Passive Power Factor Correction

- basics of passive power factor correction and the applications in which this form of power factor correction can be used.

One form of power factor correction for non-linear loads is referred to as passive power factor correction.

The non-linear loads may be in the form of bridge rectifiers or other non-linear elements that may appear at the input circuitry of the load.

Passive power factor correction, as the name implies uses passive circuit techniques to apply the power factor correction to any non-linear circuit.


Harmonics

One of the main issues with non-linear loads is that they introduce harmonics onto the main or line supply. Often power supplies only take current when the smoothing or reservoir capacitor needs charging, and therefore the current pulses occur at a specific point within the sinusoidal voltage waveform.

Non-Linear Load Current for Electronic Power Supply
Voltage and Current for Non-Linear Load
such as an Electronic Power Supply

The short current bursts are rich in harmonic content, and typically contain much higher levels of 3rd, fifth, 7th and other odd order harmonics. Therefore for a 50Hz supply, the first major harmonic of interest is the 3rd harmonic at 150Hz and then the others are at 250Hz, 350 Hz, etc.

Harmonics generated by a non-linear load
Harmonics generated by a non-linear load


Filter passive power factor correction

In view of the fact that the first major harmonic of interest is the third harmonic, a simple low pass filter is able to provide a sufficiently high degree of attenuation to sufficiently to provide a return to a near sinusoidal waveform using a simple low pass filter.

Low pass filter response for passive power factor correction
Low pass filter response for passive power factor correction

By arranging the cut-off frequency of the filter to be just above the fundamental frequency, the optimum attenuation of harmonics can be achieved.

The basic form for a low pass filter is likely to be of the format shown below. Either a Π like that below, or a T network may be used.

Low pass filters for passive power factor correction
Low pass filters for passive power factor correction


Filter passive power factor disadvantages

Possibly the simplest way of providing passive power factor correction is to use a filter.

Much of the power factor correction required by non-linear loads is the restoration of the waveform shape. As waveform distortion results in harmonics being generated, it is possible to remove these and restore the required sinusoidal waveform by using a filter.

The filter reduces the harmonic content of the current waveform and as a result it makes the load look like a linear device.Using a filter, it is possible to bring the power factor back to almost unity, but there are several disadvantages to passive power factor correction.

  • Size of filter components:   The passive power correction filter normally utilises a large-value high-current inductor and a high-voltage capacitor, in view of the frequencies involved. Mains or line AC supply is typically 50 or 60Hz dependent upon the country or location. These large value inductors and capacitors, which must also be able to withstand the line voltage with any spikes that may occur. This makes these components bulky.
  • Cost of filter components:   In view of their performance and size, the cost of these components is high, especially when compared to those components normally used within computers, monitors and the like for which they would often be used.
  • Worldwide operation:   Many items such as PCs and the like need to be operable in many countries. For passive power factor correction filters this means that a mains or line-voltage range switch needs to be included within the filter circuit. The inclusion of this switch makes the appliance/system prone to operator errors if the switch selection is not properly made. It is unlikely that self-sensing techniques would be appropriate.

Although passive power factor correction seems like an attractive proposition at first sight, it has a number of disadvantages which make it unsuitable for many applications.

By Ian Poole


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