Project Report On

“POWER FACTOR IMPROVEMENT USING BOOST CONVERTER”
Submitted by BABLOO KUMAR (U06EE508) RAJ RAKESH (U06EE542) SUBHASH REDDY (U06EE569) VIKAS KUMAR (U06EE579)

B. Tech (IV) ELECTRICAL ENGINEERING
Year 2009 -2010

Under the Guidance of Mr. M. A. MULLA

Department of Electrical Engineering Sardar Vallabhbhai National Institute of Technology, Surat.

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CERTIFICATE
This is to certify that the project report titled “Power Factor Improvement USING ”submitted by Babloo Kumar (U06EE508), Raj Rakesh (U06EE542), Subhash Reddy (U06EE569) and Vikas Kumar (U06EE579) is a record of bonafide work carried out by them, in fulfillment of the requirement for the award of the Degree of Bachelor of Technology. Date: 14-05-2010

Examiner 1: ____________

Examiner 2: ____________

Examiner 3: ____________

Examiner 4: ____________

GUIDE (Mr. M. A. Mulla)

HOD (Prof. Mrs. V. A. Shah)

ACKNOWLEDGMENT
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We would like to express our deep sense of gratitude to our guide Mr. M. A. Mulla (Lecturer, EED, SVNIT, SURAT) for his valuable guidance and motivation and for his extreme cooperation to complete the project work successfully. We would like to express our sincere respect and profound gratitude to Prof. V. A. Shah, Head of Electrical Engineering Department for supporting us and providing the facilities for the project work. We appreciate all our colleagues whose direct and indirect contribution helped a lot to accomplish this project work. We would also like to thank all the teaching and non teaching staff for cooperating with us and providing valuable advice which helped us in the completion of this project.

BABLOO KUMAR (U06EE508) RAJ RAKESH (U06EE542) SUBHASH REDDY (U06EE569) VIKAS KUMAR (U06EE579)

Abstract
Today’s commonly used power converters have a poor input power factor and rich harmonic current, which deteriorates the power line quality and may interfere with other power 2

electronic equipment. This project report is targeted on the prevailing method of power factor control in industries. The present trend is to use facts (flexible ac transmission system) devices. The static var compensator is a thyristor based facts device. To improve the input power factor of current power converters, stringent input power factor regulations such as IEC 1000 have recently been enacted. Therefore, power factor correction techniques have been very popular topics in recent years’ power electronic research. Because the addition power factor converter will increase the cost of the overall system, the integrated single-stage power factor correction techniques become attractive especially in low-power cost-effective applications.

Contents Chapters 1. Introduction……………………………………………….…………..1
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2. Power Factor 2.1. Power Factor……………………………………………………….2 2.2. Disadvantages of Low Power Factor……………………………...2 2.3. Benefits of Power Factor Correction…………………………… 3 2.4. Need for Power Factor Correction………………………………3

3. Power Factor Correction 3.1. Various Methods of Power Factor Improvement………………5 3.2. Capacitor Banks…………………………………………………5 3.3. Synchronous Condensers………………………………………..6 3.4. Thyristor Controlled Reactors…………………………………..6 3.4.1. Principle of Operation…………………………..……….8 3.5. Static VAR Compensator 3.5.1. Principle of Operation………………… …………………..11 3.5.2. Connection……….…………………………………….......11 3.5.3. Modeling and Simulation………………………………...12 3.5.4. Advantages……………………………………….………...15 4. Boost Converters 4.1. Boost Converter………………………………………………….16 4.2. Circuit Analysis…………………………………………………..16 4.3. Modes of Operation 4.3.1. Continuous Mode………………………………………….17 4.3.2. Discontinuous Mode………………………………………19 5. Power Factor Correction Using Boost Convertors 5.1. PFC Boost Pre-regulator………………………………………….21 5.2. Modes of Operation 5.2.1. Discontinuous Mode……………………………………….23 5.2.2. Continuous Mode…………………………………………23 5.3. Power Factor Correction Circuits……………………………….24 6. Current Mode Control for PFC 6.1. Average Current Control……………………………………..26 6.2. Variable Frequency Peak Current Control……………………..27 6.3. Hysteresis Control……………………………………………29

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7. MATLAB Simulations 7.1. Basic Boost Implementation……………………………………31 7.2. Power Factor Improvement By Boost Converter With Firing MOSFET By PWM……………………………….32 7.3. PFC With Boost Convertor By Firing MOSFET With Voltage and Current Closed Loop Control…………….35 7.4. Hardware Implementation of Boost Convertor Using LM3524…………………………………………………..38

