MICROCONTROLLER BASED VARIABLE GATE VOLTAGE

FOR MOSFETs

NURIPAIZAIL BIN AHMAD

UNIVERSITI MALAYSIA PAHANG

“I hereby acknowledge that the scope and quality of this thesis is qualified for the

award of the Bachelor Degree of Electrical Engineering (Electronics)”

Signature : ______________________________________________

Name : RAMDAN BIN RAZALI

Date : 17 NOVEMBER 2008

MICROCONTROLLER BASED VARIABLE GATE VOLTAGE

FOR MOSFETs

NURIPAIZAIL BIN AHMAD

A thesis submitted in fulfillment of the

requirement for the award of the degree of

Bachelor of Electrical Engineering (Power System)

Faculty of Electrical and Electronic Engineering

Universiti Malaysia Pahang

NOVEMBER 2008

ii

“All the trademark and copyrights use herein are property of their respective owner.

References of information from other sources are quoted accordingly; otherwise the

information presented in this report is solely work of the author.”

Signature : ____________________________

Author : NURIPAIZAIL BIN AHMAD

Date : 17 NOVEMBER 2008

iii

Specially dedicated to

My beloved mother, brothers

Supervisor, lecturers, staffs and

Friends

Thousands of thanks for helping, attentions,

Supports and encouragement.

iv

ACKNOWLEDGEMENT

I am grateful to Allah SWT, the most powerful and the merciful on His

blessing of giving the opportunity to complete this project successfully. I wish to

express my sincere appreciation to my project’s advisor Mr. Ramdan Bin Razali for

his guidance, encouragement and supports.

I am also thankful to my housemates, lecture mates and friends for their

helping hand, encouragement and knowledge sharing. Throughout the period of

accomplishing this project, those persons are very helpful on providing me lot of

brilliant ideas. Other than that, they are also always helping me on troubleshooting

the circuit.

My special thanks are also goes to my beloved mother, brothers and sisters.

They are never stop from supporting me on whatever I am doing. They are always

motivating me to work hard and give me spiritual inspiration. With their concern,

everything is going to be good and smooth. Thanks to all.

v

ABSTRACT

Metal Oxide Semiconductor Field Effect Transistor, MOSFETs are important

and expansive switching devices in many power electronic circuit applications.

MOSFETs require adequate pulses at its gate terminal in order to work properly. The

amount of current going through the source and drain terminals is controlled by the

magnitude pulses supplied. Normally, the magnitude of gate pulses is fixed and set to

a nearly maximum allowable current of MOSFET. The problem with fixed type gate

pulses is whenever overload occurs; the MOSFETs may experience very high current

and thus working beyond their safe operating area (SOA). This situation might

destroy the MOSFET. This project is implement base on PIC16F84A to generate

PWM. The IR2109 use provide variable magnitude of PWM pulses and then to drive

the based gate of the MOSFET. IR2109 is selected because of its ability to withstand

high voltage input.

vi

ABSTRAK

Metal Oxide Semiconductor Field Effect Transistor, MOSFET merupakan

suis elektronik yang amat penting dan mahal. Ianya banyak digunakan dalam aplikasi

litar elektronik kuasa. MOSFET memerlukan dedenyut yang mencukupi pada

terminal gate untuk membolehkannya berfungsi dengan sempurna. Jumlah arus yang

melepasi terminal source dan drain dikawal oleh magnitud dedenyut yang

dibekalkan kepadanya. Normalnya, magnitud dedenyut yang dibekalkan di terminal

gate adalah malar dan ditetapkan kepada arus maksimum yang boleh diterima oleh

MOSFET. Masalah dengan magnitud dedenyut ini adalah, apabila berlaku lebihan

beban, MOSFET akan berhadapan dengan jumlah arus yang besar melebihi

kemampuannya dan menyebabkan ia beroperasi di luar zon selamat. Situasi ini akan

menyebabkan MOSFET musnah. Projek ini akan menggunakan aplikasi PIC 16F84A

untuk menghasilkan dedenyut manakala IR2109 digunakan untuk menghasilkan

dedenyut yang berubah-ubah magnitudnya. Dedenyut ini akan digunakan untuk

mengawal MOSFET. IR 2109 dipilih kerana kebolehannya untuk beroperasi dengan

nilai voltan yang tinggi.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF ABBREVIATIONS xiii

