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LMR16030: EMI filter

Genius 9880 points
Part Number: LMR16030

Hi,

Regarding the application notes SNVA755 and SNVA489C, I have designed an input filter but i am not sure about component values. As you can see below, like in the SNVCA489C, i did not use an damping electrolytic capacitor and i am not sure how this will effect the PSU. Could you please check the values and confirm if these component values are OK to go?

Thank you

Regards,
May

  • Hello May,

    What is the ESR of the electrolytic which would serve as the damping resistance Rdamp? Eq7 in SNVA755 

  • Hi Marshall,

    Did you meant the EQ7 in SNVA489C because there is no ESR formula in SNVA755 and this is the part confuses me. In SNVA755, if you check the Section 2.4, it says ''In this case, in order to reduce the cost, no electrolytic is used.'' Which means there is no damping resistor. So i am not sure if i can apply the same thing on my design or follow the every step on SNVA489C and use an electrolytic capacitor.

    Switcher-ii PSU RevA.rar

    One more thing, since i am going to use a desktop adapter ( like laptop adapter) as source, i've added bulk capacitor at the PCB input side. You can see on my design files. Do you think it is a good idea using that with EMI filter?

    Thanks,

    Regards,
    May

  • Hello May,

    My apologies, eq7.

    You want to put in close proximity to the input of the buck. As close as possible to the ceramic input capacitors of the buck.

    Your damping cap would be Cin*4

    Rdamp= sqrt(4.7uH/Cin)

    Cin for IC1 is ~10u

    You could have a single input filter and use Cin-toal (both converter Cin) for the above calculations

    In the case you use a ceramic damping capacitor then you can use a discrete resistor in series.

    In the case of a large damping capacitance, it may make more sense to use a hybrid polymer electrolytic with a discrete resistor in series. This gives you best of both words in a reduced solution size, lower cost, and no tempCo worry.

  • Hi Marshall,

    Just received response from customer, please see details below.

    've updated my design according to your suggestions. I did use AN-2162 Excel Sheet Calculator. Input 19V and output +/-12V 2A. Calculator showed 0,9u Cf, I used 2,2uF (it drops to 1,6uF with DC Bias) and at least 40uF Cd and i used 82uF as a damping cap. ESR is calculated as 0.8ohm so i've used a resistor in series with damping aluminum capacitor. Also added a few 100nF HF filter capacitors.

    I am adding design files, could you please check the schematic and component placement especially the input caps section. It would be amazing if you can take a look. 

    Switcher-ii PSU RevB.rar

    Thanks.

    Regards,
    May

  • Hello May, the design looks good.

    I would recommend you to add a few placeholders in parallel with C3 in the case you need to add more cap.