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#1
Start by
Ernest
09-08-2013 10:09 PM

Is there a high reliability long life energy storing core?

I learned at Ridley Engineering’s excellent workshop that many MPP cores have issues with achieving long life at elevated temperatures. I am trying to get life test data from a number of manufactures but in parallel I am looking into ferrite cores. For fly-back transformer/inductor I need a small reliable energy storing toroidal core. Initial search has found a gapped ferrite but I am worried about its reliability because ferrite is very brittle and single gap will put stress on the ferrite if the coefficient of thermal expansion does not exactly match that of the ferrite. I would feel safer with two gaps at 180 degrees because identical material would expand equally in each gap creating minimal stress and as a bonus each air gap would be shorter having less fringing. There is still the issue of stress added by the winding process and stress induced by the wound assembly with either impregnation or potting applying stress espically during thermal cycles.

I would appreciate comments on gapped ferrite toroidal or distributed gap cores. The switching frequency is over 200kHz and will see extreme temperature cycles that will span -40C to over 120C, while needing high reliability and long life up to 20 years. I am working with a previously designed power supply that the customer does not want to make major changes too.
09-08-2013 10:10 PM
Top #2
Charlie
09-08-2013 10:10 PM
Depending on whether cost is an issue then gapped amorphous may be worth considering. It is pretty robust mechanically being a cut tape wound core. There have been recent discussions on this - try to see past the rather disappointing tone from a certain person.
09-08-2013 10:11 PM
Top #3
John
09-08-2013 10:11 PM
I agree with Charlie... Gapped amorphous won't be quite as efficient as a better material, but it will definitely hold up well to the thermal cycling and high temperature operation.
09-08-2013 10:11 PM
Top #4
Peter
09-08-2013 10:11 PM
I'm very surprised to hear the statements about MPP, like any powder core materials, are known for having no long-term effects at high, elevated temperatures. I think maybe you’re hearing complaints about iron powder cores, like the ones from some manufacturers. These are very different than powder core that we work with, Iron powder has very high loss and exhibits thermal aging (i.e. the degradation of permeability over thermal cycles). Powder core materials like MPP and Kool Mu are rated for use at 200C continuous, and that’s only because the coating is rated to 200 C. The material itself is usable up to its curie temperature of 500C.

In my opinion ferrites are not good for designs across wide temperature ranges or where the environment is harsh. As you say, ferrites are very weak and thermally unstable. I can forward a bulletin about designing a flyback with powder core materials, which should help answer your questions. I hope this helps.
09-08-2013 10:12 PM
Top #5
Ray
09-08-2013 10:12 PM
Yes, indeed, it is the iron powder cores that were in the Micrometals article, not MPP.

I think the message is that any time you are using a core material with a binder, it is important that the manufacturer understands the thermal aging issues, and has generating lifetime data so they can be designed in properly.

The article showed great variability from one manufacturer to another.
09-08-2013 10:13 PM
Top #6
Pau
09-08-2013 10:13 PM
Gapped amorphous might have quite big losses at 200kHz. I agree with Peter and I am also surprise, MPP, Kool mu and HighFlux are known for having excellent performance with temperature and no aging effects.
I've made also designs for automotive with ferrite material in flyback transformers which worked pretty well withstanding all temperature and cycling test. Maybe key point is choosing right ferrite material, like 3C95 from Ferroxcube (very good temperature stability).
09-08-2013 10:13 PM
Top #7
Ernest
09-08-2013 10:13 PM
Yes, Magnetics Inc. says there cores are good to 200C. The problem MicroMetals discusses is with powered Iron but the problem is caused by the binder and binders are used in the MPP cores as well. I am trying to get test data from Magnetics Inc. We used Magnetics Inc. MPP cores over 10 years ago on Space Shuttle and they are still working fine but were used in inductors de-rated so far they run cool and have not logged a lot of actual run time. I am probably going to use MPP core as it has already been designed in and layout is done with it. Thank you for all the responses, I am still watching if someone has a breakthrough.
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