INTRODUCTION

The demand for power, which has increased tremendously over the last few decades, has forced the power engineers to establish reliable network in order to supply quality power to the consumer. Over the years lot of research has been carried out for the supply of quality power to the consumers. This research got a tremendous boost with the strides made in the miniaturization of the electrical industry. The power electronic devices are very versatile devices capable of delivering power as high as 10KW. These devices are capable of working at frequencies in the range of hundreds of KHz and at the same time the control being only at the gate terminal of the devices, which makes these devices easily controllable.

Research in improved power quality utility interface has gained importance due to stringent power quality regulation and strict limits on total harmonic distortion (THD) of input current placed by standards such as IEC 61000-3-2 and IEEE 519-1992. This has led to consistent research in the various techniques for power quality improvement. Research into passive and active techniques for input current wave shaping has highlighted their inherent drawbacks. Passive filters have the demerits of fixed compensation, large size and resonance whereas the use of active filters is limited due to added cost and control complexity.

An ac to dc converter is an integral part of any power supply unit used in the all electronic equipments. So all the PFC methods have been received a great attention and a number of techniques have been developed. There are two types of PFC’s. 1) Passive PFC, 2) Active PFC. The active PFC is further classified into low frequency and high-frequency active PFC-depending on the switching frequency. Among these PFC’s we will get better power factor by using high-frequency active PFC circuit. These converters can operate in Continuous Current Mode – CCM, where the inductor current never reaches zero during one switching cycle or Discontinuous Current Mode - DCM, where the inductor current is zero during intervals of the switching cycle.

In DCM, the input inductor is no longer a state variable since its state in a given switching cycle is independent on the value in the previous switching cycle. The peak of the inductor current is sampling the line voltage automatically. This property of DCM input circuit can be called “self power factor correction” because no control loop is required from its input side. In DCM the input current is nearly sinusoidal waveform with constant duty cycle switching. Another merit of this mode is that the turn of the switch occurs at

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zero current switching (ZCS).Therefore the switching loss of this converter is very low.

Most applications requiring ac-dc power converters need the output dc voltage to be well regulated with good steady-state and transient performance. The circuitry typically favored until recently (diode rectifier—capacitor filter) for the utility interface is cost effective, but it severely deteriorates the quality of the utility supply thereby affecting the performance of other loads connected to it besides causing other well-known problems. In order to maintain the quality of the utility supply, several national and international agencies have started imposing standards and recommendations for electronic equipment connected to the utility.

POWER FACTOR CORRECTION ISSUES THAT HAVE TO DO WITH POWER FACTOR CORRECTION

BACKGROUND

Conventional single phase ac-dc utility interface

Conventionally, the utility interface of a low power single phase off-line ac-dc converter typically consists of a simple uncontrolled rectifier feeding a bulky filter capacitor. An input filter is usually present in the ac side in commercial products to reduce the electromagnetic interference (EMI) due to the converter. The bulk capacitor, Cin, is designed to maintain the ripple in the dc bus to an acceptable level and also to meet the "hold-up time" requirements. The circuit draws narrow input current pulses around the line voltage peaks.

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Fig: conventional converter & its input waveform

Power factor: Power factor (PF), is defined as the ratio of the average power to the apparent power. Mathematically, it is represented as:

Power factor reflects how effectively the given source power is utilized by the load. It has been assumed that the input voltage is sinusoidal with low distortion. Here, Vs is the rms input voltage, Is is the rms input current, is the rms value

of fundamental input current, and is the phase angle of the fundamental

current. The ratio is the distortion factor; its value reflects the effect due to input

current harmonics. The cosine of the angle O is the displacement power factor; its value reflects the conventional power factor when there is no distortion.

PFC Techniques

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PASSIVE PFC TECHNIQUES

The power line disturbances caused by the proliferation of phase controlled and diode rectifier circuits were of concern even in late 70s. The definition of power factor for nonlinear circuits and passive techniques for improving it are presented in an early literature. Currently, passive techniques remain attractive for low power PFC applications. It has been reported that power factor as high as 0.98 can be achieved using passive PFC techniques. The following sub-sections discuss a few of the passive PFC arrangements.

INDUCTIVE FILTER

The Figure below shows a well-known scheme with an inductor inserted between the output of the rectifier and the capacitor. The inclusion of the inductor results in larger conduction angle of the current pulse and reduced peak and rms values. For low values of inductance the input current is discontinuous and pulsating. Typical PF achieved in discontinuous mode operation (DCM), with practical values for the inductor, is in the range of 0.65 to 0.75. Better power factor (PF) is achievable by using a larger value of the inductance and pushing the operation to continuous conduction mode (CCM). However, it is shown that even for infinite value of the inductance, the PF cannot exceed 0.9 for this arrangement.

Figure: Conventional Rectifier Circuit with inductive filter.

