CHAPTER 1

Introduction

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Chapter

Introduction

1.1 BACKGROUND

1.1.1 Power System & UPS

The Electrical Power system should ensure the following characteristics

230 phase-to-neutral voltage with an error of + 5% to – 5%.

50 Hz frequency with an error of + 1% to – 1%.

Sinusoidal waveform of the voltage.

Continuity of supply.

All of the above mentioned things are supplied by an Electric Utility,

normally. However, under certain conditions, the power supply is blocked.

Uninterruptible power supplies are, normally, designed to deal with such

situations. They ensure the continuity of supply. They also find the

applications in case of load shedding / load management. The normal UPS

does not ensure sine wave output waveform. This problem is cured in a

sine wave UPS.

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1.1.2 The Use of Uninterruptible Power Supplies

While not limited to protecting any particular type of equipment, a UPS is

typically used to protect ‗computers, data centers, telecommunication

equipment‘ or other electrical equipment where an unexpected power

disruption could cause injuries, fatalities, serious business disruption or

data loss. UPS units range in size from units designed to protect a single

computer without a video monitor (around 200 VA rating) to large units

powering entire data centers, buildings, or even cities.

1.2 COMPARISON OF UPS & GENERATOR

A UPS differs from an auxiliary or emergency power system or standby

generator in that it will provide instantaneous or nearly instantaneous

protection from input power interruptions by means of one or more attached

batteries and associated electronic circuitry for low power users, and or by

means of diesel generators and flywheels for high power users.

The other problem is voltage and frequency variation of the generator. The

voltage and frequency of the power produced by a generator depends on

the engine speed. The speed is controlled by a system called a governor. 3 | P a g e

Some governors are mechanical, and some are electronic. The job of the

governor is to keep the voltage and frequency constant, while the load on

the generator changes. This may pose a problem where, for example, the

startup surge of an elevator can cause short "blips" in the frequency of the

generator or the output voltage, thus affecting all other devices powered by

the generator.

1.3 DIFFERENT TYPES OF UPS

1.3.1 Offline UPS

Also called a "standby UPS", this unit doesn't act until a disruption in the

electrical current is identified. After this happens, the battery within the UPS

begins supplying the current in the utility's absence. When the UPS closes

the transfer switch and begins to supply power, there's a brief period during

which power is unavailable. While offline UPS are inexpensive compared to

other UPS solutions, some types of sensitive equipment can't work properly

with this inherent limitation.

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Figure 1.1 – Offline UPS

1.3.2 Online UPS

An online UPS maintains a constant connection from the UPS battery to

the equipment that needs power. Electricity is converted from AC to DC

and then converted back to AC before delivery. The continuous link

between the online UPS battery and the equipment not only prevents any

brief loss of power (like that experienced with offline UPS), but helps

managing voltage irregularities. While this type of UPS usually costs more

and is less energy-efficient, its reliability is an important factor for many

mission critical applications.

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Figure 1.2 – Online UPS

1.3.3 Ferro-Resonant UPS

This type of UPS is very similar to an offline UPS. It works in the same

manner. The most significant difference is the use of an internal

transformer within the Ferro-resonant UPS. The main limitation of an offline

UPS is the brief power loss between the time the transfer switch is closed

and the battery begins supplying power. The Ferro-resonant UPS

transformer is designed to resolve that issue. Ideally, the transformer holds

enough energy to cover the momentary power loss. Because equipment

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that's used for power factor corrections reacts poorly to the transformers,

this type of UPS is seldom used today.

1.3.4 Line Interactive UPS

Line interactive UPS attempts to correct some of the problems of an offline

UPS. Like an online UPS, this unit maintains a continuous connection

between the battery and the output. However, the primary source of the

electrical current is still the AC input. In the event the primary source

becomes unavailable, the transfer switch is opened, allowing an inverter

and converter to charge the battery before delivering the current. While

they're more expensive than an offline UPS, they still have difficulty

regulating voltage sags and surges

.

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Figure 1.3 –Line interactive UPS

1.4 CLASSIFICATION W.R.T OUTPUT

1.4.1 Square Wave UPS

The output of this kind of inverter is square wave which is most easily

generated. Half bridge can be used to produce this output with minimal

control circuitry. They are the cheapest of the lot.

1.4.2 Modified Square Wave UPS

This kind of inverter‘s output is better than the square wave but much

worse than pure sine wave because it still contains harmonics. They are

also referred to as quasi- square or stepped-square.

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1.4.3 Pure Sine Wave UPS

The output of this kind of inverter is pure sine wave and hence free of any

harmonics. The advantages of not having harmonics will be discussed

later.

Figure 1.4 demonstrates all these waveforms over each other.

Figure 1.4 – Various Output Waveforms

GREEN--- PURE SINE WAVE

BLUE --- SQUARE WAVE

RED --- MODIFIED SQUARE WAVE

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1.5 DISADVANTAGES OF SQUARE WAVE

1.5.1 Voltage Spikes

Transient (short term) over-voltage i.e. spikes or peaks: Causes wear

or acute damage to Electronic equipment.

The abrupt change from negative peak to positive peak produces

discontinuities in the operation of sensitive elements of the circuit.

Memory loss, data error, data loss and component stress

1.5.2 over Voltages

Increased voltage for an extended period of time causes light bulbs to

fail.

The heating and wear of components due to excessive voltage.

A UPS having such problems cannot be used where voltage

regulation is crucial.

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1.5.3 Harmonic Distortion

Multiples of power frequency superimposed on the power waveform

cause excess heating in wiring and fuses.

Distortions superimposed on the power waveform cause electro-

magnetic interference.

Figure 1.5 –Square wave along with its components

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1.6 INTERNATIONAL STANDARDS AND LOCAL REQUIREMENTS

International standards for a UPS are given as

Output must be a pure sine wave and voltage regulation must be less

than +/- 5%.

Back up time of at-least 15-20 minutes.

An international regulatory authority for UPS specify batteries where

acid present in the form of gel and batteries are sealed therefore,

requires no maintenance.

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While our local demands are:-

A cost effective product.

A design which is immune to elements.

Long back up time of around 2-3 hours due to current power crisis.

