Inverter.

DESIGN AND CONSTRUCTION OF 1.5KVA INVERTER CIRCUIT

BY ALADE DAMOLA VICTOR ENG 0011185

A PROJECT REPORT SUBMITTED TO THE DEPARTEMENT OF ELECTRICAL ELECTRONIC, FACULTY OF ENGINEERING.

UNIVERSITY OF BENIN BENIN CITY

NOVEMBER 2006

i

DESIGN AND CONSTRUCTION OF 1.5KVA INVERTER CIRCUIT

BY ALADE DAMOLA VICTOR ENG 0011185

A PROJECT REPORT SUBMITTED TO THE FACULTY OF ENGINEERING UNIVERSITY OF BENIN. BENIN CITY.

IN PARTIAL FULFMENT OF THE REQUIREMENT FOR THE AWARD OF THE BACHELOR OF ENGINEERING DEGREE IN ELECTRICAL AND ELECTRONICS ENGINEERING (B.ENG)

ii

NOVEMBER 2006

CERTIFICATION

This is to certify that this project work was carried out by ALADE DAMOLA VICTOR in the Department of Electrical and Electronic Engineering, University of Benin, Benin City

Prof. Edeko F. O.

Engr. Onohaebi S. O.

Supervisor

H. O. D. Elect/Elect

Dept.

Date: -----------------------

Date:

--------------------------

iii

DEDICATION

This project work is dedicated to the Almighty God for His loving kindness and fulfillment of His word towards me during my years of study in this university. Also to my beloved wife and my little boy for their encouragement, understanding, love and support during the course of my study.

iv

AKNOWLEDGEMENT

I wish to express my immense gratitude to the Almighty God for His protection, journey mercy, wisdom and understanding that has kept me throughout this project work. My sincere gratitude also goes to Prof. F. O. Edeko, for his keen supervision in making this work a success. I also wish to express my warm heartfelt to entire lecturers and staff of Electrical / Electronic Department for their time and support in the course of this project work. My sincere appreciation also goes to my immediate family and the extended family for their understanding and encouragement all the way. May God bless you all.

Benin, November 2006

v

ABSTRACT

The aim of this project work is to design a 1.5KVA inverter circuit capable of inverting 12Volts d.c. from a car battery to 220volts a.c. In the design, some basic electronic components are employed. In generating frequency of 50hertz a bipolar transistor BC107 in conjunctions with resistors and capacitors were used to achieve an astable multivibrator circuit. The capacitors were carefully selected after the necessary design criterion, considering the frequency of pulse in question. The selection of the resistors is at the range of that to generate a high current from the multivibrator. The 12volts d.c from the battery supply biasing voltage to the transistor. Having generated high current pulse train. The two output of the multivibrator are fed into the source of two parallel MOSFET IRFZ44. The MOSFET are four in all with each two in push pull arrangement to achieve current amplification. IRFZ44 has drain current of 51A, this helps in achieving the required high output current. Also adequate caution was taken during the soldering and connection of components. The MOSFETs are screwed to heat sink to take care of heat dissipation. Output of IRFZ44 is being fed to secondary winding of the transformer and the 220volts a.c. output is obtained from primary winding of the transformer. The transformer is designed to carry 1500watt load. With 60A.h fully charged car battery the inverter was tested with laptop for almost three

vi

hours. However the inverter was tested with tape recorder, 60watt bulb and television set. All this items worked for some hours.

Table of Contents. Title page

---------------------------------------------------------------------

Certification

-------------------------------------------------------------

Dedication -------------------------------------------------------------------Acknowledgement Abstract

i ii

iii

-----------------------------------------------------

-------------------------------------------------------------------

iv v

Table of contents ------------------------------------------------------------

vi – vii

Chapter 1:Introduction 1.1

Aims and Objectives---------------------------------------------------- 2.

1.2

Scope of work

---------------------------------------------

1.3

The Block Diagram

----------------------------------------------

Chapter Two:

2 – 4. 4- 6.

Literature Review and Analysis of Components.

2.0.

Literature Review ----------------------------------------------------

6 – 7.

2.1.

Component Analysis-------------------------------------------------

7 – 8.

2.2.

Pulse Width Modulation -------------------------------------------

8 – 10.

2.3.

Multivibrator

-----------------------------------------

10 – 11.

2.4.

Oscillators

----------------------------------------------------

11 – 13.

2.5.

Operational Amplifier

---------------------------------------------

vii

13 – 17.

2.6

Transistors

---------------------------------------------

2.7

MOSFET

2.8

MOSFET Drivers -----------------------------------------------------

2.9

Capacitor and Capacitance

17 – 20.

------------------------------------------------------------

2.10 Transformer

20 – 25. 26 – 28.

-------------------------------------

29 – 30.

----------------------------------------------------

30 - 41

Chapter 3: Design 3.0.

Designs

------------------------------------------------------------

41.

3.1.