LIST OF APPENDICES xiv

1 INTRODUCTION 1

1.1 Overview 1

1.2 Problem Statement 2

1.3 Objective 3

1.4 scope of Project 4

2 LITERATURE REVIEW 5

2.1 The Construction of MOSFET 5

2.2 Modes of Operation of MOSFETs 7

viii

2.2.1 Cut-off or Sub-threshold or Weak Inversion Mode

7

2.2.2 Triode Mode or Linear Region (also referred to as the Ohmic Mode)

9

2.2.3 Saturation Mode (also referred to as the Active Mode)

10

2.3 MOSFET’s driver circuit 12

2.3.1 Introduction 12

2.3.2 Bipolar Totem-Pole driver 12

2.3.3 Integrated Circuit Gate Driver 13

3 METHODOLOGY 15

3.1 Overall System Design 15

3.2 Power supply 16

3.2.1 Full Wave Rectification Using Center-tapped Transformer

18

3.2.2 Voltage Regulation 20

3.3 Peripheral Interface Controller (PIC) 22

3.4 Driver Circuit 25

3.5 MOSFET 28

3.6 Problems and Solutions 29

4 RESULT AND ANALYSIS 30

4.1 Introduction 30

4.2 Gate Driver Output 30

4.3 PIC 16F84A 31

4.4 DC Power Supply 32

5 CONCLUSION AND RECOMMENDATION 37

5.1 Conclusion 37

5.2 Future Recommendations 37

5.3 Costing and Commercialization 38

REFERENCES 39

APPENDIX A 40

APPENDIX B1 42

ix

APPENDIX B2 59

APPENDIX C 63

x

LIST OF TABLES

TABLE NO. TITLE PAGE

3.1 Various type of IC regulator 20

3.2 Switch and duty cycle 24

3.3 Pin definition of IR2109 27

3.4 Maximum rating of IRF 740 28

3.5 Problems and solution 29

4.1 Relationship of gate voltage to gate-to-source

voltage.VGS

35

xi

LIST OF FIGURES

FIGURE NO. TITLE

PAGE

1.1 Problem statement 3

2.1 Cross-section and circuit symbol of an n-type

Metal-Oxide-Semiconductor-Field-Effect-

Transistor (MOSFET)

6

2.2 MOSFET drain current vs. drain-to-source voltage

for several values of VGS − Vth.

9

2.3 Cross section of a MOSFET operating in the linear

(Ohmic) region.

10

2.4 Cross section of a MOSFET operating in the

saturation (active) region.

12

2.5 Totem-pole gate driver 13

2.6 Example of IC gate driver 14

3.1 Block diagram 16

3.2 DC power supply 17

3.3 Center-tapped transformer full-wave rectifier 18

3.4 Positive half AC cycle 19

3.5 Negative half AC cycle. 19

3.6 Example of RC filter 21

3.7 3.7: Actual power supply 21 21

3.8 Pin diagram of PIC16F84A 22

3.9 PIC connection 23

3.10 IR2109 driver 25

3.11 Pins diagram 25

xii

3.12 Gate driver connection 26

3.13 (a) Input, IN (b) Output, HO (c) Output, L 27

3.14 IRF 740 28

4.1 Output of PIC 16F84A (50% duty cycle) 31

4.2 Output of PIC 16F84A (25% duty cycle) 32

4.3 Gate driver output, 34

4.4 Graph of Gate-to-source voltage, VGS versus gate

voltage

36

xiii

LIST OF ABBREVIATIONS

MOSFET - Metal oxide semiconductor field effect transistor

PWM - Pulse width modulation

PIC - Peripheral interface controller

IC - Integrated circuit

VGS - Gate-to-source voltage

VTH - Threshold voltage

IR - International Rectifier

xiv

LIST OF APPENDICES

APPENDIX TITLE

PAGE

A PIC Programming 40

B1 Datasheet PIC 16F84A 42

B2 Datasheet IR 2109 59

C Cost of project 63

xv

1  

CHAPTER 1

INTRODUCTION

1.1 Overview

MOSFET is a power electronics device that operated with high frequency and

high efficiency switching application in electronics industry. MOSFET is categorized

as Field Effect Transistor, FET family. FET technology was invented in 1930, some

20 years before the bipolar transistor. The first signal level FET transistors were built

in the late 1950’s while power MOSFETs have been available from the mid70’s.

Today, millions of MOSFET transistors are integrated in modern electronic

components, from microprocessors, through “discrete” power transistors [1].

