Power Factor Correction Using Boost Converter Technique Engineering Essay

This paper researches a design a single phase rectifier with improved power factor by using the boost converter technique. This paper presents the use of boost converter technique can improve the power factor. By designing the needs of the techniques, the overall Power Factor (PF) would be improved to the expectation. The low power factor is caused by non-linearity of the input current. Boost converter is one of method of re-shaping the input waveform to be same pattern with the sinusoidal input voltage. The connected controls that act as a Power Factor Correction (PFC) circuit. The results were compared without PFC and passive PFC and active PFC. Finally the simulation results are shown to verify the performance of this modified the PFC.

Keywords : rectifier , boost converter , active PFC , passive PFC , power factor correction (PFC) , power factor

Introduction

In most electrical and electronic power supplies, the AC input is rectified is connected directly after the diode rectifier bridge. The AC mains are common and extended source of energy. DC power supply is needed for operation of electronic equipments such as in computers, televisions, monitors and others. A simple circuit consisting of diode rectifiers and bulk capacitor can possible to obtain DC voltage. The problem of the input circuit, the high harmonic distortions on the line and excessive peak input currents are produced and the result of power factor in the system is poor. [1] In order to improve these problems, many power factor corrections of ac dc converters are presented to achieve high power factor and low harmonics distortion by modifying the input stage of the diode rectifier and the filter capacitor circuit.

Power factor is defined as the ratio of the real power (P) to the apparent power (S) or cosine for pure sine wave for both current and voltage that represent s the phase angle between the current and voltage waveform. Figure 1.0a shows the power factor triangle.[2] Power factor is measure how efficiently electrical power is consumed. The power factor can vary between 0 to 1. It can be either lagging (inductive) or leading (capacitive). The poor power factor result in reduced efficiency which increases cost electricity.[2] Many utilities companies call upon penalties for the low power factor. When a converter has less than unity power factor, it means that the converter absorbs apparent power that is higher the active power it consumes. A higher VA rating than the load needed from the power source should be rated.

Figure 1.0a: Power factor triangle

The boost converter most widely used topology for achieving power factor correction. The figure shows the PFC boost converter circuit. [3] The process of reshaping the input current is done by boost converter. The bulk energy storage capacitor sits on the output side of the boost converter rather than just after the diode bridge. The control circuit for this project used low-cost components and satisfactory results.

Figure 1.0b: relationship of Ac input voltage and current

Figure 1.0b is show the waveform of relationship between Ac input voltage and current. The waveform is due the purely resistive non linear load.

SYSTEM DESRIPTION

A single phase rectifier with boost converter is shown in figure 1. Boost converter is used for reshaping input current.

Figure 1.1a: Boost PFC converter

Figure 1.1b: Voltages and currents waveforms

Figure 1.1a and 1.1b shows that the basic configuration of rectifier that uses Boost converter technique as PFC with its respective voltages and currents. The input current would be highly non-linear for rectifier without PFC. It happened when the capacitor is having large value. [4] The boost converter involves a switch, a diode, an inductor and a capacitor. The switch and diode are depending on the voltage and current stress that they must handle for all the possible operating conditions. The boost converter also called step up converter which is the output voltage always greater than the input voltage.

POWER FACTOR CORRECTION

Power factor correction (PFC) is one of the methods to improve the low power factor of a system by using suitable devices. There are two type of the PFC namely the passive and active power factor correction. The aim of the power factor correction circuits is to make the input of the power supply behave like purely resistive or a resistor.[4,5] By using the passive PFC, it can achieve the objective of the low power factor applications. The addition of a filter inductor which is connected to the series with the input circuit, the passive elements is introduced to improve the line current. This passive element will increased to a value of 0.7 approximately. The achievement is degraded with the smaller values of inductance. The passive PFC is more suitable at lower power levels.

The active power factor correction results are more suitable option for achieving the nearest unity power factor and the sinusoidal input current waveform. The active PFC design functions by controlling the input current in order to make the current waveform behave like the supply voltage. A converter and switching frequencies higher than the AC line frequency is added between the output of the diode bridge rectifier and the bulk capacitor in the active power factor correction.

NON ISOLATED

ISOLATED

BOOST/BUCK, BUCK+BOOST

PWM RECTIFIER

FLYBACK FORWARD PWM RECTIFIER

HIGH FREQUENCY

LOW FREQUENCY

P.W.M

RESONANT

Singe phase OFFLINE PDC circuit

ACTIVE PFC

PASSIVE PFC

Figure 1.2: singe phase offline PFC topologies.

PASSIVE POWER FACTOR CORRECTION

The AC-DC converter comprises of a full bridge rectifier followed by a large filter capacitor for the input stage. The input current for the rectifier circuit comprises of the largest discontinuous peak current pulses. The high distortion of the input current occurs due to the diode rectifier conduct for the short period.[7]

By using this passive power factor correction, the value of the power factor can be increased of the 0.7 approximately. Increasing power factor due to the idea of passive PFC which is to filter out the harmonic currents by using low pass filter.

Thus, this passive power factor correction has the following main advantages and disadvantages:

Advantages — it has a simple structure, it is reliable and rugged. The cost was very low and the high frequency switching losses are not sensitive to noises.