Transformer Design

---------------------------------------------

41 – 48.

3.2.

Design of multivibrator ---------------------------------------------

49 – 50.

3.3

Wire gauge table ----------------------------------------------------

51 – 53.

3.4

Complete circuit diagram ……………………………………………

53 - 54

Chapter 4 Construction, Testing and Result 4.1.

Construction-----------------------------------------------------------

55.

4.2

Fabrication of Circuit Board

55.

4.3.

Packaging ------------------------------------------------------------

55.

4.4

Testing

56.

4.5

Testing of Mosfet -------------------------------------------------------

4.6

Testing of Bipolar Transistor ----------------------------------------

4.7

Transformer Testing

4.8

Test Result --------------------------------------------------------------

4.9

Engineering Bill of Quantities & Evaluation

Chapter 5

-------------------------------------

------------------------------------------------------------

-----------------------------------------------

58 - 59 59 – 60. 60.

-----------------

Conclusion and Recommendation

5.1

Conclusion

5.3

Recommendation ------------------------------------------------------

Appendix

57.

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----------------------------------------------------------------------viii

61. 62

60.

References -------------------------------------------------------------------

ix

63

Chapter 1 INTRODUCTION 1.0

Inverter Bases: Basically it is required of the power utility company to supply continuous,

uninterruptible power to appliance and sundry usage. But many a times even in developed countries where this is achievable the need of basic inverter still finds its usage. An inverter allows the use of 220v electrical appliances from a car battery or a solar battery. It must therefore supply a voltage that corresponds to an rms of 230volts sine wave like the household supply or similar. Sine -wave voltages are not easy to generate. The advantage of sinewave voltages is to soft temporal rise in voltage and the absence of higherfrequency oscillation portions, which cause unwanted counter forces on engines, interferences on radio equipment and surge currents on condensers. On the other hand, very simply switches e.g. electronic valves like mosfet transistors can generate square-wave voltage. The efficiency of a square wave inverter is higher than that of appropriate sine-wave inverter due to its simplicity. With the help of a transformer the generated square wave voltage can be transformed to a value of 230volts or even higher as in the case of radio transmitter. One of the fundamental challenges to face in generating square wave voltage is the ability to obtain a perfect upper and lower threshold value of the duty cycle. This must be adequately looked into in order to obtain square wave signal that is free of chatter effect. Of course chatter effect will lead to lose of 1

useful voltage and this may be of great influence after amplification stages. Also the higher

Percentage of the duty cycle obtained the better, this is of great important because the output transistor is either on or off, not partially on as with normal regulation, so less power is wasted as heat and smaller heat-sinks can be used. The present day electronic circuit uses appropriate pulse width modulation (PWM) circuitry to generate the pulse-wave voltage.

1.1 Aims and Objectives: The main aim of this project is the design and construction of 1.5 KVA inverter circuit. The multivibrator circuit will be design to generate a high current pulse waveform of frequency 50 hertz. The outputs of this shall be feed to a push pull amplifier configured IRFZ44. The output a.c is achieved via a step up transformer. The input 12V d.c shall be from car battery.

1.2

Scope of work: Since d.c. are be inverted to a.c. signal in inverter circuit it applications

comes majority in mobile office, mobile-fan and in picnic environment. This is because simple or portable 12v car battery or 12v dry cell are only required alongside with the inverter circuit to achieve a.c. voltage for turning on appliances like television, laptop, useful model computer accessories, games, compact disc player and so on. This versatile nature of an inverter power supply has made it to evolve through technological developments. Over the 2

time some simple and inexpensive circuit that can produce a dual (positive and negative) voltage from a

single supply input extremely useful for powering op amp and other circuits that require a dual voltage from a single battery has been developed. This circuit operates at an input voltage from around 5v to 20v and produces an output from ±2.5v to ± 10v. However, in powering some high-powered appliances we have to careful design the circuit to develop higher current compensate for the higher power requirement. Recall that power (p) = IV. Since our voltage is stable at 220v to 230v the only physical factor is the current. Hence the choice of appropriate Component to handle this higher current is another challenge when it comes to high power inverter circuit. In overcoming this either ladder network of bipolar transistor is used or some view stages of mosfet. Mosfet has better advantages over bipolar transistor due to its inherent construction advantage. Mosfet dissipate lower heat than its equivalent bipolar transistor. The output power is ultimately the function of the transformer; the transformer design has to put into consideration the power rating of the inverter. Circuit. In achieving this care will be taking in transformer winding and the gauge of copper wire to be used. The focus of this project is to generate 230v from a 12v battery. Attention shall be given to the required output power of 1.5.KVA, reliability of the whole unit, safety to the user and appliances. Also maintainability while in

3

use shall also be taking care off. Another main issue to be considered is the issue of economic aspect of the unit.

