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Old 06-21-2019, 05:54 AM   #1
Hitto
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Default Bypassing Passive PFC In ATX Power Supplies

I'm planning to remove and bypass the passive PFC transformer/coil on an ATX power supply.

I don't care about reduced efficiency. I'm worried, howewer, about any problem in terms of increased ripple in the primary rail.

Does the removal of the passive PFC has negligible downsides in terms of increased ripple?

Thank you.
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Old 06-21-2019, 11:15 AM   #2
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Default Re: Bypassing Passive PFC In ATX Power Supplies

i doubt it effects the ripple.
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Old 06-21-2019, 03:08 PM   #3
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Default Re: Bypassing Passive PFC In ATX Power Supplies

But it reduces harmonics, correct? It looks that it may have some benefits in terms of less stress placed to the components placed onto the primary (based onto the following topic: https://www.badcaps.net/forum/showthread.php?t=67663).
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Old 06-21-2019, 03:31 PM   #4
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Default Re: Bypassing Passive PFC In ATX Power Supplies

passive PFC is just a big fat coil.
So its only a bridge you need.
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Old 06-23-2019, 09:33 AM   #5
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Default Re: Bypassing Passive PFC In ATX Power Supplies

That's good to hear.

If you have more informations about any possible drawback (I'm going to upgrade the primary capacitors from 560uF to 820uF and the goal is to combine as many improvements as possible) feel free to write them.
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Old 06-23-2019, 11:58 AM   #6
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Default Re: Bypassing Passive PFC In ATX Power Supplies

You MIGHT experience slight bulging on some PSUs. YMMV depending on how good was the PSU built.

The one I fixed in that thread you mentioned managed to somehow make a Hitachi HP3 cap (820uF 200v if my memory serves me right) slightly bulge. The PSU is still running fine, although I sold the PC I installed the PSU in and bought a GTX750Ti.
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Old 07-06-2019, 08:29 PM   #7
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Default Re: Bypassing Passive PFC In ATX Power Supplies

Quote:
Originally Posted by Hitto View Post
But it reduces harmonics, correct? It looks that it may have some benefits in terms of less stress placed to the components placed onto the primary (based onto the following topic: https://www.badcaps.net/forum/showthread.php?t=67663).
No, that would be APFC which puts more stress on the primary caps.

PPFC (large line-connected inductor) *shouldn't* put more stress on any of the primary components. However, the reasons I say *shouldn't* is because there are cases where PPFC can stress the primary caps. Like all inductors, a PPFC coil may produce significant inductive kickback in the event of sudden power loss. Thus, if you live in an area where you get frequent power outages and power dips (or very dirty power), then it may be possible for the PPFC coil to put more stress on the primary caps. But in general, that is very rare. So I suggest you leave it, as you get added benefits of extra filtering from noise on the line input and better PF.

Last edited by momaka; 07-06-2019 at 08:33 PM..
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Old 07-11-2019, 06:35 AM   #8
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Default Re: Bypassing Passive PFC In ATX Power Supplies

Quote:
So I suggest you leave it, as you get added benefits of extra filtering from noise on the line input and better PF.
It should also limit inrush current, isn't it?

By the way, this power supply has no NTC. Which should be a practice for increased efficiency but I do not like that.

I am planning to add a 2.5 Ohm NTC: I know that the value is usually higher (8-10 Ohm) but it is just for reducing a bit the inrush current.

The rating of the NTC is 5.5A but I do not know if it refers to the instantaneous current or the operating current.

Last edited by Hitto; 07-11-2019 at 06:39 AM..
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Old 07-11-2019, 02:14 PM   #9
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Default Re: Bypassing Passive PFC In ATX Power Supplies

NTC has nothing to do with efficiency, once the NTC gets hot the resistance will go down but will not be 0 Ohms so you will have power loss on the NTC.
The purpose of BTC is to limit the inrush current when power is first applied to the circuit, once that is over the NTC should be bypassed so it is no longer in the circuit.
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