Conclusion…………………………………………………………………..40 References……………………………………………………………………41

:: LIST OF FIGURES ::
FIGURE NO. 2.1 3.1 POWER FACTOR TRIANGLE PFC USING CAPACITOR BANK 1 NAME OF THE FIGURE PAGE NO. 2 5

3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.1 4.2 4.3 4.4 4.5 5.1 5.2 5.3 5.4 5.5 6.1 6.2 6.3 6.4 6.5 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10

THYRISTOR CONTROLLED REACTOR MATLAB MODEL OF THYRISTOR CONTROLLED REACTOR SIMULATION OF TCR AT FIRING ANGLE 90 SIMULATION OF TCR AT FIRING ANGLE 126 STATIC VAR COMPENSATOR MATLAB MODEL OF SVC SIMULATION OF SVC AT FIRING ANGLE 90 SIMULATION OF TCR AT FIRING ANGLE 126 BOOST CONVERTER BOOST CONVERTER CIRCUIT DIAGRAM TWO CONFIGURATIONS OF BOOST CONVERTER CURRENT AND VOLTAGE WAVEFORMS OF BOOST CONVERTER IN CCM CURRENT AND VOLTAGE WAVEFORMS OF BOOST CONVERTER IN DCM FLYBACK ACTION IN INDUCTOR PFC BOOST CONVERTER DCM OPERATION CCM OPERATION TYPICAL WAVEFORMS INA POOR PF SYSTEM BOOST PFC USING AVERAGE CURRENT CONTROL BOOST PFC USING PEAK FREQUENCY CONTROL INPUT CURRENT WAVEFORMS(ON TIME CONTROL) INPUT CURRENT WAVEFORMS(OFF TIME CONTROL) INPUT CURRENT WAVEFORMS(HYSTERESIS CONTROL) BASIC BOOST TOPOLOGY MATLAB SIMULATION FOR PFC IN BOOST TOPOLOGY USING PWM WAVEFORMS OUTPUT CURRENT AND VOLTAGE WAVEFORMS PFC USING CLOSED LOOP CONTROL WAVEFORMS FOR INPUT VOLTAGE AND CURRENT OUTPUT VOLTAGE OF BOOST CONVERTER TOTAL HARMONIC DISTORTION TOP VIEW OF SG3524N IMPLEMENTATION OF BOOST CONVERTER

7 8 9 10 12 12 13 14 16 16 17 17 19 21 22 23 24 25 27 28 28 29 30 31 32 33 34 35 36 37 37 38 39

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Chapter 1: Introduction
Power Factor Improvement is the growing issue of concern. Within power quality framework, one of the important aspects is reactive power control. Consumer load requires reactive power that varies incessantly and increases transmission losses while affecting voltage in the transmission network. To prevent unacceptably high voltage fluctuations or the power failures that can result, this reactive power must be compensated and kept in balance. This function has always been performed by passive elements such as reactors or capacitor, as well as combination of the two that supply inductive or capacitive reactive power. The more quickly and precisely the reactive power can be compensated, the more efficiently the various characteristics of transmissions can be controlled. Since most loads in modern electrical distribution systems are inductive, there is an ongoing interest in improving power factor. The low power factor of inductive loads robs a system of capacity and can adversely affect voltage level. As such, power factor correction through the application of capacitors, synchronous Alternators, TCR SVC , Power Electronic DC-DC convertors etc. is widely practiced at all system voltages. As utilities increase penalties they charge customers for low power factor, system performance will not be the only consideration. The installation of power factor correction circuits improves system performance and saves money. 2

In order to ensure most favourable condition for a supply system from engineering and economical standpoint it is important to have power factor as close to unity as possible. Linear loads with low power factor (such as induction motors) can be corrected with a passive network of capacitors or inductors. Non-linear loads, such as rectifiers, distort the current drawn from the system. In such cases, active or passive power factor correction may be used to counteract the distortion and raise power factor. The devices for correction of power factor may be at a central substation, or spread out over a distribution system, or built into power-consuming equipment.

Chapter 2: Power Factor 2. 1.Power Factor
Power Factor is the ratio between the real power (kW) and apparent power (kVA) drawn by an electrical load. It is a measure of how effectively the current is being converted into useful work output and a good indicator of the effect of the load current on the efficiency of the supply system. Poor power factor results in increase load current draw that causes additional losses in the supply and distribution systems. Power factor can also be measured as the cosine of the phase difference between the voltage and the current, however, where the current is distorted such as with electronic equipment loads, this may not be a true indication of the power factor. Power factor can be can be determined as follows: Power Factor = Active Power (kW)/Apparent Power(kVA)……………..Eqn 1.1

Fig2.1. Power Factor Triangle Power factors range from zero (0) to unity (1) with a typical power factor being between 0.8 and 0.95. The power factor can also be leading or lagging depending on whether the load is predominantly capacitive or inductive in nature. 3

Poor power factors are typically due to the effect of inductive or capacitive loads such as with a motor or with long cables providing capacitive coupling. Poor power factor due to distorted current waveforms such as with high harmonic content caused by electronic equipment cannot normally be corrected with PFC alone and will typically require complex or costly filtering.

