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SEPIC Converter oscillation

Discussion in 'Electronic Design' started by QQ, May 3, 2007.

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  1. QQ

    QQ Guest


    I am trying to simulate a SEPIC converter in HSPICE. I find that there
    is an oscillation at the switch node (drain of the mosfet) as well as
    the second node (other end of the flying capacitor) but the output is
    stable and at the correct value. The sinusoidal noise rides on the
    switching waveform. Any idea what is the cause of this noise?

    I suspect the noise may be because of some kind of resonance between
    the inductors and capacitors but I am not sure what exactly or how to
    eliminate it. Any help in understanding this is much appreciated.

  2. MooseFET

    MooseFET Guest

    I assume this is a high frequency thing on the drain of the MOSFET.
    It is likely the capacitance of the MOSFET and the inductor

    Is it really a problem? It may just be the normal sort of ugly wave
    form you see in switchers. If this is the case, don't try to get rid
    of it. Anything you do to get rid of it makes a new and real problem.
  3. Joerg

    Joerg Guest

    If it's not a ringout with a fast decay but a stable oscillation this
    might fly into the OP's face at the EMC lab. When they give the thumbs
    down for EMC cert.
  4. MooseFET

    MooseFET Guest

    I don't think this is what the OP was talking about. It is fairly
    hard to make a switcher have a sustained oscillation above the
    switching rate. It it was below the switching rate, he would likely
    have said something about the pulses being modulated or the like.
  5. Joerg

    Joerg Guest

    If it's way below the switching rate some of it usually show up in the
    output. Until a couple years ago I thought the same, that it is hard to
    sustain oscillation on a switcher. Until one fine day where a client
    asked me to see what can be done to improve the EMI of a system so it
    would pass at the EMC lab. Found a nice stand of "trees" around 260MHz.
    Before disassembling much I asked their engineers about clock rates and
    such. The consensus was that there wasn't anything that could generate
    this. Turns out it was the switch mode supply. Singing like a bird.
    Forgot which manufacturer but it was one of the big famous ones.
  6. MooseFET

    MooseFET Guest

    The "stand of trees" effect can be a burst oscillation on the
    switching edge. It can also be just the selection from the existing
    spectrum by something resonant. I don't think the OP was talking of
    either of these. He was looking with a scope. This sort of thing is
    hard to see.

  7. Joerg

    Joerg Guest

    I also chased it with a scope, an EMCO near field kit and a wideband amp
    in front of the scope. It was oscillation. Didn't bother the
    functionality of the loop though but I guess they had designed a bit "on
    the edge" there.
    I really don't remember, this was just one of the layers to peel in that
    case. Like usual it ends up with a laundry list of changes. That power
    supply was outta there afterwards. We didn't want to fix the problems of
    an OEM supplier, just informed them about it. Of course, I had offered
    to redesign it for them later but haven't heard back ;-)
  8. MooseFET

    MooseFET Guest

    Yes that is a scope, in the same sort of way a formula 1 car is still
    a car. I have never seen a problem like what you had but can imagine
    how it could happen if there was enough wrong with the design and

    I once had a DC-DC converter of my own design act very strange. It
    turned out that when the layout was done a large plane area was
    connected to the drain of the MOSFET instead of ground. You could see
    harmonics of the 150KHz switching frequency on a scope probe held more
    than a foot away from the PCB. It made so much EMI it messed with
    other supply circuits on the same PCB.

  9. Joerg

    Joerg Guest

    Yep, but the fix was easy: Switched manufacturers, problem gone.

    The most embarrassing scenario is when you turn on the new super-duper
    design and the lab radio goes weeeeeooouuu ... phshhhhhhht....
  10. MooseFET

    MooseFET Guest

    On May 5, 11:39 am, Joerg <>
    [.... about switchers ....]
    You must have lived a charmed life.

    KABLAM FIZZZKKKKKKA-POOF and throwing the casings of the capacitors
    clear across the lab is by far more embarrassing.
  11. Joerg

    Joerg Guest

    Oh, I had my share of those. The first one was as a teenager when I was
    still blissfully unaware that even large electrolytics have their limits
    on the ripple current you can burden them with. Transformer-less tripler
    from 230VAC since a transformer just wasn't in the budget. Cranked out a
    stiff 910VDC, sweet. Just don't ever plug in the power cord reversed. In
    Europe those weren't polarised so a Sharpie dot had to suffice. Turned
    it on, fired up the amp. 500W out, great, 700W, 900W, 1200W, yeehaw!
    Then the fluorescents in my room dimmed in a weird fashion. Even more
    weird because I knew for sure there was no dimmer. KAFOOMP!

    One of the caps went straight into the ceiling, taking out a big chunk
    of plaster, then returned from orbit still hissing. Or rather, its
    wrinkled can returned because the rest had come out of it and was
    raining down as fluffy snow. I guess it had celebrated its last seconds
    by mutating into rocket fuel. Luckily I wasn't with my head over the
    amp, else I'd be disfigured now. Had to mix plaster, get up on a ladder,
    then re-paint the whole ceiling. Oh, and my shirt had lots of burn holes
    in it and so did my bed and the carpet and a chair and... Mom was not
    enthused about my hobbyist activities that day.
  12. DJ Delorie

    DJ Delorie Guest

    "I'm pretty sure we shouldn't be breathing the yellow smoke."

    Been there :)
  13. Joerg

    Joerg Guest

    The real stench came from tantalums. They let of a greenish cloud. Cured
    me pretty much from ever using them.
  14. krw

    krw Guest

    I've used kabillions of tantalums. The only time I ever had a magic
    stinky (and violent) smoke release was when the dummy populating the
    cards plugged them in backwards. We had some pretty strict quality
    standards for 'em though.

    If you want stink, try frying a selenium rectifier. ...eewwwww!
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