1.3

The Block Diagram: This project block diagram explains the various basic unit/stages. The

source voltage is the 12v battery, which shall supply the biasing voltage for the electronic components and also serving as the input voltage for the transformation component. The first major stage is the frequency generation stage. This consist of a 555 timer pulse width modulation arrangement with carefully selected RC arrangement to cater for 50HZ voltage frequency required by appliances (using Nigeria Power Standard). Here, pulse-wave signal are generated via the oscillator circuit. Closely following frequency-generating stage is the duty cycle shaper stage where the lower and upper threshold are maintained and corrected for the pulse-wave voltage. Here the possible effect of chatter is been taking care of:

12V Battery

Frequency Generation

Power Driver (Optional)

Power Amp.

Feed Back

4 Figure 1:

Block Diagram of the Inverter Circuit.

Power Transformation

The power driver stage involves the amplification of the current to achieve the goal of higher power. This is achievable with the use of two stages MOSFET. Here

the advantage in field effect transistor (FET) most especially MOSFET shall be explored. The final main stage is the power transformation stage, which mainly comprises of step up transformer. The transformer should be carefully designed to accommodate the required power of 1.5KVA. Current and voltage transformation should be carefully doctored during design of the transformer. Also proper winding and the former compatibility are necessary to prevent unnecessary noise from the circuit. The battery and the earthing technique in the unit provide a feedback network for effective output control. For better smooth power i.e. to take off harmonies and smoothen the waveform. The block diagram for the inverter circuit above is figure 1. The scope or aim of this project is to generate 220v with 1.5KVA power rating from a 12v battery source.

5

Chapter 2

LITERATURE REVIEW AND ANALYSIS OF COMPONENTS 2.0

Literature Review: Power inverter is exactly the opposite of power converters. A power converter

is an electronic device that converts a.c voltage to d.c. Voltage to a.c voltage. In this work we are converting store energy in form of d.c. Voltage in a battery to a.c. voltage using appropriate circuiting and step up transformer. Power inverter as source of power source means that the output a.c has the same frequency as the utility power supply (i.e. 50Hertz) in Nigeria. The generation of the rights frequency and high waveforms is of high importance. There are inverters that use a square wave and step waveforms instead of sine waves. Those other waveform is easier and economical than sine wave. For sine wave it is expensive and more complicated in generating. Different load response at difference degree of performance with square or sine wave. Some response like inductive loads. So the type of waveform used depends on the load or target. For this project square wave is employed and the target is common electronic appliances, computer system and other picnic gadget. 6

The inverter basically involves a signal generating stage with desired square waveform and frequency of 50HZ. The signal is inverted to form two alternate waveforms (pulses) usually employed in push pull fashion or configuration. This inverted alternate pulses then allows to drive two sets of power devices either transistor in ladder network (depending or power aiming at) or power MOSFET.

The power transistor or MOSFETs then drives current (i.e. d.c. source) through the step up transformer. The current is driven through the transformer alternately by the two stage of MOSFETs, emf is generated at the output of the Step-up transformer.

2.1

Component Analysis: The basic component in this inverter circuit comprises of i.c. CD 4047 or

better still multivibrator circuit. But the advantage of using the i.c. is that enough current will be generated, hence aiding the output power. Also the capacitor and resistor to form the tune circuit for frequency generation is of very importance. A careful designed R.C circuit we give appropriate frequency together with i.c. in use. Again i.c. LM324 further help to shapes the output pulse from the frequency generation stage. The upper and lower threshold value of the duty circuit is maintained at this stage. In other words chatter effect is eliminated with help of this device. Then the output of the above is subjected to current amplification with the aid of transistor or power mosfet. If transistors are employed the collector 7

is shorted to collector in order to prevent heating i.e. thermal runaway. If we employ MOSFET (as in the case of this project. The push pull arrangement is employed to aid the power development. Finally, the MOSFET output goes to the carefully designed step-up transformer. This transformer essentially determines power-handling capability of the inverter circuit. The secondary must have thicker gauge of wire at to give tolerant for its current handling capacity. Here the transformer is designed to accommodate 1.8.KVA even though we required 1.5KVA. Now a little review of the components and devices employed shall be analyzed vise a vise their function in the circuit. 2.2 Pulse Width Modulators: A pulse width modulator is a device or circuit that has an oscillatory circuit that generates or produces a train of pulses having widths that arc proportional to the level of the amplifier input signal. When the input signal level is small, a series of narrow pulses is generated and when the input level is large a series of wide pulses are generated figure 2.1 shows some typical outputs of a pulse width generator or modulator used in voltage regulator application. As the input signal increases and decreases, the pulse width increases and decreases in direct proportion. Figure 2.1.(b) also shows how a pulse width modulator can Ref. be constructed using a saw tooth generator and voltage comparator.

