LLC converter with additional external magnetising inductor is not an LLC any more?

[treez]

Hello,
We wish to have an additional ‘magnetising inductor’ external to our LLC transformer. However, we are already using a “section wound” LLC transformer with a high leakage inductance term.
Therefore, this having the external ‘magnetising inductor’ is not possible, because with an additional external ‘magnetising inductor fitted across the LLC transformer primary, it is simply not an LLC converter any more…do you agree?

LTspice simulation of such an LLC converter, and schematic, ie with the additional external ‘megnetising inductor’ is attached.

 

[FvM]

Therefore, this having the external ‘magnetising inductor’ is not possible, because with an additional external ‘magnetising inductor fitted across the LLC transformer primary, it is simply not an LLC converter any more.

Why particularly? The statement makes no sense.

 

[treez]

When an external, additional “magnetising inductance” is placed in parallel with the intrinsic magnetising inductance of an LLC transformer, it must be placed directly across the intrinsic magnetising inductance…….but if the LLC transformer has a big leakage inductance, then you cannot place the “external magnetising inductance” directly across the “intrinsic magnetising inductance”, but instead you end up placing it across the combination of the intrinsic leakage inductance and the intrinsic magnetising inductance, and in so doing, you end up with something that is no longer an LLC converter.

As you know, its often desirable to have a low henry value magnetising inductance in an LLC transformer, and sometimes this is achieved more easily by adding in an external “magnetising inductor”. This is especially so with high power LLC converters. Easy Peasy has discussed this before, also.

 

[Easy peasy]

ideally the series component of the L in LLC is outside the transformer (purely in series with), thus with a section wound transformer Treez is leaning towards being correct...

 

[FvM]

Any combination of parallel and series inductances in a transformer can be mapped to a basic LLC transformer with primary and secondary leakage and main inductance. It's just a matter of calculating it right.

Of course there must be some amount of series inductance before the parallel inductor.

Another question is about the purpose of parallel inductance in the LLC design. It can be used to vary the voltage ratio with variable frequency. Or the transformer can be operated near series resonance frequency with fixed voltage ratio, utilizing the main inductance only for soft switching.

 

[treez]

I think the simulation of post#1 shows what I mean…..if you click on the “external magnetising inductor” you can see that its current is sinusoidal. As you know, this is not how it should be in a LLC resonant converter.
The Magnetising current in an LLC transformer should be nearly a triangular waveform. The magnetising current in the “external magnetising inductance” of the LLC converter in the simulation of post#1 is not a triangular waveform.

As we know, what I am calling the “external magnetising indutance” is not actually that strictly speaking, but I cannot think of a better word for it….of course, the “external magnetising inductor” as shown in post#1 is not coupled at all with the secondary.

I am not saying that the converter of post#1 cannot be made workable, I am just saying that it is not an LLC converter any more, and all the equations for it would need deriving, -in other words, the equations in AN-4151 do not apply to the converter of post#1.

 

[FvM]

I see what you mean. I keep however my point that the setup can be mapped to the same equivalent circuit. I would b.t.w. suggest to refer to a complete LLC equivalent circuit which also reflects the secondary leakage inductance.

If the current through the main inductance in the basic LLC is actually triangular depends on the amount of (uncompensated) secondary leakage. But if the transformer is operated in the series resonance point with no load, the input current will be triangular.

Similarly, the current through a parallel inductor that is connected on a tap between external and internal leakage as in your circuit won't be exactly triangular. But if you adjust the circuit to series resonance, the unloaded LLC transformer input current will.