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ISOW7841: Common Mode Choke recommendation

Part Number: ISOW7841

Dear Isolation team,

my customer would like to know which Common Mode Chokes he should select for the power lines of ISOW7841. The application report http://www.ti.com/lit/an/slla368a/slla368a.pdf describes how to reduce the radiated emissions but does not state any type of common mode choke which is suitable for that.

Please help by providing a recommendation for a common mode choke.

Best regards

Cevin

  • Hi Cevin,

    Thank you for checking with us on common-mode choke recommendations. Any common-mode choke with an impedance of 2kΩ or higher at 100MHz should help here. One of such examples include a choke DLW5AHN402SQ2 from MuRata.

    The need of common-mode choke, ferrite bead, Y2 cap or an inter-layer stitching cap would depend on the emissions standard that customer is trying to meet and also on the PCB dimensions. Hence I would like you help me with the below information to help you make the best recommendations to improve emissions performance. Thanks.

    1. What is the emissions standard that customer need to comply with? Is it CISPR 22 Class A?
    2. What are the input / output voltages that customer would like to configure on ISOW?
    3. What is the maximum load going to be on ISOW’s VISO?
    4. What is the application and what are PCB rough dimensions?


    Regards,
    Koteshwar Rao
  • In reply to Koteshwar Rao:

    Hi Rao,

    1. our device is classified as an equipment of group 1, class B, CISPR 11.
    2. The ISOW device will be configured to Input=5V and Output=5V.
    3. The max. average load expected on VISO would be 200mW. Also peak loads can also occur for a time of max. 10ms in a range of 500 - 1000mW.
    4. The application is a medical device and the PCB rough dimensions are ca. 155 x 70mm.


    Best regards,
    Alexandru
  • In reply to Alexandru Mixich:

    Hi Alexandru,

    Thank you for sharing the additional details related to your application. Since the requirement is for CISPR 11 Class B, it is recommended to have the common-mode chokes (on both input and output power) along with an inter-layer stitching capacitor. The stitching capacitor implementation has been discussed in detail in the App Note SLLA368A which Cevin had already referred in his post. The alternative to common-mode choke are the ferrite beads with similar impedance but it is important to have the ferrite beads on both VCC and GND.

    If your application involves quite a few cables and see 60MHz to be much higher in amplitude then you may need a Y2 SMD cap across the isolation barrier between GND1 & GND2.

    Unfortunately, the emissions cannot really be predicted. It depends a lot on the application, the PCB dimensions and also largely on the DUT test setup in emissions chamber. If there is a metal casing on end product, this keeps emissions to very minimal.

    I hope the above points help you build optimum PCB to meet emissions requirement. Let me know if you have any questions, thank you.


    Regards,
    Koteshwar Rao