As long as total gap is 0.35 mm you can do any combination of gaps on each core half (ie 0 + 0.35 mm, 0.175 mm + 0.175 mm etc).
i m also doing experiment on my transformer by altering gap . i know one reason for introducing gap is that it helps in preventing core saturation but if ne other thing is there then kindly let me know.
You are correct the air gap prevents the core from saturating. Experimenting can be interesting but for your final design I'd strongly recommend you follow PI Expert or PI Xls for guidance on the gap size. The gap will determine the value of the flux density (and therefore at what primary current core saturation will occur) as well as primary inductance (and therefore power delivery).
See AN16 page 20 for the mathematical relationships (this is for TOPSwitch but applies to any flyback):
This does make some sense. If the gap is larger than specified then the value of the primary inductance will be much lower than designed for. As output power is proportional to 0.5 x Lp x Ip^2 x f the TOPSwitch will still try to regulate the output by increasing the primary peak current. As the energy dissipated in the clamp (transorb in your case) is proportional to 0.5 x Ll x Ip^2 x f (where Ll is the leakage inductance) then the energy dissipation in the clamp can increase significantly.
Other possibilities are: leakage inductance too high, clamp voltage too low (need to be ~1.5 times the value of VOR). The TOP249Y is a large device so I'd expect a leakage inductance value <1% of the value of the primary inductance.
To help understand more specifically can you provide the schematic and transformer design and measured primary inductance and leakage inductance.
If you don't have an LCR meter then this AppsTV segment might help you:http://www.powerint.com/en/design-support/appstv#4
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