2.2 Disadvantages of Low Power Factor

1. KVA rating of the electrical equipments increases due to low power factor as power factor is inversely proportional to the KVA rating of the equipment. This increases the size and cost of the equipment. 2. Conductor size increases. To transmit the same amount of power at low power factor at constant voltage needs to carry high current. So to keep the current density constant conductor area increases. 3. Copper loss of the equipment increases. 4. Voltage regulation becomes poor. Current at low lagging power factor causes greater voltage drop in alternators, transformers and transmission lines causing to have low power supply at the receiving end. 5. Handling capacity of the equipment decreases because the reactive component of current prevents the full utilization of the installed capacity.

2.3 Benefits of Power Factor Correction (PFC)
1. Electricity tariff savings. 2. Avoidance of Network Service Provider (NSP) penalties for low power factor, including restricted access to more suitable tariffs (minimum of 0.9 for large and high voltage supply establishments in most states). 3. Reduced losses. 4. Reduce power drawn from distribution systems, optimum sizing of electrical infrastructure. 5. Stabilized site voltage levels by reducing the inductive effect of the connected load. The payback for PFC installations can be very reasonable and should not be over looked when considering PFC for existing installations

2.4 Need for Power Factor Correction
New Works, Upgrades And Refurbishments. Power factor correction shall be provided under the following circumstances for new, upgraded or refurbished buildings: 1. To meet the NSP requirements for minimum power factor. 2

2. At defense establishments with a high voltage tariff, any new building refurbished or upgraded building with a power factor less than 0.9. 3. Where the feasibility assessment determines a worthwhile cost benefit or greenhouse reduction measure. Where assessment of the natural power factor confirms it will remain within the prescribed range (e.g. above 0.9) and it is unlikely that the facility will require PFC at a later stage, PFC or provision suitable space is not required. Where assessment of the natural power factor cannot confirm suitability, however, there is some uncertainty as to the need for PFC, the PFC equipment may be omitted provided adequate space is allowed in the design of the building to incorporate PFC equipment as a future requirement. This would also apply to buildings where it would be reasonable to assume that PFC may be required at a later stage. When allowing for future PFC installations the designer shall make all practical provisions for the installation and connection of the future PFC equipment. Comprehensive Maintenance Contract (CMC) or Comprehensive Maintenance Services (CMS) contractor or design consultant shall monitor buildings not provided with PFC during the defects liability period to confirm suitable power factor performance. Where the performance is found to be unsuitable during the defects liability period, PFC shall be installed and commissioned prior to completion of the project. Existing Installations PFC shall be considered for existing buildings to comply with the NSP requirements for minimum power factor to avoid disconnection of supply, costly penalties, tariff restrictions or where the feasibility assessment determines a worthwhile cost benefit or greenhouse reduction measure. The funding of power factor correction works for existing buildings shall be in accordance with regional funding and prioritising arrangements. New Equipment Equipment performance, both individual performance and the cumulative effect of non PFC equipment needs to be considered as part of the design and also for equipment specifications. Ensure that all equipment meets appropriate standards for harmonic content and that the equipment power factor performance is considered to avoid the need for PFC or expensive filtering in the first instance.

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Chapter 3: Power Factor Correction 3.1. Various Methods of Power Factor Improvement
Industrial loads, which normally operate at poor power factor, are induction motor, arc and induction furnaces. Fluorescent tubes, fans etc also operate at low value of power factor. All these loads working at low power factors need large amount of reactive power which results in reduced voltage level at the load terminals. A low voltage at consumer terminals is undesirable as it leads to the impaired performance of their utility devices. The various methods of power factor improvement are as under: 1. Use of capacitor banks. 2. Use of synchronous condensers. 3. Use of thyristor controlled devices. 4. Use of DC-DC converters

3.2. Capacitor Banks
A bank of capacitors is connected across the load. Since the capacitor takes leading reactive power, overall reactive power taken from the source decreases, consequently system power factor improves.

Fig3.1. PFC Using Capacitor Bank 2

Advantages of using capacitor banks 1. They have low losses. 2. They require little or no maintenance as there is no rotating parts. 3. They can be easily installed as they are light and do not require foundation. 4. They can work under ordinary atmospheric condition. Disadvantages of using capacitor banks 1. They have short life span of 8-10 years. 2. They get easily damaged if exceed the rated value. 3. Once damaged, they have to be removed as their repairing is uneconomical.

