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Input rectifier cap

Discussion in 'Electronic Design' started by Hammy, Jun 9, 2010.

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  1. Hammy

    Hammy Guest

    If I calculated a 120Hz, 1.3Arms ripple current for an input rectifier
    cap for a flyback, how could I calculate the core temp based on a
    given maximum ambient temperature (50C)?

    I'm interested in using one Panasonic 270uf 105C cap rated at 1.42
    Arms @120Hz.

    There's no way that the commercial SMPS's I've looked at are using the
    105C rms rateing to select the input cap. Most I've taken apart have
    either one cap or two in series for a switch doubler including the
    350W SMPS powering my computer. They must be increasing the rms
    handling capability of a cap based on a reduced operating temperature
    then the 105C specified in the datasheet.

    Basso's book mentions a multiplier for determining max rms current for
    reduced operating temperature. I've looked at several datasheets for
    caps and haven't seen one. Is there a rule of thumb? Also is there a
    reasonable estimate for including the converters high frequency rms
    contribution to the capacitor?
  2. Grant

    Grant Guest

    A lot of them only want 12 months life -- don't care what happens after
    warranty expires :(
    Odd you don't have datasheet numbers for this, it was on datasheets
    I looked at a few weeks ago. Look for a table of multipliers buried
    in the details, the tables are small, perhaps easy to miss?

    An example. Recently I worked through ripple current adjusted for
    both temp. and frequency for two different brands of caps and the
    multipliers are a little different. These are 85'C caps.

    I built a bank of caps for about 600A ripple current for <45'C, but
    it's down to about 350A at 85'C (~500A at 60), bank is 20 x 10mF/50
    plus 60 x 4700/50.

    First brand is 1.73x < 45'C and 1.5x at 60'C, second is 1.55x <45'C
    and 1.30x at 60'c (both 1.0x at 85'C).

    Multipliers for frequency varied more, first is 1.08x for 10kHz,
    second is 1.20x for 10kHz (both 1.0x at 120Hz).

  3. Hammy

    Hammy Guest

    Mine has been going for 8yrs now. I doubt it could deliver 350W
    continuous power.

    I also noticed with mine when cleaning it , it has no Common mode
    choke!! This explained why a TV operating off the same line would get
    snowy when the PC was on simultaneously. Oddly enough it has all these
    regulatory approval stickers and it's a Antec supposedly.
    Your right it is there. I can double the rms current for 70C ambient.
    I always thought the 105C specification was for core temperature not
    ambient. I have been grossly derateing my caps! Oh well live and
    learn. :)
    Christ what's it for an industrial egg friar. ;-)
  4. Grant

    Grant Guest

    A small Splat! (stored energy) welder, only ~25 Ws max. Built for
    occasional use with the cheapest caps I could find ;) Yet to make the
    power dump switch, I'm thinking 16 x 75A MOSFETs in parallel might
    survive the experience, otherwise I could play safe and use a big
    stud mounted SCR I got in a box here.

  5. Phil Allison

    Phil Allison Guest

    "Hammy the Halfwit "
    ** Look at the data.

    ** The ripple current data for lower ambients is in the table on page 1 of
    the pdf - it doubles at 70C .

    But a 3000 hour life is no good to anyone - so the usual practice is to use
    the rated rms ripple for 105C and keep the ambient down to no more than 65C.

    With just a bit of luck, the electro will then outlast the equipment.

    ..... Phil
  6. Phil Allison

    Phil Allison Guest

    ** HUH ??????

    The impedance of a high voltage ceramic cap is waaaaayy more than an electro
    at SMPS frequencies.

