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1/32 brick dc-dc converter cookbook

Discussion in 'Electronic Design' started by mook johnson, Nov 22, 2012.

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  1. mook johnson

    mook johnson Guest

    http://www.murata-ps.com/newsletter/e-specifier_2012_01_26.htm

    There are several companies that make the same 1/32 brick power supply
    with planar transformers. I'm sure there is a appnote/cookbook or
    reference designs on the design of these small supplies but I can't find
    them.

    In my application I cant use the commercial parts as the parts are not
    rugged enough so we'd like to build a custom one.

    Anyone know of a public domain reference design or appnote on making these?

    Thanks.
     
  2. legg

    legg Guest

    I couldn't qualify these for consumer apps, at the advertised ratings,
    based simply on their own thermal imaging.

    What do you mean by rugged enough?

    At these power densities, even the 'experts' trip over their own
    salesmen.

    RL
     
  3. Tim Williams

    Tim Williams Guest

    Cool. First time I used planar transformers, I got about 10W through a
    transformer the whole size of that module. Helps to use more than two
    layers, of course. They're really pushing it, and getting only 75C temp
    rise for that much power density isn't bad.

    If they're getting too hot (as legg observes), they should run a hell of a
    lot cooler with derating. They appear to be ceramic capacitor construction,
    so they should have fine lifetime at reasonable temperatures.

    Planar transformer design works like any other kind, helps to have a PCB
    editor that supports and facilitates their design of course.

    Tim
     
  4. legg

    legg Guest

    One of the issues is isolation - the insulator temperature index being
    limited to that of the printed wiring means there are certification
    issues.

    Others involve PTC of loss in ferrites (above ~85C) and fets (always).
    I'd want to see two timestamped images that didn't vary appreciably.

    It doesn't matter for the supply itself, if it runs cooler when
    derated. It would allow use in a specific product, where suitably
    derated - but thats not what you pay for in a pre-qualified
    subassembly.

    I expect that any certifications may specify an airflow, but I've
    never done this with products that didn't generate their own....
    Sticky issue.

    RL
     
  5. mook johnson

    mook johnson Guest


    I'm mainly interested in what these designs look like in the inside so I
    can get an idea of how they get them so small. Our current rugedized
    constructed power supply module is 10X bigger. I'd like to see if there
    are any tricks they are using beside pushing components to the ragged
    edge to make the supply smaller. even at 5X this size it would be a
    substantial improvement in our present one.
     
  6. legg

    legg Guest

    Rolling your own is a decision that has to be justified, in the first
    place. If you can't beat them, the alternative is to use their
    product. High reliability situations may even preclude the use of a
    single converter. Like I said - define ruggedness.

    Can you stick to telecom bus dc inputs? Safety agencies recognize
    these as being isolated, so that only basic isolation is required for
    SELV at the system output. This may not be important for printed
    transformer winding use, but that use is a decision that needn't be
    predetermined.

    Second consideration is topology selection for magnetics and silicon
    utilization, over the functional operating range. Unless you have
    bright ideas, you're going to be using two magnetic components, with
    the isolation transformer operating under reversing flux levels. You
    may be able to get away with murder at lower power levels, but the
    closer you get to competitive power densities, the less room there is
    for the 'big semi - small heatsink' solution.

    The single transformer employment in your OP showed an attempt at
    resonant transfer, judging from the output ripple waveforms. This may
    have worked fine for higher output voltages and lower turns ratios,
    but you could see the results of trying to stretch it to low output
    voltages and higher output currents.

    Switched snubbing and synchronous rectification are the most common
    methods of reducing size of the parts used, even for single-magnetics
    type flybacks, in spite of their potential complexity. These can be
    stretched and massaged for the application. There's a plethora of
    literature out there covering component selection for those circuits.

    RL
     
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