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ISOW7841EVM: ISOW7841EVM

6054416
Prodigy 20 points
Part Number: ISOW7841EVM

Dear TI support

We have some EMI problem with the test board ISOW7841EVM

The CM noise around 120Mhz-150Mhz can be attenuated by stich capacitor as suggested in your EMI application note. 

However, the DM noise of 50Mhz-70Mhz is very difficult to be reduced (input is 5V and output is 5V as well)

The test board we received from TI cannot comply with the CISPR 25 above 30Mhz

We have tried the ferrite beads BLM18BD252SH1*2 and 0603 capacitor 10uF *1 to become a T type filter on the input side. But these tialsdont give much impact on EMI performance. 

May I confirm if you could provide us with some practical solution to reduce the DM noise (of course CM noise as well if possible.)

Best regards

 

  • 0
    TI__Guru 54255 points

    Hi,

    Sorry to hear about the emissions issue and thank you for sharing details related to the issue you are facing.

    CISPR 25 is a more stringent standard to meet than the CISPR 22 whose results are shared in the application note SLLA368C. Is it possible for you to share the emissions test results that you have observed?

    To reduce emissions around 60MHz, we recommend you to use common-mode chokes (CMC) at both input and output power supply pins of ISOW as shown in the image below.

    The ferrite beads on I/O lines are optional, you can also choose to use >1kΩ resistors instead if the datarate is not very high. Please make sure all the I/O signals are referred to the GND points after the CMC and not to ISOW device GND pin. ISOW device GND & GNDISO pins should stay local to device and all the signal I/Os should only refer to the GND point that is after the CMC.

    The CMCs should have an impedance of >1kΩ at 60MHz, I am listing below some of the example CMC part numbers that we recommend.

    1. CMC - 200mA, 15kΩ - TDK - ACT1210-510-2P

    2. CMC - 200mA, 20kΩ - TDK - ACT45B-510-2P

    3. CMC - 200mA, 10kΩ - Wurth - 744235510

    Let us know if you have any questions, thank you.

    Regards,

    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 20 points

    Hi

    Many thanks for your suggestion.

    Please see the EMI results from your test board ( this is total noise from either plue or negative line of input side)

    I personally believe the noise around 60Mhz is due to the DM noise mostly, and its third harmonics around  180Mhz.

    These two are the real problem for us.

    Kind regards

  • 0 in reply to 6054416
    TI__Guru 54255 points
    Hi,

    Thank you for sharing your emissions test results on ISOW7841EVM. Is it also possible for you share a photograph of your test setup so that I see how is the board connected to power supply and/or other components during the test.

    Our tests have indicated that the 60MHz and many other harmonics get suppressed when the suggested common-mode chokes are used. This does indicate that these frequency components are radiating as common-mode noise otherwise the common-mode chokes wouldn't have been effective.

    Please do try to test your board with the suggested common-mode noise and let us know your findings. It is best if those chokes where to have a footprint on the PCB. Having them hanging connected onto EVM might not suppress the emissions completely. Let me know if you have any other questions, thank you.


    Regards,
    Koteshwar Rao