sop midsem report

7/31/2019 SOP Midsem Report

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STUDY ORIENTED PROJECT

APPLICATIONS OF POWER ELECTRONICS

(POWER FACTOR CORRECTION)

BY: G.SRITEJA REDDY

2009AAPS071H

UNDER THE GUIDENCE OF MRS.MADHURI BAYYA

BITS-PILANI HYDERABAD CAMPUS


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ACKNOWLEDGEMENT

I would like to extend my gratitude and sincere thanks to my project guide

Mrs.Madhuri Bayya madam, Department of Electrical Engineering for valuable

guidance and continuous supervision. I would like to express my special

gratitude and thanks to Mr.U.Madhava Rao Sir, for teaching me the concepts

of power electronics. My special thanks for my parents and my friends for their

continuous encouragement and support.


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ABSTRACT

Power Factor, the ratio between the real or average power and apparent

power forms a very essential parameter in power system. It is indicative of

how effectively the real power of the system has been utilized.

With rapid development in power semiconductor devices, the usage of power

electronic systems has expanded to new and wide application range that

include residential, commercial, aerospace and many others. Power electronic

interfaces e.g. switch mode power supplies (SMPS) have proved to be superior

over traditional linear power supplies. However, their non-linear behaviour

puts a question mark on their high efficiency. The current drawn by the SMPSs

from the line is distorted resulting in a high Total Harmonic Distortion (THD)

and low Power Factor (PF).

Hence, there is a continuous need for power factor improvement and

reduction of line harmonics. This project aims at studying different possible

power factor correction circuits and comparing their efficiencies.

In the second part of the project a DC television circuit has been designed. The

simulation part of the circuits is attached.


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Table of Contents

1. Introduction1.1 Power Factor1.2 Harmonics1.3 Effect of harmonics on power quality.

2. Power Factor correction2.1 Sources of poor power factor2.2 Energy balance in PFC circuits2.3 Passive and Active PFC converters

3. Role of DC-DC Converters3.1 Basic Circuit topologies for Active Power factor correctors3.2 Boost Converter3.3 Buck-Boost Converter3.4 Boost Converter for Power Factor Correction

4. Control Principles of DC-DC Converters4.1 Peak current control4.2 Average control4.3 Hysteresis control4.4 Borderline control4.5 Discontinuous current control

5. Modified Circuits5.1 Three-Level Boost power factor correction converter5.2 Modified Buck-Boost converter


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6. DC Electrical Systems6.1 Television6.2 PSpice Simulation

Conclusion

List of References


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CHAPTER 1: INTRODUCTION

1.1 Power Factor:

Power factor is defined as the cosine of the angle between voltage and current

in an ac circuit. There is generally a phase difference between voltage and

current in an ac circuit. cos is called the power factor of the circuit. If the

circuit is inductive, the current lags behind the voltage and power factor is

referred to as lagging. However, in a capacitive circuit, current leads the

voltage and the power factor is said to be leading.

In a circuit, for an input voltage V and a line current I,

VIcos the active or real power in watts or kW.

VIsin - the reactive power in VAR or kVAR.

VI- the apparent power in VA or kVA.

Power Factor gives a measure of how effective the real power utilization of the

system is. It is a measure of distortion of the line voltage and the line current

and the phase shift between them.

Power Factor=Real power (Average)/Apparent power

Where, the apparent power is defined as the product of rms value of voltage

and current

Linear Systems:

In a linear system, the load draws purely sinusoidal current and voltage, the

current and voltage; hence the power factor is determined only by the phase

difference between voltage and current.

i.e. PF=cos


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Power Electronic Systems:

In power electronic system, due to the non-linear behaviour of the active

switching power devices, the phase angle representation alone is not valid. A

non-linear load draws typical distorted line current from the line. The PF of

distorted waveforms is calculated as below:

The Fourier representation for line current is and line voltage vs are given by,

is = IDC+ Isnsin(nt+)

vs=VDC+Vsnsin(nt+)

The line current is non-sinusoidal when the load is nonlinear. For sinusoidal

voltage and non- sinusoidal current the PFcan be expressed as

Kp

Where, cos is the displacement factor of the voltage and current. Kp is the

purity factor or the distortion factor.

