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multiphase switching advantages

Discussion in 'Electronic Design' started by Jamie Morken, Mar 26, 2008.

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  1. Jamie Morken

    Jamie Morken Guest


    Is there any advantage to doing multiphase switching (ie. a
    3 phase stepdown buck) with synchronous switching, or is it
    only advantageous to use interleaved switching?

    I know the input and output ripple will be reduced only with
    interleaved switching, but I am not sure if the inductor requirements
    will be lessened with synchronous switching or not. I was thinking
    that since an inductors size is proportional to inductance*current^2,
    that a 3 phase synchronous or interleaved switcher would have 1/3rd
    the overall inductance capacity requirement of a single phase switcher.

    Is this correct? I know if you parallel inductors the inductance goes
    down but I am not sure if that is important here.

  2. MooseFET

    MooseFET Guest

    The two are basically the same thing. Interleaved switching is what
    multiphase converters do and single phase ones don't.

    There are multiple advantages:

    Both the input and output ripple are shifted up in frequency by N.

    The amplitude of both ripples are reduced by N or more.

    The ripple in the ground plane is reduced by a lot more than N and
    this can help a lot if there are analog circuits sharing this plane.

    The Nyquist of the control loop typically is increased by N.

    The EMI radiation may be reduced by as much as N.

    With the sharing of part of the working inductor, a slightly smaller
    total amount of core may be needed. The shared part needn't be as
    good at the high harmonics as the seperate parts, but this is too
    little of an effect to matter.
  3. Jamie Morken

    Jamie Morken Guest

    What about a multiphase converter that does synchronized switching?
    Would that be the same performace as a single phase converter? (ie.
    no reduction in the input/output voltage ripple)

    I am just curious if there is any benefit at all for a multiphase
    converter that has synchronous switching (not synchronous rectification)
    over a single phase converter.

  4. MooseFET

    MooseFET Guest

    Please explain exactly what you mean by synchronous switching.

    If you mean all the devices switch at the same time, the converter is
    not multiphase.

    If you mean synchronous with some external pulse then multiphase can
    have a large advantage there. It is a fairly common trick to time the
    DC-DC converters switching at the point where some other part of the
    system doesn't mind the noise so much or to make the DC-DC converter's
    noise alias to a frequency you don't care about in an ADC. In these
    cases, a multiphase converter can make things a lot easier by allowing
    the DC-DC converter to run at a lower frequency.
  5. Jamie Morken

    Jamie Morken Guest

    Yes that is what I meant, all devices switching at the same time.
    I just was wondering if the overall inductor core volume would be
    lessened in that case rather than a single switch, since the
    separate inductors for 2 phase for example would each carry half
    the current and inductor volume increases with current^2.

  6. MooseFET

    MooseFET Guest

    That isn't multiphase. It is just paralleling smaller converters.
    The core requirements etc are not any easier to meet except you could
    perhaps a practical consideration. If you can find the smaller cores
    that exactly meet the requirements but at three times the size you
    can't and have to go up to a bigger model then there is an advantage.
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