Voltage Saw

Output

8 Figure 2.1

time

-

Saw tooth Generator

+ Voltage Comparat or

Figure 2.1(b) A pulse width modulator does not necessarily have to be driven by a saw tooth generator, any waveform could be used. Pulse width modulator can also be constructed by driving an oscillator by another oscillator circuit. This produces a pulse output that has width dependent on the driving signal of the first oscillator. Majority of nowadays inverter circuits are of the pulse width modulator (PMW) type. This technique varies the conduction time of the switching. In PWM control, the power supplied is be switched on and off very rapidly. The d.c. voltage is converted to a square-wave signal, alternating between fully on (nearly 12v) and zero. By adjusting the duty cycle of the signal (modulating the 12V pulse, hence the ‘PMW’) i.e. the time fraction it is “on” the average width of the

power can0V be varied. Voltage

20% Duty Cycle 12V 0V 50% Duty Cycle 12V

0V

9 80% Duty Cycle

time

The following advantages are derived from PWM; the output transistor is either on or off, not partly on as with normal regulation, so less power is wasted as heat and smaller heat-sink can be used, the full power pulsing action will run fans at a much lower speed than an equivalent steady voltage.

2.3

Multivibrator: A multivibrator consists of two inverters joined together as show in fig 2.3

below. The output of one is joined to the input of the other. The way the combined circuit behaves depends on the way in which the inverters are coupled together.

Inverter o/p

1/P Inverter B

A 1/p

0/P

Figure 2.3: Two Inverters Connected to make a Multivibrator

Multivibrator are two stages switching circuits in which the output of the first stage is fed to the input of the second vise visa. When one output is high the second is low i.e. their output are complimentary. Switching between the two logic levels is so rapid that the output voltage waveforms are square. The 10

term multivibrator arises from this, since a square wave consists of a large number of sine waves with frequencies that are odd multiples of the fundamental. There are three types of multivibrator (monostable, bistable, and Astable.) depending on the

feedback element used. For this project RC feedback is used to achieve the Astable multivibrator.

2.4 Oscillators: Discussion on waveform generation cannot be completed without talking about oscillators. A device without an oscillator either does nothing or expected to be driven by something else (which probably contain an oscillator). Apart from the obvious case of signal generation. Every oscillator has at least one active device be it a transistor or even the old valve. This part of discussion shall be confine to LC oscillators or oscillator basics. At turn on, when power is first applied, random noise is generated within our active device and then amplified. This noise is fed back positively through frequency selective circuits to the input where it is amplified again and so on. A state of equilibrium is reached where the losses in the circuit are made good by consuming power from the power supply and the frequency of oscillation is determined by the external components, be they inductors and capacitors (L.C.) or a crystal. The amount of positive feed back to sustain oscillation is also determined by external components. 2.4.1 Hartley Oscillator: 11

The figure 2.3 below shows the Hartley oscillator. It is about the simplest form of oscillator.

L1 R C

Load L2

Figure 2.3 Schematic of a Hartley Oscillator

2.4.2 Collpitts Oscillator: The basic Collpitts oscillator circuit looks as shown in figure 2.4 below and you can observe some similarities.

C1

R

L1

Load C2

12

Figure 2.4 Schematic of a Collpitts Oscillator

If you consider positive feedback is applied to compensate for the losses in the tuned circuit, the amplifier and feedback circuit create a negative resistor.

When Z1 and Z2 are capacitive, the impedance across the capacitors can be estimated from a formula which I may not go into here, hie as well as XC1 and XC2. Suffice to say it can be shown that the input impedance is a negative resistor in series with C1 and C2. And the frequency is in accordance with

F0

=

1 2π

.

[LC1 C2 / (C1 + C2) ]½

2.4.3 Quart Crystal Oscillator: For real stability there is no substitute for a crystal oscillators. This uses a piece of quartz (same chemical glass, silicon oxide) that is cut and polished to vibrate at a certain frequency. Quartz is piezo electric, so acoustic waves in the crystal can be driven by an applied electric field and in turn can generate a voltage at the surface of the crystal equivalent circuit contains two capacitors, giving a pair of closely spaced (within 1%) series and parallel resonant frequencies. The quartz crystal’s has high Q factor and good stability makes it a natural source for oscillator control, as well as high filters. Just like L.C. oscillator, the crystal’s equivalent circuit, provides positive feedback and gain at the resonant frequency leading to sustained oscillations.

13

2.5

Operational Amplifiers:-

The op-amp is basically a differential amplifier having a large voltage gain, very high input impedance and low output impedance. The op-amp has a inverting or (-) input and non-inverting or (+) input and a single output. The op-

amp is usually powered by a dual polarity power supply in the range of ± 5volts to +15 volts. A simple dual polarity power supply is shown in the figure below which can be assembled with two 9volts batteries.

