What is a 'demagnetization coil' ?
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Posted by Sven Wilhelmsson on June 5, 2007, 12:45 pm
Please log in for more thread options 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' ? -- Sven Wilhelmsson http://home.swipnet.se/swi | |||||||||||||||||||
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Posted by Eeyore on June 5, 2007, 3:01 pm
Please log in for more thread options Sven Wilhelmsson wrote: To remove remanent magnetism that might otherwise result in core saturation. Graham | |||||||||||||||||||
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Posted by Sven Wilhelmsson on June 5, 2007, 3:32 pm
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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! Thanks, -- Sven Wilhelmsson http://home.swipnet.se/swi | |||||||||||||||||||
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Posted by Traver on June 6, 2007, 11:25 pm
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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 | |||||||||||||||||||
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Posted by Sven Wilhelmsson on June 7, 2007, 3:53 am
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On Wed, 06 Jun 2007 20:25:43 -0700, Traver wrote: > 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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>
> 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' ?