Transformer application/design question

Discussion in 'The Projects Forum' started by Artifex, Oct 20, 2011.

  1. Artifex

    Thread Starter New Member

    Oct 20, 2011
    2
    0
    Hi everyone,

    I've got an application that requires a small (400 mm^2) footprint board-mount transformer that can take a 5VDC, low-current (<1A) square-wave signal and output a 3000Vp-p, 500kHz signal in the tens of mA (5W max output power).

    Used for generating an abnormal glow discharge in a microcavity.

    Does anyone know of a particular type of transformer that would be ideal for the application? I've found some current-sensing units that may work, but I don't have a grip of the differences between the types. Efficiency is key.

    Thanks taking the time to read.
     
  2. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi Artifex,

    5Watts / 3000V = 1.67mA max. secondary current with 100% efficiency.

    Since you wanted "tens of mA", will less than 1.67mA be enough?

    20mA at 3000V would be 60 Watts of secondary power. 60W / 5V = 12 Amps of primary current. A 400mm^3 transformer would have a hard time doing 5W.

    500KHz is too high for this application, 15KHz would be more reasonable. Use a flyback design transformer with a turns ratio of approximately 1:600.

    I'll try some simulations to see what is possible. Can you re-state your power requirements? Is there a higher primary voltage you can use?

    Regards,
    Ifixit
     
  3. Artifex

    Thread Starter New Member

    Oct 20, 2011
    2
    0
    Thanks for the quick reply!

    It seems I flubbed the calculation for output current; corrected, I think the actual target would be 3.33mA (5W/1500V). The fact that it's an oscillating signal with +/- 1500V amplitude (3000V peak to peak) leads me to use 1500V in the current calc.

    So I'm actually looking for a 1:300 turns ratio transformer. 500kHz is the most optimal frequency we've tested so far; anything below 100kHz is not enough for the target discharge characteristics.

    Primary voltage is fixed at 5V as a requirement. Input current can be as high as 2A. So, up to 10W in, 5W out, but efficiency is a primary design driver, so I'd say 10 in/5 out is a worst case scenario.

    Again, thank you for your time. :)
     
  4. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi Artifex,

    Forget I suggested a flyback transformer, I missed the p-p after the 3000 and was thinking you needed 3000V DC:rolleyes:

    A pulse transformer design would be best with a center-tapped primary and isolated secondary. Getting 3000V p-p at 100KHz to 500KHz range will be challenging in a small transformer package with a 400mm^2 base. How tall can it be?


    Questions:
    1. Can you tolerate a bigger footprint?
    2. Is the signal a sinewave, or a squarewave?
    3. Is the load constant, or will it vary?
    4. If it varies, by how much do it vary?
    5. Is the load resistive, capacitive, or inductive?
    The secondary needs to be wound with as little inter-winding capactance as possible so as to have a high self-resonance well above the operating frequency... unless it can be resonate with the load???

    BTW 3000Vp-p is 1072V RMS if the signal is a sinewave. 5W / 1072VRMS is 4.7mA, or if current is important then; 1072VRMS x 3.33mA is 3.57Watts. Please clarify.

    Regards,
    Ifixit
     
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