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Power Supply with transformer 300V

Discussion in 'Electronic Design' started by Dotinho, Mar 8, 2007.

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  1. Dotinho

    Dotinho Guest

    Hello, i have i power supply of Siemens in one olde machine, and i
    have to repair it, but i don't know how.

    the essencial, the 1 step is transform 220 VAC to 315VDC and then a
    MOSFET will pulse the 300VDC at 50KHz to a transformer that out 5V at
    secudary, the problem is that tranformer giver out 8V and don't work..

    Anyone can tell me if i change the frequency,. the output volt will
    decrease?

    Thanks.
     
  2. Dan H

    Dan H Guest

    Not likely that freq is problem

    More likely:

    The power supply expects a load at the output that will drop the
    voltage or

    There is a control circuit at the output that feeds back to control
    the voltage and the feedback control is not working.

    Dan
     
  3. Robert Baer

    Robert Baer Guest

    I have seen a few circuits that do preliminary regulation by
    switching the supply voltage like that, but usually SCRs or triacs are
    used, so it might be a good thing to double check not only that part but
    everything around it.
    Do you have a schematic; if not, scribble one up aqnd document each
    part as best as you can.
    Check each part to see if they are within spec, especially that "MOSFET".
    I am guessing that the "MOSFET" is always on, and that the second
    transformer sees the full 300V sinewave from the first one.
    Then again, you did say 315VDC, which (to me) means that the circuit
    may be closer to what i had seen.
    First is a transformer, Primary: 220VAC,Secondary: 300VAC.
    Second is most likely a bridge rectifier where at least 2 components
    are SCRs that are switched, allowing the output to be adjusted from zero
    to 315VDC.
    Now it is possible that 4 diodes are used and a MOSFET is in the
    bridge acting as a series adjustable resistor to the "load", but that
    useage (as a series resistor) is extremely unlikely to say the least.
    If indeed it is a MOSFET, it most likely is used as a timed and
    synchronized switch to the load and there is no filtering on the 315VDC
    source (might be some at thhe load).
    So now one has a semi-regulated and adjustable DC to feed some kind
    of a load.
    It is not clear as what that load is; surly DC *cannot* be applied
    directly to a transformer!
    Usually, that DC is used to drive a switcher that drives the second
    transformer, and the second one is dinky compared to the first
    transformer. That was done in a high votage bench supply that was
    precicely (via a helipot) adjustable from 0 to 3KV (i think; it has been
    over 30 years).

    Maybe what the layout is like the following:
    First is a transformer, Primary: 220VAC,Secondary: 300VAC.
    Second is most likely a bridge rectifier where at least 2 components
    are SCRs that are switched, allowing the output to be adjusted from zero
    to 315VDC.
    Now it is possible that 4 diodes are used and a MOSFET is in the
    bridge acting as a timed and synchronized switch to the load (second
    transformer).
    So now one has a semi-regulated and adjustable AC to feed that
    transformer.

    No matter what the case is, it is clear that the switching components
    (SCRs, Triacs, MOSFETs) are "shorted" and thus the full sine wave is
    being applied to that second transformer.
     
  4. Dotinho

    Dotinho Guest

    Robert Baer escreveu:

    Thats true, but the transformed are mailfunction, becoused i already
    tested the other part, and that tranformer don't produce anymore, and
    i'm trying to get another solution, you understand?

    thanks..
     
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