flywheel diode

Thread Starter

Cerkit

Joined Jan 4, 2009
287
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??
 

mik3

Joined Feb 4, 2008
4,843
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.
 

Joe&Steph

Joined Jun 12, 2009
1
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!
 

mik3

Joined Feb 4, 2008
4,843
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!
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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