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Tantalum caps

Discussion in 'Electronic Design' started by Marco Trapanese, Sep 21, 2011.

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  1. A friend of mine, a good old analog engineer always talks about using
    tantalum caps as bypass in power supply. I wonder if that's still a good
    practice and if they needs a 100n in parallel to improve the
    high-frequency behavior.

    I try to avoid electrolytic caps (Al) if I don't need a large capacity.
    Instead I prefer to use ceramic (up to several uF) because they *should*
    have a better frequency response.

    Where is the truth? I'm talking about standard power supply filter, near
    an LDO or even a small DC/DC.

    Thanks
    Marco
     
  2. Phil Allison

    Phil Allison Guest

    "Marco Trapanese"
    ** It never was good advice and still is not.

    Fitting a 100nF cap in parallel is totally superfluous in most cases.

    ** A ceramic or film cap of a few uF will have a lower ESR than an electro
    ( tant or Al ) of the same value. But a 100uF ( or higher ) low ESR electro
    will cost way less and do the job of bypassing way better.

    ** Tants are so fragile that if there is ANY chance of a large current
    spike, a short or the application of reverse voltage of even 1 volt - then
    use an Al electro. Also it is wise to derate the voltage by a factor of 2
    or 3 times.

    Fact is, most tantalums will not stand repeated shorts when charged to their
    rated voltage while Al electros laugh at such treatment.

    A jaded engineer once said: " Tantalum caps are perfectly reliable, long as
    you put a fuse in series and a reverse diode in parallel".


    .... Phil
     
  3. Il 21/09/2011 14:24, Phil Allison ha scritto:

    The advice you're talking about is the use of tantalum cap or the 100n
    in parallel?


    Ok, I got it.


    Good point.
    So, changing the question: when would you use a tantalum cap?


    LOL :)

    Marco
     
  4. Phil Allison

    Phil Allison Guest

    "Marco Trapanese"
    ** Both.

    All a parallel 100nF cap will do is create a narrow impedance dip at about
    5 MHz.

    ** They are sensibly used where the DC supply voltage is low, there is no
    chance of a charge or discharge current spike nor a polarity reversal AND
    the small size is crucial. Eg a mobile phone.

    Low leakage apps are often touted for tants - however leakage is not
    constant with temp and suffers big sample to sample variations, so best use
    a film cap for that too.

    A jaded engineer once said:

    " Tantalum caps are perfectly reliable, long as you put a fuse in series and
    a reverse diode in parallel ".

    He avoided them entirely.


    .... Phil
     
  5. Nico Coesel

    Nico Coesel Guest

    I avoid tantalum because they are very prone to shorts. They also may
    burn a hole in a PCB if you are unlucky.
    Ceramic is a good choice but sometimes the ESR of a ceramic is too
    low. For example: I like to use LM1117 LDO's but I need to put a 0.5
    Ohm resistor in series with the 10uf ceramic at the output.
     
  6. Il 21/09/2011 15:44, Phil Allison ha scritto:

    Ok, thank you!
    Marco
     
  7. IME, it's less of a problem where you have internal relatively wimpy
    power supplies.

    When you give the customer power supply connections or allow hot
    plugging of PCBs it's more likely to show up.
     
  8. Failure to put 15" of 10-mil wide 1/2 oz copper in series with the
    cap.
     
  9. Joerg

    Joerg Guest

    It usually doesn't. Tants can be fine for a few months. Then one fine
    morning the birds start to chirp, fresh coffee smell wafts through the
    lab ... tsst ... *POW* ... a smoke alarm starts blaring, a stench wafts
    through the rooms and *POP* .. *BAM* ...

    A VP of production once said "And then it was like popcorn".
     
  10. Joerg

    Joerg Guest

    We had them derated. Yet still ...

    Bottomline is that I don't trust those things. For timer purposes, ok,
    but even there I prefer ye olde CD4060 or 74HC4060 over tantalums.
     
  11. Fred Bartoli

    Fred Bartoli Guest

    Joerg a écrit :
    I've designed some unusual power supplies boards with about 100 (yep,
    one hundred) 33uF/25V tantalums, for bypassing +/-15V supplies. 100A RMS
    into the caps.
    The customer produced more than 2K boards to my knowledge which are
    running almost 24/24 since 10 years and I've yet to hear of a
    catastrophe scenario.
    Of course it was carefully designed :)
     
  12. Joerg

    Joerg Guest

    1A ripple each and only 40% derated? With such small capacitors that is
    like Russian Roulette :)


    You were very lucky. I have seen designs where they weren't. And the
    shirt of a guy with dozens of tantalum holes in it. Luckily he wore a
    T-shirt underneath and it didn't go through.

    Could you explain that some more? What's the secret trick?
     
  13. Rich Grise

    Rich Grise Guest

    I haven't done a whole lot of analog, but a "rule of thumb" I've used on
    most of my digital connect-the-dots boards has been about a 10 uF tantalum
    at the point where the power enters the board, another at the opposite
    corner, a 1uF tant. for each row of chips, and a preferably .1 uF
    monolythic at each chip if I have any on hand or can grab a handful from
    the stockroom when nobody's looking. ;-) Otherwise, .01 discs.

    Hope This Helps!
    Rich
     
  14. Rich Grise

    Rich Grise Guest

    John, why do your power supplies have such high dV/dT?

    Thanks,
    Rich
     
  15. Nico Coesel

    Nico Coesel Guest

    I guess you never worked in a repair department :) Besides that
    tantalums may explode when mounted wrong. Last year one blew up right
    in my face when testing a board which just came in from the assembly
    company. Fortunately I wear glasses otherwise I would have suffered
    serious eye injuries.
     
  16. Joerg

    Joerg Guest

    Yo, Rich, in case you do another one of those: 1uF and 10uF ceramic have
    been invented quite a while ago. Que estan mucho mejor. Oh, and they are
    cheap.
     
  17. John S

    John S Guest

    Never use a tantalum. They suck. Or, actually, they blow!
     
  18. Rich Grise

    Rich Grise Guest

    I've never seen a blown-up tantalum cap, and until I came hear I'd never
    even heard of one.

    Thanks,
    Rich
     
  19. Phil Allison

    Phil Allison Guest

    "Rich Grise"
    ** Most designers shied away from them by the late 1970s - because of bad
    experiences.

    The horrible things are like red flags to repairers, soon as you see even
    ONE on a PCB you say to yourself " well, there's the problem ".



    ..... Phil
     
  20. Guest

    Guest Guest

    Yep. We used to use tantalums for everything, until we had a board with
    a 470uF 16V tant across the power-in jack, for bulk decoupling. The PSU
    was 9V at a couple of amps but the short-circuit transient curent was
    much higher, about 18 amps for a few tens of ms IIRC. Fortunately all
    the "*BANG* + smoke + flames on PSU connection" incidents happened
    during initial testing rather than in front of the customer. We
    replaced it with an aluminium electro - no more problems.

    Even when run under very benign conditions they can go leaky - we had
    another board which used the same tant to hold up the supply for some
    CMOS RAM during battery replacement. Current was limited to a milliamp
    or so. A few boards came back with no power to the RAM, the cap
    measured a few hundred ohms. Remove the cap from the board and apply
    rated voltage (ramped up slowly) from a bench PSU -> nice fireworks
    display.

    These were all AVX TPS series. Anyone have any better experience with
    other brands?

    R.
     
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