ii

VOLTAGE MODE CONTROL FOR BUCK CONVERTER

MARAN PILLAI A/L THANDAYITHABANI

This thesis is submitted as partial fulfillment of the requirement for the award of the

Bachelor of Electrical Engineering (Power Systems)

Faculty of Electrical & Electronics Engineering

Universiti Malaysia Pahang

JUNE 2012

vi

ABSTRACT

This project is about the study, design and a practical implementation of

voltage mode control of buck converter. Buck converter is a stepdown switch mode

power supply where its output voltage is always less than the input voltage. UC3573

is used in this project as a PWM control IC where it provides pulse signal to the gate

terminal of the MOSFET. The output voltage of buck converter is changed when

there is changing in the input voltage and/or the load. To cater the problem, voltage

mode control (VMC) scheme is applied in this project. This controller has a simple

hardware implementation and flexibility. PSpice simulations are performed before

hardware implementation. A prototype buck converter with voltage mode control is

built on printed circuit board (PCB). The prototype buck converter maintains the

output voltage when there is changing in the input voltage and/or the load.

vii

ABSTRAK

Projek ini adalah mengenai reka bentuk dan pelaksanaan yang praktikal

tentang kawalan mod voltan penukar buck. Penukar buck adalah bekalan kuasa mod

suis stepdown di mana voltan keluaran adalah sentiasa kurang daripada voltan input.

UC3573 digunakan dalam projek ini sebagai PWM kawalan IC di mana ia memberi

isyarat denyut ke terminal get MOSFET. Voltan keluaran penukar buck berubah

apabila terdapat perubahan dalam voltan input dan / atau beban. Untuk menampung

masalah, kawalan mod voltan (VMC) skim digunakan dalam projek ini. Pengawal ini

mempunyai pelaksanaan perkakasan yang mudah dan fleksibiliti. Simulasi Pspice

dilakukan sebelum pelaksanaan perkakasan. Penukar buck prototaip dengan kawalan

mod voltan yang dibina di atas papan litar bercetak (PCB). Prototaip penukar buck

mengekalkan voltan output apabila terdapat perubahan dalam voltan input dan / atau

beban.