3.3. Synchronous Condensers
In electrical engineering, a synchronous condenser (sometimes synchronous compensator) is a specialized synchronous motor whose shaft is not attached to anything, but spins freely. Its purpose is not to produce mechanical power, as other motors do, but to adjust electrical conditions on the local electric power distribution grid. Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed to support the grid's voltage or to maintain the grid's power factor at a specified level. The condenser’s installation and operation are identical to large electric motors. Increasing the device's field excitation results in its furnishing magnetizing power (kVAR) to the system. Its principal advantage is the ease with which the amount of correction can be adjusted. The energy stored in the rotor of the machine can also help stabilize a power system during short circuits or rapidly fluctuating loads such as electric arc furnaces. Large installations of synchronous condensers are sometimes used in association with high-voltage direct current converter stations to supply reactive power. Advantages and Disadvantages Unlike a capacitor bank, the value of reactive power can be continuously adjusted. However, the synchronous condenser does have higher losses than a static capacitor bank. The motor windings are thermally stable to short circuit current and faults can be easily removed. They produce noise and have high maintenance cost. Most synchronous condensers connected to electrical grids are rated between 20 MVAR and 200 MVAR and are hydrogen cooled.

3.4. Thyristor Controlled Reactors
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Static thyristor controlled reactors are connected in parallel with load for the control of reactive power flow. With increase in the size of industrial connected loads, fast reactive power compensation has become necessary. For such loads, thyristor controlled reactors are now becoming increasingly popular.

Fig.3.2. Thyristor Controlled Reactor

3.4.1.

Modelling and Simulation of TCR

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Fig.3.3. Matlab Model of Thyristor Controlled Reactor

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Fig.3.4. Simulation Result at Firing Angle 90 Degrees

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Fig.3.5. Simulation Result at Firing Angle 126 Degrees

3.5. Static VAR Compensator
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A Static VAR Compensator (or SVC) is an electrical device for providing fast-acting reactive power compensation on high-voltage electricity transmission networks. SVCs are part of the Flexible AC transmission system device family, regulating voltage and stabilizing the system. The term "static" refers to the fact that the SVC has no moving parts (other than circuit breakers and disconnects, which do not move under normal SVC operation). Prior to the invention of the SVC, power factor compensation was the preserve of large rotating machines such as synchronous condensers. The SVC is an automated impedance matching device, designed to bring the system closer to unity power factor. If the power system's reactive load is capacitive (leading), the SVC will use reactors to consume VARs from the system, lowering the system voltage. Under inductive (lagging) conditions, the capacitor banks are automatically switched in, thus providing a higher system voltage. They also may be placed near high and rapidly varying loads, such as arc furnaces, where they can smooth flicker voltage. 3.5.1. Principle of Operation Typically, a SVC comprises a bank of individually switched capacitors in conjunction with a thyristor-controlled air- or iron-core reactor. By means of phase angle modulation switched by the thyristors, the reactor may be variably switched into the circuit, and so provide a continuously variable MVAr injection (or absorption) to the electrical network. In this configuration, coarse voltage control is provided by the capacitors; the thyristor-controlled reactor is to provide smooth control. Smoother control and more flexibility can be provided with thyristor-controlled capacitor switching. The thyristors are electronically controlled. Thyristors, like all semiconductors, generate heat, and deionized water is commonly used to cool them. Chopping reactive load into the circuit in this manner injects undesirable odd-order harmonics, and so banks of high-power filters are usually provided to smooth the waveform. Since the filters themselves are capacitive, they also export MVARs to the power system. 3.5.2. Connection Generally, static VAR compensation is not done at line voltage; a bank of transformers steps the transmission voltage (for example, 230 kV) down to a much lower level (for example, 9.5 kV).This reduces the size and number of components needed in the SVC, although the conductors must be very large to handle the high currents associated with the lower voltage.

Fig.3.6. Static VAR Compensator 2

3.5.3. Modelling and Simulation

Fig.3.7. Matlab Model of SVC

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Fig.3.8. Simulation Result at Firing Angle 90 Degrees

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Fig.3.9. Simulation Result at Firing Angle 126 Degrees

3.5.4. Advantages
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The main advantage of SVCs over simple mechanically-switched compensation schemes is their near-instantaneous response to changes in the system voltage. For this reason they are often operated at close to their zero-point in order to maximize the reactive power correction they can rapidly provide when required. They are in general cheaper, higher-capacity, faster, and more reliable than dynamic compensation schemes such as synchronous condensers.