VrmsI1rms

VrmsIrms

______Cos Cos =

I1rms_____

Irms= Kp cos

FIG 1: a) Waveforms of input current and voltage b) harmonics in input current


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Another important parameter that measures the percentage of distortion is

known as the current total harmonic distortion (THDi) which is defined as

follows:

Hence the relation between Kpand THDiis

1.2 Harmonics:

Switching converters of all types produce harmonics because of the non-linear

relationship between the voltage and current across the switching device.

Harmonics are also produced by conventional equipment including:

1) Power generation equipment (slot harmonics).2) Induction motors (saturated magnetics).3) Transformers (over excitation leading to saturation).4) Magnetic-ballast fluorescent lamps (arcing).5) AC electric arc furnaces.

All these devices cause harmonic currents to flow and some devices, actually

directly produce voltage harmonics.

1.3. Effects of harmonics on power quality:

The contaminative harmonics can decline power quality and affect system

performance in several ways:


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1) Conductor loss and iron loss in transformers increase due to harmonicsdecreases the transmission efficiency and causes thermal problems.

2) The odd harmonics in a three phase system overload of the unprotectedneutral conductor.

3) High peak harmonic currents may cause automatic relay protectiondevices to mistrigger.

4) Excessive current in the neutral conductor of three-phase four-wiresystems, caused by odd triple-n current harmonics (triple-n: 3rd, 9th,

15th, etc.). This leads to overheating of the neutral conductor and

tripping of the protective relay.

5) Telephone interference and errors in metering equipment.6) The line rms current harmonics do not deliver any real power in watts to

the load, resulting in inefficient use of equipment capacity (i.e. low

power factor).

7)

Harmonics could cause other problems such as electromagneticinterference to interrupt communication, degrading reliability of

electrical equipment, increasing product defective ratio, insulation

failure, audible noise etc.

CHAPTER 2: POWER FACTOR CORRECTION:

2.1 Sources of poor PF:

Poor power factor caused by reactive linear circuit elements results as the

current either leads or lags the voltage, depending on whether the load looks

capacitive or inductive.

In most off-line power supplies, the AC-DC front end consists of a bridge

rectifier followed by a large filter capacitor.


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In this circuit, current is drawn from the line only when the peak voltage on the

line exceeds the voltage on the filter capacitor. Since the rate of rise and fall of

current is greater than that of line voltage, and the current flows

discontinuously, a series of predominantly odd harmonics are generated.

It is these harmonics that cause problems with the power distribution system.

The power factor of the system can be improved slightly by either adding

series inductance with the line or decreasing the value of the holdup capacitor,

which will lengthen the conduction angle.

FIG 2(a) Traditional poor power factorcurrent either leads or lags the voltage

FIG 2(b) : Improvement of power factor


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However, both these methods severely limit the amount of power that can be

drawn from the line.

2.2 Energy Balance in PFC circuits:

Let vl(t) and il(t) be the line voltage and line current respectively. For an ideal

PFC unit (PF=1), we assume

Vl(t)=Vlmsin lt

Il(t)= Ilmsin lt

where Vlm and Ilm are the amplitudes of line voltage and line current

respectively. The instantaneous input power contains the real power (average

power) component and an alternative component with frequency 2l.The

working principle of a PFC circuit is to process the input power in such a way

that it stores the excessive input energy when instantaneous power Pin is

greater than the power demanded Po. The excessive input energy, wex(t) is

given by

The excessive input energy is stored in the dynamic components (inductor and

capacitor) of the PFC circuit.

2.3 Passive and Active PFC Correctors

2.3.1 Passive PFC

Harmonic current can be controlled in the simplest way by using a filter that

passes current only at line frequency (50 or 60 Hz).Harmonic currents are

suppressed and the non-linear device looks like a linear load. Power factor can


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be improved by using capacitors and inductors i.e. passive devices. Such filters

with passive devices are called passive filters.

Disadvantages:

1) They require large value high current inductors which are expensive andbulky. A passive PFC circuit requires only a few components to increase

efficiency, but they are large due to operating at the line power

frequency.

