LLC design tool

Hello. What is meant by Ln: Ln=Lm/Lr or Ln=Lp/Lr (Lp=Lm+Lr) ?
The flow-chart shows the formula Mg(Ln,Q,f) for Ln=Lp/Lr, not for Ln=Lm/Lr.

Hi daa,

Can you tell me what MPS device you are looking at? I’m not sure what device or what flow chart you are looking at.

Brendan Schoemehl
Field Applications Engineer
Monolithic Power Systems | MPS Now

This formula appears in many methods for calculating LLC, and everywhere under Ln is meant Lp/Lr, but in your LLC design tool Ln=Lm/Lr

Hi daa:

This is a matter of nomenclature. The Lm in our flow chart is the transformer’s magnetizing inducatnce of the Main Primary winding inductance. Lm for us stands for “Magnetizing” Inductance of the transformer. This is the inductance of the primary winding of the LLC transformer. Some other companies call this same Magnetizing inductance as Lp where p stands for “Primary”. Both Lp and Lm are same.

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I have an external choke 100uH and a transformer with Lp = 500uH and Ls = 10uH. What will be the magnetizing inductance, 490 or 590, for you design tool?

Hi daa:
Please measure the Leakage inductance of the Main Primary winding. You can do this by shorting all the other windings (Primary Bias and all secondary windings) together. Then subtract this measured leakage inductance from 500uH to get the Magnetizing Inductance (Lm) value. Also add this measured leakage inductance to the 100uH external choke you have to get the Lr value to use in the formula and the design tool. Please note that the Leakage inductance and the Magnetizing inductance of the transformer are in series . So the formula is Lp = Lm +Lleakage. So when you measure the Primary Inductance you are getting the addition of these two inductors. You have to extract the magnetizing inductance by measuring the leakage inductance. This is another reason why we use the Lm as Magnetizing inductance in our formula and not the Lp since the Lp includes the leakage inductance which needs to be added to the Lr (external resonant choke). I hope this helps.

Thanks. For my example Lr=110uH (Lext+Lleak), Lm=490uH (Lprim-Lleak). Right?

Yes correct. I am assuming here that Ls (10uH) is the Inductance measured across the Main Primary winding with all other windings shorted and Lp (500uH) is the inductance measured across the Main Primary winding with all other windings open circuit. The Lm= 490uH and Lr = 110uH based on this assumption .