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CHAPTER 2

Theory Related To Project

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Chapter 2

Theory Related to Project

2.1 BASIC STRUCTURE

The basic structure of a UPS or inverter can be thought of a switching

device and a D.C. source with input and an output. The following block

diagram illustrates this concept.

Figure 2.1 - Block diagram of UPS

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2.2 MAJOR COMPONENTS OF UPS

The list of major components is as under.

1) Main supply.

2) Inverter (DC to AC).

3) Rectifier (AC to DC).

4) Control Circuit.

5) Filter.

6) Re-Chargeable Batteries.

7) Transformer.

8) By Pass Switch

9) Microcontroller.

The detail of each component is discussed as under:

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2.2.1 Main Supply

Main supply in our case can be fed to one or two components of the UPS.

In this case the supply being used is AC single phase 220V rms with

frequency of 50 Hz.

2.2.2 Inverter

The inverter circuit is the most important component controlled by the

control circuit which will be discussed later. Inverter circuit converts the

D.C. power from batteries to A.C. power. It consists of switching devices

which are arranged in different arrays.

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Figure 2.2 - H-bridge configuration

(A Famous Inverter Topology)

The output can be of desired magnitude and frequency. The output voltage

could be fixed or variable at a fixed or variable frequency. A variable output

voltage can be obtained by varying the input D.C. voltage and maintaining

the gain of inverter constant. On the other hand if the D.C. input voltage is

fixed and it is not controllable, a variable output voltage can be obtained by

varying the gain of the inverter, which is normally accomplished by PWM or

pulse width modulation control within the inverter. The inverter gain may be

defined as ratio of the A.C. output voltage to D.C. input voltage.

2.2.3 Rectifier

A rectifier is an electrical device that converts alternating current (AC) to

direct current (DC), a process known as rectification. Rectifiers have many

uses including as components of power supplies and as detectors of radio

signals. Rectifiers may be made of solid state diodes, vacuum tube diodes,

mercury arc valves, and other components.

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Figure 2.3 - Rectification Process

When only one diode is used to rectify AC (by blocking the negative or

positive portion of the waveform), the difference between the term diode

and the term rectifier is merely one of usage, i.e., the term rectifier

describes a diode that is being used to convert AC to DC. Almost all

rectifiers comprise a number of diodes in a specific arrangement for more

efficiently converting AC to DC than is possible with only one diode.

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2.2.4 Control Circuit

The control circuit comprises of a micro-controller coupled with relays and

other switches for detection and switching of the power supply. Its basic

parts are

A microcontroller.

Relays.

2.2.5 Filters

It is sometimes desirable to have circuits capable of selectively filtering one

frequency or range of frequencies out of a mix of different frequencies in a

circuit. A circuit designed to perform this frequency selection is called a

filter circuit, or simply a filter. A common need for filter circuits is in high-

performance stereo systems, where certain ranges of audio frequencies

need to be amplified or suppressed for best sound quality and power

efficiency

A practical application of filter circuits is in the ―conditioning‖ of non-

sinusoidal voltage waveforms in power circuits. Some electronic devices

are sensitive to the presence of harmonics in the power supply voltage, and

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so require power conditioning for proper operation. If a distorted sine-wave

voltage behaves like a series of harmonic waveforms added to the

fundamental frequency, then it should be possible to construct a filter circuit

that only allows the fundamental waveform frequency to pass through,

blocking all (higher-frequency) harmonics.

2.2.5.1 Low-Pass Filters

A low-pass filter is a circuit offering easy passage to low-frequency signals

and difficult passage to high-frequency signals. There are two basic kinds

of circuits capable of accomplishing this objective, and many variations of

each one:

2.2.5.1.1 Inductive Low-Pass Filters

Figure 2.4 - Inductive low-pass filter

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The inductor's impedance increases with increasing frequency. This high

impedance in series tends to block high-frequency signals from getting to

the load. The response of an inductive low-pass filter falls off with

increasing frequency. The inductive low-pass filter is very simple, with only

one component comprising the filter. The inductive low-pass filter is often

preferred in AC-DC power supplies to filter out the AC ―ripple‖ waveform

created when AC is converted (rectified) into DC, passing only the pure DC

component. The primary reason for this is the requirement of low filter

resistance for the output of such a power supply. The inductive low-pass

filter does not require an extra resistance in series with the source. In the

design of a high-current circuit like a DC power supply where additional

series resistance is undesirable, the inductive low-pass filter is the better

design choice.

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2.2.5.1.2 Capacitive Low-Pass Filters

Figure 2.5 - Capacitive low-pass filter

The capacitor's impedance decreases with increasing frequency. This low

impedance in parallel with the load resistance tends to short out high-

frequency signals, dropping most of the voltage across series resistor R.

The response of a capacitive low-pass filter falls off with increasing

frequency.

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The capacitive filter is with only a resistor and capacitor needed for

operation. Despite their increased complexity, capacitive filter designs are

generally preferred over inductive because capacitors tend to be ―purer‖ reactive components than inductors and therefore are more predictable in

their behavior. By ―pure‖ It means that capacitors exhibit little resistive

effects than inductors, making them almost 100% reactive. Inductors, on

the other hand, typically exhibit significant dissipative (resistor-like) effects,

both in the long lengths of wire used to make them, and in the magnetic

losses of the core material. Capacitors also tend to participate less in

―coupling‖ effects with other components (generate and/or receive

interference from other components via mutual electric or magnetic fields)

than inductors, and are less expensive.

A capacitive low-pass filter requires an extra resistance in series with the

source. If low weight and compact size are higher priorities than low

internal supply resistance in a power supply design, the capacitive low-

pass filter are used. One frequent application of the capacitive low-pass

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filter principle is in the design of circuits having components or sections

sensitive to electrical ―noise.‖ AC signals sometimes can ―couple‖ from

one circuit to another via capacitance (Stray) and/or mutual inductance

(Stray) between the two sets of conductors.

2.2.5.1.3 Cutoff Frequency

All low-pass filters are rated at a certain cutoff frequency. That is, the

frequency above which the output voltage falls below 70.7% of the input

voltage. In a simple capacitive/resistive low-pass filter, it is the frequency at

which capacitive reactance in ohms equals resistance in ohms. In a simple

capacitive low-pass filter (one resistor, one capacitor), the cutoff frequency

is given as:

When dealing with filter circuits, it is always important to note that the

response of the filter depends on the filter's component values and the

impedance of the load. If a cutoff frequency equation fails to give

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consideration to load impedance, it assumes no load and will fail to give

accurate results for a real-life filter conducting power to a load.

