PSU question: what is PFC Power Factor Correction?

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Robin Henderson

ungelesen,
19.04.2002, 00:30:4219.04.02
an
Hi again,

got another PSU question: what is PFC? What does it do? Is it necessary?

thanks again!

cheers
Robin


Hawk

ungelesen,
19.04.2002, 12:10:2519.04.02
an

"Robin Henderson" <Rob...@hotmail.com> wrote:
> Hi again,
>
> got another PSU question: what is PFC? What does it do? Is it necessary?
>
> thanks again!

A PFC front-end in a power supply causes the supply to look like a resistive
load to the AC line. It effectively puts the input current in phase with the
line and makes the input current look sinusoidal. The main objective of PFC is
to minimize the noise and distortion that can be induced on the AC line by heavy
loads. Instead of pulling spikes of current at the peaks of the line, you are
pulling a nice sinusoidal current.

Generally the PFC functionality is incorporated in some sort of a boost
front-end which allows you to run through a wide range of input voltage and
frequencies. Usually these boost sections generate a 400VDC bulk voltage and
the switching output converters run off of this 400V bulk. A PFC boost has no
need for a 110VAC / 220VAC set switch on the unit, it will automatically run
properly with just about any AC input (90-264 VAC 45-440Hz). The exact range
of operation will vary across different makes/models.

The convential PC power supplies that have an AC line set switch use a voltage
doubler configuration to get the high bulk voltage needed to run the output
converters.

The nice thing about PFC boosters from a sales point of view...is that you
design one power supply that will operate anywhere in the world. All you have
to do is send them the correct input cord, instead of designing a special power
supply for each potential input voltage/frequency.


PFC in a small desktop PC power supply probably doesnt make a whole lot of sense
for a couple of reasons.

1. The average 300-400W supply is not pulling much current from the line.

2. Most people don't move their desktops from country to country so being able
to handle wide AC line voltages/frequencies is a non-factor.


(*>


Robin Henderson

ungelesen,
20.04.2002, 10:56:3720.04.02
an
>
>The nice thing about PFC boosters from a sales point of view...is that you
>design one power supply that will operate anywhere in the world. All you
have
>to do is send them the correct input cord, instead of designing a special
power
>supply for each potential input voltage/frequency.

So is this the same as a 'switching' power supply?

>
>
>PFC in a small desktop PC power supply probably doesnt make a whole lot of
sense
>for a couple of reasons.
>
>1. The average 300-400W supply is not pulling much current from the line.
>

So does that mean there's no need to base a buying decision on whether the psu
has pfc feature?

And does that still apply 420-470W psus? (still deciding which wattage to go
for)

thanks heaps for your reply, cheers

Robin


Hawk

ungelesen,
20.04.2002, 16:17:4820.04.02
an

"Robin Henderson" <Rob...@hotmail.com> wrote:

> So is this the same as a 'switching' power supply?

I dont think that you could find a PC power supply using linear regulators for
the main high current power rails. Linear regulators are far less efficient,
need larger heatsinks, and cost more than a switching design.


> PFC in a small desktop PC power supply probably doesnt make a whole lot of
> sense for a couple of reasons.
>
> 1. The average 300-400W supply is not pulling much current from the line.
>
> So does that mean there's no need to base a buying decision on whether the psu
> has pfc feature?
>
> And does that still apply 420-470W psus? (still deciding which wattage to go
> for)

I personally wouldn't spend more money just to get a PC supply with PFC. As the
end user you are not going to gain any advantage in having it.

PFC become more of an important issue in an industrial setting where you may
have many large pieces of equipment connected to the same power grid. You
wouldn't want some high powered piece of equipment causing sags and dips in the
line that could cause problems for other machines. For a home user it's a
non-factor.


(*>


Robin Henderson

ungelesen,
20.04.2002, 19:47:0720.04.02
an
>> So is this the same as a 'switching' power supply?
>
>I dont think that you could find a PC power supply using linear regulators
for
>the main high current power rails. Linear regulators are far less efficient,
>need larger heatsinks, and cost more than a switching design.

