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2mm core gap is too big for flyback?

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grizedale

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I've asked a similar question before, and the answer that came was that there would be fringing losses with the gap which is in the centre of the ETD core

**broken link removed**

The SMPS is 90-265VAC input and output = +/-40V, 4A.

Can any one quantify the losses associated with this large gap?

Am i wrong to opt for a 2mm gap, is it just way too big?
 

It practically doesnt matter wether it is .5mm or 2mm. The transformer need to be wound to a specific value and thus value is adjusted with the gap settings of the cores. ultimately the goal is to get the particular micro henry value and to get it the gap may vary a lot. Or you need to adjust the coil winding a bit so that the gap can be altered.
Cheers
 

A large air gap doesn't necessarily increase losses. You'll get more losses from more leakage inductance, but your core will transfer energy more efficiently. Getting a quantitative answer requires much more information than you gave.
 
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A flyback with a 2mm gap is surely folly?

A large gap will have large fringing fields ,and so the stored energy in the gap is coupling away to other things such as the windings
 

you need to add card or tape on the former around the gap to keep the wire away from the fringing flux - if you do this the wires will not cook in the middle of the bobbin - especially if you run peak flux higher than 50mT
 
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Hello

If you used valid design formulas and the result says 2mm, then that should be it. As a practical idea, for that particular power rating it doesn't sound too exaggerated, but it may as well be that you're using a smaller core than it should be. Again, it only depends on the design. Keep in mind, though, that the core choice can prove to be quite an iterative route, so questions like yours may imply you didn't go through all the iterations.

Good luck,
Vlad
 
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