2.5.1

Inverting Amplifier:

The op-amp is connected using two resistors Ra and Rb (see figure 2.6) such that the input signal is applied in series with Ra and the output is connected back to the inverting input through Rb. The non-inverting input is connected to the ground reference or the center tap of the dual polarity power supply. In operation, as the input signal moves positive, the output will move negative and vise visa. The amount of voltage change at the output relative to the input depends on the ratio of the two resistors Ra and Rb. As the input moves in one direction, the output will move in the opposite directions, so that the voltage at the inverting input remains constant or zero volt in this case. If Ra is IK and Rb is 10k and the input is +1volt then there will be 1mA of current flowing through Ra and the output will have to move to –10volts to supply the same current through Rb and keep the voltage at the inverting input at zero. The voltage gain in this case would be Ra/ Rb or 10k / 1k = 10. Note that since the voltage at the inverting input is always zero, the input signal will see input impedance equal to 14

Ra, or 1k in this case. For higher input impedances, both resistor values can be increased.

Rb

Ra

-

+

Figure 2.6 Inverting Op Amp 2.5.2 Non-Inverting Op-Amp: The non-inverting amplifier is connected so that the input signal goes directly to the non-inverting input (+) and the input resistor Ra is grounded in this configuration. The input impedance as seen by the signal is much greater since the input will be following the applied signal and not held constant by the feedback current. As the signal moves in either direction, the output will follow in phase to maintain the inverting input at the same voltage as the input (+). + The voltage gain is always more than 1 and can be worked out from Vgain = (1

+ Rb / Ra ) -

Rb

Ra

15 Figure 2.7 Non Inverting Op Amp

2.5.3 Voltage Follower Amplifier: The voltage follower also called a buffer provides high input impedance, a low output impedance, and unity gain. As the input voltage changes, the output and inverting input will change by an equal amount + 9v

+

`

-

- 9v

Figure 2.8 Voltage Follower Amplifiers

Simple Bipolar Power Supply 2.5.4 Differential Amplifier:

Basically differential amplifier amplifies different between two input signals. Assuming V1 and V2 are the two input, output voltage Vo = Ad (V1 – V2) where Ad is the differential amplifier gain. This implies that any signal, which is common V1

-

to both inputs, will have no effect on the output voltage. V2

+

16 Figure 2.9 Schematic of Differential Amplifier

V0

However, a practical differential amplifier can not be describe by equation stated above, because the output depends not only on differential signal (Vd), but also upon the average level called the common mode signal. Vc = ½ (V1 + V2 ) Voltage gain = Ad

=

Vd V0

Vc = Acm Vd Acm = Vc Vd Common mode rejection ratio CMRR

= Acm

Ad

2.6.0 Transistors: Transistors amplify current, for example they can be used to amplify the small output current from a logic chip so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage. So the transistor is being used to amplify voltage. A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier. The amount of the current amplification is called the current gain, symbol hfe.

2.6.1 Types of Transistor: 17

There are two types of standard transistor, NPN and PNP, with different circuit symbols. The letters refer to the layers of semi-conductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon.

C

B

C

B

NPN

E

Figure 2.10 Transistor Circuit Symbols

E PNP

2.6.2 Darlington Pair: This is two transistors connected together to give a very high current gain.

Figure 2.11 Darlington Pair Here the collector is shorted to collector in order to prevent heating.

18

Figure 2.12 Transistor leads for Some Common Case Styles.

In addition to standard (bipolar junction) transistors, there are field-effect transistors, which are usually referred to as FETS. These have different circuit symbols and properties, but detail on this may not be included here.

2.6.3 Connecting a Transistor: Transistor has three leads, which must be connected the correct way round. Adequate care must be taken with this because a wrongly connected transistor may be damaged instantly when you switch on. Some transistor orientations are cleared from the PCB or strip board layout diagram, otherwise you will to refer to a supplier’s catalogue to identify the leads. Figure 2.12 below show the leads for some of the most common case styles.

B

E

C

B

E

B

C

C

B

E

C

E

T018

T092A

T092B

T039

T092C

Views are from below with the leads towards you.

B

C is the Metal Case Itself T03 Fig. 2.12 Leads for some most common case styles E 2.6.4 Soldering a Transistor:

19

Heat when soldering so if you are not an expert it is wise to use a heat sink clipped to the lead between the joint and the transistor body can damage transistor. A standard crocodile clip can be used as a temporary heat sink.

2.6.4 Heat Sinks: Waste heat is produced in transistors due to the current flowing through them. Heat sinks are needed for power transistors because they pass large currents. If you find that a transistor is becoming too hot to touch it certainly needs a heat sink. The heat sink helps to dissipate (remove) the heat by transferring it to the surrounding air.

2.7.0 MOS FET +

P

N

-

P

P

N

N

-

Fig. 2.13 MOSFET symbol

+

The full spelling of MOS FET is metal oxide semiconductor field effect transistor. It is the structure that stacked up metal, oxide and semiconductor. There are NPN types and PNP type as the semiconductor part type is called P channel. An oxide film is put to the semiconductor of NPN or PNP and metal is 20

put onto it as the gate. In case of NPN, the part of “N” is a source pole and a drain pole. In case of PNP, the parts of “P” are the poles.

