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What is power factor? What is the power factor of a switching power supply?

Power factor is the ratio of the real power to apparent power utilized. It is typically expressed as a decimal number less than 1. Real power is expressed in watts while apparent power is expressed in VA (volt-amperes). The power factor of a switching power supply depends upon what type of AC input is used: single phase, three phase, or active power factor corrected.

Single phase uncorrected switching power supplies typically have fairly poor power factor, like 0.65. This is because most units utilize a rectifier/capacitor “front end” to make a DC buss voltage. This configuration only draws current at the peak of each line cycle, creating narrow, high pulses of current which results in poor power factor.

Three phase uncorrected switching power supplies have a higher power factor, like 0.85. This is because even though a rectifier/capacitor is used to make a DC buss voltage, there are three phases which additively improves the overall power factor.

Units with active power factor correction circuitry can have very good power factor like 0.98. Essentially a front end converter is used in boost mode to make the line current draw mimic the line voltage, dramatically increasing the power factor.

Why is power factor important? Uncorrected units tend to limit what power you can get out of a particular electrical service. All line cords, circuit breakers, connectors and electrical services must be rated for the maximum current drawn. With uncorrected power supplies this tends to limit the actual power that can be provided from a particular service. When an active power factor corrected power supply is used the maximum line currents are much lower, allowing more current to be utilized by the supply and higher power supply output power can be provided by the same electrical service.

Additionally harmonic noise imposed by the power supply to the AC line can also be lower. Another benefit is active power factor correction can provide universal input voltage capability, allowing power supplies to operate worldwide on a wide range of input voltages like 88Vac to 264Vac. Power factor corrected units, they certainly have some benefits.
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