viii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

TITLE PAGE i

SUPERVISOR’S DECLARATION ii

STUDENT’S DECLARATION iii

DEDICATION iv

ACKNOWLEDGEMENT v

ABSTRACT vi

ABSTRAK vii

TABLE OF CONTENT viii

LIST OF FIGURE xi

LIST OF TABLES xiii

LIST OF ABBREVIATIONS xiv

LIST OF SYMBOLS xv

LIST OF APPENDIXES xvi

1 INTRODUCTION 1

1.1 Introduction 1

1.2 Problem Statement 2

1.3 Objectives 3

1.4 Scope of Project 3

1.5 Thesis Outline 4

2 LITERATURE REVIEW 5

2.1 Swithcing Regulators for DC-DC Converters 5

ix

2.2 Buck Converter Topology 6

2.3 Feedback and Control Circuitry Scheme 10

2.3.1 Voltage Mode Control 12

2.4 Pulse Width Modulation Circuit 14

2.5 Buck Converter Operation States 16

2.5.1 On State 16

2.5.2 Off State 17

2.6 Pspice Software 18

3 METHODOLOGY 19

3.1 Design Specification and Parameters 19

3.1.1 Power Stage Parameters 20

3.2 Power Stage Component Selection 22

3.2.1 Selection of Power Switch 22

3.2.2 Inductor 23

3.2.3 Output Capacitors 23

3.2.4 Input Capacitors 24

3.2.5 Power Diode 24

3.2.6 PWM Controller 26

3.2.7 Load 26

3.3 Compensation Scheme 27

3.3.1 Type III Compensation 28

3.4 Printed Circuit Board 31

4.1 Pspice Simulation Results 32

4.2 Small Signal Circuit Analysis 34

4.3 Hardware Testing 37

4.3.1 Breadboard Testing 37

4.3.2 PCB Design 38

4.3.3 Closed Loop Buck Converter Testing 38

4 RESULTS AND DISCUSSION 32

x

4.4 Experimental Results 39

4.5 Discussion 46

5.1 Conclusion 48

5.2 Recommendation 49

5 CONCLUSION AND RECOMMENDATION 48

REFERENCES 50

APPENDIX A 51

APPENDIX B 59

APPENDIX C 62

APPENDIX D 67

xi

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Buck Converter 6

2.2 Inductor’s voltage waveform for Buck converter 7

2.3 Inductor’s current waveform for Buck converter 9

2.4 Capacitor current waveform for Buck converter. 10

2.5 Feedback Control Loop 11

2.6 Voltage Mode Control Block Diagram 13

2.7 Signal pulse generated 14

2.8 PWM circuit 15

2.9 Oscillator Output,Vosc; Op-amp output voltage, Verror 15

2.10 PWM Output Voltage 15

2.11 General Buck Converter Schematic 16

2.12 ON State 16

2.13 OFF State 17

3.1 IRF5305 MOSFET 22

3.2 Schottky Power Diode 24

3.3 Internal Connection of UC3573 PWM IC 26

3.4 Resistive Load 26

3.5 Type III Compensation 28

3.6 Generic Type III Network 29

4.1 Complete circuit of VMC Buck converter 32

4.2 Output Voltage during load disturbances 33

4.3 Inductor current during load disturbances 33

4.4 PWM and Sawtooth waveform 34

4.5 Small Signal Circuit of VMC 34

4.6 Bias Point Analysis Calculator 35

4.7 Frequency Response Waveform (Gain and Phase Margin) 36

4.8 Top view of buck converter using breadboard 37

xii

4.9 Side view of buck converter using breadboard 37

4.10 PCB Design using EAGLE 6.0.0 38

4.11 Hardware Testing 38

4.12 (a) Ramp Voltage at pin 7 39

4.12 (b) Error Voltage at pin 2 39

4.12 (c) Output of PWM 39

4.13 Complete Hardware Testing 40

4.14 Output voltage and Diode Signal when Vin = 8V 41

4.15 Output voltage and Diode Signal when Vin = 12V 42

4.16 Output voltage and Diode Signal when Vin = 16V 43

4.17 Output voltage and Diode Signal when Vin = 20V 44

4.18 Square Wave Signal at MOSFET Gate Terminal 45

4.19 Efficiency of the buck converter (90%) 47

xiii

LIST OF TABLES

TABLE NO. TITLE PAGE

3.1 Buck converter desired design specification 20

4.1 Results from experimental method 40

xiv

LIST OF ABBREVIATIONS

DC Direct current

VMC Voltage mode control

ESL Effective Series Inductance

ESR Equivalent series resistance

PWM Pulse width modulator

PCB Printed circuit board

CCM Continuous current mode

IC Integrated circuit

MOSFET Metal Oxide Semiconductor Field Effect

Transistor

Vin Input voltage

Vo Output voltage

Vramp Ramp voltage

Verror Error voltage

Vswitch Switch voltage

D Duty cycle

Ts Switching period

fs Switching frequency

xv

LIST OF SYMBOLS

% Percentage

PCB Printed Circuit Board

V Volt

L Inductor

C Capacitor

R Resistor

D Duty Cycle

∆V Voltage Ripple

DC Direct Current

fs Switching Frequency

iL Switch/Inductor Current

Vref Reference Voltage

ESR Equivalent Series Resistance

PS Power Supply

Po Output Power

CCM Continuous Conduction Mode

xvi

LIST OF APPENDIXES

APPENDIX NO. TITLE PAGE

A MOSFET Datasheet 51

B UC3573 PWM Datasheet 59

C Inductor Datasheet 62

D Diode Datasheet 67

1

CHAPTER 1

INTRODUCTION

1.1 Introduction

DC-DC converter is a power electronic circuit that changes one level of DC

voltage to another level of DC voltage, normally with a regulated output. DC to DC

converters are widely used in power supplies for electronic instruments and high power

applications such as battery charging and dc motor driver. Basically, DC-DC converter

is divided into two types of converter which are step-down converter that knows as buck

converter and step-up converter also knows as boost converter. Meanwhile for the buck-

boost converter, it can step-down or step-up the voltage, however the polarity for input

and output voltage is opposite each other. For each type of DC-DC converter, it has its

own properties including the steady state ratio between output voltage and input voltage,

the input and output current, the frequency response of the duty cycle to output voltage

transfer function and the character of the output voltage ripple.

