Re: Mike Marcum Ferrite Cores

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

Hi Matt,

On 20 Oct 2005, at 11:56, Tesla list wrote:

> Original poster: mecortner@xxxxxxxxxx
>
> -snip-
>  >You're going to hate this answer but it depends on the core
>  material. >In general, the more square the B-H curve is, the faster
>  the core >will saturate (i.e. with less applied magnetic field).
>
>  >Malcolm
>
>
> Well I guess I'll just have to build the thing
> and see what happens. That Book My friend has
> with the patent number in it wasn't at his
> shop in town, he'll check out at his place
> this week, until then I might just sketch it
> out and send it in as a JPG. Is that OK Terry?
>
> -snip-
>  >Maximum utilization of a given core is achieved by
>  >operating it with reversals (as in a half or full bridge drive
>  >configuration).
>
> Are you kidding?? I just thought that the
> magnetism travels through the material with
> little resistance no matter what direction
> it goes? If it's not too much trouble could
> you please elaborate on this a little more?
> Thanks!

Manufacturers usually issue charts showing power throughput for
different circuit topologies for a given core. The main dependency
for a given core involves copper losses - i.e. the size of the
winding window limits the amount of copper you can fit in there and
the minimum inductances you can get away with - as in fewer turns -
determines the maximum wire gauge. This is in turn dictated by the
maximum frequency the core can run at before the core (hysteresis)
losses outweigh the copper losses.
     For a single-ended forward converter which delivers power to the
load during switch conduction time, the core only gets magnetized in
one direction. Since you cannnot deliver power for 100% of a cycle
with this type of converter (the core has to recover - fall back to
near zero magnetism while the switch is off), some of the time is
effectively wasted. The same consideration applies to a flyback
converter which spends about half its time delivering energy to a
magnetic field and the other half delivering energy from the magnetic
field to the load. A half or full bridge converter can deliver power
for almost 100% of a cycle by delivering energy to the load on each
half cycle.

Malcolm