Active Power Factor Correction

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

Active power factor correction is a form of power factor correction that is used to correct the power factor for non-linear loads.

Non-linear power factor correction employs methods of power factor correction that utilise active circuitry as the name implies. In this way, active power factor correction can involve more circuitry than other methods, but can be very effective in its outcome.


Active power factor correction basics

It is often believed that the best approach to resolve a poor power factor issue is to adopt an active approach to power factor correction.

The basic building block used to provide the active power factor correction is a switch mode power supply, SMPS, boost circuit.

To provide the active power factor correction, the incoming mains AC voltage passes through a rectifier as in any power supply, and this produces a full wave rectified output. As the output from the boost circuit is higher than the incoming voltage to the rectifier, no current will flow into the reservoir capacitor unless the input voltage is above that of the reservoir capacitor.

Active Power Factor Correction Circuit Block Diagram
Active Power Factor Correction Circuit Block Diagram

Accordingly the control circuitry of the boost converter is able to detect the circuit conditions and then control the voltage and hence the timing of the current waveform to the reservoir capacitor.

In this way, it is possible for the system to adjust the boost voltage to maintain the current waveform as a sinusoid in phase with the voltage, thereby preserving the power factor.

Voltage and Current waveforms for Active Power Factor Correction Circuit
Voltage and Current waveforms for Active Power Factor Correction Circuit

To summarise, the boost switch mode converter attempts to maintain a constant DC output voltage on the reservoir capacitor while drawing a current that is always in phase with and at the same frequency as the line input voltage.

The output from the boost SMPS can then be applied to the second stage of the power supply, which is most commonly a switch mode DC-DC regulator.


Active PFC results and applications

Active power factor correction is able to provide significant benefits, although at the cost of additional circuitry. However with the high levels of integration achievable these days, the cost impact is not prohibitive even for large volume production as it can be achieved using relatively small and cheap components.

As an example of the performance that active power factor correction can provide, a typical switch mode power supply with no power factor correction may have a power factor figure of around 0.6, with passive power factor correction, it may be around 0.75, and with active power factor correction, figures better than 0.95 may be expected.

In view of the gains achievable, many computer supplies, including those used for laptops use active power factor correction.

By Ian Poole


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