A question about transformer and SMPS

[digitalheart]

I have a transformer core EER35L from a SMPS work in Flyback mode. Can i use it to design for SMPS work in Forward mode? Because, according to my knowledge, Flyback transformer need a "gap" to store energy but Forward one doesn't! And if i can do that, how does it have any effect to my design?

 

[VVV]

You can definitely use it for a forward.
You would need to remove the gap, though. Otherwise, the low magnetizing inductance will produce too high a magnetizing current. For a forward, the magnetizing current does nothing, just wasted power, because it adds to the actual secondary reflected current, so the primary and the transistor have to conduct this current, leading to high losses.

Depending on how the transformer was built, the air gap can be with shims in the outer legs, which are easy to remove, or it can have the center leg ground down. Removing that kind of air gap is not easy. It can only be done by grinding down the outer lesgs of the affected half. That can be done even with fine sandpaper, but it is extremeely messy (dusty) and you have to make sure they are flat. It can be done with diamond-coated tools, similar to the ones used to grind down the center leg at the factory. But it is unlikely you will have access to such a machine.

If you have two of these transformers, then you can build up one, using just the unaffected core halves.

 

[digitalheart]

Thanks for your very useful answer somuch! You've helped me to see my problem very clearly!And, according to your experience, Should i use what kind of transformer Core for a 250W Single-Ended Forward SMPS?

 

[VVV]

It depends on the frequency and the number of outputs, but if you were to use an ETD-39 type of core, operating at 150-200kHz, with a single output, you should be able to do it.
Multiple outputs take up more space, because of the wire exits and additional insulation layers, so they can drive the transformer size up, even though a smaller core could deliver the power.