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Hybrid capacitors

Discussion in 'Electronic Design' started by acd, Sep 17, 2012.

  1. acd

    acd Guest

    In many cases supply voltage is filtered by combinations of small (1n-100n) ceramic and larger (say 100uF) tantalum/electrolyte capacitors.
    Since each extra component requires some space around it for manufacturability I was wondering whether there are
    hybrid capacitors that combine two types (low ESL, large capacity)
    capacitors in one package. Of course, it would need a special landing pattern in order to preserve the low inductivity.
    Price would be higher and the question which combinations would
    be appropriate, but the space savings could open a marker for mobile devices, medical, etc.
    I found this article, but it discusses ceramic-only:
    http://www.ceramicindustry.com/articles/91910-pcb-space-improvements

    Maybe I missed it, and something like this exists already?

    Regards,
    Andreas
     
  2. Guest

    Including 'X' and 'A'.
     
  3. Guest

  4. rickman

    rickman Guest

    I've had this conversation with 'X' and they admit that they make their
    recommendations based on a lack of knowledge about their customer's
    designs because the designs vary across the map. Their recommendations
    are based on the worst case ever possible FPGA design with a huge dose
    of CYA thrown in.

    On the other hand, I have taken a class in high speed digital design
    with someone who used theory, then simulation and finally testing on
    boards built to test power delivery system designs and came to the
    conclusion that mixed capacitor values and a much smaller number of caps
    than are typically used are the best approach for digital designs.

    Specifically, he used an approach of actually calculating impedance at
    various frequencies compared to the required impedance calculated for
    the application. No rules of thumb, no guess work.

    Rick
     
  5. Guest

    Sure, they admit that but it really is a huge dose of CYA in even the worst
    case. I generally try to add the capacitance recommended then just no-pop a
    good share of them. It's a lot easier to add caps if the footprints are
    already there. BTW, I've never had to.
    I've seen people simulate just about every fool thing. I stopped blindly
    trusting simulations long ago. ...particularly other people's simulations.
    I've seen that done, too. It works, but so does sprinkling a few caps around.
    They're cheap.
     
  6. rickman

    rickman Guest

    Did you read the part about building a board to test the issue? He has
    verified this every way you might want.

    Depends on how many... 10-20 per board times X can be a lot of dollars.
    On the other side, if you use have too few on the board and your
    design is actually more sensitive than you expected... well, that's why
    engineering isn't just winging it.

    Rick
     
  7. Tim Williams

    Tim Williams Guest

    I once tried the power supply impedance calculator from Altera, which does
    much the same thing. I had reasonable results after a little work.

    Input rough dimensions of how much plane you're going to have, physical
    dimensions, vias, traces, that sort of thing, and it'll guess how many
    bypass caps you need.

    Of course, its first guess is something like 4 x 0603s, 9 x 0402s and some
    0201s if you can even see them, plus whatever tantalums it can find.

    After poking around a bit, I found excellent results with a single, fairly
    bulky tantalum, and a small number of 0603 0.1's per I/O bank (this was a
    middle size Cyclone).

    It's worth noting that results are almost always *WORSE* using a low-ESR
    bulk cap, like an aluminum polymer. This is because it short-circuits the
    series resonances, making things a lot worse. The ESR of a tantalum (or,
    if you can accept the variance in ESR, aluminum electrolytic) dampens it
    much better than staggered values or huge arrays of similar values.

    Tim
     
  8. Guest

    He verified that it worked. He likely didn't verify that a lesser amount
    didn't. Hell, cover the thing with caps and it'll work.
    Not *nearly* as much as too little and finding out about it a year later.
     
  9. Guest

    That's essentially what I do. On my last board, the board stuffer did it for
    me though. ;-( ...still worked fine. I won't take them off this spin,
    though.
     
  10. Guest

    Almost everything we do is 0402s, now. In fact, Murata doesn't want us using
    0603s for small (<100pF or so) caps. We don't use 0201s for reliability
    reasons, though it looks like there won't be much choice soon.
    I tend to use .1uF caps with a few at whatever capacitance is needed to have
    the SRF at the clock frequency. Of course there are larger bulk caps at the
    supplies and at really high current devices.
     
  11. rickman

    rickman Guest

    You aren't grasping the concept. He didn't build a board for some
    application and verify that it worked the way he designed it, he built a
    board just to test the functionality of decoupling caps in the analysis
    and simulation that he did. He primarily disproved the myth of the
    inductive loop of cap and power/ground pins being the determining factor
    in a system using adequate power/ground planes. Rather the power/ground
    planes couple the cap to the pins as a transmission line providing all
    the current the pins need until the wave front reaches the cap and the
    cap can supply the current. The size of and therefore the inductance of
    the loop is not directly relevant in this case.

    That is the basis of the myth of needing a cap for each power pin. Once
    you dispel that myth you can then focus on the impedance of the PDS over
    frequency and optimize the number of caps for cost and board area vs
    impedance.

    So how do you ever know you have enough if you don't do an engineering
    analysis? Do you just keep adding caps until you can't lift the board
    anymore?

    Rick
     
  12. Guest

    No, you're not grasping the "existence theorem".
    And fewer would almost surely have worked, as well.
    Only for small values of capacitance and because the ground plane *is* a cap.
    That's something that JL has been saying here for some time.
    Good Lord, you lefties really are stupid.
     
  13. Tim Williams

    Tim Williams Guest

    Why not do it yourself? The equations are simple to set up.

    I think you'll find it doesn't have a very well defined impedance, though.
    The reason is apparent when you run the numbers. Propagation is radial so
    the wave front width increases.

    More interesting than viewing it as a transmission line (which would be an
    axially driven, cylindrical or coaxial resonator structure) is its
    inductance, which follows a similar law.

    Tim
     
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