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Full Bridge SMPS With High No Load Voltage?

Discussion in 'Electronic Design' started by Jeremy Samuels, Jan 25, 2006.

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  1. In my attempt to build an inverter-style plasma cutter, I have run into
    a problem. How can a full bridge SMPS create a high (270 VDC) near
    open circuit voltage, but have a relatively low load voltage (90 vdc at
    25+ amps)? The high voltage is used to start the arc, and the lower
    voltage is used to maintain the arc. I'm planning to use a CCM PFC
    with an output of 220 vdc into a full bridge (current mode, fs=25khz).
    See http://www.northerntool.com/downloads/manuals/164686.pdf for
    similar power supply requirements.

    Thanks.
     
  2. Robert Baer

    Robert Baer Guest

    It has to act like a current source; persumedly you can alter the
    feedback to do that.
     
  3. Guest

    You could put an extra winding/diode on the output choke and feed that
    to the output, you would need another diode in the main output to stop
    it feeding back.
     
  4. Guest

    Sounds good. The other option is sense the i_out and switch the
    regulation voltage accordingly. You might find you've got almost all
    the bits in place to do that already.


    NT
     
  5. Paul Mathews

    Paul Mathews Guest

    It's not uncommon to use some parallel source of high voltage that has
    a comparatively high output impedance. When the heavy arc current
    starts to flow, the high voltage effectively collapses to the voltage
    provided by the low impedance supply. This is sometimes done with
    additional windings on the main transformer, sometimes done as a
    completely separate circuit. In either case, you can add circuitry to
    actually switch off the HV when the arc starts if you like. It may or
    may not be necessary to prevent the HV output from inhibiting LV
    circuitry through negative feedback. Many such circuits don't have
    negative feedback, but run at a conversion ratio determined by
    feedforward (line input) only.
    Paul Mathews
     
  6. budgie

    budgie Guest

    That might be an interesting exercise in stability.
     
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