    ..... Phil
  7. Grant

    Grant Guest

    Very good :) Pics here:

    Shows how much this is a hobby for me these days, not go near CAD software
    yet this century. I used to use pcad stuff in the mid-80s, on a new 25k
    PC-AT box with fancy graphics card, the 'full' 640kB installed we had to
    add the 128k extra memory for pcad to work as the PC-AT had 'only' 512k
    memory on the mobo; it was lots and lots of double decker chips, each a
    20 or so pin DIP. PC-AT complete with the first run 21MB HDDs that were
    prone failed due to contamination, we were told. Monitor was from Canada,
    with five coax leads from graphics card -- 2 colours plus the two syncs.

    Company paid $19k for a program that came on a 5 1/4 floppy disk, and the
    pcad parts library was total crap, we had to make all our own. When one
    used a large X-Y chart plotter because local PCB house didn't do NC
    drilling, no Gerber machine either.

  8. Grant

    Grant Guest

    You lumping in the tan whatever losses with ESR? Likely be an issue
    for me soon, so I plan to put temperature probes in a capacitor bank
    to derate or shutdown at high temperature -- besides, I'm curious...

  9. Hammy

    Hammy Guest

    CORNELL DUBILIER has some Java calculators for calculateing lifetime
    based on ambient temp and ripple current. Its here for anyone who
    wants to try it.

    The site seems a bit slow mainly when useing firefox. Has anyone else
    noticed CDE is slow.

    Here is an application note on how they calculate the values and how
    to use the tool. Export/Supplier Content/CDE_338/PDF/CDE_SelectingCaps.pdf

    I plugged in my ripple both 1.3Arms @ 120Hz and 0.8Arms @100kHz, for
    an inferior capacitor then the panasonic. The results were 0.44W
    dissapation, Avg core temp 63C Expected life hours 84,000 hr's!! In a
    max ambient of 50C, reasonable its only a 50W flyback no major
    heatsources and I'm mounting a small fan in the enclosure.
  10. Joerg

    Joerg Guest

    It isn't too slow here on Firefox. But the applets won't work even
    though I did enable Java and all that for this site :-(

  11. Grant

    Grant Guest

    Same here :( Download + reinstalled Java JRE for it too, no joy.

    Unless it wants the full Java development kit installed, I lost interest...

  12. Hammy

    Hammy Guest

    Yea I should have mentioned it doesn't work with Fire Fox. It does
    work with IE...... I know Blah :)

    I found it handy even if I'm not using the caps they have loaded in
    the tool it gives you an idea quickly on any similar cap.

    Or if you couldn't be bothered the application note shows how they
    make the calculations and you can just do it up in Math CAD or excel.

    I have noticed the site doesn't seem Fire Fox friendly, even
    downloading data sheets is problematic. They claim the site is FF
    compatable. I'm sure they did rigorous testing after all CDE is a
    huge multimillion $ corporation. So the only logical conclusion is we
    are doing something wrong;-)
  13. Hammy

    Hammy Guest

    For the record I do have JRE installed and it still doesn't work with
    FF, it does work with IE.

    They do recommend using either IE or Netscape for the tool. The site
    does however claim to be compatible with FF.
  14. Baron

    Baron Guest

    Hammy Inscribed thus:
    Mmm ! Works fine for me in "Opera, Firefox & Konqueror"...
  15. JosephKK

    JosephKK Guest

    No joy here. Not in wine nor in native linux. Sun Java 1.6.x both
  16. JosephKK

    JosephKK Guest

    BZZZt. The executive managers making such declaration have no such
    direct knowledge. Including the CITO and the CIO.
  17. JosephKK

    JosephKK Guest

    Geez, Netscape "died" many years ago. What is in their heads?
  18. JosephKK

    JosephKK Guest

    Possibly they are on my cookie shit list.
  19. JosephKK

    JosephKK Guest

    Ah, in the early transition era. When serious EDA was trying to
    understand how to migrate to cheap mass produced HW instead of expensive
    *nix workstations and EDA capable PC class HW was only comparatively
    economical compared to *nix workstations.
    Who else remembers TIGA cards?
  20. JosephKK

    JosephKK Guest


    Dunno, that might be in the hockey puck range of SCRs.
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