2

To achieve design objectives for line and load regulation and dynamic response,

an appropriate controller is applied to the DC-DC converter. The most common control

scheme is voltage mode control (VMC) due to its most convenient method of controlling

DC-DC converters. Buck converter works as a circuit to convert the unregulated dc

voltage input, to a regulated output voltage where its output voltage is always less than

the input voltage. However, the output voltage of buck converter is changed when there

is a changing of the input voltage and/or the load. In designing DC-DC converter for the

power converter, a tight output voltage regulation under different line and load

conditions must be obtained in order to maintain the condition of the circuit.

1.2 Problem Statement

The main reason of studying and designing a buck converter power supply is to

improve the efficiency of the desired output voltage and make all things about power

supply become better since there are several problems that occur with the conventional

power supply.

The common power supply usually lack of overall efficiency in maintaining

desired output voltage. This is because most of the common designs uses linear regulator

to control and supply power. Another drawback is the size of conventional power supply

is bigger than switch-mode power supply. They are also quite difficult to be designed

because it contains complicated integrated circuits (IC) and they also radiate more

electromagnetic interference.

3

1.3 Objectives

The objectives of the project are:

i. To study the principle operation and characteristics of buck converter.

ii. To perform cicruit simulation using Pspice software.

iii. To develop a practical voltage mode control for buck converter based

on simulation parameters and outcomes.

1.4 Scope of Project

The scopes of this project are as follows:

i. Design the closed-loop feedback of voltage mode control for buck

converter.

ii. To apply Pspice simulation to obtain the satisfied results and develop

hardware using PCB.

4

1.5 Thesis Outline

This thesis consists of five chapters. In Chapter 1, the explanation for the project

will be given in a general term. The objectives of the project will be elaborated. It is

followed by the scope of project.

In Chapter 2,describes the overview of the project elaborately. Explanation will

be focused on thereotical and simulation design. Explanation will be based on theory

and conceptual ideas. Some practical approach in this project will also be discussed.

In Chapter 3, hardware design is explained in detail under the methodology. All

input parameters will be clearly stated. Each components used for this project will be

discussed under this chapter.

In Chapter 4, the results for testing are discussed. The strengths and weakness of

the voltage mode control for buck converter will be discussed.

Chapter 5 will be the conclusion of the project. The contents include the

experience and the knowledge gained during accomplishing this project. Furthermore, a

few recommendations will also be suggested. Improvement or future enhancement will

be explained.

5

CHAPTER 2

LITERATURE REVIEW

2.1 Switching Regulators in DC-DC Converters

Switching regulator is used widely in DC-DC conversion. It offer three main

advantages compared to a traditional linear power supplies. First, switching efficiency

can be much better than linear. Second, because less energy is lost in the transfer,

smaller components and less thermal management are required [2]. Third, the energy

stored by an inductor in a switching regulator can be transformed to output voltages

that can be greater or smaller than the input. However, linear regulators provide lower

noise and higher bandwidth; their simplicity can sometimes offer a less expensive

solution. A switching regulator is a circuit that consists of power switch, an inductor,

and a diode to transfer energy from input to output. It can be rearranged to form a step-

down (buck), step-up (boost), or an inverter (flyback)[4]. The buck converter topology

as shown in Figure 2.1 is used switching regulator as a switching power. In summary,

because of its versatility, efficiency, size and cost, the switching regulator is preferred

in most high power applications.

6

Figure 2.1: Buck Converter

2.2 The Buck Converter Topology

The buck converter or step-down switch mode power supplies can also known

as a switch mode regulator. Popularity of a switch mode regulator is because of its

fairly high efficiency and compact size and a switch mode regulator is used in place of

a linear voltage regulator at relatively high output [5]. Linear voltage regulators tend to

be inefficient. Since the power devices used in linear regulators have to fritter a fairly

large amount of power, they have to be adequately cooled, by putting them on heat

sinks and the heat is transferred from the heat sinks to the surrounding air either by

natural convection or by forced-air cooling. Heat sinks and provision for cooling makes

the regulator bulky and large. In applications where size and efficiency are critical,

linear voltage regulators cannot be used.

7

The basic idea behind the buck converter is that by rapidly opening and closing

the switch, short bursts of current are allowed to flow through the inductor to the load.

Another feature to describe buck converter is the relationship between the dc input

voltage Vin and the dc output voltage Vout [6]. The relationship is a simple function of

the duty cycle of the switch. It is a simplest power stage topology that converts a higher

input voltage to a lower output level. Figure 2.1 below show the basic buck converter

circuit where Vs is input voltage, S is the swich, D is the diode, L is the inductor, C is

the capacitor, RL is the load and Vo is output voltage.