2.2.5.2 Band-Pass Filters

Applications where a particular band, or frequencies need to be filtered

from a wider range of mixed signals. Filter circuits can be designed to

accomplish this task by combining the properties of low-pass and high-pass

into a single filter. The result is called a band-pass filter. Creating a band

pass filter from a low-pass and high-pass filter can be illustrated using

block diagram.

Figure 2.6 - Block diagram of a band-pass filter

Series combination of these two filter circuits is a circuit that will only allow

passage of those frequencies that are neither too high nor too low. The

general idea of combining

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Figure 2.6 - Block diagram of a band-pass filter

Series combination of these two filter circuits is a circuit that will only allow

passage of those frequencies that are neither too high nor too low. The

general idea of combining low-pass and high-pass filters together to make

a band-pass filter has certain limitations. Because this type of band-pass

filter works by relying on either section to block unwanted frequencies, it

can be difficult to design such a filter to allow unhindered passage within

the desired frequency range. Both the low-pass and high-pass sections will

always be blocking signals to some extent, and their combined effort

makes for an attenuated (reduced amplitude) signal, even at the peak of

the pass-band frequency range.

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The capacitive and inductive band-pass filters are shown in the figures 2.7

& 2.8.

Figure 2.7 - Capacitive band-pass filter

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Figure 2.8 - Inductive band-pass filter

2.2.5.3 Single Tuned Filters

This filter allows only a single frequency to pass. This filter is very useful,

owing to the fact that it only allows the desired frequency to pass on and

blocks the remaining frequencies. The details of this type of filter are given

in chapter 3.

2.2.6 Rechargeable Batteries

The battery plays a very important part in the un-interruptible power supply

system. The basic two types of batteries that are used are as under

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2.2.6.1 Wet Batteries

The most common type of this kind of battery is lead-acid battery, which is

widely used because of its availability in different Voltages and Ampere-

hours configurations.

2.2.6.2 Dry Batteries

The most common application of this kind of battery is in less power

required areas like computer supplies etc. The most common type of

battery is simply Nickel metal hydride battery.

A lot of work is being done on some other kind of batteries like lithium-

polymer, lithium-ion batteries, and nickel-cadmium batteries. Because they

are very handy and are maintenance free.

2.2.7 Transformers

A transformer is a device that transfers electrical energy from one circuit to

another through inductively coupled electrical conductors. A changing

current in the first circuit (the primary) creates a changing magnetic field; in 30 | P a g e

turn, this magnetic field induces a changing voltage in the second circuit

(the secondary). By adding a load to the secondary circuit, one can make

current flow in the transformer, thus transferring energy from one circuit to

the other.

The secondary induced voltage VS, of an ideal transformer, is scaled from

the primary VP by a factor equal to the ratio of the number of turns of wire

in their respective

i

By appropriate selection of the numbers of turns, a transformer thus allows

an alternating voltage to be stepped up — by making NS more than NP —

or stepped down, by making it less.

Transformers are some of the most efficient electrical 'machines', with

some large units able to transfer 99.75% of their input power to their output.

Transformers come in a range of sizes from a thumbnail-sized coupling

transformer hidden inside a stage microphone to huge units weighing

hundreds of tons used to interconnect portions of national power grids. All

operate with the same basic principles, although the range of designs is

wide.

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If the secondary coil is attached to a load that allows current to flow,

electrical power is transmitted from the primary circuit to the secondary

circuit. Ideally, the transformer is perfectly efficient; all the incoming energy

is transformed from the primary circuit to the magnetic field and into the

secondary circuit. If this condition is met, the incoming electric power must

equal the outgoing power.

Pin coming = IPVP = Pout going = ISVS

Giving the ideal transformer equation

Figure 2.9 - Ideal transformer

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2.2.8 By-Pass Switch

The purpose of by-pass switch is to connect the load to either A.C. supply

or A.C. power from the inverter. By-pass switch basically by-passes the

inverter circuit if A.C. power from the mains is active. There are many types

of by-pass switches, the most common of which are.

Relays

Thyristors

Most common are the relays. A relay is an electrical switch that opens and

closes under the control of another electrical circuit. In the original form, the

switch is operated by an electromagnet to open or close one or many sets

of contacts. Because, a relay is able to control an output circuit of higher

power than the input circuit, it can be considered to be, in a broad sense, a

form of an electrical amplifier.

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Figure 2.10 - Circuit symbols for relays

2.2.9 Microcontroller

Microcontroller is normally used for providing the gating signals to the

bridge circuit. There is a variety of microcontrollers available. However, an

8051 serves the purpose, in a very nominal cost, so it was used. The

details are given in chapter 3.

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2.3 SWITCHING DEVICES FOR BRIDGE CIRCUIT

Following devices are used for the switching purpose in the bridge circuits.

1. Bipolar Junction Transistors (BJTs)

2. Metal-oxide Field Effect Transistors (MOSFETS)

3. Insulated Gate Bipolar Junction Transistors (IGBTs)

2.3.1 Bipolar Junction Transistors (BJTs)

The bipolar junction transistor is the mostly used switching device in low

price switching mode power supplies. It is a three-terminal device

constructed of doped semiconductor material and may be used in

amplifying or switching applications. Bipolar transistors are so named

because their operation involves both electrons and holes.

Although a small part of the transistor current is due to the flow of majority

carriers, most of the transistor current is due to the flow of minority carriers

and so BJTs are classified as 'minority-carrier' devices.

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A BJT consists of three differently doped semiconductor regions, the

emitter region, the base region and the collector region. These regions are,

respectively, p type, n type and p type in a PNP, and n type, p type and n

type in a NPN transistor. Each semiconductor region is connected to a

terminal, appropriately labeled: emitter (E), base (B) and collector (C).

The base is physically located between the emitter and the collector and is

made from lightly doped, high resistivity material. The collector surrounds

the emitter region, making it almost impossible for the electrons injected

into the base region to escape being collected, thus making the resulting

value of α very close to unity, and so, giving the transistor a large β. A

cross section view of a BJT indicates that the collector–base junction has a

much larger area than the emitter–base junction.

