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Active PFC and square wave supply

Discussion in 'Electronic Design' started by Pimpom, Jun 27, 2013.

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  1. Pimpom

    Pimpom Guest

    I started using a Tagan 700BZ PSU in my computer a couple of months
    ago - my first PSU with active PFC. The UPS is a basic one that has
    served me well for 10 years, and outputs a square wave on battery.
    While setting up this particular combination, I briefly wondered how
    the PFC circuit would react to working with non-sinusoidal power but
    then forgot about it - until it died a couple of days ago. It blew the
    fuse on the UPS and shows a short at the AC input. One of the two
    paralleled 20N60C3 MOSFETs was a dead short.

    The UPS battery - a 70Ah car battery - is of the same age as the UPS
    (10 yrs) and has little backup power left. So I always shut my
    computer down quickly in the event of a power outage, of which there
    were dozens during the 2 months I've been using this PSU.

    The PSU died while I was in another room. I didn't notice a power
    outage or fluctuation during that time but can't be 100% certain. Is
    it likely that there was an outage and the PFC transistor was finally
    killed by the square-wave supply?

    To put it another way, if I repair the PSU, is it likely to go poof
    again unless I change my UPS to a more sophisticated one? I'd never
    done a detailed study of practical active PFCs before and could well
    miss something with a quick analysis.

    (I did some online reading about active PFCs and non-sinusoidal
    supplies after my PSU died but they're mostly end-user discussions and
    are all inconclusive).
     
  2. An interesting test would be to see what a PFC does when you feed it plain
    old DC.

    Has anybody exploded any switching power supplies of any sort by just
    feeding them DC, filtered or with 120Hz ripple? They should int theory
    love that, but I'm not sure about the autoranging device they use before
    the filter caps that either rectifies or doubles the AC input.
     
  3. miso

    miso Guest

    My advice is use a double conversion true sine UPS or none at all. With
    a double conversion UPS, you will know soon enough if everything is
    compatible when the fecal matter hits the fan. That is, line and battery
    operation are similar since the inverter is always running.

    If you go through
    http://www.xbitlabs.com
    website, they test power supplies run from a crappy UPS. just to see how
    much power they can deliver before the UPS protection circuitry cuts in.
    Some supplies really piss off the UPS!

    The drawback to double conversion is the thing is running 24 and 7. And
    it makes noise. Oh, and they aren't cheap.

    The advantage to double conversion is you have no brownout issues, line
    to battery switchover is fast since the inverter is already running, and
    the surge suppression is as good as it gets since they are filtering DC
    prior to the inversion.
     
  4. Pimpom

    Pimpom Guest

    You're right in that my UPS is not true square wave but has a
    dead time in between transitions. My son has been using a PSU
    with active PFC for more than a year and his 800VA UPS ($45 with
    a 9Ah internal battery) outputs the same type of waveform except
    that mine has noticeably steeper edges. Maybe that's what makes
    the difference. Some of my friends are also using PSUs with PFC
    and, judging from what they paid for their UPSes, I'm sure none
    of them is a sinusoidal type.

    The focus here is on how the PFC components bear up to a
    non-sinusoidal input rather than on the actual PF correction.
    Similarly, although my computer needs less than half of the PSU's
    700W rating, I used it because it returned good performance
    figures in reviews and I got a used unit cheap.
     
  5. I am pretty curious about this, but not enough to test it on my computer.

    Modern computers do lack AC blowers, real power switches and LCDs lack
    degaussing coils, so I decided to see what I switching power supplies I
    can explode with DC.

