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SN6501 EMI Problems

Juerg Albrecht
Prodigy 40 points
Other Parts Discussed in Thread: SN6501, SN6505B

Dear Support Team

I am using a SN6501 configured like Fig. 47 in the datasheet with a 760390014 transformer (Wuerth) to get a isolated 8V from a 3.3V Supply. The circuit works well, but I have some problems to satisfy the EMI requirements. The signal analyzer shows peaks at all harmonics of the switching frequency. Do you have any suggestions or application notes for reducing the peaks?

Thank you in advance

Jürg

  • 0
    TI__Intellectual 2200 points
    Hi Juerg,

    Could you explain your setup to measure EMI? What specs do you have to meet?
    You could consider using SN6505B, which has the same clocking frequency but has Spread Spectrum modulated on top. This could help reduce the peaks.

    Rgds,
    Anant
  • 0 in reply to Anant Kamath
    Prodigy 40 points

    Hi Anant,

    Thank you for your response. I have to meet the EN 55022 / CISPR 22 standard at Level B.

    The SN6505B could be a good option, but the price is really high.

    After some experimentation I have reduced the peaks with 2 ceramic capacitors. One 100nF/50V at the center tap on the secondary winding and one 100pF/1kV across the transformer. Now I can fulfill the specs.

    Kind regards

    Juerg

  • 0 in reply to Juerg Albrecht
    TI__Intellectual 2200 points
    Hi Juerg,

    I am glad you are meeting your specs. I understand how the capacitor across the transformer would help. Regarding the 100nF/50V at the centre tap of the secondary, did you connect this between the output and the centre tap?

    Would you mind dropping me an email at anantkamath@ti.com ?

    Rgds,
    Anant
  • 0 in reply to Anant Kamath
    Expert 1915 points

    Hi.

    I have same EMI Issue.

    How did you connect 100nF and 100pF?

    Please let me know details.

  • 0 in reply to Jake Jung
    Prodigy 40 points

    Hi

    I am using the SN6501 configured like Fig. 47 in the datasheet
    and I have connected 100nF from the secondary center tap to 0V.

    The 100pF was not a good option at all.
    It raised many other issues.

    The main reason for my issue was the couple capacity of the transformer.
    Now I use a customer specific transformer with separated windings.
    So the couple capacity is much smaller and the EMI level is reduced by more than 15dB.

    On the other hand the leakage inductance of this transformer is higher compared to the original transformer,
    so the switching waveform of the SN6501 is not perfect any longer.
    As I am only needing small power, this is not an issue for me.
    I am not sure whether this solution is valid when needing more power.

    I hope this helps, regards

    Juerg

  • 0 in reply to Juerg Albrecht
    Expert 1915 points

    Dear Juerg.

    Thank you so much.

    But I can not see the image.