Transistor controls an output current by the input current. However, in case of FET, it controls an output current by input voltage (electric field). The input current doesn’t flow. To handle an MOS FET, it needs an attention. Because the oxidation insulation film is thin, this film is easy to destroy in the high voltage of the static electricity and so on. +

Metal

Drain

Oxide

Semiconductor

Drain

N

P D

D

P

Gate

N

Gate G S

N

Source

G P

Source

N channel 1 FET

S

P channel 1 FET

Fig. 2.14 MOSFET structure 2.7.1 The Operation Principle of P-N Junction Diode: First of all, I will explain about the operation principle of P-N junction diode simply. 21

The N-type semiconductor has Electrons (Negative) and the P-type semiconductor has Electron holes (Positive).

When the positive voltage to the side of p-type and the negative voltage to the side of N-type applied respectively, the electron in N-type is pulled with the positive voltage on the side of P-type and the electron flows through to the Ptype beyond the boundary of the semiconductor. Also the hole in P-type is pulled with the negative voltage on the side of N-type and the hole flows through to the N-type. In this way, the electric current flows through the semiconductor. As the opposite case, when the positive voltage to N-type and the negative voltage to P-type are applied respectively, the electron in N-type is pulled with the positive voltage on the side of N-type and the hole in the P-type is pulled with the negative voltage on the side of P-type. In this case, the electron in the semiconductor doesn’t move through the boundary and the electric current doesn’t flow.

2.7.2 The Operation Principle of MOSFET: The semiconductor part of MOS FET consists of NPN or PNP. So, when not applying voltage to the gate, the electric current doesn’t flow between drain and source. When positive voltage is applied to the gate of the N-channel MOS FET, the electrons of N-channel of source and drain are attracted to the gate and go into the P-channel semiconductor among both. With the move of these electrons, it becomes the condition like spans bridges for electrons between

22

drain and source. The voltage to apply to the gate controls the size of this bridge.

D

D

N N G

G

P

P

Open OR

N

N

S

Same as S S

Fig. 2.15 mosfet operation In case of P-channel MOS FET, the voltage is opposite but does similar operation. When negative voltage is applied to the gate of P-channel MOS FET, the holes of P-channel of source and drain are attracted to the gate and go into the N-channel semiconductor among bith. With the move of these holes, a bridge for holes is spanned and the electric current flows between drain and source.

D D P G

G

N

N

Open P

OR N

S

Same as S S

23 P

Fig. 2.16 Holes and electron movement.

Because there is an oxide film between gate and semiconductor, the electric current doesn’t flow through the gate. An electric current flow between drain and source is controlled only with the voltage when is applied to the gate. +

+

S S G

P

H

L

G

P

G

G

N

N

S

S

Fig. 2.17 principle of operation in mosfet.

2.7.3 The Operating Principle of C-MOS FET: C-MOS FET is the abbreviation of complementary MOS FET. C-MOS FET is the circuit which combined a P-channel MOSFET and a N-channel MOSFET. When the input is an L level, the P-MOS FET becomes ON condition and when 24

the input is H level, the P-MOS FET becomes ON condition. At the C-MOS FET circuit, the N-MOS FET and the P-MOS FET do the operation that is always opposite.

The important characteristic of this circuit is that the comparatively big current can be controlled. When the input becomes an L level, the output is Connected with the power supply by the P-MOS FET and becomes H level. Also, When the input becomes H level, the output is connected with the grounding by the N-Mos FET and becomes an L level. Phase of the input and the output is opposite. The drain current of MOS FET is cut off even if the gate voltage doesn’t become 0 V. It may depend on the kind of the FET but a drain current is cut off when the gate voltage is lower than 1V or 2V. So, in case of C-MOS circuit, PMOS FET and N-MOS FET and N-MOS FET don’t become ON condition at the same time.

2.7.4 Choosing a MOSFET: In choosing a mosfet is better we first examine mosfet parameters that determines factors to be considered. (i)

On resistance Rds (ON) is the resistance between the source and drain terminals when the MOSFET is turned fully on.

(ii) Maximum drain current Id(max) is the maximum current that the MOSFET can stand passing from drain to source. It is largely determined by the package and Rds (on). (iii) Power dissipation (Pd) this is the maximum power handling capability of the MOSFET, which depends largely on the type of package it is in. 25

(iv) Linear derating factor is how much the maximum power dissipation parameter above must be reduced by per 00, as the temperature rises above 250C. (v)

Avalanche energy (EA) is how much energy the MOSFET can withstand under avalanche conditions. Avalanche occurs when the maximum drain-to-source voltage is exceeded and current rush through the MOSFET. This does not cause permanent damage as long as the energy (power x time) in the avalanche does not exceed the maximum.

