POWER FACTOR CORRECTION
Increasingly, switched mode power supplies (SMPS) are designed with an active power factor correction (PFC) input stage. This is mainly to meet new regulations aimed at restricting the harmonic content of the load current drawn from power lines. Both users and power companies benefit from PFC, as does the environment.
Power Factor Correction (PFC) can be defined as the reduction of the harmonic content, and/or the aligning of the phase angle of incoming current so that it is in phase with the line voltage. Mathematically, Power Factor (PF) is equal to Real Power divided by Apparent Power. The basic concept is to make the input look like a pure resistor. Resistors have a power factor of 1 (unity). This allows the power distribution system to operate at maximum efficiency, which reduces energy consumption.
Non-PFC power supplies use a capacitive filter at the AC input. This results in rectification of the AC line, causes high peak currents at the crests of the AC voltage. These peak currents lead to excessive voltage drops in the wiring and imbalance problems in the three-phase power delivery system. The full energy potential of the AC line is not utilized. Nonlinear peak currents also distort output voltage and create harmonics. There is an international standard for controlling harmonics (IEC100-3-2) and PFC is mandatory for home appliances consuming 70W or more power in EU nations as of January, 2001.
PFC circuits are classified into two types: active and passive.
Passive PFC uses passive elements such as a ferrite core inductor on the input source to create a countering reactance. While easily applied to the existing power circuitry without much modification, the power factor is low (60 - 80%), the AC input must be chosen (115VAC / 230VAC), and the harmonics produced from the difference between the capacitance and the inductance are hard to control. Significant electromagnetic noise can result.
Active PFC uses switching regulator technology with active elements such as IC, FET and diodes, to create a PFC circuit This circuit has a theoretical power factor of over 95%, reduces total harmonics noticeably, and automatically adjusts for AC input voltage. However, it requires a complex EMI filter and an input source circuit, and is more costly to build.
The benefits of high PF for the user comes from the reduced AC current drawn by high PF PSUs, not in any savings from electricity bills, except in the case of commercial utility users who do pay for V(oltage) x A(mperes). There are two broad consequences:
Less stress on the AC electrical wiring: The lower current drawn by a high PF power supply means that there is less stress on the electrical wiring of the building. This can be a big plus in the case of older building with lower capacity AC wiring. It is certainly easy to see the benefits in a enterprise setting where dozens or hundreds of PCs are drawing power. If the total current load from the IT department could be reduced by 30% or more, this would be very signficant in direct electricity savings, reduced airconditioning cost, and possible avoidance of building AC re-wiring.
Lower UPS costs: Lower current draw also means that smaller capacity Uninterruptible Power Supply (UPS) units can be used. As UPS units are priced in direct proportion to their current capacity (VA), a PF of 0.98 versus one of 0.6 can traslate into a 40% reduction in purchase cost. Again, in an enterprise setting with hundreds or thousands of PCs, the savings can be very significant.
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