2) Only less than 0.9 PF can be achieved.3) THD is high.4) The output is unregulated and sensitive to circuit parameters.5) Optimization of the design is difficult.

2.3.2 Active PFC:

An active approach is the most effective way to correct power factor of

electronic supplies. Here, we place a DC-DC converter (boost converter)

FIG 3: Series tuned LCharmonic filter PF corrector.


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between the bridge rectifier and the main input capacitors. The converter tries

to maintain a constant DC output bus voltage and draws a current that is in

phase with and at the same frequency as the line voltage.

Working principle:

The incoming line voltage passes through a bridge rectifier that produces a full

wave rectified output. No current flows into the holdup capacitor unless the

line voltage is boosted above the voltage present in the holdup capacitor. This

allows the control circuit to adjust the boost voltage to maintain a sinusoidal

input current. The control circuit uses the input voltage waveform as a

template, to maintain a sinusoidal input current.

Hence,

The control circuit:

1) Measures the input current, compares it to the input voltage waveform, and

adjusts the boost voltage to produce an input current waveform of the sameshape.

2) It monitors the bus voltage and adjusts the boost voltage to maintain a

coarsely regulated DC output.

FIG 4: Correcting the poor power factor associated with electronic power supplies

requires an active approach in which a control circuit adjusts a boost voltage to

maintain a sinusoidal input current.


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Active PFC Functions:

1) Active wave shaping of the input current.2) Filtering of the high frequency switching.3) Feedback sensing of the source current for waveform control.4)Feedback control to regulate output voltage.

CHAPTER 3: ROLE OF DC-DC CONVERTERS

Power electronic converters are essentially required when we need to convert

electricity from one form to other. They form an interface between the source

and load side.

In the last several years, the massive use of single phase power converters has

increased the problems of power quality in electrical systems.

High-frequency active PFC circuit are preferred for power factor correction.

Any DC-DC converters can be used for this purpose, if a suitable control

method is used to shape its input current or if it has inherent PFC properties.

The DC-DC converters can operate in Continuous Inductor Current Mode

CICM, where the inductor current never reaches zero during one switching

cycle or Discontinuous Inductor Current Mode - DICM, where the inductor

current is zero during intervals of the switching cycle.

In CICM, different control techniques are used to control the inductor current.

Some of them are (1) peak current control (2) average current control (3)

Hysteresis control (4) borderline control.


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3.1 Basic circuit topologies of Active Power Factor Correctors

Many circuits and control methods using switched-mode topologies have been

developed. The active PFCs employ six basic converter topologies

1) Buck Corrector

2) Boost Corrector

3) Buck-Boost corrector

4) Cuk, Sepic and Zeta Correctors

We go for boost corrector which is one of the most important high power

factor rectifiers from a theoretical and conceptual point of view. It is obtained

from a classical non-controlled bridge rectifier, with the addition of transistor,

diode and inductor. In this report, boost and buck-boost converters are

discussed.

3.2 Boost Converter:

It is a DC-DC converter whose output voltage is greater than input voltage. The

circuit is as shown in the figure.

FIG 5: Basic circuit of a boost converter.


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Initially when switch is open, the output voltage v0 is equal to VS. When switch

is closed, inductor charges from VS through the switch. Diode is reverse biased

and so output is isolated from input.

In steady state the time integral of the inductor voltage over one time period

must be zero,

Vd ton+ (Vd - V0)toff= 0 which gives,

Vd = (1-D)V0where D is the duty cycle and D


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Following the similar procedure of analysis as that of Boost converter, and

applying steady state analysis i.e. the time integral of inductor voltage over

one time period is zero gives,

Where, D is the duty cycle of the switch. The magnitude of output voltage

depends on the duty cycle. If DVd and if

D=0.5 gives V0=Vd.

FIG 6: Buck-Boost converter circuit


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3.4 Boost Converter for power factor correction:

It is connected between the bridge rectifier and output load.

