Re: forward converter, higher Bmax through gap?

From: John Popelish (jpopelish_at_rica.net)
Date: 07/05/04 

Date: Mon, 05 Jul 2004 14:43:31 -0400

Genome wrote:
>
> "John Popelish" <jpopelish@rica.net> wrote in message
> news:40E99070.F37F97EC@rica.net...
> | Genome wrote:
> |
> | > Perhaps you are confusing the Bfield with the Hfield.
> | >
> | > Bsat for ferrites, irrespective of the gap, is in the order of 300mT
> to
> | > 400mT, 3000G to 4000G (at 100C). If you operate the core at higher
> | > frequencies you have to reduce the value, Bmax, because core losses
> go
> | > up.
> | >
> | > Plots of the hysteresis loop are of B versus H where H is NI/Le
> ampere
> | > turns per meter.... but you may be using Oersteds.
> | >
> | > With a gap the required H field to drive the core to any given flux
> | > density is increased.
> | >
> | > However, in a transformer with a gap the magnetising inductance is
> | > reduced so ripple current increases in proportion.
> | >
> | > Although the core can support an higher H field that increase is
> negated
> | > by the higher ripple current in the windings. The equation for
> | > calculating required turns for a given Bmax doesn't make reference
> to
> | > the H field.
> |
> | (The following is a not so well thought out thought. Criticism
> | welcomed.)
> |
> | There may be a small beneficial effect of adding a bit of air gap to a
> | forward converter core that makes the extra magnetizing current
> | worthwhile. It has to do with the shape of the BH loop.
> |
> | To exaggerate the effect, assume a perfectly closed core that also has
> | a perfectly square BH loop. There will be an output the first time
> | the core is magnetized, but the squareness of the loop means that the
> | core has no ability to demagnetize itself. So the second (or third)
> | cycle of the forward converter will saturate the core. Adding a thin
> | gap linearizes the net BH loop, providing assistance in damaging the
> | core when the applied voltage is halted. The optimum gap is a trade
> | off between this benefit (and the net increase in flux swing possible
> | against the increased leakage inductance between windings and the
> | increased magnetization current. If the core material had a
> | completely flattened BH loop (no remnance) then the optimum gap would
> | be zero.
> |
> | --
> | John Popelish
>
> I think my brain just imploded, mind you I'm not too good at magnetics.
>
> I do like the idea of something providing assistance to damage the core.
> That's wicked.
>
> DNA

Damn spell checker! That should have read, "demaging the core" as in
demagnetizing the core". 

-- 
John Popelish

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