Re: What is a 'demagnetization coil' ?

On Wed, 06 Jun 2007 20:25:43 -0700, Traver wrote:

On Jun 5, 3:32 pm, Sven Wilhelmsson <sven.wilhelms...@xxxxxxxxx>
wrote:
On Tue, 05 Jun 2007 19:01:03 +0000, Eeyore wrote:
Sven Wilhelmsson wrote:

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' ?

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

Graham

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!

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


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. (??)

--
Sven Wilhelmsson http://home.swipnet.se/swi
.

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