flywheel diode

Discussion in 'Homework Help' started by Cerkit, Jan 22, 2009.

  1. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
    275
    3
    Hi. I don't really understand how the flywheel diode works in the following circuit

    O---------------O V+
    | |
    L Δ
    | |
    --------
    |
    Mosfet
    |
    O----------------OV0

    How is it that current can flow through the diode once the mosfet is switched off??
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    When the MOS is on current flows through the inductor. When the MOS switches off current stops flowing through the inductor, the magnetic field around the inductor (created by the flowing current before) collapses and a voltage is induced across the inductor due to this changing (reducing in this case) magnetic field. If you remember a changing magnetic flux induces a voltage across a coil (v=-dΨ/dt, Faraday's law). Because of this induced voltage, which its positive on the MOS side and hence the diode is placed the way it is, current flows through the diode.
    If the diode wasn't there a very high voltage would be induced (rapid change of magnetic flux) which would destroy the MOS.
     
  3. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
    275
    3
    Does current flow through the diode when the mosfet is on as well??
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    No, because it is reversed biased.
     
  5. Joe&Steph

    New Member

    Jun 12, 2009
    1
    0
    Can you give us a description as to why the back-up emf travels through the diode rather than the inductor? Is it because the diode has less resistance? Thank you!
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    The inductor is the source of the back EMF (voltage). The current caused by the back EMF flows through both the inductor and diode.
     
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