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Active PFC ?
- Thread starter Trikzy
- Start date
It is an circuit that helps reduce AC current draw, lowering your energy bill.Trikzy said:
Do you live in a European Union country? If so, then yes. Check your local regulations if it's a concern to you. Some areas now require PFC circuits in many electrical appliances.Trikzy said:
Active PFC won't lower your energy bill. The meter on the side of your house measures watts, not VA...
"Active PFC" just makes the current waveform drawn by the supply more sinusoidal in shape. The effect is that less RMS current is drawn by the power supply, but the wattage drawn by the supply remains the same. Besides, the active PFC circuit adds an extra stage to the power supply which does lower the efficiency of the supply, so technically your electric bill will go up!
"Active PFC" just makes the current waveform drawn by the supply more sinusoidal in shape. The effect is that less RMS current is drawn by the power supply, but the wattage drawn by the supply remains the same. Besides, the active PFC circuit adds an extra stage to the power supply which does lower the efficiency of the supply, so technically your electric bill will go up!
Active PFC makes your PSU more efficent by correcting Power Factor(hence the name).
power factor is when the voltage and current waveforms don't line up correctly. when the waveforms aren't aligned correctly, this is where you get VA ratings. this also means that to create the same amount of DC current on one end, it'll have to draw more current on the AC end. if the waveforms are aligned correctly, this doesn't happen and it's actually is more efficent.
power factor is when the voltage and current waveforms don't line up correctly. when the waveforms aren't aligned correctly, this is where you get VA ratings. this also means that to create the same amount of DC current on one end, it'll have to draw more current on the AC end. if the waveforms are aligned correctly, this doesn't happen and it's actually is more efficent.
With most non-PFC supplies, the filtering (caps/chokes/etc) at the input side of the supply ensure that the voltage/current waveforms at 60hz are lined up - This is the definition of "power factor" that is taught in most schools, and a non-PFC switching power supply is good by that standard.. but power factor is more complex than that. Harmonics in the current waveform drawn by a power supply also contribute to power factor.felix88 said:
In 99% of switching power supplies, the incoming AC signal is immediately rectified into DC, and that DC charges a bulk input capacitance. What ends up happening is that on the rising edge of the AC waveform, no current is drawn until the AC voltage hits the voltage on the capacitor. The current then "bangs on" until the AC voltage hits its peak and the capacitor is fully charged, and the current then drops to zero. The AC voltage then swings the other way until the negative AC voltage once again hits the input capacitor voltage and bang, the current's on again.
So the current waveform isn't sinusoidal at all - it's more of a "chopped up" sine wave which has a lot of high frequency components. The input filtering on a power supply does attenuate a lot of this and make the waveform look more sine-like, but that only works so well.
Things like AC transformers *don't* like these waveforms - not only is there wasted current in the harmonics, the high frequency current waveforms cause core loss to occur in transformers. If you have an office building full of non-PFC supplies or a large data center, this can make a transformer overheat and catch fire even if you were only running it at a fraction of its rated load! This is why the EU and other standards bodies are trying to make PFC mandatory; since more and more household devices are using non-PFC switching power supplies now, it makes things harder for the utility companies.
What Active PFC does is "fix" the shape of the current waveform.
In most implementations of active PFC, the AC input is rectified and fed into a capacitor, but the capacitor is much smaller than the usual input capacitor of a power supply. A small current-mode DC-DC converter takes that voltage and boosts it up to a higher voltage, which is then stored in a big capacitor for the rest of the power supply to use. This converter is designed so that the current it draws is directly proportional to the voltage coming in, which forces the power supply's current waveform to be sinusoidal. You do get some high frequency switching noise occuring on the current going into the supply, but a very simple filter is all that's needed to get rid of that.
This extra DC-DC converter is only so efficient - maybe 90% - and this makes active PFC power supplies less efficient. So active PFC supplies will run hotter (so use caution if you're modifying one or putting a quiet fan in one) and they will draw more electricity.
Technically, the RMS current drawn by a PFC supply will be lower than a comparable non-PFC supply.felix88 said:bah, i guess i need to go back and finish my EE degree.
i just find it odd that everything i read about PFC states that it will use less current than a comparable non PFC unit. i must be missing the point.
The overall *power* drawn by a PFC supply will likely be higher.
I
Ice Czar
Guest
a slightly more layman oriented description
PFC decoded by that master of the lay speak dan of dansdata
of course his article is roughly three times as long a gee's explaination as well
PFC decoded by that master of the lay speak dan of dansdata
of course his article is roughly three times as long a gee's explaination as well
archevilangel
[H]ard|Gawd
- Joined
- Jan 15, 2003
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good to know I liked dan's explanation. I just bought a enermax noisetaker 470 watt w/ active pfc recently too, only thing I dislike, are the cables are too stiff.
I liked dan's explanation. I just bought a enermax noisetaker 470 watt w/ active pfc recently too, only thing I dislike, are the cables are too stiff.