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Common SMPS failure mode?

Discussion in 'Electronic Design' started by Daniel Haude, Aug 12, 2003.

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  1. Daniel Haude

    Daniel Haude Guest

    Hey folks,

    first off, this post is somewhat OT because it isn't a design
    but rather a repair question.

    I've got this old Sun SPARC computer that's probably worth no more than
    EUR50, and that only because it's in a nice 19" case. Its power supply
    broke, and a new one costs EUR500. Unfortunately, this computer is part of
    a EUR50000,- laboratory setup, so it's indispensable, and the power supply
    will have to be fixed.

    Now while it's financially no problem to just buy a new PS (which is what
    I'm going to do anyway because there's a time factor involved), the
    thought of trashing a EUR500,- unit in which probably only a cheapo part
    needs to be replaced bugs me. However, I think that SMPSes are generally
    not worth repairing -- or downright unrepairable. I'm posting here in the
    hope that someone will pipe up and say: Oh, if this-and-this happened it's
    always this-and-this which'll have to be replaced.

    OK, the this-and-this that happened is that the P/S failed sometime at
    night while the computer was running. In the morning we found the breaker
    had tripped. We turned the breaker back on, turned on the computer, and
    the breaker tripped again. After four more attempts the internal fuse of
    the SMPS blew.

    I inspected the SMPS and found no exploded parts, and no exploded-parts
    smell. Is this thing worth looking at, and what should I look at first?

  2. Barry Lennox

    Barry Lennox Guest

    Based on my limited experience of about 4-5 SMPS and a tech I know who
    fixes them for a living, about 95% of the faults are diodes or the
    main switching transistor (or FET) Grab a meter and check them one by
    Barry Lennox
  3. James Meyer

    James Meyer Guest

    The first thing you should look for is someone with a little more
    experience than you have. I'm sure there is someone available locally who can
    take a look at the supply and fix it.

    It took you four times tripping the main breaker and then the PS fuse
    before you figured out that something was wrong?

  4. Jim Yanik

    Jim Yanik Guest

    Electrolytic caps with too much ESR,shorted diodes,open or shorted
    switching transistors.Many switchers have current-limit circuits,so if a
    diode shorted,the supply would not start,or try to start,then shut down.
    Blowing fuses would possibly indicate a primary fault such as open and/or
    shorted switcher xstrs or diodes.
    Bad electrolytics on the primary might mean not enough input line DC for
    the switcher,on the secondary would mean too much of a load causing
    Ohm out the primary switcher xstrs and diodes,use an ESR meter to check
  5. Google for the SMPS repair FAQ, it's got a lot of good information.

    You might also want to look for a used on on Ebay. I see a Sparc
    Station 20 PSU with a BIN of $9.99. and a 440W SPARC 1000 Power
    Supply, also starting at $10 (+ a lot of shipping).

    Best regards,
    Spehro Pefhany
  6. R.Legg

    R.Legg Guest

    OK, the this-and-this that happened is that the P/S failed sometime at
    The responsible diagnostic procedure, when dealing with a tripped
    breaker or a blown fuse, in any device, is to reset or replace the
    fault limiting device ONCE(!).

    Repeated reseting or replacement, after the first confirmation, merely
    adds unnecessarily to any damage that was present.

    If there had been one typical failure to check for initially, you have
    succeeded in developing a completely new train of damaged parts that
    may be unrelated to the first symptom. The lack of exploded parts and
    fire is an indication of a well-designed and safe product that was
    originally intended to be repairable under repeated single-fault
    abnormal conditions.

    A tripping breaker is the last symptom of a chain of events, that
    could have started anywhere in the product, and could be the
    culmination of a number of discrete, non-critical failure mechanisms
    accumulated over the entire life of the product. Now that fuses are
    additionally popping, this is a sign that a hard semiconductor failure
    has been achieved in the input rectifier or converter section, as
    well, through your efforts.

    If and when you send it out to qualified service personnel for repair,
    you might save yourself time and money by describing fully the latter
    events. This will alter their estimate of any costs involved, and may
    affect your decision re repair or replacement.

  7. ....Which you can then sell to the desparate customer at the BARGAIN PRICE of
    USD 375.99 ;-)

    BTW: Auctioning off the *defect* one on Ebay will probably pay for the
    shipping since Ebay does not give a rat's arse about complaints!
  8. Daniel Haude

    Daniel Haude Guest

    On 12 Aug 2003 09:55:25 -0700,
    Are you suggesting I did something wrong?

  9. Often the fan freezes up, then things overheat and the heat destroys
    something. But if you have a hard short, the rectifier or other
    components very close to the incoming AC line are usually where the
    problem is. Could be a bad FW bridge, or even a bad capacitor or
    inductor. Your ohmmeter can tell you what's shorted.
    @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
    ###Got a Question about ELECTRONICS? Check HERE First:###
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at>
    Don't be ripped off by the big book dealers. Go to the URL
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  10. Daniel Haude

    Daniel Haude Guest

    On Tue, 12 Aug 2003 05:03:28 -0700,
    It is. Very nice paper!

    BTW, thanks to everybody for the suggestions. I've indeed found a pair of
    dead IRF840 switching transistors in the main 5V branch. The +/-12V
    branch, which runs independently off the same primary DV voltage, works
    fine. Now I'm just waiting for the new transistors. It looks as if I could
    turn a EUR750 purchase (the 500 turned out to be bogus) into a EUR5

    BTW, when testing the supply I looked at the voltage across the main
    storage cap while slowly turning up the AC input voltage. At first the cap
    voltage followed the input, but upon reaching about 50V AC input, the cap
    voltage suddenly jumped to 400V (accompanied by a short 2A input current
    surge) and stayed there throughout the input voltage range. Is this a PFC
    circuit at work?

  11. Daniel Haude

    Daniel Haude Guest

    On Wed, 13 Aug 2003 16:44:05 +0000 (UTC),
    Quite true. I omitted the lightbulb, relying on the 2A breaker and the
    ammeter in the isolated variac. Of course that doesn't replace a real
    current limiter like a resitor or lightbulb, but it looks as if I got away
    with it. Do you think you could keep me out of your "serious idiot" box,
    considering that I used an isolation variac instead of plugging the supply
    straight into mains?

  12. Of Course We Do - Think of the sheer entertainment value, the tense moments
    before the switch is thrown, the thrill of the sight and smell of a new
    innoncent engineers finest work creating a neat fireball on the workshop
    desk acompanied by a muffled "BOOM" from the sub-basement .......
    Ahhh.......makes it all worthwhile, does it not!!
  13. R.Legg

    R.Legg Guest

    Safety Glasses, gentlemen, please.

    Tinted lenses might also avoid Zoro-esque permanent markings on the
    backs of your retinas.

  14. Da Man

    Da Man Guest

    Don't forget to look for dried out capacitors - that's another big one, but
    they don't short usually.
  15. N. Thornton

    N. Thornton Guest

    Do you think you could keep me out of your "serious idiot" box,
    A variac is not an isolating transformer. Try not to fry yourself.

    Regards, NT
  16. Daniel Haude

    Daniel Haude Guest

    On Sun, 17 Aug 2003 14:16:04 +0200,
    Don't worry. It is. Still charges the storage cap to 400V, isolated or

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