What is wrong when the Unit Under Test is off

Discussion in 'General Electronics Chat' started by superway, Oct 12, 2011.

  1. superway

    Thread Starter Active Member

    Oct 19, 2009
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    0
    Hello,

    It's very rare when I see this problem and I still don't know why.
    During my power supply unit testing, there is no failure and no problem.
    But when the Unit finished testing and powered down, my inverter board inside of unit blew up and smoked. Does anyone know why?

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    A part shorted out.

    Seriously, it'll be hard to divine what happened without so much as a schematic.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    You have given very little information to help anyone comment, but I could make a few general observations.


    1. Is the output load removed before the input power is disconnected? Can the converter handle this, for instance in terms of duty cycle and output voltage rise?
    2. How are the switching devices controlled during and immediately after shut-down? Is there any chance of mishandling e.g. shoot-through?
    3. Is energy stored in any capacitor or inductor that could be released in a destructive manner during or after shut-down?
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Some power supplies have a failure mode that happens when they are turned off with no load: the output capacitor holds it's charge longer then the input capacitor and the regulation stage gets back biased and fails catastrophically.

    So on these rare occasions, is there no load connected? Does it ever happen when turned off with a large load still connected?
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Similar horrors can arise in a SMPS if the switching devices cannot be held "parked" in a suitable state for long enough, particularly if the input power is lost suddenly. For instance, output switching devices can be a real menace:

    1. If they come on accidentally, or worse half-on in the dying moments while the output filter is still charged. A lot of the energy gets dumped say in some poor MOSFET, which promptly blows up.
    2. If a low-side device is supposed to stay on to dump the charge, but the bias keeping it that way does not last long enough. It slowly turns off, starts to dissipate, and here again the sparks fly
     
  6. Lundwall_Paul

    Member

    Oct 18, 2011
    220
    19
    Keep the load connected until testing is complete and the supply source has been disconnected.

    Never hot sawp a load!
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    That is good policy, provided that the user requirement does not dictate otherwise. Some applications may require hot swapping, load dumps, output or input open / shorted...
     
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