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Motherboard inductors

Discussion in 'Electronic Design' started by John Kimble, Apr 5, 2005.

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  1. John Kimble

    John Kimble Guest

    Hi all,

    Just a short question on something that's been bugging me for a
    while...

    On motherboards, it is common to see various inductors placed around
    the board. What, in general, is the purpose of such components? It was
    my understanding that inductance is something that is generally
    undesirable in digital systems that are switching at high clock
    frequencies (hence the need to a heirarchy of decoupling capacitors).
    Are they used for clock generation?

    Stephen
     
  2. Guest

    Guest Guest

    You're right. Those inductors are slowing your motherboard WAY down.
    The manufacturers put them in there for their low-end boards and if you
    pay a lot more, they simply take them out and presto, instant speed demon.
    That way they make a profit because they don't have to build two versions
    with a lot of different parts.

    Jim
     
  3. They are part of the switch mode power supply circuitry, used to
    convert from one supply voltage (e.g. 5V) to another (e.g. 1V).
     
  4. Hi, one.
    Can't have that.
    In general, to store some energy and give it up later. A
    great many more detailed purposes fall within that rubric.
    Au contraire. Without inductance, signals would never
    get anywhere. Wave propagation would be halted.
    Yes, indirectly. Take them out and any number of
    clocks will stop being generated, as the power forms
    they help condition or generate collapse.
     
  5. Pooh Bear

    Pooh Bear Guest

    Almost invariably as part of voltage step down on board power supplies
    needed to generate the low voltages needed for modern CPUs and GPUs.

    No.


    Graham
     
  6. Are you talking about soldered on components, or serpentine traces?
     
  7. Paul Burke

    Paul Burke Guest

    Resistance is futile! Capacitance is immoral!
     
  8. Fred Bloggs

    Fred Bloggs Guest

    Unbelievable garbage response...
     
  9. John Devereux wrote...
    Just now my 3.3V is running at 3.15V, and my Vcore = 1.52V, for an
    AMD Athlon64 3200+ processor. This beast's current draw varies
    dramatically as a function of whatever program is executing, a buck
    switching regulator located on the PCB must be able to handle this.
    This switcher is sourced from the +12V supply. The peaks currents
    are 70A, 120A or whatever, so the switcher is broken into multiple
    sections in parallel, four is common. There may be another supply
    for the ram or other chips. That's why you see so many inductors.
     
  10. Tim Shoppa

    Tim Shoppa Guest

    Out of curisosity, is this breaking-into-sections for form-factor
    reasons (bigger inductors would stick up to high and block plug-in
    boards or otherwise stick outside of the ATX form factor), for PCB
    reasons (for high currents you'd like to have thicker traces but
    instead you compromise to thinner copper because most traces are logic
    not power), component price reasons (sweet spot for inductors or
    semis), or something else?

    I'm completely flabbergasted that umpteen-layer ATX motherboards are
    availble for $30 with all that stuff already on it.

    Tim.
     
  11. Graham Orme

    Graham Orme Guest

    There are two reasons for dividing the load among multiple stages. It is
    much easier to design a 30 Amp switcher than a 120 Amp one and the
    efficiency is likely to be much higher. The main reason is that in a
    buck converter current is only drawn for a small proportion of the time.
    When converting from 12 V to 3 V the switching device conducts for 25%
    of the time. This means you need capacitors with enormous ripple current
    ratings. By running four stages in synchronism but each phased 90
    degrees after the previous one, the input and output capacitors see
    almost constant current.

    You're probably right about the thickness of the traces. 120 Amps is a
    lot for a pcb.
     
  12. Mike Monett

    Mike Monett Guest

    Graham Orme wrote:

    [...]
    Especially for writing ascii text messages to send to a newsgroup:)

    Mike Monett
     
  13. Ted Edwards

    Ted Edwards Guest

    While I have had a fair bit of experience with sitch mode, I haven't
    seen on of these mother boards. Two questions:
    Is what you are calling a a "buck converter" what I have always called
    a switching regulator? It sounds like it - i.e in your example, switch
    on 25% connecting 12V to LC filter?

    This means you need capacitors with enormous ripple current
    I assume, then, that four switches feed four inductors from one input
    capacitor and one output capacitor. Neat idea! The caps should see
    only minor switching transients.

    Ted
     
  14. Joel Kolstad

    Joel Kolstad Guest

    A buck converter is a "switching regulator" that specifically has a lower
    output voltage than input voltage. There's also boost converters,
    inverting, and buck-boost, which have self-evident meanings.
    Well, when you're talking about e.g., 100A, even with multi-phase converters
    you still have to pay attention to the capacitors' ripple current specs...
    :)
     
  15. Tim Shoppa

    Tim Shoppa Guest

    Makes perfect sense. My experience with buck regulators is at the
    much-lower-current end (a few amps, not hundreds!)Thanks, Graham!

    Tim.
     
  16. Graham Orme

    Graham Orme Guest

    Yes you do, but the capacitors have a much easier life. With four stages
    each running at 30 Amps the current through the first inductor might go
    from 25 to 35 Amps during the time it is being energised. Then the
    second inductor does the same and so on. The current drawn from the
    supply is a sawtooth waveform of 30 Amps d.c. plus or minus 5 Amps. The
    input cap' only sees the +/- 5 Amp bit.

    A single stage with the same percentage ripple would draw zero current
    before the inductor is energised. This would jump to 100 Amps when the
    switching device turns on and then rise to 140 Amps before returning
    rapidly to zero for the remaining 75% of the cycle. Now THAT is what I
    call ripple current.

    The output cap' also has a much easier time of things.
     
  17. Winfield Hill wrote...
    A followup zoomed photo is on a.b.s.e., "Re: ASUS P4 mobo photo,
    for s.e.d. "Re: Motherboard inductors" - asus_p4c800.jpg"
     
  18. Winfield Hill wrote...
    The inductors are wound with three paralleled #14 wires, for a
    calculated Rdc = 0.8 milliohms (and measured, below 1 milli-ohm).
    We know the inductor's ac resistance may be higher due to skin and
    proximity effects, so this issue bears more detailed examination.
     
  19. legg

    legg Guest

    Ignoring the big toroids used in the buck regulators, there are other
    inductors on your board - smd beads gathered generally around the I/O
    ports to assist in obtaining electromagnetic compatability.

    RL
     
  20. Alan Turner

    Alan Turner Guest

    Hi Win,

    Thanks very much for the description of this power supply Win! I found it
    very interesting. Posts like this one make s.e.d really worth the reading
    time :)

    Regards,
    Alan
     
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