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UCC28782: A ringing current flows between low side turn-off and high side turn-on

Kaji@PAN
Mastermind 7795 points
Part Number: UCC28782

Tool/software:

Hello,

I have previously made some inquiries in the thread below.
I would like to ask a follow-up question regarding question 2 of that inquiry.
In your response you pointed out that there is a possibility of self-resonance between the primary leakage inductance and the primary winding capacitance.
In order to reduce ringing, I believe that the primary leakage inductance will have to be reduced, since the primary winding capacitance cannot be reduced.
Is my thinking correct?
Also, are there any other ways to reduce the ringing?

Best regards,

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1521882/ucc28782-question-about-the-operation-of-active-clamp-flyback

  • +1
    TI__Mastermind 40520 points

    Hello Kaji-san, 

    Both the primary leakage inductance Llk and the winding capacitance Cw are parasitic components to the transformer design that are dependent on winding geometry.  So, theoretically,  both can be changed (reduced or increased) but not necessarily independently or economically. 
    I think probably they are inverse of each other: 
    Tighter winding structure improves magnetic coupling and reduces Llk, but that increases Cw.  Spreading the windings out and avoiding overlaps reduces Cw but increases Llk. 

    I think that reducing primary turn and using a larger core can reduce Lkl and reduce Cw, but may increase the overall size of the transformer.  

    Note: Since this current can be measured outside of the transformer and it is ringing at ~29MHz, I think a ferrite bead on one of the primary winding pins can damp out this ringing. 
    Be aware that the damping is a power loss, so the bead will heat up.  Make sure it does not over heat. 

    Regards,
    Ulrich