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Measurement of the inductance of the LLC resonant circuit.

Hardware design
March 11, 2021 by Angel 89

I would like to ask about the measurement of magnetizing inductance and leakage inductance. I saw Lp=Lm+Llkp and Lr=Llkp+Lm||(n^2*Llks) in Fairchild's LLC article

Among them, Lm is the magnetizing inductance, Llkp is the primary leakage inductance, and Llks is the secondary leakage inductance. The article says that Lp can be measured directly for secondary side open circuit, and Lr can be measured for short circuit. However, I have studied flyback circuits and read books about magnetic components. Tell me, the secondary open circuit measurement is Lm, and the short circuit measurement is leakage inductance, so how should I understand the measurement in their article.

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Connor Posted on March 11, 2021

The measurement instructions in Fairchild's article are correct. For the flyback circuit, the secondary disconnection measurement is Lm, because the leakage inductance is small and there is no detailed description.

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Taylor Posted on March 11, 2021

The flyback transformer requires a relatively small leakage inductance, usually less than 5% of Lm. Because of the accuracy of the transformer inductance, it can be approximated that the secondary side is open, and the measured primary inductance is Lm. LLC transformers (especially integrated transformers), the leakage inductance will be relatively large, this time can not ignore the leakage inductance. Therefore, the leakage inductance is measured by the short-circuit secondary, and Lm is equal to the measured inductance LP of the disconnected secondary minus the measured inductance Lr of the short-circuited secondary.

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Ashley Posted on March 11, 2021

The definition of LM in Fairchild and the literature you read may be different. You need to be clear about the value of your LP and LM. I suggest you check the English name of each value.

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