MOSFET are widely used in analog and digital circuits. The MOSFET is

composed of a channel of n-type or p-type semiconductor material and is accordingly

called an NMOSFET or PMOSFET (also commonly nMOSFET, pMOSFET) [2].

MOSFET is three terminal devices. The terminals are gate, drain and source.

2  

As stated earlier, MOSFET is one of FET family. FET is different with

Bipolar Junction Transistor, BJT. The major different between them are BJT is a

current-controlled device, whereas FET is a voltage-controlled device. In other

words, for BJT collector current, IC is a direct function of the level of base current, Ib.

For the FET the drain current, ID will be a function of the gate-source voltage, VGS.

In each case the current of the output circuit is controlled by a parameter of the input

circuit-in one case a current level and in the other an applied voltage [3]. From this

statement, in order to make the MOSFET function the adequate voltage has to be

supplied to its gate. The focus of this project is to variable the gate voltage that being

supplied to the gate of the MOSFET.

1.2 Problem Statement

Normal MOSFET that used nowadays has a fix value of gate voltage. Other

than that, it was also set nearly maximum allowable current of MOSFETs. The

problem with fix variable gate voltage are, whenever overload occurs, the

MOSFETs may experience very high current and thus working beyond their Safety

Operating Area (SOA). This situation of overloading might destroy the MOSFET.

 

 

 

 

 

 

 

 

3  

 

 

Figure 1.1: Problem statement

In this project the variable gate voltage of MOSFET was implemented. The

advantages with variable gate voltage are the current flowing through the MOSFET

can be variable, prolong life of MOSFET and the MOSFET can be implement to

drive low and high current loads.

1.3 Objective

The objectives of this project are:

I. To find a method on how the variable magnitude pulse gate could be

implemented.

II. To understand the switching behaviour of MOSFETs.

4  

1.4 scope of Project

The scopes of work of this project are:

I. Set up the power supply:

The power supply that being used are 5V and 15V. 5V power supply is

used to supply the voltage to Programmable Integrated Controller, PIC

whereas 15V power supply is used to ON the MOSFET.

II. Write the program of PIC:

PIC is used to produce Pulse Width Modulation, PWM by coding the

PIC.

III. Set up the driver circuit for MOSFET:

Driver circuit is used to drive the MOSFET. In other to function,

MOSFET require adequate pulse that fed by driver circuit.

 

 

 

 

 

 

 

 

 

 

 

5  

CHAPTER 2

LITERATURE REVIEW

2.1 The Construction of MOSFET

The n-type Metal-Oxide-Semiconductor Field-Effect-Transistor (nMOSFET)

consists of a source and a drain, two highly conducting n-type semiconductor

regions, which are isolated from the p-type substrate by reversed-biased p-n diodes.

A metal or poly-crystalline gate covers the region between source and drain. The

gate is separated from the semiconductor by the gate oxide [4]. The basic structure of

an n-type MOSFET and the corresponding circuit symbol are shown in Figure 2.

 

 

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7  

semiconductor interface are concentrated in a thin (~10 nm thick) "inversion" layer.

By now, most MOSFETs are "enhancement-mode" devices [4].

While a minimum requirement for amplification of electrical signals is power

gain, one finds that a device with both voltage and current gain is a highly desirable

circuit element. The MOSFET provides current and voltage gain yielding an output

current into an external load, which exceeds the input current, and an output voltage

across that external load which exceeds the input voltage [4].

The current gain capability of a Field-Effect-Transistor (FET) is easily

explained by the fact that no gate current is required to maintain the inversion layer

and the resulting current between drain and source. The device has therefore an

infinite current gain in dc. The current gain is inversely proportional to the signal

frequency, reaching unity current gain at the transit frequency [4].

The voltage gain of the MOSFET is caused by the current saturation at higher

drain-source voltages, so that a small drain-current variation can cause a large drain

voltage variation [4].

2.2 Modes of Operation of MOSFETs

2.2.1 Cut-off or Sub-threshold or Weak Inversion Mode

This phenomenon is occurring when gate-source voltage, VGS is less than

threshold voltage, Vth. According to the basic threshold model, the transistor is

turned off, and there is no conduction between drain and source. In reality, the

Boltzmann distribution of electron energies allows some of the more energetic

electrons at the source to enter the channel and flow to the drain, resulting in a

subthreshold current that is an exponential function of gate–source voltage. While

the current between drain and source should ideally be zero when the transistor is

being used as a turned-off switch, there is a weak-inversion current, sometimes

called subthreshold leakage [2].