Analysis of the buck converter begins by making these assumptions :

i. The circuit is operating in the steady state.

ii. The inductor current, iL is in continuous condition.

iii. The output voltage is held constant at voltage, Vo and the capacitor is very large.

iv. The switching period is T, meanwhile the switch’s closed time is DT and open

time is (1-T)DT.

Figure 2.2 : Inductor’s voltage waveform for Buck converter.[5]

8

When the switch closed,

( ) closed =

DT (2.1)

When the switch opened,

( ) opened = - (

(1-D) T (2.2)

The net change in inductor current over one period is zero,

( ) closed + ( ) opened = 0 (2.3)

Vo = Vs D (2.4)

Note that the average current must be the same as the average current in the load resistor.

Thus, since the average capacitor current must be zero for steady- state operation:

IL = IR =

(2.5)

The value ( ) is referred to the ( ) opened and the maximum value of the inductor

current is

I max = IL +

= Vo [

–

] (2.6)

9

The minimum inductor current is,

Imin = IL -

= Vo [

+

] (2.7)

The voltage ripple is determined by,

=

(2.8)

Figure 2.3 shows the inductor current waveform for the buck converter which is in

continuous current mode. The value is always positive and greater than zero.

Figure 2.3 : Inductor’s current waveform for Buck converter.[5]

The average inductor current must be the same as the average current in the load resistor

[2]. The capacitor current waveform was shown in figure 2.4.

10

Figure 2.4 : Capacitor current waveform for Buck converter.[5]

2.3 Feedback and Control Circuitry Scheme

One of the key objectives in designing a controller for the power converter is to

obtain tight output voltage regulation under varying load and input voltage condition.

Feedback and control circuitry can be carefully nested around converter circuits to

regulate the energy transfer and maintain a constant output within normal operating

conditions. The implementation of feedback and control circuitry to the converter

consists of two purposes which are to adjust the duty ratio to a required value in order

to provide a fixed output and to ensure the stability where the power converter will

return to the desired operating point after some disturbances are applied.

As shown in Figure 2.2, a power supply feedback loop can be interpreted in

terms of small-signal linear equivalent gain blocks. The appended to certain gain

11

blocks represents that the gain varies as a function of frequency. The open-loop gain, T,

is defined as Equation 2.1 which is the total gain around the entire feedback loop:

T(s) = K EA ∙ K MOD ∙ K PWR ∙ K LC ∙K FB (2.1)

Closed-loop gain, G, defines as Equation 2.2 where the output vs. control input

relationship, with the loop closed:

G(s) =

(2.2)

Figure 2.5 : Feedback Control Loop [6]

12

Open-loop gain T is normally very much greater than 1 when it is at low

frequencies so that closed-loop gain G approaches the ideal 1/KFB. At higher

frequencies, mostly because of the low-pass filter characteristic KLC(S), T will be

decreased. The frequency where T has decreases to 1 (0dB) is indicated as the

crossover frequency, fC. In a power supply voltage control loop, G(s) represents the

power supply output vs. the reference voltage. KFB is normally a simple voltage

divider. For example, if VREF is 2.5V, a 2:1 divider (KFB = 0.5, G = 2) as the results

VOUT = 5Volts.

2.3.1 Voltage Mode Control

The key feature of this control is the existence of a feedback loop which keeps

track of the output voltage variation and adjusts the duty cycle. Furthermore, in this

control scheme, the difference between the output voltage vc, and a reference signal,

vref, is processed by a compensation network which generates a control signal,

vcon[1]. This control signal tells how the duty cycle has to be changed in order to give

the best transient dynamics for the desired output. This control signal is compared with

a periodic ramp signal, Vramp(t), to generate a pulse width modulated signal which

drives the switch.

In the voltage mode control scheme, the converter output voltage is sensed and

subtracted from an external reference voltage in an error amplifier. Then the error

amplifier produces a control voltage that is compared to a constant amplitude saw tooth

waveform. After that the comparator produces a PWM signal that is fed to drives of

controllable switches in the dc – dc converter. Furthermore the frequency of the PWM

signal is the same as the frequency of the saw tooth waveform. The advantage of this

scheme is simple hardware implementation and flexibility[3].