Disadvantages — it create harmonic, it has poor power factor, it produces high losses and it reduces power maximum power capability from the load. In the passive PFC, the use of the inductor in the input circuit is simply used.

.

ACTIVE POWER FACTOR CORRECTION

An active PFC is a power electronic device designed to control the amount of power drawn by a load and obtain a power factor as close as possible to unity. In an active PFC, the function was by controlling the input current and make the current behave like supply voltage. There are two classes of the active PFC solutions which are the low frequency and the high frequency active power factor correction.

METHODOLOGY

The simulation process is carried out by using PSIM software. The PSIM simulation model for overall system is shown in figure 2.0. The simulation circuit can be divided to three sections which are the main source (input), the PFC (rectifier) and the load (output).

Figure 2.0: PSIM simulation model for overall system

The software simulation process is divided into three steps which are non linear load, passive PFC and active PFC (boost converter).

Start

Without PFC.

Simulation process using PSIM

With PFC by using active filter (boost converter)

With PFC by using passive PFC

1

2

3

Figure 2.1: flowchart of the simulation process

First, a non-linear load without PFC is simulated for proof of concept using PFC. Figure 2.1.1 shows the flowchart of the process of the first simulation step.

1

Circuit modeling

Connect a non linear load

Desired result

End

Yes

No

Circuit simulation

Figure 2.2: flowchart without PFC circuit

In the next step, passive PFC is added in the circuit. The passive PFC which is additional to the diode bridge rectifier. The passive elements are introduced to improve the nature of line current. As the voltage increases, the sizes of PFC components increase too. Figure 2.1.2 shows the flowchart of the process of the second simulation step.

2

Circuit modeling

Connect a non linear load

Add passive element to the circuit

Desired result

End

Yes

No

Circuit simulation

Desired result

End

Yes

No

Circuit simulation

3

Circuit modeling

Connect a non linear load

Add boost converter after bridge diode in the circuit

Add control circuit

Figure 2.1.2 : flowchart for third simulation

Figure 2.1.2: flowchart for second simulation

In the third step, an active PFC replaced the passive PFC. The boost converter is added in this circuit. An active PFC can control the amount of power drawn by a load and obtains the power factor as close as possible to unity. Then

the control circuit also added to the boost converter. Mosfet is used in the boost converter circuit which is a switching.

RESULT AND DISCUSSION

Without Power Factor Correction

For the non linear load which is the circuit without the PFC , the power factor is low compares to the the using of power factor correction. From the waveforms , as we can see the power factor is 0.6 from 0.1 seconds onwards. The shape of input current is not likely the the shape of input voltage. It it because there are some disturbance happened in system.

Figure 3.1 : without PFC

Figure 3.2 : PF , Vin and Iin waveforms

With Passive Power Factor

The first step of the simulation is designed for non linear load. In the next step, a passive element which is the inductor is added to improve the nature of the line current. From these figure, the power factor increased to 0.7. The shape of input current also not behave like the input voltage. But it was better compare to using without PFC.

Figure 3.3: Passive Power Factor Correction

Figure 3.4: PF, Vin and Iin waveforms

With Active Power Factor Correction by using Boost Converter

In this step, the boost converter replaced the passive element which is using the Mosfet as the switch. Through the simulation, the power factor is improved to 0.9 from 0.1 seconds onwards.

The process of reshaping the input current happened. The shape of input current also likely of the shape input voltage. From this simulation, by using the boost converter is obtained to get the power factor as close as possible to unity.

The main of AC input voltage is rectified to the boost converter that mainly consisting of an inductor, Mosfet, a power diode, and a bulk capacitor. The boost converter is one of the high switching frequency topologies.

Figure 3.5: PF, input current and input voltage waveforms

Types

Power factor

Without PFC

0.6

Passive PFC

0.7

Acvtive PFC

0.9

Table 1: the value of power factor from simulation

The table 1 shows that the power factor increase from 0.6 to 0.9. We get these values from the simulation of the three circuits. The rectifier with power factor correction was developed to realize the performance of this project. The power factor improvement by using active PFC was get good result compare to using passive filter. It is because the active PFC is more complex than passive PFC. the power factor correction makes the load look likely resistive element compare to a nonlinear load one without PFC.

CONCLUSION

In conclusion, the simulation results matched to the theories involved. This paper presents a research about power factor improvement by using power factor correction. The use of the power factor correction is to improve the low power factor and to make the input of power supply behave like to the purely resistive. The PFC circuitry that the controls the Boost converter is having the limitation when the loads current are smaller. The circuit will avoid the AC current to flow exceeding to the load demand. From the simulation results , the power factor is almost unity and purely sinusoidal input current followed the input voltage.

ACKNOWLEDGMENT

I would like to sincere thank you to my supervisor PM Pauziah bt Mohd Arsad , deparment of Electrical Engineering , UiTM Shah Alam for providing me the necessary guidance to carry out this project. I would like to take this opportunity to thank her for her constant support and guiding me throughout my work.

Besides, I also would like to sincere thanks to my parents because always support me. Then thanks to all my friends who are help me during my project was running.