Chapter 4: Boost Converters 4.1. Boost Converter
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A boost converter (step-up converter) is a power converter with an output DC voltage greater than its input DC voltage. It is a class of switching-mode power supply (SMPS) containing at least two semiconductor switches (a diode and a transistor) and at least one energy storage element. Filters made of capacitors (sometimes in combination with inductors) are normally added to the output of the converter to reduce output voltage ripple. A boost converter is sometimes called a step-up converter since it “steps up” the source voltage. Since power (P = VI) must be conserved, the output current is lower than the source current.

Fig.4.1. Boost Convereter

4.2 Circuit Analysis
Operating principle

The key principle that drives the boost converter is the tendency of an inductor to resist changes in current. When being charged it acts as a load and absorbs energy (somewhat like a resistor), when being discharged, it acts as an energy source (somewhat like a battery). The voltage it produces during the discharge phase is related to the rate of change of current, and not to the original charging voltage, thus allowing different input and output voltages.

Fig.4.2. Boost Converter Circuit Diagram The basic principle of a Boost converter consists of 2 distinct states: ➢ In the On-state, the switch S is closed, resulting in an increase in the inductor current. ➢ In the Off-state, the switch is open and the only path offered to inductor current is through the fly back diode D, the capacitor C and the load R. This result in transferring the energy accumulated during the On-state into the capacitor. ➢ The input current is the same as the inductor current as can be seen . So it is not discontinuous as in the buck converter and the requirements on the input filter are relaxed compared to a buck converter.

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Fig.4.3. Two Configurations Of A Boost Converter, Depending On The State Of Switch S 4.3. Modes of Operation
There are basically two modes of operation. 1. Continuous Mode 2. Discontinuous Mode.

4.3.1 Continuous Mode

Fig.4.4 Voltage and Current Waveforms of Boost Converter Operating In Continuous Mode When a boost converter operates in continuous mode, the current through the inductor (IL) never falls to zero. Above figure shows the typical waveforms of currents and voltages in a converter operating in this mode. The output voltage can be calculated as follows, in the case

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of an ideal converter (i.e. using components with an ideal behaviour) operating in steady conditions: During the On-state, the switch S is closed, which makes the input voltage (Vi) appear across the inductor, which causes a change in current (IL) flowing through the inductor during a time period (t) by the formula:

At the end of the On-state, the increase of IL is therefore:

D is the duty cycle. It represents the fraction of the commutation period T during which the switch is ON. Therefore D ranges between 0 (S is never on) and 1 (S is always on). During the Off-state, the switch S is open, so the inductor current flows through the load. If we consider zero voltage drop in the diode, and a capacitor large enough for its voltage to remain constant, the evolution of IL is:

Therefore, the variation of IL during the Off-period is:

As we consider that the converter operates in steady-state conditions, the amount of energy stored in each of its components has to be the same at the beginning and at the end of a commutation cycle. In particular, the energy stored in the inductor is given by:

So, the inductor current has to be the same at the start and end of the commutation cycle. This means the overall change in the current (the sum of the changes) is zero:

Substituting

and

by their expressions yields:

This can be written as: …………………Eqn 7.1 This in turns reveals the duty cycle to be: …………………Eqn 7.2

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From the above expression it can be seen that the output voltage is always higher than the input voltage (as the duty cycle goes from 0 to 1), and that it increases with D, theoretically to infinity as D approaches 1. This is why this converter is sometimes referred to as a step-up converter.

4.3.2 Discontinuous mode

Fig.4.5 Voltage and Current Waveforms of Boost Converter In Discontinuous Mode In some cases, the amount of energy required by the load is small enough to be transferred in a time smaller than the whole commutation period. In this case, the current through the inductor falls to zero during part of the period. Although slight, the difference has a strong effect on the output voltage equation. It can be calculated as follows: As the inductor current at the beginning of the cycle is zero, its maximum value DT) is (at t =

During the off-period, IL falls to zero after δT:

Using the two previous equations, δ is:

The load current Io is equal to the average diode current (ID). As can be seen on figure 4, the diode current is equal to the inductor current during the off-state. Therefore the output current can be written as:

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Replacing ILmax and δ by their respective expressions yields:

………..Eqn 7.3 Therefore, the output voltage gain can be written as flow: ………………………………...….Eqn 7.4 Compared to the expression of the output voltage for the continuous mode, this expression is much more complicated. Furthermore, in discontinuous operation, the output voltage gain not only depends on the duty cycle, but also on the inductor value, the input voltage, the switching frequency, and the output current.