The inductor may also be introduced on the ac side. The position of the inductance will not affect the PF in DCM operation. Under CCM operation, however, the circuit behavior itself will be different depending upon the location of the inductance. For instance, the presence of an infinite inductance on the ac side (i.e. CCM operation) will result in zero input current and zero voltage across the bulk capacitor, theoretically. However, the same inductance on the dc side will result in rectangular blocks of current in the input with the bulk capacitor charging up to the average value of the rectified input voltage. For lower power levels, the distribution line inductance will itself act as a good filter. For an office plug point (15 A), the line inductance is typically in the range of 1 to 4 mH.. An estimate of this value can be obtained from the assumption that the ac side reactance Xs (=ωLs) is 5% of the ratio of nominal rated voltage to the maximum current rating of the plug point.

In the scheme shown in figure below, a small filter capacitor Cs is connected across the input terminals of the circuit. The line inductance (not shown in figure) and C, forms the first stage LC filter. Therefore higher order harmonics of

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the line frequency will undergo greater attenuation (typically 80 dB) resulting in better harmonic performance. It is reported that even for relatively small values of the inductance, a PF of 0.86 is attainable, which is a considerable improvement over the no- capacitance case.

Figure: Rectifier circuit with input capacitor Cs

RESONANT INPUT FILTER

The Figure shows the series filter arrangement for power factor correction, which results in good power factors as high as 0.94. Thus, harmonic performance is also good. However, the power factor depends upon the resonant quality factor which is load dependent. Here the band pass filter is designed with a centre frequency equal to the supply frequency. The quality factor "Q" determines the bandwidth and hence the harmonic content of the supply current. High "Q" (narrow bandwidth) will result in reduced harmonic content and close to unity power factor. This circuit arrangement is popularly used in applications where the supply frequency is high. One such application is the space platform, with supply frequency up to 25 kHz.

Figure: Rectifier circuit with series resonant filter

Use of parallel resonant filter for PF improvement has also been suggested. With this arrangement power factor close to 0.95 is achieved. The filter is tuned to offer very high impedance to the third harmonic component (the most predominant). The high value parallel resistor is added to damp out circuit oscillations. The figure for such arrangement is shown below.

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ACTIVE POWER FACTOR CORRECTION

The active PFC technique, which involves the shaping of the line current using switching devices such as MOSFETs (metal oxide semiconductor field effect transistors) and IGBTs (insulated gate bipolar junction transistors), is a result of advances in power semiconductor devices and microelectronics. For low and medium power ranges up to a few kilowatts (<5 kW), MOSFETs are by far the popular choice for PFC because of their switching speed, ease of driving and ruggedness. BJTs and more recently IGBTs are used for high voltage medium power applications which MOSFETs are unable to contend with owing to their large on-state resistances. For achieving good input current wave shaping using active techniques, typically the switching frequency should be at least an order of magnitude greater than 3 kHz (= 50 x 60 Hz = 50th harmonic of line frequency). With modern advances in MOSFETs and IGBTs, this is feasible.

The use of active PFC techniques results in one or more of the following advantages.

Lower harmonic content in the input current compared to the passive techniques.

Reduced rms current rating of the output filter capacitor.

Near unity power factor (0.99) is possible to achieve with the Total Harmonic Distortion (THD) as low as 3-5%.

For higher power levels active PFC techniques will result in size, weight and cost benefits over passive PFC techniques.

The following sub-sections present the recent advances in single phase active PFC techniques. The active PFC techniques have been classified into two broad categories in this survey.

(1)Active PFC techniques with poor load dynamics. These have been referred to in this paper as "Conventional active PFC techniques". They are typically followed by a dc-dc downstream converter which caters to the demands of the load.

(2)Active PFC techniques with fast load dynamics. Here, the PFC unit is capable of meeting the fast dynamic requirement of a typical load.

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Conventional active PFC techniques

Here, there are basically two approaches. One approach is to use current-source-type circuit, in which the PFC acts as a current source feeding the load. Though the voltage source type circuits are more popular, the current source type circuits are useful in certain niche applications. In the following subsections both types of circuits and schemes are discussed.

Topologies:The current-source-type PFC converter is usually of buck type. However,

for the voltage-source-type PFC converter, any one of the basic dc to dc converter switching cells, such as buck, boost, buck-boost and Cuk converter, can be used. Among these, the boost and the buck-boost topologies are more popular. In the following subsections the notable features of these topologies (both current and voltage source types) and certain critical issues related to these topologies are presented. These topologies are described briefly below and the diagrams are given below. These power factor correction circuits are of conventional type and are useful in certain applications.

Figure: Circuit variations of the boost PFC topology. Single-switch (a), two-switch (b), four- switch (c), and half-bridge (d) boost PFC's.

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THE BOOST POWER FACTOR CORRECTION CONVERTER

It is an ac-dc converter which operates at DCM and soft-switching with partial resonant method. The partial resonant method operates at zero current switching (ZCS) and zero voltage switching (ZVS).

The figure shows the proposed ac-dc boost converter. The proposed ac-dc boost converter consists of step up inductor, switches S1, S2.The proposed converter switches operates at soft switching condition by partial resonant condition by control of constant duty cycle. The turn on of the switches is done under zero current switching (ZCS) and turn off of the switches is worked under zero voltage switching (ZVS) by partial resonant method. Therefore the proposed converter is operated with high efficiency.

