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What is a 'demagnetization coil' ?

Discussion in 'CAD' started by Sven Wilhelmsson, Jun 5, 2007.

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  1. In the context of fly-back converters, what is a 'demagnetization coil'?

    I would think of it as a coil used to actively demagnetize the core.
    Sometime it seem to denote a coil that is passively de-energizing the
    core during the demagnetization phase in order to e.g. protect the
    driving transistor.

    So, what exactly is the purpose of a 'demagnetization coil' ?
     
  2. Eeyore

    Eeyore Guest

    To remove remanent magnetism that might otherwise result in core saturation.

    Graham
     
  3. So the coil must *actively* give a kick to remove remanence ??
    And in cases when the coil is passive, the term is wrongly used.
    If that interpretation is false, please correct me!
    Thanks,
     
  4. Marra

    Marra Guest

    A classic example is the degausing coils in a TV to to demagnetize the
    screen.
    If a magnetic area builds up it can distort the electron gun beams.

    Its usually on for a second at power up and is in series with a
    thermistor to turn it off.
     
  5. Traver

    Traver Guest

    When you drive a transformer winding with a voltage, you magnetize the
    core. If you drive it in one direction too long, it will saturate,
    meaning that it no longer acts like a transformer (or an inductor in
    the case of a flyback) and the winding will appear shorted, usually
    destroying parts in the driving circuit. So the trick is not to drive
    too long in one direction and then drive in the other direction. But
    many topologies can't drive the tranformer in the other direction.
    Topologies like the flyback let the transformer "reset" on its own.
    The problem is if you just drive the winding and let it go (a truly
    passive reset), there is no place for current to go with the power
    switch (mosfet) off and the primary winding will produce a huge
    voltage trying to reset, also destroying parts. Here is where the
    reset winding comes in. It is usually a tap off the primary with a
    diode to ground specifically used to dump reset energy fro the
    transformer. This is an active reset.

    One of the posts was already on the right track but I wanted to
    elaborate. I have never heard it called a demagnetization coil before,
    just a reset coil.

    Traver
     

  6. Thanks for a good answer. However, what you describe above is the case I
    would call 'passive', because the coil does not *actively* kick energy
    into the core to remove remanence. The term 'reset coil' or 'protective
    coil' would be fine in this case.
    What I would like to know is whether the term 'demagnetization coil' would
    require something more, like a circuit (capacitor, diode) to bounce back
    energy in order to actively remove remanence. Or maybe the term
    'demagnetization coil' shouldn't be used at all in the context of flyback
    converters, because it implies a wrong idea. (??)
     
  7. Traver

    Traver Guest

    I agree. True "active" reset circuits have a controlled swtich like a
    mosfet
    turning on purposely to drive energy out of the core. Although the
    "passive"
    reset is cheaper and easier.
     
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