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Jul 9 2018

What Is Power Factor Correction (Compensation)

In the fifties of the last century, an improved method was proposed for the low efficiency of power supply caused by the different phases of voltage and current of AC electrical appliances with inductive load. 

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Figure 1 The voltage and current waveforms under inductive load

Due to the different phase of voltage and current, the burden of power supply line becomes heavier and the efficiency of power supply line is decreased, which requires that a capacitor should be connected to the AC electrical appliances to adjust the phase characteristics in voltage & current.

For example: 

At that time, the required 40W fluorescent lamp had to be connected in parallel with a 4.75μF capacitor. With capacitors connected to inductive loads, the characteristic of current leading voltage on capacitors are used to compensate for the characteristic of current lagging voltage on inductors to make the overall characteristics more closer to a circuit containing just resistance, thus improving the efficiency, and this method is called power factor correction/compensation. The power factor of alternating current can be expressed in term of cosine function value (cosφ), where φ is the phase angle between supply voltage and load current.

Since the 1980s, a large number of high efficiency switch-mode power supplies have been used in electrical appliances. The switching power supply after rectified usually uses a filter capacitor with large capacity, so it is a capacitive load that the electrical appliance drives, which causes sawtooth ripples at both ends of the electrical appliance under a 220v supply due to the charge and discharge of the filter capacitor.

The minimum voltage on the filter capacitor is far from zero, which is not much different from its maximum value (ripple peak). According to the unidirectional conductivity of the rectifier diode, the rectifier diode is turned on because of the forward bias only when the instantaneous value of the AC line voltage is higher than the voltage on the filter capacitor; and when the instantaneous value of the AC input voltage is lower than that of the filter capacitor, the rectifier diode is turned off due to the reverse bias.

That is to say, in each half cycle of the AC line voltage, the diode is only turned on near its peak. Although the AC input voltage still maintains a sinusoidal waveform substantially, the AC input current has high amplitude spikes, as shown in Figure 2. This severely distorted current waveform contains a large amount of harmonic waves, causing a severe drop in power factor of the line .

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Figure 2 Previous sinusoidal waveform suffered high amplitude spikes

In the positive half cycle (1800), the conduction angle of the rectifier diode is much less than 1800 or even as low as 300-700. 

Due to the requirement of the load power, a very large on-current is generated during a very narrow conduction angle, so that the supply current in the power supply circuit is pulsed. It not only reduces the efficiency of power supply, but also causes serious waveform distortion of AC voltage due to too less power supply line or too large circuit load (figure 3), and generates multiple harmonics, as a result that it interferes with the normal operation of other electrical appliances. This is the problem of electromagnetic interference (EMI) and electromagnetic compatibility (EMC) that we often mention.

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Figure 3 Voltage waveform distortions caused by a capacitive load

Since the electrical devices have changed from the inductive load of the past (early TV, radio and other power supplies all use inductive devices of power transformers) to the capacitive load with rectifier and filter capacitors, the power factor compensation has meant to not only solving the problem of the different phase of power supply voltage and current, but also the issues of electromagnetic interference (EMI) and electromagnetic compatibility (EMC) caused by strong pulses of power supply current.

This is the a technology developed at the end of the last century, which has background of the rapid development and wide application of switching power supply). The main purpose of the  technology is to solve the EML and EMC caused by the serious distortion of current waveform because of the capacitive load. So the modern technology PFC is completely different from the power factor compensation technology in the past. It is aimed at the distortion of the non-sinusoidal current waveform, forcing the current of the AC lines to track the transient variation track of the voltage waveform, and keeping the current and voltage in a same phase to make the system pure resistive (current waveform correction technology).

So the modern technology PFC completes the correction of current waveform and solves the problem of same phase of voltage and current.

As a result of the above reasons, for capacitive load appliances which require an power greater than 85W (some data shows more than 75W), it is necessary to add a correction circuit to correct its load characteristics so that it is more close to an resistivity one (that is, the voltage and current waveforms will have a same phase and the waveform is similar). This is the power factor correction (PFC) circuit.

What is Power Factor?


Book Recommendations

Power Factor Correction: Explaining The Meaning And Importance Of Power Factor, And Describing Methods For The Improvement Of Power Factor

This work has been selected by scholars as being culturally important, and is part of the knowledge base of civilization as we know it. This work was reproduced from the original artifact, and remains as true to the original work as possible. Therefore, you will see the original copyright references, library stamps (as most of these works have been housed in our most important libraries around the world), and other notations in the work.

by Albert Edmund Clayton

Stability of different types of power factor correction: Stability analysis of power-factor correction converters

In this book, a study for stability for the three practical controls of boost PFC converter is introduced. Each technique has been tested to determine the practical limitation for stable regions. Design guidelines are made clear for stable operation in the examined control techniques. Experimental results confirm simulation with good matching.

by Reham Haroun Mohamed


Relevant information about "What is Power Factor Correction (Compensation)"

About the article "What is Power Factor Correction (Compensation)", If you have better ideas, don't hesitate to  write your thoughts in the following comment area. You also can find more articles about electronic semiconductor through Google search engine, or refer to the following related articles:

Switch-Mode Power Supply Fundamentals (2)

The Working Principle of High-Power Adjustable Switching Power Supply

Complete Introduction and Classification of Filters and Applications

Comprehensive Explanation of Capacitors

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