter (step-up converter), steps up the input DC voltage value and provides

at output. It consists of an inductor L, capacitor C, and controllable semiconductor

switch Diode D, and Load resistance R as depicted by Figure 2-12. Capacitors are

generally added to output so as to perform the function of removing output voltage

ripple. The boost converter is one of the most important non isolated step-up converters.

Figure 2.12: A Boost Converter Circuit

Boost converter operates, assume that the inductor is charged in the previous cycle of

operation and the converter is at the steady state operation and CCM Condition.

18

Figure 2.13: The Duty Cycle for Switching Period during Steady State

Define Duty Cycle ( D ) which depends on ton and switching frequency

(2-3)

During steady state operation the ratio between the output and input voltage is

, the

output voltage is controlled by varying the duty cycle. Range of Duty Cycle .

(2-4)

( ) (2-5)

2.4.1 Analysis for switch closed ( )

Start with the power switch S is closed. Now the diode will be reversed-biased by the

capacitor voltage, hence it will act as open. The equivalent circuit is shown in

Figure 2-14. Diode is reverse-biased. Input is disconnected from the output, no energy

flows from input to output, output gets energy from capacitor, .

The inductor voltage

(2-6)

19

Since the derivative of is a +ve constant, therefore must increase linearly;

( ) (

) (2-7)

Figure 2.14: The Equivalent Circuit of Boost Converter When the Switch S Is Closed

Figure 2.15: Analysis for Switch Closed ( )

20

2.4.2 Analyses for switch open (off)

For the next cycle, the power switches S open. The condition is depicted in Figure 2.16.

Because the inductor fully charged in the previous cycle, it will continue to force its

current through the diode D to the output circuit and charge the capacitor. Inductor is

discharging, diode is forward-biased, and Input is connected to the output, energy flows

from input to output while capacitor’s energy is replenished. The output stage receives

energy from the input as well as from the inductor .

Figure 2.16: The Equivalent Circuit of Boost Converter When the Switch S Open

Figure 2-17: Analysis for Switch Opened (Off)

21

Inductor voltage

( )

Since the derivative of is a -ve constant, therefore must decrease linearly.

( )

( ) (

) ( ) ( )

Steady-state operation

( ) ( )

( ) (

) ( )

( )

In steady state the average inductor voltage is zero over one switching period known as

Volt Second Balance.

22

( )( )

( )

From the equation (2.11) average output voltage is higher than input voltage.

Inductor current:

Input power = Output power

( )

Average inductor current

(

)

( )

23

2.5 Control methods for DC-DC converters

In most of the applications it is needed to maintain the output voltage of converter

regardless - s of changes in the load or input voltage. The DC-DC power converters are

sited in middle stage of most of electrical power systems; their input is connected to

solar cells and output is connected to inverters. Both input and output sides are prone to

sudden changes in values, slow transient response increase losses in the system and leads

to dramatic reduction of efficiency.

Several methods have been proposed by researchers to control output voltage of DC -

DC converters these methods are illustrated in Figure 2.18. Classical methods (model

based) rely on mathematical model of system hence they are sensitive to changes in

transfer function. In opposite to classical methods, non-model based approaches use

intelligence techniques which are not dependent on system mathematical model.

For systems with non-constant transfer function or without mathematical methods non

model base controllers are more preferred.

Figure 2.18: Control Methods for DC-DC Converters

24

2.5.1 PID Controller

PID stands for proportional, integral, derivative are one of the most popular feedback

controller widely use in processing industry. It is easy to understand the algorithm to

produce excellent control performance.

The PID consists of three basic modes that are proportional modes, integral modes and

derivative modes. Generally three basis algorithm uses are P, PI and PID. This

controller has a transfer function for each modes, proportional modes adjust the output

signal in direct proportional to the controller output [2](M.J. Willis,1999). A

proportional controller ( ) reduced the error but not eliminated it [3](Power

Design,2008). An integral controller ( ) will have the effect of eliminating the steady-

state error but it may worsen the transient respond. The derivative controller (kd) will

have the effect of increasing the stability of the system, reducing the overshoot and

improving the transient respond.

Figure 2.19: Proportional-Integral-Derivative (PID) Controllers

In designing PID controller there are several steps to be applied in order to obtain

desired output response. It is not necessary to implement all three controllers if not

needed, if P or PI controller gives a good enough response the there is no need to

implement the derivative controller.

Controllers respond to the error between a selected set point and the offset or error

signal that is the difference between the measurement value and the set point. Optimum

values can be computed based upon the natural frequency of a system. Too 15much

feedback (positive feedback cause stability problems) causes increasing oscillation. With

proportional (gain) only control the output increases or decreases to a new value that is

5

1.5 Project report layout

This project report is organized as follows;

i) Chapter 1 briefs the overall background of the study. A quick glimpse of study

touched in first sub-topic. The heart of study such as problem statement, project

objective, project scope and project report layout is present well through this

chapter.

ii) Chapter 2 covers the literature review of previous case study of types of DC/DC

converters and based on fuzzy logic controller background and development.

Besides, also Proportional-Integral-Derivative (PID) Controllers, general

information about Boost Converter and theoretical revision on fuzzy logic control

system also described in this chapter.

iii) Chapter 3 presents the methodology used to design open loop Boost Converter and

closed loop for fuzzy logic controller and Proportional-Integral-Derivative (PID)

controllers. All the components that have been used in designing of fuzzy logic

controller are described well in this chapter.

iv) Chapter 4 reports and discuss on the results obtained based on the problem

statements as mentioned in the first chapter. The simulation results from open

loop, PID controller and the proposed of fuzzy logic controller will be analyzed

with helps from set of figures and tables.

v) Chapter 5 will go through about the conclusion and recommendation for future

study. References cited and supporting appendices are given at the end of this

project report.

60

REFERENCES

[1] Ashfaq Ahmed, 1999, Power Electronics for Technology, page(s) : 269-303

[2] Daniel W.Hart, 1997, Introduction to Power Electronics, page(s) :185-204

[3] Guanrong Chen, Trung Tat Pham ,2001,Introduction to Fuzzy Sets, Fuzzy logic, and

Fuzzy Contr ol Systems.

[4] Muhammad H. Rashid, Second Edition, Power Electronics Circuit, Devices, and

Applications, page(s) :303-323

[5] A. Rubaai, M.F Chouikha, Design and Analysis of Fuzzy Controllers for DC-DC

Converters,1994, IEEE Control, Communications and Signal Processing, page(s):

479-482.

[6] Guang Feng, Wangfeng Zhang, Yan-Fei Lui, An Adaptive current Mode fuzzy Logic

Controller for Dc to Dc Converter, Applied Power Electronics Conference and

Exposition, 2003, APEC ’03.Eigtheenth Annual IEEE,9-13 Feb,2003, vol.2,page(s):

983-989.

[7] P.Mattavelli, L.Rossetto, G.Spiazzi and P.Tenti, General-Purpose Fuzzy Controller

for DC-Dc Converter, Proceedings of 1997, IEEE Transactions On Power

Electronics, 1 January 1997, vol.12, no.1.

[8] L.A Zedah, outline of new approach of the analysis of complex system and decision

processes, 1963, IEEE Transactions System,Man, Cybern., vol. SMC-3, no.1,

page(s):146-151.

[9] Fuzzy logic toolbox 2 User Guider, The Mathworks, Inc, 2005.

[10] Power Electronics Daniel W. Hart Valparaiso University Valparaiso, Indiana

pages (s)(196-264)

Documents

DC BOOST CONVERTER USING FUZZY LOGIC CONTROLLER