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Figure 2.11 - Basic structure of an NPN Transistor

2.3.1.1 Types

i. NPN

NPN is one of the two types of bipolar transistors, in which the letters "N"

and "P" refer to the majority charge carriers inside the different regions of

the transistor. Most bipolar transistors used today are NPN, because

electron mobility is higher than hole mobility in semiconductors, allowing

greater currents and faster operation.

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NPN transistors consist of a layer of P-doped semiconductor (the "base")

between two N-doped layers. A small current entering the base in common-

emitter mode is amplified in the collector output. In other terms, an NPN

transistor is "on" when its base is pulled high relative to the emitter.

The arrow in the NPN transistor symbol is on the emitter leg and points in

the direction of the conventional current flow when the device is in forward

active mode.

Figure 2.12 - PNP & NPN Symbols

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ii. PNP

PNP transistors consist of a layer of N-doped semiconductor between two

layers of P-doped material. A small current leaving the base in common-

emitter mode is amplified in the collector output. In other terms, a PNP

transistor is "on" when its base is pulled low relative to the emitter.

The arrow in the PNP transistor symbol is on the emitter leg and points in

the direction of the conventional current flow when the device is in forward

active mode.

2.3.1.2 Working

The collector–emitter current can be viewed as being controlled by the

base–emitter current (current control), or by the base–emitter voltage

(voltage control). These views are related by the current–voltage relation of

the base–emitter junction, which is just the usual exponential current–

voltage curve of a p-n junction (diode).

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The physical explanation for collector current is the amount of minority-

carrier charge in the base region. The charge-control view easily handles

photo-transistors, where minority carriers in the base region are created by

the absorption of photons, and handles the dynamics of turn-off, or

recovery time, which depends on charge in the base region recombining.

However, since base charge is not a signal that is visible at the terminals,

the current- and voltage-control views are usually used in circuit design and

analysis.

2.3.2 IGBT

The insulated-gate bipolar transistor or IGBT is a three-terminal power

semiconductor device, noted for high efficiency. It switches electric power

in many modern appliances: electric cars, variable speed refrigerators, air-

conditioners, and even stereo systems with digital amplifiers. Since it is

designed to rapidly turn on and off, amplifiers that use it often synthesize

complex waveforms with pulse width modulation and low-pass filters.

The IGBT combines the simple gate-drive characteristics of the MOSFETs

with the high-current and low–saturation-voltage capability of bipolar

transistors by combining an isolated-gate FET for the control input, and a

bipolar power transistor as a switch, in a single device. The IGBT is used in

medium- to high-power applications such as switched-mode power supply, 40 | P a g e

traction motor control and induction heating. Large IGBT modules typically

consist of many devices in parallel and can have very high current handling

capabilities in the order of hundreds of amps with blocking voltages of

6,000 V.

The IGBT is a fairly recent invention. The first-generation devices of the

1980s and early 1990s were relatively slow in switching, and prone to

failure through secondary breakdown. Second-generation devices were

much improved, and the current third-generation ones are even better, with

speed rivaling MOSFETs, and excellent ruggedness and tolerance of

overloads.

The extremely high pulse ratings of second- and third-generation devices

also make them useful for generating large power pulses in areas like

particle and plasma physics, where they are starting to supersede older

devices like thyratrons and triggered spark gaps. They are also attractive to

the high-voltage hobbyist for generating large amounts of high-frequency

power to drive experiments like Tesla coils.

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IGBTs have the benefit that their saturation voltage is a relatively constant

3 volts even when many tens to hundreds of amperes are flowing through

the device. This makes for an inverter which has high efficiency at high

output currents. As for the gate drivers, it is not necessary to use the

special IGBT drivers that IGBT manufacturers produce for their product.

IGBTs, like MOSFETs, do not draw any continuous gate current, but do

have significant amounts for gate capacitance. This requires a circuit that

can move the required charge in and out of the gate in short periods of

time. Also, the gate drive circuit must have a provision to isolate the high

voltage of the IGBT Bridge from the logic level voltages on the inverter

control PCB. Generally, the hybrid IGBT drivers include some form of out-

of-saturation protection circuitry which may or may not be needed based on

the application.

Availability of affordable, reliable IGBTs is a key enabler for electric

vehicles and hybrid cars. Toyota's second generation hybrid Prius has a 50

kW IGBT inverter controlling two AC motor/generators connected to the DC

battery pack.

i

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2.3.2.1 Device Structure

An IGBT cell is constructed similarly to an n-channel vertical construction

power MOSFET except the n+ drain is replaced with a p+ collector layer,

thus forming a vertical PNP bipolar junction transistor.

This additional p+ region creates a cascade connection of a PNP bipolar

junction transistor with the surface n-channel MOSFET. This connection

results in a significantly lower forward voltage drop compared to a

conventional MOSFET in higher blocking voltage rated devices. As the

blocking voltage rating of both MOSFET and IGBT devices increases, the

depth of the n- drift region must increase and the doping must decrease,

resulting in roughly square relationship increase in forward conduction loss

compared to blocking voltage capability of the device. By injecting minority

carriers (holes) from the collector p+ region into the n- drift region during

forward conduction, the resistance of the n- drift region is considerably

reduced. However, this resultant reduction in on-state forward voltage

comes with several penalties.

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The additional PN junction blocks reverse current flow. This means that

IGBTs cannot conduct in the reverse direction, unlike a MOSFET. In bridge

circuits where reverse current flow is needed an additional diode (called a

freewheeling diode) is placed in parallel with the IGBT to conduct current in

the opposite direction. The penalty isn't as severe as first assumed though,

because at the higher voltages where IGBT usage dominates, discrete

diodes are of significantly higher performance as the body diode of a

MOSFET.

The reverse bias rating of the n- drift region to collector p+ diode is usually

only of 10's of volts, so if the circuit application applies a reverse voltage to

the IGBT, an additional series diode must be used. The minority carriers

injected into the n- drift region take time to enter and exit or recombine at

turn on and turn off. This results in longer switching time and hence higher

switching loss, as compared to a power MOSFET.

