Transformer for LLC converter is it gapped or not gapped?

Hello,

I am working on LLC converter, I am new to this power converter, I have a question, normally to transfer DC power the transformer needs to be gapped. This is what I typically use in Flyback type of converter. I recently heard ( this could be wrong) that LLC can use normal transformer, not necessary a gapped one. Is it correct that the LLC transformer can be designed using a non-gapped AC type transformer?

Thanks and Regards,

As the acronym LLC reveals, the converter topology involves two L elements, a series and a shunt inductance. If not using external inductors, the magnetizing inductance of the LLC transformer forms the shunt element. Respectively the transformer is build with a gap.

A tranformer without gap makes a LC series resonant converter but not a LLC.

If you are using an external magnetizing inductor, then you can get away without gapping the transformer. But if so, then you should sandwich wind it for lowest possible leakage, and also use an external "leakage inductor".

But really, this is only if your ungapped Lm is much bigger than your external Lm....because then your external Lm pretty much determines your overall Lm value. And remember, the Lm value is part of your "m" value, which you need to know pretty accurately when doing an LLC......thus you need to know your overall Lm value accurately.

The problem with ungapped Lm...is that ungapped cores have very large tolerance.

But yes...as long as you dont saturate your transformer on magnetising current, and dont overheat it with core loss, and you know your overall Lm value accurately, and you use an external magnetising inductor, then you can use an ungapped core in the transformer........but as discussed, it should be sandwich wound, otherwise, with the external Lm inductor, you dont have an LLC any more....it'd be something slightly different than true LLC.

Typical LLC ( CLL ) has a resonant capacitor in series with the transformer - hence no gapping required - the resonant cap serves double function as DC blocking cap too ...

UNLESS as stated by others - you are trying to integrate the Lr into the Tx ...