    Then things got "interesting". The laziest place to get 160or 340VDC is
    from a switching power supply. I grabbed some genericy open fram 110 watt
    unit from Artesyn and noticed it only had one, not two filter caps on the
    HV side- a single 400V fed by just a 4 pin bridge rectifier and the usualy
    noise filtering stuff. There was no autoranging chip, that that would do
    anything too useful with a single filter cap. The label on the power
    supply clearly states 100-240VAC 50/60Hz. I plugged it in and sure enough,
    there was only 164V on the filter cap. I'm going to have to guess you
    330-ish if you feed it 240. I've never seen that before. So at least for
    this power supply, it would seem feeding it any type of DC would probably
    be OK. The real test is to run this power supply off the filtered 164V
    from it's twin.

    Then it's off to see what happens to stuff like power adapter bricks and
    maybe a CFL bulb. Anybody want to vote on if a low end CFL bulb with light
    up (flames don't count in this case) on DC?
     
  6. I don't have the answer, but the middle of this page shows some types of
    PFC methods and how they work:

    http://www.fairchildsemi.com/applications/motor-control/solutions/power-factor-correction/

    My guess is whatever is cheapest to make and just passes interference
    standards is what you're going to see in a PC power supply.

    The boost one IS just a plain switching power supply anyways, so I'm not
    sure why you'd string yet another switching power supply after that.
     
  7. Fred Abse

    Fred Abse Guest

    Most, if not all PFC circuits use feedforward from the incoming waveform.
     
  8. miso

    miso Guest

    I ruined a notebook power supply with a modified sine inverter. Use that
    shit at your own risk. Like I said, get a true sine double conversion
    UPS or use nothing. You are far better off losing power than feeding a
    PC with a modified sine.

    They should call it a modified square wave! That is closer to the truth.
     
  9. Guest

    A few years back I fed a laptop "brick" power supply, 40 W nominal
    output, with 216 V DC from a string of 12 V batteries. It worked fine
    in a 5-minute test; the scope showed a lot less voltage ripple and
    somewhat less current ripple. The supply didn't seem to heat up or
    make noise any differently than it did when fed with AC. Load was a car
    tail light lamp.
    I think the one I used didn't have an auto-ranging function; I think
    it's just a single wide-range input. The nameplate says "Input:
    100-240V~ 1.0A-0.6A 50-60Hz" and "Output: 15V= 2.7A". It is for a
    1997-ish laptop.

    A lot of desktop computer power supplies have a manual 120 V / 240 V
    switch on the back that controls "rectify or double". I have heard of
    some that automatically control this decision. When used in a 230 V or
    240 V country, and there is a momentary power dip, they sometimes pick
    120 V, with predictable results.

    Matt Roberds
     
  10. josephkk

    josephkk Guest

    PFC power supplies are designed with the ASSumption of sinusoidal sources.
    The are likely fundamentally incompatable with non-sinusoidal sources.

    Alternate solution: Huge LC filter, about the size of your computer due to
    the large inductors and capacitors needed.

    ?-)
     
  11. josephkk

    josephkk Guest

    So far as i know, they would love straight DC and they do rectified ac as
    part of their normal operation. Check some of the chips data sheets, it
    is pretty clear there.

    ?-)
     
  12. josephkk

    josephkk Guest

    My bet is that CFLs will work fine on 90 to 250 V dc. Hell that is so
    interesting i might try it myself.

    ?-)
     
  13. It may take a few days to get to this project. I was also also startled to
    learn that those bi-pin U shaped compact flourescent bulbs, at least the
    pile of 13 watt ones I examined all had starters in the plastic base. The
    best part is the leads of the cap and the starter glass thingy weren't
    even soldered or crimped- just twisted together to the leads on the glass
    envelope. The caps were as least plastic film and not wax impregnated
    paper.

    I seriously though the last place you'd see a flourescent starter module
    was in the corner of an old, old hardware store.
     
  14. a ferroresonant power conditioner would be fun too, but there's no way an
    el-cheapo UPS could magnetize the core of one of those beasts and get it
    started up.
     
  15. Fred Abse

    Fred Abse Guest

    They're only "flourescent" if you break one, and white powder comes out ;-)
     
  16. Fred Abse

    Fred Abse Guest

    I got the clamp.
     
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