The input current is(t) is controlled by changing the conduction state of

transistor. By switching the transistor with appropriate firing pulse sequence,

the waveform of the input current can be controlled to follow a sinusoidal

reference. The figure shows the reference inductor current iLref , the inductor

current iL, and the gate drive signal x for transistor. Transistor is ON when x = 1

and it is OFF when x =0. The ON and OFF state of the transistor produces an

increase and decrease in the inductor current iL.

FIG 7: Boost converter for power factor

Fig 8: Inductor current waveforms


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The PFC properties of a boost converter can be estimated from the given plots:

FIG 8.1: Transistor gate drive signal x.

FIG 9.1: Harmonic content of the current waveform obtained from a rectifier circuit

FIG 9.2: Harmonic content of the current waveform of a boost PFC converter


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As can be clearly seen, the higher order harmonics are considerably reduced in

the line current by using a boost converter.

CHAPTER 4: CONTROL PRINCIPLES OF DC-DC CONVERTERS:

Control strategy for an electrical system is intended to develop a set of actions

that can detect the time evolution of electrical quantities and to impose them

to follow a desired time evolution. In general, a control algorithm can be split

into three functional sub-blocks:

1) Control Algorithm- Operates to generate reference values to the feedingalgorithm on the basis of reference values imposed to the controller.

2) Feeding Algorithm- gives the voltage or current values to impose at theconsidered system in order to follow the time evolution of the reference

values coming from the control algorithm.

3) Converter control Algorithm- provides the right sequence of firingpulses for management of the power modules based on the information

derived from control and feeding algorithm.

A dc-dc converter provides a regulated dc output voltage under varying load

and input voltage conditions. The converter component values are also

changing with time, temperature and pressure. Hence, the control of the

output voltage should be performed in a closed-loop manner using principles

of negative feedback.

Control Techniques:

4.1. Peak current control:

The basic scheme of the peak current controller is shown in Fig.


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The switch is turned on at constant frequency by a clock signal, and is turned

off when the sum of the positive ramp of the inductor current (i.e. the switch

current) and an external ramp (compensating ramp) reaches the sinusoidal

current reference. This reference is usually obtained by multiplying a scaled

replica of the rectified line voltage vg times the output of the voltage error

amplifier, which sets the current reference amplitude. In this way, the

reference signal is naturally synchronized and always proportional to the line

voltage.

Converter operates in Continuous Inductor Current Mode (CICM). This means

that devices current stress and input filter requirements are reduced.

FIG 10.2: current waveform for peak current control

FIG 10: Circuit for peak current control scheme


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Advantages:

1. Constant switching frequency.2. Only the switch current must be sensed and this can be accomplished by

a current transformer, thus avoiding the losses due to the sensing

resistor.

3. No need of current error amplifier and its compensation Network.4. Possibility of a true switch current limiting.

Disadvantages:

1. Presence of sub harmonic oscillations at duty cycles greater than 50%, soa compensation ramp is needed.

2. Input current distortion which increases at high line voltages and lightload and is worsened by the presence of the compensation ramp.

3. Control is highly sensitive to commutation noises.

4.2. Average current control:

In this scheme, the inductor current is sensed and filtered by a current error

amplifier whose output drives a PWM modulator. In this way the inner current

loop tends to minimize the error between the average input current ig and its

reference. It also works in CICM.

Advantages:

1. Constant switching frequency.2. No need of compensation ramp.3. Control is less sensitive to commutation noises, due to current filtering.


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4. Better input current waveforms than for the peak current control since,near the zero crossing of the line voltage, the duty cycle is close to one,

so reducing the dead angle in input current.

Disadvantages:

1. Inductor current must be sensed.2. a current error amplifier and its compensation network is needed.

FIG 11.1: Circuit for average current control scheme

FIG 11.2: current waveform for average current control scheme


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4.3. Hysteresis control:

In this control scheme, two sinusoidal current references IPref ,IVref are

generated, one for the peak and the other for the valley of the inductorcurrent.

According to this control technique, the switch is turned on when the inductor

current goes below the lower reference and is turned off when the inductor

current goes above the upper reference, giving rise to a variable frequency

control. It works in CICM mode.

