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.