# PFC - Passive or Active?

J

#### Justin

For a desktop PC (MicroATX) that will be around 450 watts - should I
look for a Passive PFC power supply? I researches what PFC is and the
passive is better from what I can tell.

J

#### JR Weiss

Justin said:
For a desktop PC (MicroATX) that will be around 450 watts - should I
look for a Passive PFC power supply? I researches what PFC is and
the passive is better from what I can tell.
First google response I get is:

The preferable type of PFC is Active Power Factor Correction (Active
PFC) since it provides more efficient power frequency. Because Active
PFC uses a circuit to correct power factor, Active PFC is able to
generate a theoretical power factor of over 95%. Active Power Factor
Correction also markedly diminishes total harmonics, automatically
corrects for AC input voltage, and is capable of a full range of input
voltage. Since Active PFC is the more complex method of Power Factor
Correction, it is more expensive to produce an Active PFC power supply.

The most common type of PFC is Passive Power Factor Correction
(Passive PFC). Passive PFC uses a capacitive filter at the AC input to
correct poor power factor. Passive PFC may be affected when
environmental vibration occurs. Passive PFC requires that the AC input
voltage be set manually. Passive PFC also does not use the full energy
potential of the AC line.

Frankly, I don't understand any of that. What I DO understand is that
450 Watts is a LOT of power for a computer, especially in a microATX
case. What is going to be in there -- a portable max gaming system?!?

J

#### Justin

JR said:
First google response I get is:

The preferable type of PFC is Active Power Factor Correction (Active
PFC) since it provides more efficient power frequency. Because Active
PFC uses a circuit to correct power factor, Active PFC is able to
generate a theoretical power factor of over 95%. Active Power Factor
Correction also markedly diminishes total harmonics, automatically
corrects for AC input voltage, and is capable of a full range of input
voltage. Since Active PFC is the more complex method of Power Factor
Correction, it is more expensive to produce an Active PFC power supply.

The most common type of PFC is Passive Power Factor Correction
(Passive PFC). Passive PFC uses a capacitive filter at the AC input to
correct poor power factor. Passive PFC may be affected when
environmental vibration occurs. Passive PFC requires that the AC input
voltage be set manually. Passive PFC also does not use the full energy
potential of the AC line.

Frankly, I don't understand any of that. What I DO understand is that
450 Watts is a LOT of power for a computer, especially in a microATX
case. What is going to be in there -- a portable max gaming system?!?
I just tossed that number out there since that was the de facto standard
for normal ATX.
I wiki'd PFC and got this off there:
"Passive PFCs are typically more power efficient than active PFCs – a
passive PFC on a switching computer PSU has a typical power efficiency
of around 96%, while an active PFC has a typical efficiency of about 94%."

P

#### Paul

Justin said:
I just tossed that number out there since that was the de facto standard
for normal ATX.
I wiki'd PFC and got this off there:
"Passive PFCs are typically more power efficient than active PFCs – a
passive PFC on a switching computer PSU has a typical power efficiency
of around 96%, while an active PFC has a typical efficiency of about 94%."
Active PFC might not be quite as efficient.

You can see what the various flavors do to the input waveforms.

No correction to the current waveform, is shown in this one. So this
one doesn't have power factor correction.

http://www.xbitlabs.com/articles/coolers/display/psu-methodology_8.html

The next page, has two pictures.

Passive PFC uses a choke, to counteract the "capacitive" power factor
of the switching supply.

You'll notice the active PFC current waveform, looks pretty nice. It is
almost sinusoidal. It is in phase with the voltage waveform. Effectively,
the Active PFC supply looks the same to the power company, as an incandescent
light bulb - purely resistive.

http://www.xbitlabs.com/articles/coolers/display/psu-methodology_9.html

In North America, typically home owners are not billed for reactive
power, so there is no incentive to do power factor correction. In
other areas of the world, it may be mandated, that products have
power factor correction. Which is why you'll notice subtle differences
in what products are available in North America, versus elsewhere.

Paul

G

#### Guest

Justin said:
For a desktop PC (MicroATX) that will be around 450 watts - should I
look for a Passive PFC power supply? I researches what PFC is and the
passive is better from what I can tell.
Passive PFC typically gives a power factor of around 0.80 (ideal is
1.00), while active PFC commonly gives 0.95, but power factor is of
concern to consumer uses only in regards to the size of any backup
power supply (often incorrectly called UPS) - higher power factor
means a backup with a lower VA (volts*amps) rating can be used for a

PFC doesn't improve efficiency but actually reduces it slightly, by as
much as 5%, since the related components waste some power.

You should concern yourself much, much more with the quality of the
power supply, owing to all the low quality supplies being sold. Check
reviews that measure to at least the full power rating of the product
and not merely with a computer as the load (200-300 watts).

N

#### NT

For a desktop PC (MicroATX) that will be around 450 watts - should I
look for a Passive PFC power supply?  I researches what PFC is and the
passive is better from what I can tell.
For 99% of users, pfc of any kind is totally irrelevant. It matters if
youre running the system fof homebrewed leccy or a UPS

NT

J

#### Justin

NT said:
For 99% of users, pfc of any kind is totally irrelevant. It matters if
youre running the system fof homebrewed leccy or a UPS

NT
Hi, I don't understand what that is. You mean a server?

D

#### david

Hi, I don't understand what that is. You mean a server?
I think he means self-generated electric power, or a UPS.

N

#### NT

I think he means self-generated electric power, or a UPS.
UPS = Uninterruptible Power Supply

NT