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High ESR cap ?

Discussion in 'Electronic Design' started by Mook Johnson, Jun 14, 2007.

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  1. Mook Johnson

    Mook Johnson Guest

    I have a fellow engineer that intentially uses high ESR (1 - 3 ohms)
    tantalum caps at the power input to his boards (5V, +/-12V). His claim is
    that the high ESR will help to damp ringing ans oscillation caused by all
    the ceramic caps used elsewhere on the board. Take for example the 5V rail
    the board has about 40 .1uF 805 X7R caps surving local IC bypass duty and a
    47uF 3ohm ESR tantalum right at the input.

    Does this make since to your guys?
  2. It does make sense for some linear regulators (some low drop
    out variety) and some switchers that require a minimum ESR.
    But I doubt it helps much with the high frequency ringing
    of those ceramic capacitors. Once you get a few inches from
    them, the inductance between them and the bog capacitor
    pretty well isolates their resonance from this damping.

    But the stability of the ESR is not very good over
    temperature and time, so I usually use a good, stable, low
    ESR capacitor and a fixed low value resistor in series when
    I need this effect.
  3. D from BC

    D from BC Guest

    Ringing rails?
    I've made ringers with tank circuits.

    I guess on a power rail there's the trace inductance can combine with
    the decoupling capacitance to form a tank circuit.??
    The more lossy the tank, the quicker the ring decay.
    To make a tank lossy a resistance is included in the network.

    But I still suspect something fishy going on..

    D from BC
  4. John Larkin

    John Larkin Guest

    Not really. And tantalums connected across supply rails have a bad
    habit of exploding.

  5. John Larkin

    John Larkin Guest

    Peak current (or something like i^2 * t?) is what detonates them, so
    they should be OK in a signal-type circuit. I'd guess that 10's of mA
    would be perfectly safe, but I can't quantify it any better. They do
    fail on the outputs of 3t regulators that limit the max current to,
    say, 1 amp or so.

    Polymer tantalums are apparently safe.

  6. What would you think of subjecting small SMT tantalum caps (eg.
    22uF/35V) to several hundred Hz (current limited) from ~0 to say 80%
    of voltage rating ??

    Best regards,
    Spehro Pefhany
  7. Rich Grise

    Rich Grise Guest

    Not if you install them right-side-up. ;-)

    Maybe you have supplier problems or something, but I've never seen a
    tantalum cap explode.

  8. Apparently limited to 25V or less rating, so maybe not a feasible
    fallback in this case.

    Best regards,
    Spehro Pefhany
  9. Chris Jones

    Chris Jones Guest

    Yes it does make sense to add some series resistance in some of the
    decoupling caps, to reduce the Q of any resonant circuit that is formed by
    the wiring inductance and the capacitors. This is common practice on high
    frequency integrated circuits. In order to not unduly increase the power
    supply impedance at very high frequencies, you can split the decoupling
    capacitance into two parallel capacitors, and put a resistor in series with
    only one half of the total capacitance. Choose the resistor to have a
    resistance equal to the magnitude of the reactance of the capacitor at the
    most troublesome ringing frequency. If you know the value of your wiring
    inductances then you can simulate all of this in SPICE too.

    As for using tantalum capacitors, some people here have had some bad
    experience with them failing destructively, so I would suggest perhaps
    using a normally-low-ESR capacitor with a resistor in series as a more
    predictable and reliable solution.

  10. Steve

    Steve Guest

    You've been lucky. There are military and avionics specs that require series
    resistance be used with a tantalum cap to limit destructive currents and
    reduce shrapnel.

    See section 15 in this Kemet document describing this failure mode and the
    current limiting requirement of 0.1 ohm per volt.$file/F3100ETaLdPerChar.pdf

  11. SP

    SP Guest

    It's in the application information from many manufacturers of
    tantalum caps.

    Best regards,
    Spehro Pefhany
  12. Guest

    Sounds naff to me. His design is based on a component -defect-, not a
    way to go. He should be fitting a 47uF capacitor with external damping
    Tants are horrible. I'll bet he's had 'em explode in the past and is
    rationalising this ESR resistance in terms of 'damping', when in fact
    it's simply limiting the capacitor ripple current to a safe value.
    Cap's across input supplies (dependant on circumstance) are in a prime
    location to suffer spectacular ripple currents. If they can't hack
    the ripple then leave 'em off. (The 470uF/63V 'lytic I had explode
    last week, went with a helluva bang. The confetti goes everywhere).
  13. Fred Bartoli

    Fred Bartoli Guest

    a écrit :
    Tantalus torture?
  14. whit3rd

    whit3rd Guest

    The PC board material, if you just make it lossy, will also damp
    ringing. That'd be cheaper. Heck, just a coat of paint slopped
    on the board after it's all soldered will achieve as much dielectric
    absorption as you could want. Any point-located component,
    like the ESR in a capacitor, will only make ONE POINT on
    the circuit into a node of the (presumably troublesome)
    high frequency standing waves on the wiring board.
  15. John Larkin

    John Larkin Guest

    That is absurd.

  16. qrk

    qrk Guest

    You haven't lived life to its fullest!
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