FIG 12.1: Circuit for hysteresis control scheme

FIG 12.2: current waveform for hysteresis control scheme


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Advantages:

1. No need of compensation ramp.2.

Low distorted input current waveforms.

Disadvantages:

1. Variable switching frequency.2. Inductor current must be sensed.3. Control sensitive to commutation noises.

4.4. Borderline control:

In this control approach the switch on-time is held constant during the line

cycle and the switch is turned on when the inductor current falls to zero, so

that the converter operates at the boundary between Continuous and

Discontinuous Inductor Current Mode (CICM-DICM).

The freewheeling diode is turned off softly and the switch is turned on at zero

current, so the commutation losses are reduced. The higher current peaks

increase device stresses and conduction losses and may require heavier input

filters.

The instantaneous input current is constituted by a sequence of triangles

whose peaks are proportional to the line voltage. Thus, the average input

current becomes proportional to the line voltage.


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Advantages:

1. No need of a compensation ramp.2. No need of a current error amplifier.

Disadvantages:

1. Variable switching frequency.2. Inductor voltage must be sensed in order to detect the zeroing of the

inductor current.

3. For controllers in which the switch current is sensed, control is sensitiveto commutation noises.

FIG 13.1: Circuit for borderline control scheme

FIG 13.2: current waveform for borderline control scheme


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4.5. Discontinuous current PWM control:

The internal current loop is completely eliminated, so that the switch is

operated at constant on-time and frequency.

Converter works in discontinuous conduction mode (DCM) and this control

technique allows unity power factor when used with converter topologies like

flyback, Cuk and Sepic. With the boost PFC, this technique causes some

harmonic distortion in the line current.

FIG 14: current waveform for discontinuous current control scheme


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Advantages:

1. Constant switching frequency.2. No need of current sensing.3. Simple PWM control.

Disadvantages:

1. Higher devices current stress than for borderline control.2. Input current distortion with boost topology.

CHAPTER 5. MODIFIED CIRCUITS:

5.1. Three-Level Boost power factor correction converter:

For high power or high voltage applications, the major concerns of the

conventional boost PFC converter are the inductor volume and weight, and

losses on the power devices, which affect cost, efficiency, and power-density.

A three-level boost converter uses a much smaller inductor and lower voltage

devices than the conventional boost converter yielding high efficiency and low

cost.

FIG 15: Three level Boost PFC converter.


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The voltage at the centre of the output is V0/2 which is obtained by choosing

C1=C2.

Operation Principle:

There are two regions of operation depending on whether the input voltage is

lower or higher than half of the output voltage.

Region 1 (Vin < V0/2):

In this region, boost converter charging voltage is V in and the discharging

voltage which used to be V0-Vin in a conventional boost converter, can be

chosen as V0/2-Vin.

At time t0, which is the beginning of a switching cycle, the switch S1 is turned

on and both switches are conducting. The inductor is charged with the input

voltage.

At time t1, S2 is turned off, forcing the inductor current to flow through the

bottom output capacitor C2 and the bottom diode D2. Hence, the discharging

voltage applied is V0/2-Vin.

At time t2, which is fixed at t0+Ts/2, S2 is turned on, charging the inductor with

input voltage again. At time t3, S1 is turned off and the inductor current will go

through D1, C1, and S2, discharged by V0/2-Vin again. Since the upper and lower

capacitors are alternatively used for discharging the inductor, their voltages

are theoretically balanced.


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Region 2 (Vin > V0/2):

In this region, the inductor charging voltage is V in-V0/2, and the discharging

voltage will be V0-Vin.

At time t0, which is the beginning of a switching cycle, S1 is turned on with S2

left open, the inductor current flows through S1, C2 and D2.

At time t1, S1 is turned off, forcing the inductor current to go through D 1, C1, C2

and D2. In the next half cycle, S2 repeats the above action.

FIG 16: Operation waveforms for a three level boost converter.

a) Vin < V0/2 b) Vin > V0/2


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Less Current Ripple:

In a conventional boost converter, the maximum inductor ripple which occurs

at Vin=0.5V0 is given by,

For a three level boost converter, the maximum current ripple in region 1

occurs when Vin=0.25V0 and is given by,

Clearly, inductor current ripple in three-level boost converter is one fourth of

that of conventional one. This implies for the same current ripple, three level

boost converter requires four times less inductance than the conventional one.