The additional PN junction adds a diode-like voltage drop to the device. At

lower blocking voltage ratings, this additional drop means that an IGBT

would have a higher on-state voltage drop. As the voltage rating of the

device increases, the advantage of the reduced n- drift region resistance

overcomes the penalty of this diode drop and the overall on-state voltage 44 | P a g e

drop is lower (the crossover is around 400 V blocking rating). Thus IGBTs

are rarely used where the blocking voltage requirement is below 600 V.

2.3.3 MOSFETs

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-

FET, or MOS FET) is a device used to amplify or switch electronic signals.

It is by far the most common field-effect transistor in both digital and analog

circuits. The MOSFET is composed of a channel of n-type or p-type

semiconductor material, and is accordingly called an NMOSFET or a

PMOSFET.

2.3.3.1 Circuit Symbols

A variety of symbols are used for the MOSFET. The basic design is

generally a line for the channel with the source and drain leaving it at right

angles and then bending back into the same direction as the channel.

Sometimes three line segments are used for enhancement mode and a

solid line for depletion mode. Another line is drawn parallel to the channel

for the gate.

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The bulk connection, if shown, is shown connected to the back of the

channel with an arrow indicating PMOS or NMOS. Arrows always point

from P to N, so an NMOS (N-channel in P-well or P-substrate) has the

arrow pointing in. If the bulk is connected to the source (as is generally the

case with discrete devices) it is angled to meet up with the source leaving

the transistor. If the bulk is not shown (as is often the case in IC design as

they are generally common bulk) an inversion symbol is sometimes used to

indicate PMOS, alternatively an arrow on the drain may be used in the

same way as for bipolar transistors (out for NMOS in for PMOS).

Comparison of enhancement-mode and depletion-mode MOSFET

symbols, along with JFET symbols, is given in Figure 2.13:

For the symbols in which the bulk, or body, terminal is shown, it is here

shown internally connected to the source. This is a typical configuration,

but by no means the only important configuration. In general, the MOSFET

is a four-terminal device, and in integrated circuits many of the MOSFETs

share a body connection, not necessarily connected to the source terminals

of all the transistors.

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2.3.3.2 Power MOSFET

A Power MOSFET is a specific type of Metal Oxide Semiconductor Field-

Effect Transistor (MOSFET) designed to handle large power. Compared to

the other power semiconductor devices (IGBT, Thyristor), its main

advantages are high commutation speed and good efficiency at low

voltages. It shares with the IGBT an isolated gate that makes it easy to

drive.

It was made possible by the evolution of the CMOS technology, developed

for manufacturing Integrated circuits in the late 1970s. The power MOSFET

shares its operating principle with its low-power counterpart, the lateral

MOSFET.

The power MOSFET is the most widely used low-voltage switch. It can be

found in most power supplies, DC to DC converters, and low voltage motor

controllers.

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2.3.3.3 Capacitances

In the MOSFETs datasheets, the capacitances are often named Ciss (input

capacitance, drain and source terminal shorted), Coss (output capacitance,

gate and source shorted), and Crss (reverse capacitance, gate and source

shorted). The relationship between these capacitances and theses

described below is:

Ciss = CGS + CGD

Coss = CGD + CDS

Crss = CGD

Where CGS, CGD and CDS are respectively the gate-to-source, gate-to-

drain and drain-to-source capacitances. Manufacturers prefer to quote

Ciss, Coss and Crss because they can be directly measured on the

transistor.

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2.4 MOSFET, IGBT or BJT?

A BJT is current operated device. It requires continuous base current, that

causes loading on the device (microcontroller) that is giving gating signals.

Therefore, this option is not feasible.

MOSFETs are suitable switching elements for an inverter. They are much

faster switches than IGBTs and, in many lower power cases, are less

expensive than IGBTs. The faster switching times allow for a higher

frequency PWM signal(which in this case approaches 4 kHz), which would

thus require a smaller size/cost output filter to remove the switching

frequency from the output. Also, driving MOSFETs may be easier than

driving IGBTs.

The disadvantage of MOSFETs is that their ―on‖ state resistance RDS

(on) is large enough to cause dissipation problems for high power inverters

and their saturation voltage is less stable over temperature. In addition, as

the voltage rating of MOSFETs is increased, their conduction loss

increases. However, due to fast switching capability, the MOSFETs are

preferred in this design.

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2.5 PULSE WIDTH MODULATION (PWM)

Pulse-width modulation control works by rapidly switching the power

supplied, between on and off positions. The DC voltage is converted to a

square-wave signal, alternating between fully on and zero, in such a

technique. By adjusting the duty cycle of the signal (modulating the width of

the pulse, hence the 'PWM') i.e., the time fraction it is "on", the average

power can be varied. The duty cycle of a waveform is defined as

DUTY CYCLE = (time for the function is active) / (the time period of the

function.)

Figure 2.14 – Varying Duty Cycles.

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2.5.1 Advantages

In communication, the PWM is less exposed to the noise due to its

digital nature.

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-sinks

can be used.

The power delivered can effectively be controlled by means of

controlling PWM.

The PWM can be used to effectively control the speed of motors.

2.5.2 Principle

The classical technique for producing a PWM is to compare a saw tooth

waveform with a reference waveform. (This is shown in Figure 2.15.) It

should be noted here that higher the frequency of the saw tooth waveform,

higher will be the frequency of generated PWM.

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Normally, an oscillator is used to generate a triangle or saw tooth

waveform. As it is previously mentioned that the frequency of PWM

depends upon the frequency of the saw tooth waveform so, the saw tooth

of the required frequency is generated. In case of motors, the frequencies

of 30-200Hz are commonly used. This is because of the fact that at low

frequencies the motor speed tends to be jerky, at high frequencies the

motor's inductance becomes significant and significant power is lost.

A potentiometer is used to set a steady reference voltage. A comparator

compares the saw tooth voltage with the reference voltage. When the saw

tooth voltage rises above the reference voltage, a power transistor is

switched on, and when the saw tooth voltage falls below the reference, it is

switched off. This gives a square wave output, which may have a constant

or a variable duty cycle, depending upon the nature of reference voltage.

e.g. If the reference voltage is DC then the square wave will have a

constant duty cycle, whereas in case of sinusoidal reference voltage, the

square wave will have a varied duty cycle.

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If the potentiometer is adjusted to give a high reference voltage, and the

saw tooth never reaches it, the output shall be zero. On the other hand, a

low reference voltage implies that the comparator is always on, hence

giving full power, all the time.

