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planar transformer design

Discussion in 'Electronic Design' started by [email protected], Jun 17, 2007.

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  1. Guest

    Hi, I have to design a planar transformer with the following
    specifications:

    1:1 ratio
    12A continuous RMS current in primary and secondary
    frequency 150kHz - 400kHz
    input/output voltage - 8Vp-p square wave (4Vrms)

    I don't know where to start with the designing process.

    I have calculated my S as 48VA and I chose a 3F3 ferrite double E core

    Now I seem to get stuck in the next step, do I first choose a core
    and then calculate my maximum flux density?

    Please give me some advice

    Johan
     
  2. Eeyore

    Eeyore Guest

    Planar transformers are no different to any other other aside from the method of
    construction. All the sums for the magnetics work just the same.

    Philips (Ferroxcube) had a good application note on them.

    OTOH if you don't know to design a transformer at all, that may not help you.

    Graham
     
  3. Whether the transformer is "planar" or not, what I do is:

    1. Pick a core.

    2. Pick a maximum flux density that does not produce saturation or
    excessive heat at the frequency in question, at the maximum temperature
    that the core is anticipated to achieve.

    3. Figure out how many primary turns are needed to keep the maximum flux
    density that low.

    4. Figure out how much I-squared-R loss you get at the current in
    question at the frequency in question at the maximum wire temperature
    foreseen.

    Keep in mind that the skin effect is significant at frequencies in the
    10's of KHz and higher for any wire size likely to carry 12 amps.
    Resistance of copper wire is also roughly proportional to absolute
    temperature, but the variation with temperature is less (probably closer
    to proportional to square root of absolute temperature) once the skin
    effect has boosted resistance a lot.

    There are charts and formulas for predicting resistance of a given wire
    size at a given frequency.

    I would suggest Litz wire with a high strand count for 8 volts 12 amps
    at frequencies in the 10's of KHz and higher, since that is likely to
    allow substantially smaller wire that will allow a smaller core. And I
    would try to find some data on the resistance of the Litz wire in question
    at the frequency in question.

    And after that, test and verify that the transformer does not overheat,
    since most published skin-effect-related resistance increase figures are
    for isolated straight wire and the figures can be different in a winding.

    - Don Klipstein ()
     
  4. Guest

  5. Guest

    I will use the formula V = 4.44 N A e f B 10 -8 substituting 4 for
    4.44 as I will be working with square waves. I don't know what to make
    of the turns though.

    Would it be wise to start with the windings?

    I can't just select a core? Obviously I need to calculate a suitable
    core volume?

    Thank you for all the feedback.

    Johan
     
  6. Guest

    Thanks Don. I will give that info some thought and do some
    calculations tonight. Will post my results.
     
  7. Eeyore

    Eeyore Guest

    Planar transformers use the copper foil of multi-layer pcbs for the windings.

    Graham
     
  8. Guest


    What would the duty cycle for a 1:1 transformer be? I suppose it would
    be one?

    Also, the minimum input voltage? Considering its a 4Vrms square wave?
     
  9. Guest


    I don't understand how the ferroxcube cores work. There is a core (E)
    and a plate (PLT). When I calculate my transformer's core loss density
    do I take the Ve of the E plus the Ve of the PLT into account?
     
  10. Eeyore

    Eeyore Guest

    You do it exactly like any other EI core. You can also use them in an EE configuration too.

    Try this to calaculate losses.
    http://www.epcos.com/web/generator/Web/Sections/DesignTools/Ferrites/Page,locale=en.html

    Graham
     
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