Chapter 5: Power Factor Correction Using Boost Convertors 5.1. PFC Boost Pre-regulator
Boost converter topology is used to accomplish this active power-factor correction in many discontinuous/continuous modes. The boost converter is used because it is easy to implement and works well. The simple circuit in the below Figure is a short refresher of how inductors can produce very high voltages. Initially, the inductor is assumed to be uncharged, so the voltage VO is equal to VIN. When the switch closes, the current (IL) gradually increases through it linearly since:

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Fig.5.1. Flyback Action of Inductor Voltage (VL) across it increases exponentially until it stabilizes at VIN. Notice the polarity of the voltage across the inductor, as it is defined by the current direction (inflow side is positive). When the switch opens causing the current to change from Imax to zero (which is a decrease, or a negative slope). Looking at it mathematically:

Or L times the change in current per unit time, the voltage approaches negative infinity (the inductor reverses polarity).Because the inductor is not ideal, it contains some amount of series resistance, which loads this “infinite” voltage to afinite number. With the switch open, and the inductor dis-charging, the voltage across it reverses and becomes additive with the source voltage VIN. If a diode and capacitor were connected to the output of this circuit, the capacitor would charge to this high voltage (perhaps after many switch cycles). This is how boost converters boost voltage, as shown in Figure below.

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Fig.5.2. PFC Boost Pre-regulator The input to the converter is the full-rectified AC line voltage. No bulk filtering is applied following the bridge rectifier, so the input voltage to the boost converter ranges (at twice line frequency) from zero volts to the peak value of the AC input and back to zero. The boost converter must meet two simultaneous conditions: 1. The output voltage of the boost converter must be set higher than the peak value (hence the word boost) of the line voltage (a commonly used value is 385VDC to allow for a high line of 270VACrms). 2. The current drawn from the line at any given instant must be proportional to the line voltage. Without using power factor correction a typical switched mode power supply would have a power factor of around0.6, therefore having considerable odd-order harmonic distortion (sometimes with the third harmonic as large as the fundamental). Having a power factor of less than 1 along with harmonics from peaky loads reduces the real power available to run the device. In order to operate a device with these inefficiencies, the power company must supply additional power to make up for the loss. This increase in power causes the power companies to use heavier supply lines, otherwise self-heating can cause burnout in the neutral line conductor. The harmonic distortion can cause an increase in operating temperature of the generation facility, which reduces the life of equipment including rotating machines,cables, transformers, capacitors, fuses, switching contacts, and surge suppressors. Problems are caused by the harmonics creating additional losses and dielectric stresses in capacitors and cables, increasing currents in windings of rotating machinery and transformers and noise emissions in many products, and bringing about early failure of fuses and other safety components. They also can cause skin effect, which creates problems in cables, transformers, and rotating machinery. This is why power companies are concerned with the growth of SMPS, electronic voltage regulators, and converters that will cause THD levels to increase to unacceptable levels. Having the boost preconverter voltage higher than the input voltage forces the load to draw current in phase with the ac main line voltage that, in turn, rids harmonic emissions.

5.2. Modes of Operation
There are two modes of PFC operation: 1. Discontinuous mode 4

2. Continuous mode

5.2.1 Discontinuous Mode

Discontinuous mode is when the boost converter’s MOSFET is turned on when the inductor current reaches zero, and turned off when the inductor current meets the desired input reference voltage. In this way, the input current waveform follows that of the input voltage, therefore attaining a power factor of close to 1.

Fig.5.3. Discontinuous Mode of Operation Discontinuous mode can be used for SMPS that have power levels of 300W or less. In comparison with continuous mode devices, discontinuous ones use larger cores and have higher I2R and skin effect losses due to the larger inductor current swings. With the increased swing a larger input filter is also required. On the positive side, since discontinuous mode devices switch the boost MOSFET on when the inductor current is at zero, there is no reverse recovery current (IRR) specification required on the boost diode. This means that less expensive diodes can be used. 5.2.2. Continuous Mode Continuous mode typically suits SMPS power levels greater than 300W. This is where the boost converter’s MOSFET does not switch on when the boost inductor is at zero current, instead the current in the energy transfer inductor never reaches zero during the switching cycle (Figure 10).With this in mind, the voltage swing is less than in discontinuous mode— resulting in lower I2R losses—and the lower ripple current results in lower inductor core losses. Less voltage swing also reduces EMI and allows for a smaller input filter to be used. Since the MOSFET is not being turned on when the boost inductor’s current is at zero, a very fast reverse recovery diode is required to keep losses to a minimum.

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Fig.5.4. Continuous Mode of Operation

5.3. Power Factor Correction Circuits Total Harmonic Distortion
The ratio between apparent power associated with higher order harmonics and apparent power associated with fundamental harmonic is called Total Harmonic Distortion (THD).

…….. Eqn 5.1 Where Inrms is RMS value of the n-th harmonic of the current. Any periodic non-sinusoidal current can be presented by Fourier transform.

For a periodic current waveform mentioned above:

Where Io – DC component of the current. In AC lines Io=0.