Higher efficiency and lower cost:

The capacitive turn-on loss is reduced eight times, assuming same output

capacitance for devices with different voltage ratings. The diode reverse

recovery losses are also reduced, since the reverse voltage is only half of the

output voltage. Therefore total switching loses are reduced.

5.2. Modified Single-Phase PFC AC-DC Buck-Boost Converter:

It operates in Discontinuous Conduction Mode (DCM).This converter has low

voltage stresses on the power devices than the conventional PFC AC-DC buck-


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boost converter. The complexity of control circuit is reduced in DCM mode

converters and the cost is reduced.

Mode 1: Switches S1 and S2 are turned on. The energy of the line source is

transferred to inductor L and the energy stored in the output capacitor C is

discharged to the load.

Mode 2: Switches S1 and S2 are turned off. The energy stored in inductor L is

released to the output capacitor C and the load.

FIG 17: Modified buck-boost power factor correction converter

FIG 17.1: Mode 1 of Modified buck-boost power factor correction converter


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Mode 3: Switches S1 and S2 are still turned off. The energy stored in inductor L

is empty at t = tk2. The energy stored in the output capacitor C is discharged to

the load.

Voltage gain of the modified buck boost converter is same as that of

conventional buck-boost converter and is given by,




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Voltage Stresses on Power Devices:

According to the operating principle, the voltage stresses on power devices S1,

S2, D1, and D2 are given as

Vs1 = Vm, Vs2 = V0, VD1 = Vm and VD2 = V0.

The conventional buck-boost converter on power devices S1 and D1 are given

as

VS1 = VD1 = ( Vm + V0).

Hence, the voltage stresses on the power devices of the modified buck-boost

converter are less than the conventional buck-boost converter.


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6. DC Electrical Systems

The application of DC distribution of electrical power has been suggested as an

efficient method of power delivery. This concept is inspired by the absence ofreactive power, the possibility of efficient integration of small distributed

generation units and the fact that, internally, many appliances operate using a

DC voltage. A suitable choice of rectifier facilitates the improvement of the

power quality as well as the power factor at the utility grid interface. Stand-by

losses can be largely reduced. However, because of the inherent danger

associated with DC voltages and currents, it is imperative that a considerable

amount of design effort is allocated for risk analysis and the conception of

protective devices

In this report, the architecture of a DC television is discussed. The simulations

for the circuits are done in PSpice.

6.1 Television:

The basic block diagram of a television is shown in the figure.

Fig 18: Block diagram of a television


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6.1.1 RF Section

This section consists of RF amplifier, mixer and local oscillator. RF amplifier is

used as a pre-amplifier for improving SNR. It amplifies the input composite

signal consisting of audio and video signals in separate frequency bands. Local

oscillator and mixer functions are usually combined in one stage called the

frequency converter.

The purpose of the tuner unit is to amplify both sound and picture signals

picked up by the antenna and to convert the carrier frequencies and their

associated bands into the intermediate frequencies and their sidebands.

The signal voltage or information from various stations modulated over

different carrier frequencies is heterodyned in the mixer with the output from

a local oscillator to transfer original information on a common fixed carrier

frequency called the intermediate frequency (IF). The standard intermediate

frequencies for the 25-B system are-Picture IF = 38.9 MHz, Sound IF = 33.4

MHz.

6.1.2 IF Amplifier Section

A short length of coaxial cable feeds tuner output to the first IF amplifier. This

section is also called video IF amplifier since composite video signal is the

envelope of the modulated picture IF signal.

The main function of this sections is to amplify modulated IF signal over its

entire bandwidth with an input of about 0.5 mV signal from the mixer to

deliver about 4 V into the video detector, requiring an overall gain of about

8000.


