Non-Linear Load Power Factor Correction

- basics of non-linear load power factor correction used for correcting the power factor of non-linear loads including switch mode and linear power supplies.

There are many non-linear loads that are connected to mains or line power sources.

These non-linear loads also need power factor correction, however different non-linear load power factor correction techniques are required for these loads.

Causes of non-linear load power factor

With the growing use of electronic equipment connected to the mains power system, the effect of non-linear loads connected to the overall mains or line power system can have a significant effect. Items such as personal computers, if taken singly do not affect the power factor for a generating system. However when many are used, as in the case of a large office block, where many hundreds or even thousands are used can have a major impact on the system.

In view of this, it is necessary to apply non-linear power factor correction techniques to ensure that the efficiency of the generating system is not impacted and additional charges are not incurred from the electricity supply company.

The issues with AC-DC power supplies occur as a result of the rectification process. Typically the input consists of a transformer, which has little effect on the power factor, in this case, and then a rectifier followed by a large reservoir capacitor.

Power Supply presents a Non-Linear Power factor Load
Power Supply presents a Non-Linear Power factor Load

It is found that the current is drawn only on the part of the cycle when the output voltage from the rectifier exceeds that of the filter or reservoir capacitor. The current then charges up the reservoir capacitor, and then as the voltage from the rectifier falls, current is drawn by the ultimate load from the reservoir capacitor.

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

It can be seen from this that the current waveform is out of phase with the voltage and in addition to this the waveform is far from being a sine wave. This represents distortion and introduces high levels of harmonics into the system. Accordingly non-linear load power factor correction is of great importance.


Harmonics

It has been seen that the non-linear load in the form of the rectifier with capacitor and load gives rise to current spikes. These current spikes have a rate of rise and fall that is much greater than that of the basic voltage waveform. As a result this creates a series of predominantly odd harmonics that decrease in intensity with increasing frequency.

It is actually the harmonics that cause problems with the power generation and distribution system.

A solution for non-linear power factor correction is to add some inductance in series with the supply line. This serves to attenuate the harmonics to a degree.

Another partial non-linear power factor correction solution is to reduce the value of the reservoir capacitor. When the value is reduced, the capacitor charges over a greater portion of the cycle and the current spikes are not so sharp. However it impacts the smoothing and reduces the ripple on the pre-regulated line.


EN61000-3-2

As the harmonics arising from switch mode power supplies can travel along the power lines and disrupt other services, standards have been introduced to ensure that the levels of harmonics and power factor reach acceptable levels.

Within Europe a standard EN61000-3-2 was introduced to limit the levels of harmonics being emitted onto the power lines and also ensure that the power factor is within acceptable limits.

EN61000-3-2 is applicable to all class D electronic systems. This includes, Pesronal Computers, PCs, including notebooks and PC monitors, as well as radio and TV receivers consuming more than 75 W. Class D is one of the four classes which range from A, through B, and C, to D which are detailed within the EN61000-3-2 standard. Each class has different harmonic-current limits. This standard is now accepted internationally.

In order to comply with the requirements of standards such as EN61000-3-2 and maintain high overall PF performance, power-factor correction circuitry must be incorporated into the power modules used by equipments, especially those consuming more than 75 W. Implementing power factor correction for these non-linear loads achieves a high PF number and ensures low harmonics.


Typically more sophisticated solutions for the power factor correction of the non-linear loads is required. This can be in the form of passive or active circuitry. Passive power factor correction may provide sufficient performance, but more normally active power factor correction is needed. Both forms of power factor correction for non-linear loads are addressed in the following pages.

By Ian Poole


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