What does "PFC" mean with regard to a power supply?

[ddeder]

Some power supplys have PFC while others do not. Just wondering what this is. Thanks.

 

[amdskip]

I believe it is saying it has fan control meaning the fans speed is variable depending on the temperature inside the power supply.

 

[sharkeeper]

PFC = Power Factor Correction.

-DAK-

 

[bandana163]

No fan speed control, just marketing.

 

[Mark R]

As stated it means power factor correction.

This is essentially of no benefit to you at all. It won't improve the quality of the delivered power, it won't reduce heat production and it won't save you money. There is a small amount of benefit to the electricity supplier. There is no reason why you should replace your PSU just for PFC - but if you can get PFC it won't hurt you and improves efficiency for your supplier.

 

[Mday]

PFC is useless in lower power consumption devices. it's effectiveness is negligible.

 

[sharkeeper]

This is essentially of no benefit to you at all. It won't improve the quality of the delivered power, it won't reduce heat production and it won't save you money. There is a small amount of benefit to the electricity supplier. There is no reason why you should replace your PSU just for PFC - but if you can get PFC it won't hurt you and improves efficiency for your supplier.

It may not benefit a home user with a single computer or even two on a branch circuit. However, to an enterprise user that has hundreds of servers you can bet they rely on power factor correction! Why? An off-line switch-mode power supply can draw approximately 950 Watts from a typical 110VAC wall socket protected by a 15A circuit breaker before exceeding the UL mandated limit. A simple load like a toaster can draw almost 1400 Watts. The difference between the two is due to the low power factor (PF) of the switch-mode power supply. If we correct the PF the switch-mode supply can draw about as much power as the toaster can, allowing it to power a larger load from the same 110VAC 15A wall socket.


Power factor (PF) is technically the ratio of real power consumed to apparent power (volts RMS x amps RMS), and is expressed as a decimal fraction between 0 and 1. PF is traditionally known as the phase difference between sinusoidal voltage and current waveforms. When the AC load
is partly capacitive or inductive, the current waveform is out of phase with the voltage. This requires additional AC current to be generated which isn't consumed by the load, creating I2R losses in power cables.


A simple resistor has the highest PF - one. An AC voltage across the resistor causes an AC current which is identical to and in phase with the voltage waveform. An electric motor is inductive, especially when it is starting. The current waveform lags behind the voltage waveform,
dropping the PF to well below 1. This is why many motors have 'starting' capacitors fitted to counteract the inductance,and therefore correct the PF during motor startup.


A switch-mode power supply when viewed as an AC load, is neither capacitive nor inductive, but non-linear. A switchmode supply conducts current in short pulses that are in phase with the Line Voltage. The product of (volts RMS x amps RMS) is considerable higher than the real power
consumed, and thus the PF is less than 1, typically around 0.65.


Non-unity PF can be improved with power factor correctors (PFC). The types used for switchmode power supplies 'smooth out' the pulsating AC current, lowering its RMS value, improving the PF, and reducing the chances of a circuit breaker tripping. There are two basic types of PFC:
active and passive. Active PFC is more effective, more expensive, generally integrated with the switch-mode power supply, and can achieve about 0.98PF. Passive PFC is more reliable, inexpensive, and typically corrects to about 0.90.

To determine just how much more power is available with PFC, the user needs to understand the following equation,which defines the amount of power available from the supply:

Pout = VL(RMS) x IL (RMS) x PF x Eff

For example, UL limits a system's line current to 80% of the circuit breaker's rating. For a typical 15A breaker 12A is the maximum allowed, and the best-case power available is (120VAC x 12A) = 1440Watts. A switch-mode supply with 85% efficiency and 0.65PF can only deliver (120 x 12 x 0.65 x 0.85=) 796 Watts. However, if the power factor is corrected to 0.98, the same power supply can now deliver (120 x 12 x 0.98 x 0.85)= 1200 Watts, a 51% increase!

The other component (and advantage) of PFC is the reduction of line harmonics. Line harmonics are created whenever the line current is not a pure sine wave, as is the case with a switch-mode power supply's input 'pulsed' currents. Measuring line harmonics is simply a mathematical means to describe a complex waveform's PF by resolving it into a fundamental frequency and its many harmonics. The harmonic currents do not contribute to load power, but cause unwanted heating in the wall socket, wiring and circuit breaker and distribution transformer. PFC can eliminate harmonics, eaving just the pure 'fundamental' frequency.

Switch-Mode power supplies inherently conduct line current in short pulses, which is not desirable. This less than optimum current can be quantified by measuring its PF or by its harmonic content. Poor PF causes unwanted heating in external AC circuits, which can be reduced by correcting the PF.


-DAK-

 

[buleyb]

yeah, like he said

 

[WalkingDead]

too much info...that's why I'm not an engineer material....

So, is it worth the $10-15 for the the active or passive PFC function?


Here's what Seasonic said about active PFC

 

[Auric]

It pays for itself. Higher efficiency means less power useage, less heat and thus less cooling necessary (can run slower, quieter fan). The only way it would make sense not to have active PFC is if you are a cheap builder and just want to cut inital costs.

 

[IvIaNTiS]

So for someone with one computer at home, is there any real benefits? Will I actually save money on the electric bills?

 

[thorin]

<grin> My whole system is PFC .........

Pretty F'n Cool </grin>

TGIF everyone!
Thorin

 

[Mday]

Originally posted by: Auric
It pays for itself. Higher efficiency means less power useage, less heat and thus less cooling necessary (can run slower, quieter fan). The only way it would make sense not to have active PFC is if you are a cheap builder and just want to cut inital costs.

ya, after like 100 years for the home user.

 

[Mark R]

It pays for itself. Higher efficiency means less power useage, less heat and thus less cooling necessary (can run slower, quieter fan).

But the key point is that a PSU with PFC is NOT more efficient than one without - in fact, it is probably less efficient, generating more heat and needing more cooling. I've come across a number of manufacturers who quote efficiency of 70% for their standard PSUs and 65% for their PFC PSUs.

It does however, improve efficiency in the power distribution system. As stated earlier - if you are an industrial user, this means more PCs on each power circuit and thinner cabling, potentially together with a lower power bill. For a domestic user, you pay only for the energy that you actually use - PFC makes no difference to your energy costs. In fact, the lower effeciency of PFC supplies may even inrease your bill slightly.