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TPS23754: TPS23754PWP EMI/RE Noise Issue

Part Number: TPS23754

Hi Team

We've designed our Board based on TPS23754.

During FCC Certification we're getting radiation emission at the Output and the Feedback Section.

At the FCC Testing Labs, we debugged that the Noise is generating from POE Section.

Please let us know how we can reduce the noise from our board.

Radiated Frequency:

1. 55MHz

2. 47MHz

We're attaching our schematic design and radiated emission images for your reference.

POE_Noise.zip

  • Hi Ravi,

    I recommend looking at the switch and secondary diode for any ringing at that frequency. Then you can populate/adjust the snubber.

    You can also consider increasing R201 and R533 to slow down the turn ON/OFF of the primary FET.

    In addition, since you are not using GAT2 (switching), you can connect DT to VB so that GAT2 doesn't try to switch (could be part of the problem).

  • Hi Darwin thanks for your support

    Till now we have done some changes.

    We have increased the value of R201 to 72E and below observations are made

    •  At low load(consumption in mA) - noise level is comparatively less
    • At 12V@1A- noise level increases at the output side.

    Also we tried by adding a snubber across MOSFET C=330pf, R=10E &C515, C516, C517, C518=1000pf. But still getting noise at Output and near MOSFET. 

    Please suggest what can we do to reduce the Noise. 

    Attached are the screenshots for your reference. 

  • Hi Ravi,

    I'm not sure what you mean by noise near the FET and output. Yes the switching elements will radiate noise; however, radiated noise from a standards point of view is radiation of the ethernet cable when connected. The idea is that end equipment itself is not radiating since majority will have a shield enclosure or shielded coating of the end product. Whatever noise in the end product is radiated to the RJ45 input and that noise will radiate from the ethernet cable. Is your end product enclosed during your testing and also is the enclosure shielded? 

  • Hi Darwin

    yes, the product is enclosed during testing. we got the results of our RE and CE and the results are worse. please see attached file. please suggest how we can approve0730-ISN.pdf0729-RE.pdf

  • Hi Ravi,

    If you are failing CE as well can you try adding a common mode choke in series with their ferrite bead. Youcan also try unpopulating C702 and R705 to remove the common mode current.

    Otherwise, we have to look at the layout as well as this can cause EMI issues.

  • Hi Darwin

    We have also tried by adding common mode choke in series and by removing C702 and R705. But still results are same

  • Hi Ravi,

    This is very surprising. Either the layout is bypassing the common mode choke, or there is component that we are missing that is on the board but not on the schematic(although I only got the poe schematic and the full board schematic). Can you forward this? If it not a component then it must be layout or possibly setup. Can you also send how you're taking the measurement?

  • Ravi, It has been over a week since there was activity on this thread.

    Do you still require our help?

    Thanks,

  • Ravi, Due to lack of response, I am closing this thread.

    If still requiring help from us, please reply.  A reply from your side would reopen the thread.

    Thanks,