Just wanted to know if anyone has ever had any problems with gapping EE cores for flyback converters. I wouldn't know for sure, but I think distributors can sell cores with the gap you need, and they put this gap on the center leg.

Well, I have also heard that it is possible to use paper spacers between the core halves. I have used paper spacers before for low peak primary current levels with success. But, the other day, I had some serious issues, which I believe were caused by using paper spacers. I am curious if anyone else has run into this problem.

I was building another DCM flyback, this time with a peak current of about 2 A. I was using an EE core with paper spacers placed on the outside legs, and a copper ribbon wrapped around the outside of the transformer to deal with the stray field from the outside legs. Well, in a few seconds, my FET was toast, literally. It went up in flames and the Source leg detached from the package every time. So, my suspicion was the stray field. My suspicion was confirmed after the outer winding of my transformer melted.

The problem immediately went away when I replaced the gapped core with an MPP toroidal core (wound with triple insulated wire). The converter worked as planned with the toroidal core.

So again, does anyone have similar difficulties with gapping flyback cores? From what I understand, the gap on the middle leg is shielded by the windings on the bobbin. But what is the best way to deal with the fringe field if the outer legs are also gapped? Does anyone know of any good tools to grind a gap on a ferrite core? Last weekend, I used sandpaper and water and it took me a good 3 hours to grind away 0.6mm. Even then, the gap wasn't even so I scrapped it anyways.