…………….. Eqn 5.2 2

We can also derive the relationship between PF and THD,

………………….. Eqn 5.3

……………. Eqn 5.4 Where, θ1: the phase angle between the voltage Vs (t) and the fundamental component of Is (t). Is1, rms: rms value of the fundamental component in line current. Is, rms: total rms value of line current. kdist = Is1, rms /Is, rms: distortion factor. kdisp = cosθ1: displacement factor.

Fig.5.5. Typical waveforms in a poor PF system

Chapter 6: Current Mode Control for PFC
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Over many years, different current mode control techniques were developed. Some of the very well adopted methods are: 1. Average Current Control. 2. Variable Frequency Peak Current Control. 3. Hysteresis Control.

6.1. Average Current Control
In average current control strategy, the average line current of the converter is controlled. It is more desired than the other control strategies because the line current in a SMPS can be approximated by the average current (per switching cycle) through an input EMI filter. The average current control is widely used in industries since it offers improved noise immunity, lower input ripple, and stable operation . Figure below shows a boost PFC circuit using average current control strategy. In the feedforward loop, a low value resistor Rs is used to sense the line current. Through the op -amp network formed by Ri, Rimo, Rf, Cp, Cz, and A2, average line current is detected and compared with the command current signal, icmd, which is generated by the product of line voltage signal and the output voltage error signal There is a common issue in CCM shaping technique, i.e. when the line voltage increases, the line voltage sensor provides an increased sinusoidal reference for the feed-forward loop. Since the response of feedback loop is much slow than the feed-forward loop, both the line voltage and the line current increase, i.e. the line current is heading to wrong changing direction (with the line voltage increasing, the line current should decrease). This results in excessive input power, causing overshoot in the output voltage. The square block, x2, in the line voltage-sensing loop shown in Figure below provides a typical solution for this problem. It squares the output of the low-pass filter (LPF), which is in proportion to the amplitude of the line voltage, and provides the divider (A ∗ B)/C with a squared line voltage signal for its denominator. As a result, the amplitude of the sinusoidal reference icmd is negatively proportional to the line voltage, i.e. when the line voltage changes, the control circuit leads the line current to change in the opposite direction, which is the desired situation. As it can be seen, the average current control is a very complicated control strategy. It requires sensing the inductor current, the input voltage, and the output voltage. An amplifier for calculating the average current and a multiplier are needed. However, because of today’s advances made in IC technology, these circuits can be integrated in a single chip.

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Fig.6.1. Boost PFC Using Average Current Control

6.2. Variable Frequency Peak Current Control
Although the average current control is a more desired strategy, the peak current control has been widely accepted because it improves the converter efficiency and has a simpler control circuit. In variable frequency peak control strategy, shown in Figure below, the output error signal k(t) is fed back through its outer loop. This signal is multiplied by the line voltage signal αv1(t) to form a line current command signal icmd (t) (icmd (t) = αk(t ) · v1(t)). The command signal icmd (t) is the desired line current shape since it follows the shape of the line voltage. The actual line current is sensed by a transducer, resulting in signal βi1(t) that must be reshaped to follow icmd (t) by feeding it back through the inner loop. After comparing the line current signal βi1(t) with the command signal icmd (t), the following control strategies can be realized, depending on its logic circuit:

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Fig.6.2. Block Diagram For Variable Frequency Peak Current Control Constant On-Time Control. Its input current waveform is given in Figure below. Letting the fixed on-time to be Ts, the control rules are: ➢ At t = tk when βi1(tk ) = icmd (tk ), S is turned on. ➢ At t = tk + Ton, S is turned off.

Fig.6.3. Input Current Waveforms for Variable Frequency Peak Current Control In Constant On-Time Control Constant Off-Time Control The input current waveform is shown in Figure below. Assuming the off-time is Toff , the control rules are:

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➢ At t = tk when βi1(tk ) = icmd (tk ), S is turned off. ➢ At t = tk+ Toff , S is turned on.

Fig.6.4. Input Current Waveforms for Variable Frequency Peak Current Control In Constant Off-Time Control

6.3. Hysteresis Control
Unlike the constant on-time and the constant off-time control, in which only one current command is used to limit either the minimum input current or the maximum input current, the hysteresis control has two current commands, ihcmd (t) and ilcmd (t) (ilcmd (t ) = δihcmd (t)), to limit both the minimum and the maximum of input current. . To achieve smaller ripple in the input current, we desire a narrow hysteresis-band. However, narrower the hysteresis- band, higher the switching frequency. Therefore, the hysteresis band should be optimized based on circuit components such as switching devices and magnetic components. Moreover, the switching frequency varies with the change of line voltage, resulting in difficulty in the design of the EMI filter. The input current waveform is given in the figure below. When βi1(t ) ≥ ihcmd (t), a negative pulse is generated by comparator A1 to reset them R–S flip-flop. When βi1(t ) ≤ ilcmd (t), a negative pulse is generated by comparator A2 to set the R–S flipflop. The control rules are: ➢ At t = tk when βi1(tk ) = ilcmd (t), S is turned on. ➢ At t = tk+1βi1(tk+1) = ihcmd (t), S is turned off. ➢ When, βi1(t ) = ihcmd (t ) = ilcmd (t), S stays off or on. Like the above mentioned peak current control methods, the hysteresis control method has simpler implementation, enhanced system stability, and increased reliability and response speed. In addition, it has better control accuracy than that the peak current control methods have. However, this improvement is achieved on the penalty of wide range of variation in the 1

