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DC to AC converter. - can this be done?

Discussion in 'Electronic Design' started by mook Johnson, Dec 19, 2006.

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  1. mook Johnson

    mook Johnson Guest

    I have a buddy that is struggling with a design. Heres what he's trying to
    do.
    He recieves a 45 - 200Hz 500mV p-p sine wave through a signal transformer
    and would like to buffer it and drive and high current version of the signal
    to an output. Output load cna be anywhere from open circuit to .25 ohms.

    So far he had a 6:1 transformer primary being driven by NPN-PNP totempole
    and an OPamp to elimiante the xover distortion (aka simple audio amp). It
    is desired that the secondary be allowed to have about 2Vp-p of 60-120Hz
    common mode voltage on it from external sources so it can't be tied to
    ground but it doesn not need to truelly be isolated.

    What he is attempting to do is to AC couple off both sides of the output
    coil and bring that differential signal back into to the primary side to a
    second opamp. The feedback signal will be compared by this opamp to the
    reference signal and any load dependant droop will be corrected out. with
    all the elememts in the loop (opamp 1, opamp 2+ totem, 6:1 transformer,
    etc, stability is a bit shaky. He even ran into the dreaded motorboating
    problem.

    Basically its a DC to AC linear inverter problem but with an isolated output
    and stiff voltage requirements on the output voltage.

    I have some ideas about using a isolated power supply to generate the rails
    for a power opamp and us it to buffer the signal transformer. This would
    generate quite a bit it heat even with the +/-5V rails most power opamps
    require. The low load resistance is whats challenging.

    How would you analog types go about attacking this problem?
     
  2. Tim Wescott

    Tim Wescott Guest

    The 45Hz makes it challenging, too, if you're trying to put it through a
    transformer that doesn't break your back.

    One way to approach this would be to use transformers that pass well
    beyond the desired frequencies, then condition the frequency response of
    the circuit with the feedback components. This would help with the
    instability, but it would make transformer selection difficult.

    Ultimately what you want is a circuit which has a high common-mode
    impedance and a low differential mode impedance. Your transformer idea
    is just one way to make this happen. Another way would be to build a
    pair of current drivers, with feed back coming from a differential
    amplifier. This would ease your stability difficulties (and shrink your
    circuit), yet it would still have enough pitfalls to make it really fun.

    You could mix & match, also, by using a diff-amp feedback but a
    transformer in the forward path. This would ease the need for such an
    absurd transformer that I suggested in the all-transformer solution.

    No matter what you do you'll have to deal with stability issues. This
    all sounds very doable, but not trivial.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  3. mook Johnson

    mook Johnson Guest

    Thanks. Looks like we were thinking along the same lines. Transformer has
    a shield for common mode HF noise grounding. I don't thing 120Hz will jump
    from coil to coil unless the coils are wound very badly.

    the Diff and for feedback looks promising.

    Any good reference books for stabilizing beasties like these? There is one
    example in AoE but it is grounded input and ouput which makes it somewhat
    easier.
     
  4. Tim Wescott

    Tim Wescott Guest

    Beyond a 2nd-year networks book and a 3rd-year op-amp book -- not that I
    know of. I always approach these problems with a healthy dose of first
    principals leavened if necessary by the judicious use of SPICE.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  5. Ban

    Ban Guest

    Whenever the transformer starts coming near saturation, this method will
    drive it to the rail, where it hangs until the next zero-xing. Not a good
    idea. Works only for very small levels. It also requires your amp to be
    compensated much more, because of the additional phase shift at roll-off of
    the transformer.
    But you could take the feedback from the primary and implement a negative
    output impedance which equals the combined ohmic losses of primary and
    secondary windings. This will be stable unless shorted.
    To make the negative impedance, you can put a small resistance in series
    with the primary and add an appropriate level to the voltage feedback
    signal.

    There are more possible cures, avoiding the transformer at all and also the
    floating supplies...
     
  6. Joerg

    Joerg Guest

    Just bought running shoes from Alpine Design and this model was called
    "Analog". Seriously. So I guess I qualify ;-)

    If the size of the transformer is a concern one could take a different
    tack: Build a switch mode power supply, basically taken from a
    TI/Unitrode app note or similar. Make sure its regulation bandwidth is
    well in excess of 200Hz, usually not a big deal.

    Now transfer the power at over 100kHz, rectify at the output, easy on
    the output cap value, load that pretty good so the cap will bleed down
    fast enough for a 200Hz upper limit and provide the usual
    TL431+optocoupler scheme for the feedback.

    You would modulate in via the FB input of you regulator. It's not really
    trivial to build a switcher that goes from truly zero to full scale but
    it's been done before. SEPIC is what I usually do but that's not
    isolated. Flyback or push-pull may be your ticket because IIUC the
    output ground side needs to be floating to inject that other 2Vpp signal.

    There is a series of really tiny lab bench supplies from Lascar
    Electronics. Don't know if they can be hacked to modulate the output but
    AFAIK they are fully isolated and quite cheap:
    http://www.lascarelectronics.com/
     
  7. Tim Wescott

    Tim Wescott Guest

    It sounds like the OP needs to source AC to his load; this rules out a
    'regular' switcher. One could consider a switcher with synchronous
    rectification, although things would get complex that way.

    I wonder if one could use a class D amplifier chip intended for
    subwoofers? You'd have to apply external feedback, but that should be
    straight forward. You may even have to power it from an isolated
    supply, but you'd still have something that was built of individually
    straightforward blocks.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  8. Does the value of the output load move quickly or slowly?
    If slowly perhaps your colleague could keep the overall
    setup above but rectify/smooth both input and output,
    compare DC's, and use the average DC error to adjust AC
    error. Time constants will be in the order of seconds.
     
  9. Mook Johnson

    Mook Johnson Guest

    output can move fairly rapidly. Not a step changes but it will not be
    smooth lazily varying load.

    The switching power supply and the ClassD are intereseting but switching
    noise will be an issue as we are making high resolution measurements in the
    vacinity.

    I appreciate the input. Really gives me some ideas for tackling this
    problems on similar projects.
     
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