(vi) Peak diode recovery dv/dt is how fast the intrinsic diode can go from the off state (reverse biased) to the on state (conducting). It depends on how much voltage was across it before it turned on. Hence the time taken, t= (reverse voltage / peak diode recovery). (vii) Drain-to-source breakdown voltage Vdss is the maximum voltage that can be placed from drain to source when the MOSFET is turned off. (viii)Thermal resistance Ojc this is the function of the heat sink. (ix) Gate threshold voltage Vas(th) is the maximum voltage required between the gate and source terminals to turn the MOSFET on. It will need more than this to turn it fully on. (x) Forward transconductance gfs; as the gate-source voltage is increased, when the MOSFET is just starting to turn on, it has a fairly linear relationship between Vgs and drain current. This parameter is simply ((Id/Vgs) in this linear section. 26

(xi) Input capacitance Ciss is the humped capacitance between the gate terminal and the drain terminals. The capacitance to the drain is the most important.

2.7.5 Power and Heat: The power that the MOSFET will have to contend with is one of the major deciding factors. The power dissipated in a MOSFET is the voltage across it times the current going through it. Even though it is switching large amounts of power, this should be fairly small because either the voltage across it is very small (switch is closed – MOSFET is on), or the current going through it is very small (switch is open-MOSFET is off). The voltage across the MOSFET when it is on will be the resistance of the MOSFET, Rds (on) times the current going through it. This resistance, Rds (on), for good power MOSFETs will be less than 0.02ohms. The then the power dissipated in the MOSFET is P

=

ia2 Rds on

Another source of power dissipation in MOSFET occurs when the MOSFET is switching between states. For a short period of time, the MOSFET is half on and half off. However, the MOSFET is only dissipating this for the short period of time that the MOSFET is switching between states. Any power dissipate above 1 watt requires that the MOSFET is mounted on a heat sink. Power MOSFETs comes in a variety of packages, but normally have a metal tab, which is placed 27

against the heat sink, and used to conduct heat away from the MOSFET semiconductor.

2.8.0 MOSFET Driver: To turn a power MOSFET on, the gate must be set to a voltage at least 10volts greater than the source terminal (about 4volts for logic level MOSFETs). This is comfortably above the Vgs (th) parameter. On feature of power MOSFETs is that they have a large stray capacitance between the gate and the other terminals, Ciss. The effect of this is that when the pulse to the gate terminals arrives, it must first charge this capacitance up before the gate voltage can reach the 10volts required. The gate terminals then effectively do take current. Therefore the circuit that drives the gate terminal should be capable of supplying a reasonable current so the stray capacitance can be charge up as quickly as possible. The best way to do this is to use a dedicated MOSFET driver chip. Often you will see a low value resistor between the MOSFET driver gate terminals. This is to dampen down any ringing oscillations caused by the lead inductance and gate capacitance which otherwise exceed the maximum voltage allowed on the gate terminal. It also slows down the rate at which the MOSFET turns on and this can be useful it the intrinsic diodes in the MOSFET do not turn on fast enough.

2.8.1 Paralleling MOSFETs: Mosfets can be placed in parallel to improve the current handling capability. Simply join the gate, source and drain terminals together. Any 28

number of MOSFETs can be parallel up. But note that the gate capacitance adds up as you parallel more MOSFETs, and eventually the MOSFET driver will not be able to drive them.

2.9.0 Capacitor and Capacitance: A capacitor consists of two conductive electrodes or plates, separated by an insulator. The capacitors capacitance © is a measure of the amount of charge (Q) stored on each plate for a given potential difference or voltage (v) which appears between the plates.

C

Charge Plate Area

=

Ø V

+∅ -∅

A

Electric Field E

Fig.2.18Plate Separation d

The capacitance is proportional to the surface area of the conducting plate and inversely proportional to the distance between the plates. It is also proportional to the permittivity of the dielectric (that is, non-conducting) substance that separates the plates.

29

Impedance: The ratio of the phase voltage to the phase current is called the impedance of a capacitor and is given by: Zc

=

Where

-J 2 fc

=

Xc

=

-JXc

I is the capacitive reactance, WC

W

=

2 π f is the angular frequency

F

=

frequency

C

=

capacitance in farads, and

J

=

√-1 is the imaginary unit.

Hence

Xc

=

1 . 2π ƒ c

F

=

1 . Xc2π c

This implies that for a specific frequency (e.g. 50Hz). The capacitance (from impedance) and the capacitor value is specific. This relation helps generating frequency.