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6.1.3 Video Detector

Modulated IF signals after due amplification in the IF section are fed to the

video detector. The detector is designed to recover composite video signal and

to transform the sound signal to another lower carrier frequency. This is done

by rectifying the input signal and filtering out unwanted frequency

components. A diode is used, which is suitably polarized to rectify either

positive or negative peaks of the input signal. An L-C filter is used instead of

the usual RC configuration employed in ratio receiver detectors to avoid undue

attenuation of the video signal while filtering out carrier components.

6.1.4 Video Amplifier

The picture tube needs video signal with peak-to-peak amplitude of 80 to 100

volts for producing picture with good contrast. With an input of about 2 volts

from the detector, the video amplifier is designed to have a gain that varies

from 40 to 60. A contrast control is essentially the gain control of the video

amplifier. A large contrast makes the picture hard, whereas a low value leaves

it weak or soft.

6.1.5 Picture Tube

The picture tube or kinescope serves as the screen for a television receiver and

is a specialized from of cathode-ray tube. A luminescent phosphor coating

provided on the inner surface of its face plate produces light when hit by the

electrons of the fast moving beam.

For colour picture tubes the screen is formed of three different phosphors and

there are three electron beams, one for each colour phosphor. The three


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colours red, green and blue produced by three phosphors combine to produce

different colours.

The cathode is indirectly heated and consists of a cylinder of nickel that iscoated at its end with thoriated tungsten or barium and strontium oxides.

These emitting materials have low work-function and when heated permit

release of sufficient electrons to form the necessary stream of electrons within

the tube.

The grids that follow the control grid are the accelerating or screen grid and

the focusing grid. These are maintained at different positive potentials with

respect to the cathode that vary between 200 V to 600 V.

The composite video signal that is injected either at the grid or cathode of the

tube, modulates the electron beam to produce brightness variations of the

tube, modulates the electron beam to produce brightness variations on the

screen. The current in the deflection coils is modulated such that the electron

beam scans the entire screen.

Fig 18.1: Various parts of a picture tube


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6.1.6 Sound Section

Sound signal after separation from the composite signal in the video detector

is fed to intermediate frequency amplifiers for amplification.

After amplification it is given to FM demodulator for recovering the audio

signal. The output signal is proportional to the deviations from carrier

frequency. Then the signal is amplified using audio amplifiers and sent to the

loud speaker.

Typical Circuits used in the television

1.RF amplifier


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2.Audio amplifier

3.Video amplifier


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DC voltages required

The RF amplifier needs 12V supply whereas audio amplifier needs about

9V supply. The video amplifier needs 15V supply. FM demodulator needs

about 2.3V. The picture tube requires about 200V.

So a 48V DC line is suitable for a television and this voltage can be

converted to other voltages required by the circuits.

6.2 PSpice Simulation

1.Buck converterVarious DC voltages have been generated from 48V supply. To get

15V, the duty cycle of the pulse is given as 0.3125. The corresponding

inductor, capacitor, and resistor values are shown in the picture.

Fig 19: Block diagram showing DC voltages required for various circuits


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Fig 20: PSpice circuit diagram and waveforms of a buck converter


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2.Boost converterHigher voltage is generated using a boost converter. Here duty

cycle used is 0.72. This voltage is used in picture tube.

Fig 21: PSpice circuit diagram and waveforms of a boost converter


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CONCLUSION:

The main objective throughout the project has been to improve the power

factor with simultaneous reduction in input current harmonics. Power factor

correction has become essential for effective use of input power and the

circuits with Choppers provide a solution for this purpose. In this report,

traditional power factor circuits and their operational principles are discussed.

Different control schemes for the control circuit along with their advantages

and disadvantages are discussed. Finally, some modifications are done in the

traditional circuit to improve certain parameters like efficiency, reduction of

voltage and current stresses etc.


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LIST OF REFERENCES:

1. Rashid M., Power Electronics Handbook.2. Parillo.F: High performances Power Factor Correction Systems(PFC).3. Power Electronics and applications by Ned Mohan.4. Report on Three level boost power factor correction converter by

Michael T.Zhang, Lee.

5. Modified Buck-Boost converter by Lung-Sheng Yang.6. Wikipedia.7. Monochrome colour television by R.R.Gulati.

sop midsem report

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