Figure 2.15 – PWM Generation.

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2.5.3.2 Power Delivery

PWM can be used to reduce the total amount of power delivered to a load

without losses normally incurred when a power source is limited by

resistive means. This is because the average power delivered is

proportional to the modulation duty cycle. With a sufficiently high

modulation rate, passive electronic filters can be used to smooth the pulse

train and recover an average analog waveform.

High frequency PWM power control systems are easily realizable with

semiconductor switches. The discrete on/off states of the modulation are

used to control the state of the switch which correspondingly controls the

voltage across or current through the load. The major advantage of this

system is that the switches are either off and not conducting any current, or

on and have no voltage drop across them. The product of the current and

the voltage at any given time defines the power dissipated by the switch,

thus no power is dissipated by the switch. Realistically, semiconductor

switches such as MOSFETs or BJTs are non-ideal switches, but high

efficiency controllers can still be built.

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2.5.4 Problems Associated With Non-Proper Use Of PWM

Output has low power.

Output voltage & frequency fluctuates from the required output wave.

Problem of noise, when load is varied.

The output wave shape varies undesirably, under the load.

The components are heated and power dissipation is large

The Harmonics are added into the output Waveform.

The Spikes and transients may occur.

Time varying characteristics on output.

Back currents, lags and sags due to inductive load.

2.5.5 PWM Generation Techniques

2.5.5.1 Single Pulse Width Modulation

In such a modulation, the PWM is generated by comparing a DC level and

a Single Triangular Wave, hence a rectangular pulse is acquired. It

provides a reasonable efficiency, but has many problems as described

preciously.

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Figure 2.16 – Single PWM

2.5.5.2 Multiple Pulse Width Modulation

It is generated by comparing a DC level and Multiple Triangular Wave we

get many rectangular pulses. It is improved version of the above, It is 83%

Efficient but have many problems as discussed previously.

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2.5.5.3 Sinusoidal Pulse Width Modulation

It is generated by comparing a Reference Sine Wave with relatively high

frequency Triangular wave. It is better than above methods but has low

fundamental harmonic and the more switching a cycle increases the losses

and the power is lost in heating.

The Harmonics are present in the output wave & noisy Characteristics with

load.

Figure 2.17 – Sinusoidal PWM

Figure 2.17 – Sinusoidal PWM

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2.5.5.2 Multiple Pulse Width Modulation

It is generated by comparing a DC level and Multiple Triangular Wave we

get many rectangular pulses. It is improved version of the above, It is 83%

Efficient but have many problems as discussed previously.

2.5.5.3 Sinusoidal Pulse Width Modulation

It is generated by comparing a Reference Sine Wave with relatively high

frequency Triangular wave. It is better than above methods but has low

fundamental harmonic and the more switching a cycle increases the losses

and the power is lost in heating.

The Harmonics are present in the output wave & noisy Characteristics with

load.

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Figure 2.17 – Sinusoidal PWM

2.5.5.4 Impulse-Sine Product Technique

In such a technique, the PWM is achieved by means of microcontroller

programming. This is the proposed technique and is discussed in depth in

chapter 3.

2.6 INVERTER

As transistors and various other types of semiconductor switches have

been developed these days they have been incorporated into inverter

circuit designs.

There are many different power circuit topologies and control strategies

used in inverter designs. Different design approaches address various

issues that may be more or less important depending on the way that the

inverter is intended to be used. 59 | P a g e

Inverters normally use H-bridge configurations. However, the voltage level

at which, the H-bridge is operated can be varied.

The normal inverters convert the DC into AC at 12 Volts. After this

inversion, the newly made AC is stepped up by means of a transformer.

The advantages of such an approach is that bridge construction is easy, as

it is not exposed to high voltages. The disadvantages accompanying such

an approach are

The transformer steps up the harmonic content.

The size and weight of the transformer increases considerably, as the

capacity of inverter is increased.

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To overcome such problems, in our design, the conversion of DC to AC is

done at output voltage of the UPS. The battery voltage is first boosted by

means of a DC-DC converter and then given to, a specially designed, H-

bridge. This arrangement overcomes the previously mentioned

disadvantages. The main disadvantage of such an approach is that the

bridge circuit becomes too complex.

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CHAPTER 3

Implementation of Project

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Chapter 3

Implementation of Project

3.1 MAIN PARTS OF UPS

The main parts of our UPS are

Charging Circuit

Inverter

Control Circuit

The block diagram of the UPS is as under:

Figure 3.1 – Block Diagram of UPS

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3.1.1 Charging Circuit

The charging circuit serves to charge the battery, whenever charging is

required. It is mainly consisting of a transformer that will step down the

incoming 230 V, to 14 V. This stepped down voltage is then applied to the

terminals of the battery through the control circuit. The charging circuit will

remain ON, until the battery is fully charged. This is a very simple circuit

and can easily be constructed.

3.1.2 Inverter

An inverter is a circuit that coverts DC to AC. This circuit is the heart of our

UPS; as the quality of Sine Wave, mainly depends upon this circuit. The

main components and their sequence is shown with the help of a block

diagram as under.

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Figure 3.2 – Block Diagram of Inverter

So, the main parts to be implemented are

DC – DC Converter

Microcontroller (in terms of programming)

H – Bridge

Filter

3.1.2.1 DC – DC Converter

This circuit boosts the voltage from 12 V to 300 V DC. The circuit consists

of a pulsating mechanism that gives delayed pulses of very high frequency

(in tens of kHz). These high frequency pulses are then passed through a

Ferrite Core Transformer1. The transformer steps up the voltage to the

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pulses of about 300 V. These pulses are supplied to the load through a

capacitor.

1 A

Due to shortage of time, the DC-DC Converter was bought from the

market. The rating of the converter is 250 Watts.

3.1.2.2 Microcontroller

The microcontroller used is 8051. It is a well-tested design, introduced in its

original form by Intel in 1980. The development costs of this device have

now been lowered to a very low value, and prices of modern 8051 devices

now start at less than US $1.00. At this price, one gets a performance of

around 1 million instructions per second, and 256 bytes of on-chip RAM. It

is also provided with 32 port pins and a serial interface. The 8051‘s profile

(price, performance, available memory, serial interface) match the needs of

many embedded systems very well. As a result, it is now produced in more

than 400 different forms by a diverse range of companies including Philips,

Infineon, Atmel and Dallas. Sales of this vast family are estimated to have

the largest share (around 60%) of the micro-controller market as a whole,

and to make up more than50% of the 8-bit microcontroller market. Its

various versions are used in various embedded products from simple toys 66 | P a g e

to high-tech automotives, because it is an excellent device for the

construction of many embedded systems.

