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LM5022-Q1: Current Limiting with the LM5022-Q1 in a Flyback Application

Nicole Leuenberger
Prodigy 120 points
Part Number: LM5022-Q1

Dear all,

I'd like to find the right values for the resistors RSNS, RS1 and RS2 in regards to current limiting with taking the slope compensation into account. The formulas in the datasheet of the LM5022-Q1 on page 21 are for a Boost Converter only.

Formulas

Typical Application (that's what the formulas are valid for)

I am not quite sure on how they change these formulas in case I have a flyback converter with the transmission ratio Np/Ns. Can you please provide me with these formulas (36 and 40) for a flyback converter? For example for the following reference design: PMP9334, www.ti.com/.../tidrui5.pdf

Thank you very much in advance for your assistance.

Best regards

  • 0
    TI__Mastermind 21720 points

    Hi Nicole

    The current limit equation can be derived as shown below. This is true regardless of the topology. To simplify the equation the Vslope term can be removed.

    For a current mode control architecture the add slope compensation should be equivalent to the down slope of the inductor times the current sense gain. It is okay to have more slope compensation than a factor of 1. For a boost converter this is:

     

    Which yields

    In the datasheet K=(3*D/fsw), which is there to give margin and help fit a majority of the applications while keeping the equation fairly simple. Back solving for Rsense is how the equation in the datasheet is derived.

    Following the same approach for the flyback the equation is:

    K is just a factor of slope compensation, 1 is optimal, increasing the value will add more slope compensation. K=(3*D/fsw) can still be used as well.

     

    The second equation should stay the same for the flyback converter as the slope compensation scheme does not change with topology.

    Please let me know if you have any questions. 

    Thanks,

     

    Garrett