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LM5141-Q1: How to get the EMI filter formula

user5859485
Prodigy 200 points
Part Number: LM5141-Q1
Other Parts Discussed in Thread: LM5146-Q1

hi all:

       I read datasheet of LM5141-Q1 , it`s good device for us. For the EMC test I want to know how to derive the formula of EMI filter. could you show me some article or information ? 

thank you so much!

following will show the formula from datasheet. page 27.

  • 0
    TI__Mastermind 49647 points

    Hello,

    Take a look at the EMI filter design section of the LM5146-Q1 datasheet. There are also some app notes on EMI referenced in the collateral section of that datasheet (near the end). In particular, you can review app note snva489 (Simple success with conducted EMI from DC-DC converters). Also, take a look at this EMI article series: http://www.how2power.com/other/EMI_Guide.php.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 200 points

    hi Timothy:

          Thank you for your reply!

          I get some documents to understand significance of formula , but something confuse me. could you help me describe following formula :

    Cfb : I think the formula derive from Fc=1/(2*pi*sqrt(LC)) . How to understand |Att|db / 40 ? And why let 10^|Att|db/40 equal to Fc*(2*pi*sqrt(LC)) ? Fc is f_ cutoff.

    Cfa: How to infer this formula?

    Thank you so much!

    from snva489c.pdf     page 6.

        

  • 0 in reply to Timothy Hegarty
    Prodigy 200 points

    hi Timothy:

          Thank you for your reply! that`s useful !

          I get some documents to understand significance of formula , but something confuse me. could you help me describe following formula :

    Cfb : I think the formula derive from Fc=1/(2*pi*sqrt(LC)) . How to understand |Att|db / 40 ? And why let 10^|Att|db/40 equal to Fc*(2*pi*sqrt(LC)) ? Fc is f_ cutoff.

    Cfa: How to infer this formula?

    Thank you so much!

    from snva489c.pdf     page 6.

  • 0 in reply to user5859485
    TI__Mastermind 49647 points

    The first formula ensures that the resonant frequency of the EMI input filter is at least one decade below the switching frequency. The second formula is derived based on a -40dB attenuation profile of the LC filter.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 200 points

    thank you Tim !

         could you show me some detailed information , I don`t infer the second formula.

    I think that :

           LC filter have -40db/dec , so if we get |Att|db then we can get : |Att|db - 40db = 20log(Av)-20log(100)=20log(Av/100).  don`t infer the 10^|Att|db/40.

    Thank you so much!

  • 0 in reply to user5859485
    TI__Mastermind 49647 points

    Here is the derivation:

    user5859485 said:

    thank you Tim !

         could you show me some detailed information , I don`t infer the second formula.

    I think that :

           LC filter have -40db/dec , so if we get |Att|db then we can get : |Att|db - 40db = 20log(Av)-20log(100)=20log(Av/100).  don`t infer the 10^|Att|db/40.

    Thank you so much!

  • 0 in reply to Timothy Hegarty
    Prodigy 200 points

    hi Tim:

        Thank you so much!