Figure 3.3 - A Typical 8051The language used for the programming of the microcontroller is ―C-

language‖, and the software used for compiling and making HEX file is

―Keil‖.67 | P a g e

3.1.2.1.1 Sinusoidal Pulse Width Modulation (SPWM)

A sinusoidal pulse width modulation is a modulation, in which the duty cycle

of the pulses vary, according to the value of Sine function. The traditional

technique for the generation of SPWM is to compare a sinusoidal function

with a triangular wave, as explained in chapter 2.

However, in this project the SPWM is achieved by the programming of an

8051 microcontroller. The technique is known as ―Impulse-Sine Product.

3.1.2.1.2 Impulse-Sine Product Technique

In this technique, first of all, an impulse train of suitable frequency is

selected. The frequency chosen is 4 kHz. It is worth noting here that the

magnitude of impulse is unity. The impulse train is then multiplied with Sine

function of required frequency, which in this case is 50 Hz. Let us call this

funtion ―Eeta, as shown in Figure - 3.4.

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Figure 3.4 – Eeta

Now, for one period of Sine function the number of impulses comes out to

be 80 i.e. 40 impulses for each half cycle. Thus for each half cycle we have

40 values of the product function of Sine and impulse train function. These

40 values are separated by a time period of 250 micro seconds.

The next step is to adjust the duty cycle of these 40 values or impulses. To

make the resultant PWM a function of Sine, the duty cycle at any point is

equal to the value of ―eeta‖, at that point.

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In this way, the SPWM can be achieved through Impulse-Sine Product

technique. The calculations are done in MATLAB.

3.1.2.1.3 Microcontroller Programming

The main issue in such type of programming is a precise delay of time in

the order of micro-seconds. There are two methods of producing time

delays in a microcontroller that are as under:

Through nested loops

Through built-in timers

The former method is an approximated method for generating delays,

whereas the latter one is an accurate method[2]. Therefore, for the SPWM

programming, we have relied on the second method.

All members of 8051 family have at-least two 16-bit built-in timers, that is

Timer 0 and Timer 1. These timers are used for producing an accurate time

delay. In 8051 family, the timers are incremented after 12 oscillator cycles.

This means that if a 12 MHz crystal is used, then the timer will be

incremented 1 million times in a second.

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Now, as the timers are 16-bit so if the timer has an initial value of 0 then

after 65.35 ms, the timer will reach its maximum value and will overflow,

causing an overflow flag to become high. Actually, this behavior is utilized

for creating the accurate time delay.

To generate the time delay

The required initial value is calculated.

The overflow flag is cleared.

The value is loaded into the timer, and the timer is started.

The timer will be incremented, on a rate depending upon the

frequency of the crystal.

The timer will indicate the end of the delay, by setting the overflow

flag.

3.1.2.1.3.1 Important Definitions

TMOD SFR

This special function register has four, four bits for both timers. Timer one

has 4 MSB bits, whereas the Timer zero has the LSB bits. Out of each

timer‘s 4 bits, the two LSBs must either have decimal 1 or 2 loaded in them

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for hardware delays. The two MSBs must be cleared as they indicate the

Gate and Counter mode which are not required in hardware delays.

THx, TLx, TFx & TRx

THx & TLx are the two 8-bit registers associated with each timer. TFx is the

overflow flag related to each of two timers. TRx must be given value of 1 to

start a timer.

Timer 1 & 0 have respectively, 1 & 0 in place of x.

ET0 & ET1

These are respectively given zero value for having no interrupts in timer 0

and timer 1.

3.1.2.2.4 Microcontroller Detailed Circuit

The detailed circuit diagram of the microcontroller circuit is shown in Figure

3.5. The other components include a 7805 voltage regulator IC. For

Buffering purposes, 74245 IC is used. And a blinking LED is used to

indicate the proper operation of microcontroller.

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Figure 3.5 – Microcontroller Detailed Circuit

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3.1.2.3 H- Bridge

H-Bridge is used to give bidirectional output from a unidirectional input. An

H-bridge has applications in robotics where it is used for running the DC

motors in forward and backward directions. An H–bridge consists of four

switches, a supply source and the load. The switches are turned ON in

pairs of two, in one cycle. While the remaining two switches must be at

OFF position in that cycle, otherwise a short circuit would occur.

The simplified diagram of the H-bridge is given as:

Figure 3.6 –A Simple H-Bridge

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It is very clear from the Figure 3, that the direction of the output can be

controlled by turning ON two switches at a time, while keeping the other

two OFF. The directions can be shown through the given table.

Table 3.1 – Switching of a Simple H-Bridge

Switches at ON position OUTPUT DIRECTION

S 1 S 3 From left to right

S 2 S 4 From right to left

S 1 S 2 Not allowed (Due to occurrenceS 3 S 4

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Proposed H-bridge Design

The main thing in the design is that the H-bridge will work on more than

300 Volts. The circuit diagram is shown in Figure 3-7.

The MOSFETs used are IRF 740. They are rated 400V and 10A. Opto-

isolators (4N35) are used to optically isolate the microcontroller circuit from

the Power H-bridge circuit. 555 Timer IC is used for forced commutation

and removal of inherent inversion that occurs due to the use of opto-

isolators.

The capacitors, that e used with the upper two MOSFETs, act as a dummy

battery; so as the potential difference across the gates of the MOSFETs is

greater than the potential difference across Drain with respect to the

Source.

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Figure 3.7 – H-Bridge Circuit Diagram

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3.1.2.4 Filter

There are a lot of high frequency components in the output, due to high

frequency switching. Therefore, filtering is required to remove such high

frequency components.

3.1.2.4.1 Low Pass Filter

A low pass filter can be used to effectively filter out undesirable high

frequency components. A low pass filter can be made by connecting an

inductor in series, and a capacitor in parallel. As the frequency is

increased, the inductive reactance of the inductor is increased, so there is a

hindrance in the flow of high frequency components of current. Similarly,

the capacitive reactance decreases with the increase in frequency and as

the capacitor is connected in parallel so the high frequency components

are given with a by-pass path.

