[Genome]

| Genome wrote:
|
| > | > | But what about power inductors wound on a bobbin core. Isn't that
| > | more or less a pot core with a whopping big gap?
| > |
| > | --
| > |
| > | Tim Wescott
| > | Wescott Design Services
| > | http://www.wescottdesign.com
| >
| > How is that relevant to the latest telephone bill?
| >
| > DNA
| >
| >
| Well, you were discounting the value of gapping a core, yet the
| bobbin-core inductors seem to give most of the advantages of a gapped
| core -- much higher resistance to core saturation, more current per
| volume, better ability to talk to every other conductive loop on your
| board, etc.
|
| --
|
| Tim Wescott
| Wescott Design Services
| http://www.wescottdesign.com

Oh, how does that relate to forward converter transformers?

DNA

 

[John Popelish]

but you need a new bh curve made by the ferrite manufactor or by yourself to determince how the hysterese loop changed right?

Just found this page that shows the graphical method to alter the
material BH loop to show the effect of a gap.

http://www.magmet.com/tapewound/gap2.html

In a self resetting forward converter application the core resets only
to the point on the top side of the loop crosses the B axis at H=0.
The example shows a pretty significant reduction in this residual flux
with the consequent lowering of the slope of the loop (decrease in
effective permeability).

 

[Tim Wescott]

| Genome wrote:
|
| > | > | But what about power inductors wound on a bobbin core. Isn't that
| > | more or less a pot core with a whopping big gap?
| > |
| > | --
| > |
| > | Tim Wescott
| > | Wescott Design Services
| > | http://www.wescottdesign.com
| >
| > How is that relevant to the latest telephone bill?
| >
| > DNA
| >
| >
| Well, you were discounting the value of gapping a core, yet the
| bobbin-core inductors seem to give most of the advantages of a gapped
| core -- much higher resistance to core saturation, more current per
| volume, better ability to talk to every other conductive loop on your
| board, etc.
|
| --
|
| Tim Wescott
| Wescott Design Services
| http://www.wescottdesign.com

Oh, how does that relate to forward converter transformers?

DNA

The original question didn't mention transformers (or telephone bills,
either). I assume it was about inductors because of the mention of
cores and gaps, but the OP could have meant magnetic material to be used
with rare-earth magnets for all I know.

You know more about SMPS than me, or I wouldn't ask. At this point I know:

1. You can build a fine SMPS with an inductor or transformer based on a
pot core or toroid, no gaps.
2. You can build a fine SMPS with a bobbin core inductor (but watch the
16-bit ADC circuit right next door).
3. Gapped cores are a custom order so they _can't_ be universally cool.

I was just hoping to learn something useful.

 

[John Popelish]

The original question didn't mention transformers (or telephone bills,
either). I assume it was about inductors because of the mention of
cores and gaps, but the OP could have meant magnetic material to be used
with rare-earth magnets for all I know.

You know more about SMPS than me, or I wouldn't ask. At this point I know:

1. You can build a fine SMPS with an inductor or transformer based on a
pot core or toroid, no gaps.
2. You can build a fine SMPS with a bobbin core inductor (but watch the
16-bit ADC circuit right next door).
3. Gapped cores are a custom order so they _can't_ be universally cool.

I was just hoping to learn something useful.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

The rule of thumb is that if the inductive component also has to act
as an energy storage device (DC filter inductor, flyback (or voltage
boost) converter, then you need at least some gap, because ferrite
stores very little energy. The gap also makes the inductance much
less dependent on the exact permeability of the core material, so
precision and especially stability is easier.

If you just want coupled turns then an ungapped core may be just fine,
since this configuration has the highest coupling between different
windings (they all share more of the same flux) and the magnetizing
current will be lowest. A forward converter is an example of the
second type, except that a small gap helps it to reset the flux to a
lower value when the forward drive is off. If you have a
symmetrically driven transformer, the might still serve the purpose of
reducing the start up surge, if the startup has the same polarity as
the last half cycle before shut down. In that case, either the core
saturates or is running at no more than about half of saturation flux
peak.

 

[Genome]

| I was just hoping to learn something useful.
|
| --
|
| Tim Wescott
| Wescott Design Services
| http://www.wescottdesign.com

Sorry about that.

As I've said my magnetics design theory isn't too hot. I ssumed the OP
was talking about the transformer, I still think that's the case.

The only advantages I can see for gapping a forward converter
transformer are...

1) Gives a degree of 'free' slope compensation in peak current mode
controllers. The magnetising ripple adds to the output ripple in the
sense element giving an additional upslope to the current sense
waveform.

2) Sharpens the waveform transitions
a) Useful when using the output waveform for synchronisation of
downstream converters or an active leading edge modulated post
converter.
b) Improves low level regulation in such a converter stage.

I don't think the impact on leakage inductance is that great if at all.
The gap is placed under the windings. Leakage inductance is more a
factor of winding geometry.

I don't think the gap has any impact on the operating flux density of
the core regarding its remanance. There is nothing to be gained. I might
be skating on thin ice on that one though.

Cheers

DNA

 

[Graham Orme]

| If spell checkers do nothing else for me, they do encourage me not to
| use abbreviations any more.
|
| --
|
| Tim Wescott
| Wescott Design Services
| http://www.wescottdesign.com

Always check your spell checker.

The trouble with spell checkers is that they can't tell when you've left
out any of the ingredients, or mispronounced the incantation. Perhaps
they should be renamed "spelling checker".

 

[Julius Junghans]

The original question didn't mention transformers (or telephone bills,
either). I assume it was about inductors because of the mention of
cores and gaps, but the OP could have meant magnetic material to be used
with rare-earth magnets for all I know.

im in the process to build a transformer for the forward converter, but hes right...i didnt mentioned it.
next time i think more before posting, but its really an interesting discussion...isnt it?

 

[Julius Junghans]

Just found this page that shows the graphical method to alter the
material BH loop to show the effect of a gap.

http://www.magmet.com/tapewound/gap2.html

great site, have to update my mozilla version for the formulas.
i assume it is also usefull for e cores, or just for toroidal cores?

 

[John Popelish]

On Mon, 05 Jul 2004 17:15:22 -0400


great site, have to update my mozilla version for the formulas.
i assume it is also usefull for e cores, or just for toroidal cores?

Certainly. Toriods are just used as an example of the technique.