On 1/22/21 8:19 AM, Jeff Anderson wrote:
On Thu, Jan 21, 2021 at 01:36 PM, Miro, N9LR wrote:
That would be asking for trouble, like having BJT (bipolar transistors) in
parallel. On different frequencies, temperatures, flux one is destined to take
more then the other, and that one heats even more and takes even more on
itself
I'm wondering if there is any empirical data or theoretical derivations showing that paralleling ferrite cores of different materials is a bad idea.
From a heating perspective, it seems to me that if calculations show either core, stand-alone, does not overheat in the spec'd application, then there should not be a heating issue if the two cores are combined (unless overall common-mode resistance *drops* below the common-mode resistance presented by either core itself, but then what would be the point of paralleling the two cores?)
From my own measurements, the common-mode resistance of paralleled 43 and 31 cores looks pretty good (see the attached chart). From this measured data (taken on an HP 8753C) I don't see any reason why these two cores should not be paralleled. If anyone has different data, I would very much like to see it. After all, it's always possible my measurements are in error, or that I'm looking at the issue incorrectly.
Thanks,
- Jeff, k6jca
I think you'd need to look at the magnetic circuits. You've got two cores with different permittivities, so the flux will distribute unequally. So the real question is whether there are "pathological" frequencies and power levels that might get you into trouble with saturation or loss.
This is sort of different than, say, a core made of a mixture of the two ferrites.
Imagine a rope and a piano wire in parallel, with the rope being very slightly shorter in the no load condition. As you load it, the pianowire will take more and more of the load, since it's got a higher spring constant. Now make it an oscillatory load and both the rope and the wire being lossy.? It's not obvious at first glance what the behavior will be.
For anyone really wanting to get into this, find a copy of E.C. Snelling's "Soft Ferrites", preferably the 2nd edition. He probably covers mixed cores somewhere in there.