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A typical low pass filter is shown in the Figure 3.8.

Figure 3.8 – Low Pass Filter

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3.1.2.4.2 Single-Tuned Filter

This filter presents very low impedance, for a single resonating frequency.

The impedance for all other frequencies is very high, so only a single tuned

frequency is allowed to pass. This filter is made by connecting the inductor

and the capacitor, in series with load, as shown in Figure 3.9.

Figure 3.9 – Single Tuned Filter

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The resonating frequency is given as:

Where,

f = Resonating frequency.

L = Inductance of Inductor.

C = Capacitance of Capacitor.

In this project, single-tuned filter is used.

3.1.2.4.3 Calculations

The frequency is 50 Hz that should be allowed to pass. The capacitance

should be as minimum as possible, because with the high capacitance

there is more chance of over voltages in the circuit.

Thus, to minimize the capacitance, the inductance shall be as high as

possible. The maximum value of inductance, available in laboratory is

100mH.

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For these values, the capacitance comes out to be 101.23F, through the

previously mentioned formula.

3.1.3 Control Circuit

The control circuit mainly consists of relays for the overall control of the

circuit. There is a principle relay for the switching between main supply and

the UPS supply. The actuating circuit of this relay is energized by the

output of the transformer, that is used for the charging of the battery. Two

other relays are used for disconnecting the charging circuit, when the

battery has been charged completely and turning ON/OFF the inverter

circuit.

The control circuit also controls two panels of LEDs. Both the panels are

displayed on the outer side of the casing, to indicate various operations and

situations in the UPS. There is a buzzer that will beep, once the battery has

reached to the minimum level of charging.

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First panel of LEDs indicate various levels of charging of the battery. The

LEDs in the second panel show:

Main Supply

UPS Supply

Charging of battery

Full charge of battery

Microcontroller working properly.

The control circuit along with the microcontroller circuit is shown in the

Figure 3.10.

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Figure 3.10 - Control and Microcontroller Circuit.

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CHAPTER 4Simulation & Results

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Chapter 4Simulation & Results

The chapter includes the simulation results and practical results of the

project. The goal was to create output waveform with less harmonic content

i.e. near to the sine wave. Firstly the Simulation results are given and

secondly the practical results.

4.1 SIMULATIONS

The software used for simulation purpose is ―Proteus‖. 4.1.1 PWM Signals

The code was made on the ―Keil‖. Once the errors were removed, the

software created the hex file from the code. This hex file is then used in the

Proteus for simulations. On Proteus ATML 89C51 is selected. Various

settings, such as crystal frequency, were also set. The hex file created was

loaded in the microcontroller. The microcontroller‘s corresponding output

was checked on a virtual CRO. The frequency of the output was verified

from the simulation results, by fixing the output waveform. The simulation

results are shown in the Figures 4.1 & 4.2.

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Figure 4.1 - Digital Oscilloscope Output

The PWM signals are enlarged in the figure below. These are the gating

signals which are to be given on the gates of MOSFETs of the H-bridge.

Owing to the fact that duty cycle, of PWM, is varying according to the Sine

function; the output of the bridge will be a sine wave under load.

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Figure 4.2 – PWM Simulation (Enlarged)

4.2 RESULTS

4.2.1 PWM Signals

Figure 4.3 shows the PWM signals on the CRO. It can be easily depicted

from the figure that the two signals comes alternately. Also both of them

increase in width while going towards their centers and their width

decreases while going towards their margins, which indicates that it‘s a

correct sinusoidal pulse width modulation.

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Figure 4.3 - PWM Signals on CRO

In the figure 4.4 the scale of the CRO is readjusted, to enlarge the PWM

signal. Again it is seen that the signals increases in width while going

towards their centers and their width decreases while going towards their

margins.

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Figure 4.4 - PWM Signals on CRO (Enlarged)

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4.2.2 Hardware Various circuits and hardware are shown as under:

Figure 4.5 – Control & Microcontroller Circuit

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Figure 4.6 – H- Bridge

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Figure 4.7 – DC – DC Converters

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Figure 4.8 – UPS with all components assembled.

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4.2.3 Output Voltage Waveform

Figure 4.9 shows the output voltage waveform when a small load is

connected

Figure 4.9 - Output with a Small Load connected

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The voltage waveform with the increased load, is shown in Figure 4.10.

It can be seen that with the increase in the load, within prescribed limits,

the waveform will become more and closer to sine wave

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Conclusion

We started the project with the objective to practically design, fabricate and

test a Sine wave UPS. We have successfully achieved our objective, with

some discrepancies, especially in the testing part. We have implemented

the proposed design of the UPS. There were many difficulties. Hardware of

the H-bridge created huge problems. We gave a lot of time, to bring it to the

working condition. Programming was tough, but did not create large

problems. Initially, the microcontroller did give some problems, but after

that it was all right. It was noted that several problems were only due to the

fault in the breadboard. The DC-DC converter also showed minor

problems, but eventually we managed to create a sine wave, that was our

ultimate target.

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Future Recommendations

Due to lack of much time, and unavailability of high valued inductor we did

not practically filter the output. We, therefore, recommend to practically

filter the output in future.

We focused on creating a sine wave, the power rating was not our focus.

We recommend that the power rating of the given design be increased.

That can be achieved by increasing the rating of DC-DC converter.

We also recommend that the DC-DC converter be designed, from scratch,

by the students.

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References

1. A.Ali Qazalbash, Awais Amin, Abdul Manan and Mahveen Khalid, ―Design and Implementation of microcontroller based PWM technique for Sine Wave inverter‖ in POWERENG 2009, Lisbon, Portugal, March 18-20, 2009

2. Michael J. Pont, Embedded C, Pearson Education limited, 2002, ISBN 0-201-79523-X, Page 116

3. Transistor‖,available on-line at http://en.wikipedia.org/wiki/Transistors

4. Muhammad H. Rashid, Power Electronics: Circuits, Devices and Applications

5. Thomas L. Floyd, Electronic Devices

6. Francisco C. De La Rosa, Harmonics and Power Systems

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