High Voltage Flyback

Discussion in 'The Projects Forum' started by joeyd999, Dec 23, 2014.

  1. joeyd999

    Thread Starter AAC Fanatic!

    Jun 6, 2011
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    I am developing a 2000V flyback converter that operates from 2 'AA' batteries. It's driven by a PIC18F23K20 running a PID routine, and CCP2 to control the charge/discharge timing. It can run both constant voltage and constant current.

    It is currently up and running. At constant voltage, I am getting +/- 0.5% regulation into a 15uA load from 2.0 to 3.0V supply.

    Constant current regulation is much better (there are reasons for this!).

    Not looking for any help. It is an interesting project, and I just wanted to share. Happy to answer any questions!
     
  2. strantor

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    Oct 3, 2010
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    Is this a James Bond style "Pen of Death"?
     
  3. #12

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    Cattle prod for a hamster?
     
  4. joeyd999

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    No. Very low current. Used for a measuring device. Sorry, I cannot reveal the purpose. The available energy is so low that you can actually touch the high voltage node and it'll give your finger a nice jolt (been there, done that)...but nothing close to dangerous.
     
  5. ebeowulf17

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    Sounds like a portable version of the hipot tester I use at work.
     
  6. joeyd999

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    I suppose I could use it for something like that, or, at least, an insulation resistance tester. There are other possible applications as well: photomultiplier tubes, gas ionization, etc.

    But no, that's not it.

    I thought you guys would be more interested in the how, as opposed to the why.
     
  7. NorthGuy

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    Jun 28, 2014
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    I wonder how many stages you have and what's the efficiency level.
     
  8. joeyd999

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    Stages? I don't know what you mean.

    I am driving a single flyback transformer. 100 turns on the primary, 6000 turns on the secondary, and a 60 turn feedback secondary. It's about 1" square and 0.5" tall.

    So far, I am running at about 20% efficiency (a lot of power is lost to leakage inductance and winding resistance -- can't be helped). It is good enough for the application, and I don't know if I can do better.
     
  9. joeyd999

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    The transformer is similar to this:

    [​IMG]
     
  10. ebeowulf17

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    Aug 12, 2014
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    Can't speak for anyone else, but I avoideded the how simply because there's so little hope I would understand anyway. This stuff is all fascinating to me, but I'm working at a very basic level right now - I only recently learned of the "joule thief" circuit and still can't wrap my head around how it really works.

    Anyway, your posts are always very interesting, even if they are way beyond me. Thanks for sharing!
     
  11. joeyd999

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    The theory is quite simple:

    A flyback transformer is nothing more than a dual winding inductor. The inductor's magnetic field is charge through the primary, then discharged through the secondary, with a voltage multiplication (and current division) equal to the turns ratio of secondary over primary. Actually, quite a bit simpler than an AC transformer!

    EDIT: I say simpler because during the charge cycle, you only need to consider the primary, and during the discharge, only the secondary. An AC transformer requires consideration of both windings at all times.

    Additional voltage gain is achieve through the transformer's flyback effect. The voltage on both windings instantaneously jump to a much higher voltage than the original excitation voltage as the magnetic field begins to collapse, again proportional to turn ratio.

    A third winding can be added that reflects the output voltage to a much lower (proportional) "sense" voltage that can be used as an input to the control logic.

    Flybacks work well with constant "off" time and variable "on" time. If D (duty cycle) is defined as on-time/off-time, the output voltage is, IIRC, approximated by (Vexcite)(turns-ratio)*(D)/(1-D), or something like that.

    Sorry, I've already had a few glasses of wine tonight. My memory will be much clearer in a couple of days (and I'll be able to understand my notes again).

    Merry Christmas
     
  12. ebeowulf17

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    Merry Christmas! Thanks for the primer. I'll try to let that sink in for a while until the next lesson, when that wine is out of your system!
     
  13. foolios

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    Feb 4, 2009
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    how can you use it to hi-pot test?
     
  14. #12

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    You can't. That's not what it was designed for.
     
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