Figure: The proposed ac-dc Boost converter

MODES OF OPERATIONThere are four modes of operation during one switching cycle. At the

beginning the current flowing through the inductor Lr is zero. Both the switches remain in off condition and the capacitor charges up to the same voltage at the dc output side. The equation of input voltage and output voltage of full bridge rectifier is given below.

MODE1: ( )Mode 1 begins with the turning on of two switches. The input voltage

and capacitor voltage are added and applied to the inductor. Now the capacitor starts discharging through inductor forming a series LC resonant circuit.

MODE1 operation of boost converter

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Thus at this mode the turning on off the switches occur at zero current switching (ZCS).Under ZCS the equation of and is given by;

Where,

When = 0, the inductor current at the end of this mode is given by;

MODE2: Mode 2 begins when voltage across Cr becomes zero and diodes d1

and d2 begins to conduct. The inductor current equation is linearly increased given by;

This mode ends when both the switches turn off and the inductor current is given by the equation;

MODE3:

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Mode 3 begins by turning of both the switches at same time. The current flows through path -d1- -d2, thus charges the capacitor. This mode takes of a series resonant circuit. Turning of both the switches occurs at ZCS condition when voltage across capacitor is zero. When voltage across capacitor reaches “ ”, then the diode d3 start conducting. The inductor current at this mode is assumed to be a constant value , because of short period of this mode.

MODE4: Mode 4 begins when diode d3 start conducting and the inductor

current flows into the load .Here the inductor current decreases linearly is given by the equation:

This mode ends when the inductor current reaches Zero value.

SIMULATION RESULTS

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A diode rectifier bridge can be used to supply the input AC voltage. The PFC converter is constructed by use of a boost chopper circuit with a switching device in the dc side of the diode bridge rectifier circuit. A sinusoidal current waveform in phase with the ac line voltage and the constant dc voltage can be obtained from the PFC converter.

The proposed converter is analyzed with the help of PSIM simulation. The input to this simulation is given below

Input ac voltage =100 volt(rms) Output dc voltage =300 volt Switching frequency =50 kHz Duty cycle=30%

Components used in the converter are listed as: Filter inductor Lf=3mH Resonant inductor Lr=0.75uH Filter Capacitor Cf=5nF Resonant Capacitor Cr=91nF Output capacitor =2000 Resistive load R=400

In MODE 1 the waveforms for inductor current and voltage across

capacitor is given below. This plot shows the soft switching across switch S1.

In MODE 2 the waveforms for inductor current and capacitor voltage is shown below. This plot of voltage and current shows the soft switching across switch s2.

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The waveform below shows the input and output voltages of the PFC boost converter.

The input waveform for discontinuous current mode (DCM) is shown below.

The input ac voltage waveform and the sinusoidal input current is given below

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Input ac voltage waveform

Sinusoidal input current waveform

CONCLUSION

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From simulationThe input current waveform got to be sinusoidal in proportion to supply

voltage waveform with constant duty cycle i.e. 30% operation. The DCM method reduces the complicacy of designing control circuit. Therefore the input current waveform got to be sinusoidal without any control circuit .The circuit switches operate under partial resonant soft switching method, thus it increases the efficiency of the circuit. The proposed converter achieves unity power factor with high efficiency from the above result.

Limitations of PFCThe main switch Si operates in ZVS mode and reverse recovery of D1 is

gradual. Hence the switching losses of Si and D1 decrease notably. The turn-off

loss of S2 is similar to that Si, while its turn-on loss is caused by discharge of the

body capacitor and transient voltage dropping. Since S2 only conducts a short

period, its power rating can be much smaller than S1's and so does its body

capacitor. As a result, the discharge loss caused by body capacitor of S2 in a soft-

switched converter will be much smaller than Si in its hard-switched

counterpart.

By suppressing the diode recovery current and utilizing the energy stored in the main switch's body capacitance, the soft-switching technique reduces PFC's loss obviously when switching frequency is not very high. As penalty, the soft switching makes converter be more complex.

The loss of auxiliary switch relates to its on-time period. Constricting this on-time period too much will increase switching loss notably. There is an on-time lower limit for the auxiliary switch in the soft-switched PFC. As a result the soft-switching technique makes PFC decrease efficiency notably when the frequency is close to its limitation.

To ensure efficiency of PFC the equivalent PWM on-time period requires much longer than on-time period of auxiliary switch. For AC220-240VAc input and 385-400VDC output PFC, widely variable duty ratio causes the converter to reach above-mentioned limit. The effect of increasing CCM boost PFC's switching frequency by soft-switching technique is not obvious. It is not realistic that the switching frequency could be as high as DC/DC converter's level (1MHz), though maybe it is useful to enhance the efficiency by employing ZVS auxiliary switch.

On the other hand, the limitation in the soft-switching PFC will not be a serious problem if the input voltage is lower (e.g. 85-120VAc) or the output voltage is higher (e.g. higher than 400VDC). Alternatively by changing the

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topology of PFC to enlarge minimum duty ratio, the soft switching can also avoid being affected by this limitation.

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