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UCC256303: how to design LLC when we used external Lm?

Ann_Lien
Genius 14430 points
Part Number: UCC256303

Tool/software:

Hello expert,

My customer is using external Lm choke. They would like to know how to design core size / turns number to make core have enough saturation performance and Lm inductance? Do we have application note or any document to explain it? Thanks a lot!

Best regards,

Ann Lien

  • 0
    TI__Guru* 86266 points

    Hello,

    Your inquiry is under review.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86266 points

    Hello,

    Could you verify your customers question?

    The Lm is generally the magnetizing inductance of the transformer.  The LLC does not use and output filter that can be referred to as a choke.

    Regards,

  • 0 in reply to Mike O'
    TI__Genius 14430 points

    Hello Mike,

    Customer would like to have external Lm design. They would like to know how to design this external Lm. Is there external Lm(addtional) adding on LLC topology? If yes, they would like to know how to design it. Thanks!

    Best regards,

    Ann Lien

  • 0 in reply to Ann_Lien
    TI__Guru* 86266 points

    Hello Ann,

    We are not recommending the UCC25303 devices for new designs.  I would have the customer switch over to a UCC25640x LLC controller or the UCC25660x LLC controller. 

    When it comes to designing the transformer,  Lm is the magnetizing inductance in the transformer and cannot be designed separately from the transformer.

    The resonant inductor Lr can be design separately or the customer can choose to use the leakage inductance of the transformer as the resonant inductor.

    When it comes to selecting the Lm and Lr ratio of the transformer that is based on the gain that you need for the design.  The following link will bring you to an application note that describes how to select the Lm and Lr ratio.


    Most Frequently Asked Questions About the UCC25640x LLC Resonant Controller.pdf

    The following application note will describe how select Lm to achieve ZVS.

    https://www.ti.com/lit/an/slua923/slua923.pdf

    Once you have the Lm, Lr and Np/Ns you can send this information to a magnetic design house and have them design the magnetics for you and your customer. 

    If your customer is designing their own magnetics, there are application notes at Mag-inc that your customer might find useful in their design process.  There application engineers may also be able to help with the magnetic design process.

    https://www.mag-inc.com/Design/Design-Tools

    Regards,