So what is the difference between a switching power supply as opposed to a pfc
power supply?

And is linear regulation the same as the 'voltage doubler' you mentioned
earlier?

So in summary, for a say 420-470W PC psu, which of the following do I look for
then?
- switching, or
- voltage doubler, or
- pfc, or
- linear regulation, or
- max combined +3.3V, +5V, or
- max combined +3.3v, +5V, +12V, or
- max current rating on each individual rail
(ie, is max combined rating more important than the
max rating for individual rail? I can find a psu
with high individual rail current rating, but
has the same (or slightly lower) combined rating
than another psu that has a lower invidiual
rail max rating)

I don't know much about psu, so please explain and correct me if I'm using the
terms and definitions wrong.

Thanks again, cheers

Robin

Hawk

ungelesen,
20.04.2002, 23:40:0020.04.02
an

"Robin Henderson" <Rob...@hotmail.com> wrote:
> So what is the difference between a switching power supply as opposed to a pfc
> power supply?
>

Power Factor Correction is what makes the power supply look like a resistive
load to the AC line. It is really a separate subject from the type of
converters that are inside of the supply. In my former post I was saying that
units with PFC usually have a switching boost converter to generate this high
voltage instead of a simple voltage doubling circuit. The switching output
converters are a separate entity.

> And is linear regulation the same as the 'voltage doubler' you mentioned
> earlier?

No. A voltage doubler is used in the average PC power supply to create a high
internal bulk voltage from low line (120VAC). The switching output converters
use this high DC bulk voltage as their power source.

A linear regulator is simply a type of voltage regulator (as opposed to
switching regulators). A linear regulator can be thought of as kind of a
dynamic resistance between the input voltage and output that varies its value to
maintain a steady output voltage. The main disadvantage of a linear regulator
is that it has to dissipate a fair amount of power. Power = voltage x
current.....so the larger the voltage differential between input and output, and
the higher the current level the more power the regulator has to dissipate.

A switching regulator chops the input voltage into a controlled square wave that
is averaged out with an output filter made up of a correctly chosen combination
of output inductor and capacitors. The advantage is that you dissipate far less
power in the regulator as the duty cycle is altered to maintain regulation.
There are periods where the converter FET's/Transistors are off...


> So in summary, for a say 420-470W PC psu, which of the following do I look for
> then?
> - switching, or
> - voltage doubler, or
> - pfc, or
> - linear regulation, or
> - max combined +3.3V, +5V, or
> - max combined +3.3v, +5V, +12V, or
> - max current rating on each individual rail
> (ie, is max combined rating more important than the
> max rating for individual rail? I can find a psu
> with high individual rail current rating, but
> has the same (or slightly lower) combined rating
> than another psu that has a lower invidiual
> rail max rating)

So my recommendation is....

You don't need power factor correction...a voltage doubler input will work just
fine so don't pay extra for PFC.

Total combined power can be somewhat deceiving...you are really concerned that
each rail will be able to supply enough current (and consequently power) to
power all of the devices that need that rail. A simplified exaggerated example
would be:

(Keep in mind that I am making these numbers up for sake of explanation....they
may not be anywhere near reality)

===Power Supply A===

3.3V @ 40A 132W
12V @ 10A 120W
5V @ 25A 125W

377W Total Power

===Power Supply B===

3.3V @ 40A 132W
12V @ 2A 24W
5V @ 50A 250W

406W Total Power

===Your System Requires===

3.3V @ 34A
12V @ 8A
5V @ 20A


In this example....if you were to look at total power you would think that Power
Supply B is the better choice. But if your system is going to require more than
2A from the 12V channel...it's not going to work. Power Supply A would in fact
be the better choice in this example, because it is able to deliver the required
amount of power on all rails...and it happens to be rated 29W less overall.

The only way to know what you really need is to add up the power requirements of
all the hardware you intend to use, and make your decision based on that. I
think that you will find that most 300W and up supplies will be adequate for
todays PC's...but you need to be sure that you dont fall into the trap that I am
trying to illustrate with my example.

Sorry if I have overcomplicated this...I just got going and couldn't stop...ha
ha

(*>


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