LLC resonant converters (Why the second inductor?)

[AstRii]

Hello,

I've been looking into high efficiency power supplies and discovered the LLC topology.


I have read through some Infineon design guides but what I fail to understand is why the second "leakage" inductor? The transformer already has a primary coil with significant inductance, why is there a need for another inductor?
And why is there no need for the second inductor in DRSSTC topology?

[Egg]

There are a lot of reasons why LLC is better than just LC in series. But mainly, it is used because it is more efficent, easier to operate on zvs and it doesnt require extra parts because both inductors are already present in the transformer (when the transformer is built for this purpose.)
Here is a good article on the topic.
https://arxiv.org/pdf/2305.18132.pdf

[AstRii]

That's a very nice article, thank you for that.

So if I understand that correctly.. The leakage inductor is there to keep the resonance at a reasonable frequency when there is high load on the transformer (Lm = shorted)?

Does the converter run at fixed frequency and the power is regulated by load itself, by making Lm smaller so that the converter runs closer and closer to resonance?

[erlendse]

Actually they move in frequency.

They generally NEVER operate at the resonant point, but do get closer to it when more power is demanded. That is by design when picking components.

The controller datasheets do give quite some design hints on how to design them.