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CCFL Piezo xfrmr @ 200 KHz or higher ?

Discussion in 'Electronic Design' started by Martin Davis, Apr 16, 2005.

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  1. Martin Davis

    Martin Davis Guest

    Hello folks,

    I have been looking high and low without success
    for a source for a CCFL type Piezo transformer that
    has a resonant frequency at 200KHz or higher.

    5-10 volts in, and anywhere from 700- 2KV out
    @ 1 ma or more.

    Anyone know of such an animal ?

    Thanks in advance
    Martin
     
  2. Robert Baer

    Robert Baer Guest

    You expect a chunk of ceramic to handle two watts or so??
    Why piezo, and why that frequency?
    Use a standard CCFL transformer (Cooper/Bussman comes to mind), toss
    the Royer design; use low Rds(on) FETs with a driver and bypass the tap.
    Drive near output resonance frequency; at 10meg load i got over 2KV
    pk-pk and resonance was roughly 250-300KHz.
     
  3. Martin Davis

    Martin Davis Guest


    Sorry, actually the power requirement is very low.
    ..1ma is what I meant to say. Also the input voltage
    isn't a critical requirement either. Vin can be anywhere from
    10V to 50V
    Why piezo?
    1. Less radiated RF
    2. A standard CCFL has to much capacitance between
    the windings and the core to achieve the level
    of floatingness I desire.
    (sure floatingness is a word!, really) :)

    Why that frequency?
    Much easier to filter. In fact, I had already goosed
    up a standard CCFL inverter to 380Khz and was pleased
    to discover that at that frequency little or no
    filtration seemed to be required at all for my relatively
    high impedance adjacent circuitry.
    Interesting. Thanks for the tip. When I goosed up my CCFL
    inverter made by TDK, I got piss poor output which I assumed
    was due to core losses. Perhaps I'll dork around with it some
    more and try your suggestions and see what I get.

    However, I still want the output to be floating more so than
    I'll probably be able to obtain with a hotrodded magnetic
    xfrmr.
    But at least it'll get me in the game until I can locate
    a USA supplier for a 168kHz piezo which AFAIK is about the
    peak frequency being made to date. Hopefully that'll be high
    enough to solve the induced noise problem

    Thanks again for the advice.
    Martin
     
  4. James Meyer

    James Meyer Guest

    A magnetic inverter or a normal piezo device powered from a completely
    separate battery should be able to achive an extremely high degree of
    floatability. A rechargable battery isolated with a relay to allow recharging
    when the device is connected to a power source but nominally "off", should be
    do-able.
    Piezo devices are usually operated at a resonant point in their
    response. That's why the choice of operating frequencies is limited and not
    adjustable.

    Jim
     
  5. Martin Davis

    Martin Davis Guest

    Thanks,
    yes, I have already considered the battery option at least
    for now in the experimental phase of this project.
    Also have considered using a a low voltage low frequency
    piezo inverter in place of the battery to supply power to
    and float a high frequency-hotrodded magnetic CCFL inverter





    So far, the highest resonant frequency I've seen for piezo
    CCFL type transformers is in the neighborhood of 160-170kHz.
    Do you think this is an upper boundry inherent in the material?
    Anyone know ?

    Thanks again
    Martin
     
  6. James Meyer

    James Meyer Guest

    The piezo elements have to get thinner as their resonant frequency goes
    up. Perhaps there is a point where the element gets too thin to support the
    voltage required. A thinner element might also be less able to withstand the
    heat generated. The efficiency can't be 100%. At least some heat is generated
    within the element.

    Jim
     
  7. Joel Kolstad

    Joel Kolstad Guest

    Robert,

    Just curious... what don't you like about the Royer design? Jim Williams
    seems to have done an awfully good job advocating its usage (although I
    realize his classic app note is now pushing something like a decade old).

    ---Joel Kolstad
     
  8. Martin Davis

    Martin Davis Guest

    Perhaps he was shunning the Royer for my purposes which
    for now is to jack up the operating frequency of an old
    magnetic CCFL inverter.

    I wasn't able to stabilize the operation at any whimsically
    chosen abitrary frequency. Regardless of how I tried to tune
    the Royer circuit, it seemed to favor 84kHz or 385kHz.

    It would be a flip of the coin which of these two states it
    would come up in when power was applied.

    Rather than try to resolve this problem and since I needed to
    get more power anyway, I finally did pretty much as he said
    and tossed the Royer in favor of an externally clocked control
    circuit with beefier chippers

    Martin
     
  9. Joel Kolstad

    Joel Kolstad Guest

    Thanks Martin, I'll keep those potential problems in mind.

    ---Joel
     
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