its for a 600W full bridge transformer.
If you don't gap, you need too many turns to get below Bsat.
If you use that many turns, then it just increases the leakage term in the transformer.
This is not acceptable.
The leakage delays the rise of I(pri) and robs us of duty cycle, -we need to know as accurately as possible what leakage we are getting, and we just cant do that if we don't gap.
There are other reasons too why we want the gap.
Such as the problem of primary magnetising current staircasing up to Bsat during transients where the error amplifier is railed high, and because of the leading edge blanking time of the control chip, the Imag can staircase high as discussed....we cannot have this happening with ungapped cores as the I(mag) can fly away into saturation levels....hence we need the gap.
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The following two threads have already discussed gapping of full bridge smps transformers….
https://www.edaboard.com/threads/341980/
https://www.edaboard.com/threads/340530/
..These threads don’t go into enough depth to really realise the problem of ungapped bridge transformers
If you actually go about the process of making out the transformer manufacturing document in relation to the design, it becomes obvious that having a small gap is best.
Our 600w full bridge smps is as follows
pout=600w
vout=380v
vin=390v
fsw=100khz
core = etd44 3c90 with 0.2mm shimmed gap
NP = 36 NS = 58
...if you go about this design, you see that you cannot do it as well without a "small" gap.....unless you use a much bigger core.
This thread also becomes a discussion on gapping, after about 15 posts
https://www.edaboard.com/threads/348624/
.....the subject of staircasing of primary current was raised here (due to the leading edge blanking time), but glossed over.
The subject of how core remenance flux is helped by gapping was also explained, but again glossed over.