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Opto-couplers in smps...

Discussion in 'Electronic Design' started by Steve, Feb 1, 2007.

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  1. Steve

    Steve Guest

    Hi all,
    Here we go again with another smps question.
    As you may have read earlier, I'm trying to compensate a smps-design.
    The error amp is located on the secondary-side. The error signal is
    interfaced to primary side through an opto-coupler. And this opto is
    what makes me concerned. Since optos has a wide coupling ratio, I'm
    afraid, at some point, end up with excessively low loop gain and
    eventually makes the loop crossover frequency say hello to the filter
    resonance and the thing will go up in smoke. Can this be the case? Do
    I have to worry about this.
    How to minimize the effects of "wild optos"?

    Best regards,
    Stefan
     
  2. Anirban

    Anirban Guest


    Sureshot way : Use a pulse width modulator to transfer the amplitude
    information across the optocoupler; average it out on the other side
    or better still, use the time domain information. That way the
    optocoupler is always operating either at full power or zero. And
    hence highly INsensetive to varying gains. Or you could use a Hall
    effect DC current sensor (also called a DC Current Transformer / DCCT)
    to Ohmically isolate the two sections and still transmit the required
    data.

    --- Anirban
     
  3. Paul Mathews

    Paul Mathews Guest

    Simply specify an optocoupler with a more tightly spec'd coupling
    ratio. They exist for just this reason.
    Paul Mathews
     
  4. It changes with time.


    Best regards,
    Spehro Pefhany
     
  5. Joerg

    Joerg Guest

    Way to go, at least when riding close to the cliff in terms of loop
    stability.

    Another simple method is to use a little RF transformer instead of the
    opto coupler and transfer a carrier where the amplitude is proportional
    to the input signal. I often use 13.56MHz for this kind of stuff, less
    worries at the EMC lab :)
     
  6. Paul Mathews

    Paul Mathews Guest

    Of course, I agree that you have to design around the non-ideal
    characteristics of optocouplers, but they are out there in the world,
    functioning reliably in the billions of units. The drift due to aging
    is toward less coupling, and is one of many factors that the designer
    must consider when deciding how much margin to require.
    Transformer isolation is a good alternative, and it is often a
    requirement in space and avionics electronics, with optos not allowed.
    In addition to using various pulse width feedback schemes, there are
    plenty of examples of amplitude feedback thru transformers, including
    one in the TI/Unitrode Seminar Notes, IIRC.
    Paul Mathews
     
  7. Steve

    Steve Guest

    Does anyone of you have an example of how to implement a pwm feedback
    using opto?
    How to integrate the pulses to achieve a correct "copy" of the
    original error signal.
    This must require a carrier frequency more than 10 times the switching
    frequency of main converter?

    I appreciate any advice.
     
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