Maximum gap size for flyback transformer core?

[Flyback]

What would you say is the maximum gap length for use in the centre leg of the following epcos 49/25/16 core?
Its for a 330W flyback for Class D guitar amplifier supply. (vin = 90-265VAC)

I want to have a 2mm gap in the centre leg (only) , but is this too big?

https://www.epcos.com/inf/80/db/fer_07/etd_49_25_16.pdf

.. the overall centre core length is 35.4mm, as you can see from the datasheet.

 

[ronsimpson]

2mm is about the max I would go for center leg gap.
How will you make the center leg short? You can't grind the leg.
You can have the gap on all legs. It will not be as quite. But......
If you have to have more than 2mm, get the 2mm gaped core and add a little spacer to the outside legs.

 

[Flyback]

we've had centre legs ground down for us before. (just the centre leg)

 

[MrAl]

Hello,

Yes you can gap just the center leg, but that's more difficult for a custom design. It's much simpler to run a piece of insulation across the flats of the whole thing. Yes, the effective gap is then twice as much as the thickness of the insulation so you'd then have to go with 1mm if you wanted 2mm for example. Yes better shielding with only the center leg gapped.

The maximum gap really depends on the number of turns on the primary and the primary voltage as well, not just the core size. Together these things must be able to support the required primary voltage in normal operation.

 

[ronsimpson]

I have tried several ways of grinding on a center leg. Results is a shattered core. I leave that to the manufacture. There probably is a way but I don't have it.

If you know how do grind down glass, then try it.

 

[MrAl]

Hi,

I've ground and cut cores before, but using a small 1 inch diamond wheel on a high speed Dremel. Hard to get super accurate though.

I had a lot of fun testing some cores and then cracking some of them in a vise, then gluing back together with super glue and then retesting. Even very small air gaps have a large effect on the permeability of the whole construction.