Technology Primer - PFC for Switch Mode Power Supply Technology

Tech Primer

SMPS have become the technology of choice for most of today AC to DC power supply requirements, because of their relatively low cost, high efficiency and small form factor, compared to more traditional passive linear solutions. One disadvantage of the SMPS however is the lower power factor and Harmonic content generated by their non liner behaviors (switching).

Power Factor is the ratio of real power in watts (W) to apparent power (product of RMS current and RMS voltage) in VA (volt-amperes) and is represented as a decimal fraction between 0 and 1. The components apparent power traditionally arise from the effects of relative loads (capacitors and inductors) on an AC power supply, which cause the current to either lead (capacitive) or lag (inductive) the AC voltage. A power factor of 1 indicates a purely resistive load with the voltage and current waveforms in phase. When the current and voltage are out of phase, the reactive power increases and the apparent power, the vector sum of real and reactive power, increases. this increase in reactive power causes less efficient power transmission, less of the power is used to do real work. This is a undesirable situation, not least because the utilities companies charge per unit of apparent power consumed.

In the case of switched mode power supplies the situation is somewhat different, as it is the non-linearity of the current waveform rather then a phase shift which causes problems. Switched mode power supplies as the name suggests are constantly switching, causing a pulsed current profile. The current waveform isn't out of phase with the voltage waveform, but it is now no longer sinusoidal, and this has the effect of creating harmonics which propagate back though the supply lines. This generation of harmonics causes energy to be lost and as before the ratio of the real and apparent power (Irms * Vrms) gives a measurement of performance as a Power factor rating for the device. Transmission of such harmonics on the mains is undesirable as it can interfere with the operation of other devices. To prevent this, most regions have regulations which define acceptable harmonic current emissions that devices must meet, the European standard EN61000-3-2 for example. Power Factor Correction techniques for SMPS fall into 2 categories, Passive control and active control. Passive techniques employ a low pass filter between the AC/DC rectification stage and DC/DC converter stage to filter the harmonic components. This technique is less expensive and typically more reliable than active PF correction techniques due to lower component count and lack of need for any switching in the PFC circuit, but doesn't provide as high a PF correction. Active PFC is usually achieved by introduction of a boost converter between the AC rectification stage and DC/DC conversion stage. The boost converter provides continuous input current that can be shaped through use of a multiplier and average current mode control to achieve a near unity power factor.

Other Resources

• AN-42047 Power Factor Correction (PFC) Basics
• Why Switch mode Power Supplies Need PFC
• Specifying the Best PFC Topology for Your Power Supply
• Switched-mode power supply
• Active PFC for Electronic Power Supplies
• Line Current Harmonic Analysis of Active PFC Circuits
• Power factor - Wikipedia, the free encyclopedia
• Guidelines to the standard EN 61000-3-2