2.10.0 Transformer A transformer is a device for stepping down, stepping up or isolating AC voltage or signals.

30

Transformer Symbol Isolation Transformer

Step Down Transformer

Step Up Transformer

Vpri = Vsec

Vpri > Vsec

Vpri < Vsec

Primary Coil

Primary Coil

Primary Coil

Secondary Coil

Secondary Coil

Secondary Coil

Fig. 2.19 Schematic Symbol

Transformer type Symbol Européen Transformer

Iron Transformer

Ferrite Core Transformer

Tuned Slug Ferrite Core Transformer

Symbol

Primary Coil

Secondary Coil

Primary Coil

Secondary Coil

Primary Coil

Secondary Primary Coil Coil

Secondary Coil

Fig. 2.20 Types of transformer symbol 2.10.1 Transformer Theory: A transformer usually consists of 1 primary coil and 1 or more secondary coils wound around a common metallic core (iron for low frequency, and ferrite for high frequency). Some very high frequency transformers are air cored only. A transformer works by inducing a voltage from the primary coil to the secondary coil. When an alternating voltage is applied to the primary coil, and alternating magnetic field is created around this coil since it acts as an 31

electromagnet. Since the secondary coil is in the alternating magnetic field, a voltage on the secondary

coil depends on several factors: the ratio of primary to secondary turns (often just called the turns ratio), the core material, the driving frequency and coupling. The most important utility of transformers is to convert voltages. With AC (which is supplied by the electricity grid), the voltage is converted several times between the large electrical generators and your house. At this point is usually around 120V AC or 240 V AC depending on where you live. When you plug in an electrical appliance, it may require a different voltage or voltages to operate, these appliances will usually use a transformer to convert) To obtain several voltages, transformers can either have several secondary with different winding ratios or a single tapped secondary (output wires are connected to several places along the secondary coil, allowing the number of turns to be selected).

2.10.2 Magnetic Shunt: Shunts can be used to limit current or regulate voltage. This regulation is accomplished by inserting a ferrous magnetic shunt into the transformer core, such that the magnetic flux from the primary winding has an alternate (although High impedance) path around the secondary winding. As the current draw on the transformer secondary winding increases, more primary magnetic flux diverts through the magnetic shunt. Selecting different materials with suitable saturation characteristics it is possible to make transformers with a range of regulation functions. 32

Transformer Types Multitap

Isolated Secondaries Secondary Coil

Primary Coil

Secondary Coil

Primary Coil

Secondary Coil

Fig. 2.21Type of Transformer:

2.10.3 Autotransformer Another type of transformer is known as the autotransformer.

Primary Coil Secondary Coil

Fig.2.22 Autotransformer It consists of a single tapped primary where the center tap is common to both primary and secondary (not isolated). Some of the turns on the coil are used variable autotransformer is known as a variac. A variac is a single coil with a turn: secondary turns to be altered easily.

2.10.4 Current Transformer: 33

Yet another type of transformer is the current transformer. Central to all of the AC power transducers is the measurement of current. This is accomplished using a current transformer (CT), a “Donut” (toroidal) shaped core through which

is threaded the wire whose current is to be measured. Current transformers are designed to produce either an alternating current or alternating voltage proportional to the current being measured.

2.10.5 Ferroresonant Transformer: These transformers use a tank circuit composed of a high-voltage resonant winding and a capacitor to produce a nearly constant average output with a varying input. The Ferro resonant approach is attractive due to its lack of active components, relying on the square loop saturation characteristics of the tank circuit to absorb variations in average input voltage. The Ferro resonant action is a flux limiter rather than a voltage regulator, but with a fixed supply frequency it can maintain an almost constant average output voltage even as the input voltage varies widely. Ferroresonanat transformers output either non-sinusoidal (CVN type transformers) or sinusoidal wave shapes (CVS type transformers).

2.10.6 Potential Transformer: Potential transformer are used by the electrical industry for smaller power applications and. They are a step-down transformer and range in wattage. This is a common Westinghouse 1500VA 12,000V transformer.

34

2.10.7 Properties of A Transformer: Transformers have many properties due to construction. The ideal transformer is lossless, but the losses in a transformer will be elaborated later as well. Here we have a simplistic model of our idealized transformer. N1 and N2 are the number of turns for each winding, e1 and e2 are the voltages in the windings, and O is the flux.

Ô 1

3 +

+

+

+ N1

N2

-

-

-

2

4

Fig. 2.23 Model of idealized transformer. With a load attached to the secondary we now see the currents h and h. 35

Φ i1 V1

i2

3

1 +

+

+

+

V2 N2

N1

Load

-

-

-

-

2

4

Fig. 2.24 Idealized transformer with load in the secondary. Primary induced E.M.F.

dΦ e2 = N dt 2

e1

dΦ = N 1 dt υ

e1

Proportionality of the Transformer 1 is the transformation Radio. Voltage and current relationship

=

N1 =

υ2 e2 N2 v1 ί1 = V2ί2

Changing Flux relationship Φ = the

, And secondary induced E.M.F

ί1 =

, where

a

= a ί2

Φm sin wt, where w = 2π ƒ which is

angular frequency in radians/second, f is the source frequency, t is the time at which the measurement takes place, and Φm is the magnetizing Flux density. 36

Load Impedances

Z2 = Induced Primary E.M.F. e1

And the R.M.S. value E1

V1

1

V1

= I2

11

=

2

= a

=

1

2

I1

a2

N1wΦ m costwt,

Z1

N1 ω Φm