LLC X-former airgap and number of windings?

[TroelsM]

Hi.

This is probably a stupid question, but here we go.

I'm by no means good or experienced in SMPS or LLC for that matter, so I rely on excel-calculators and formula´s found online to design my LLC transformer.

Example: https://www.infineon.com/dgdl/an-1160.pdf?fileId=5546d462533600a40153559a85df1115
See "step 6" on page 13.

All of the calculators I have found will calculate the primary windings based on max-bus-voltage, ferrite-type(max field and cross-section) and duty-cycle.
Thats the same as for a "normal" smps without airgap.

But, I faintly remember that the airgap will change the properties for the magnetic path and that the ferrite+gap can tolerate a higher field than the ferrite alone.

Basically I would guess that the LLC transformer can be constructed with fewer windings than the calculator says, because the calculator do not include the airgap.

So, what am misunderstanding?

Kind regards

 

[Silvio]

@ TroelsM
All I can tell you is that it is true that a core with an air gap will take a higher field saturate as compared with a core with no gap. On the other hand if a gap is present in the core more windings are needed to get to the desired inductance compared to a core with no gap.
An LLC trafo is somewhat different and usually they create the needed leakage inductance in the main trafo itself by separating the windings and make them loose coupled. The construction is usually done with the primary and secondary windings side by side instead of on top of each other with a divider on the center of the bobbin to separate them.

Silvio

 

[TroelsM]

HI Silvio.

Yes the LLC-transformer is often implemented with windings side-by-side, but there is ALSO an airgap (usually placed under the primary or between prim and sec)

Troels

 

[Silvio]

@TroelsM
Referring to the air gap there may be some misunderstanding here.
I was referring to the gap in the center leg of the ferrite core. This will minimize the AL value thus you need to wind more turns to get to the necessary inductance.

Leaving space between windings I believe is there to create leakage inductance by loosening the coupling between them. It could be done by putting windings side by side or leaving space between primary and secondary by adding some thick tape for example when placing them on the bobbin.

For tight coupling we usually split the windings and sandwich them together to get maximum coupling possible, on the contrary to create leakage inductance loose coupling of the windings will create it. The amount of leakage inductance has to be experimented with various methods explained.

Measuring can be done by firstly shorting out all the secondaries together and measuring the primary inductance. This should be zero any inductance left will be the leakage inductance. However even in tightly coupled arrangements there is also a little leakage. Nothing is perfect in this world.

Regards Silvio