switching frequency. It is also possible to improve the hysteresis control in a constant frequency operation, but usually this will increase the complexity of the control circuit.

Fig.6.5. Input Current Waveform of Hysteresis Control

Chapter 7: MATLAB SIMULATIONS
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The following Matlab simulations have been implemented and the corresponding outputs obtained are shown below

7.1 Basic Boost Implementation

Fig.7.1. Basic boost topology Observations : 1)Input DC voltage : 12V 2) Output DC voltage : 47.87 3) Duty ratio : 75%

7.2. Power Factor Improvement By Boost Converter With Firing
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MOSFET By PWM

Fig.7.2. Matlab simulation for PFC with boost convertor (Firing by PWM)

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Fig.7.3. (I) Modified Input Current (III) Current after rectification

(II) Input Voltage (IV) Voltage after rectification

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Fig.7.4. (I) Output Current with resistive load (II) Output Voltage with resistive load Observations: 1. Peak input current: 70.2 A 2. Peak input Voltage: 100V 3. Average output voltage – 74.06V 4. Average output current – 24.69A

7.3. PFC With Boost Convertor By Firing MOSFET With Voltage And
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Current Closed Loop Control

Fig 7.5: PFC using Closed Loop control

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Fig 7.6 :

(I) Input Voltage in Pink color and current in Yellow color (II) Input voltage (III) Current after rectification

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Fig 7.7: Output voltage of the Boost convertor

Fig 7.8: Total Harmonic Distortion Observations: Peak input Voltage = 163V Peak input Current=145.6A Average Output voltage from boost converter=220v Total harmonic distortion=0

7.4. Hardware Implementation of Boost Convertor Using LM3524
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Many Integrated Chips are available in the market these which have all the circuits in build in them. Some of the Integrated Chips of same sort are 1. LM3524 2. SG3524N 3. LM5001 4. UC5696 Every IC has its own application of power factor improvement. All the above IC’s are used with Boost convertor for power factor correction. We have used SG3524N with the boost convertor. Features of SG3524N ➢ ➢ ➢ ➢ ➢ ➢ ➢ ➢ ➢ Fully interchangeable with standard LM3524 family 1% precision 5V reference with thermal shut-down Output current to 200 mA DC 60V output capability Wide common mode input range for error-amp One pulse per period (noise suppression) Improved max. duty cycle at high frequencies Double pulse suppression Synchronize through pin 3

Fig.7.9 Top View of SG3524N

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v Fig.7.10. Implementation of Boost Convertor Input voltage – 12V Output Voltage – 52.5V Calculations: Duty Ratio (D) = [ 1-(Vin/Vo) ] = 0.77

Rf = 100k ohm Fosc = 212.765 kHz L1= 1.65mh C0=4.7microF I0 max = 15mA

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Conclusion
The key factor is that power factor correction and most other concepts are not new from the point of view of formal circuit theory. The question is how the problem can be best understood from the basics and then tackled in the best possible way. PFC is rapidly becoming a mandatory feature in AC power sources because IEC 6100-3-2 requires the use of PFC circuits. Active and passive PFC circuits are designed to bring the PF of a system closer to unity (PF = 1.0). While no system is 100% efficient, most PFC technology makes the power factor of a system greater than 0.95. Highly efficient electrical systems have the advantage of supplying less current to drive a load. This is beneficial to customers that have low power factor problems because utilities sometimes charge penalties for low power factor. While cost savings from PFC on small AC sources isn’t nearly as noticeable as money saved from PFC on large systems, in the long run PFC will provide reduced costs for high energy consumers.

References
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Power Factor Correction Circuits by Issa Batarseh, Ph.D.and Huai Wei Ph.D.university of central florida USA. Book on Power Electronics by Dr. P.S. Bimbhra Book on Power Electronics by Ned Mohan Power Factor Correction basics from www.fairchildsemi.com Circuit theory and design of Power Factor Correction circuits by Prof. Chi. K. Tse,department of electronics and information engg., Hongkong polytechnic university

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