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In my circuit, RFID card sometimes works sometimes not. it depends on how I put the card on RFID receiver. Usually it does not work when I put around the smps transformer. I guess SMPS transformer affect RFID card. I cannot change transformer and RFID receiver location. What do you suggest to remove SMPS transformer effect?

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    \$\begingroup\$ Get some flexible (and cutable) ferrite screening material from ferroxcube or Wurth and stick it on top of the SMPS. \$\endgroup\$
    – Andy aka
    Sep 10 '18 at 11:18
  • \$\begingroup\$ Do you mean like this digikey.com/product-detail/en/laird-signal-integrity-products/… \$\endgroup\$
    – mehmet
    Sep 10 '18 at 11:29
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    \$\begingroup\$ Yes, try it out. It may solve the problem and it may not but fingers crossed! \$\endgroup\$
    – Andy aka
    Sep 10 '18 at 11:43
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If the SMPS really influences the RFID receiver, it is likely to be inductive coupling. This can be tested by removing the RFID receiver from its original place and using a short cable (3-5 inch) to space it away from the SMPS module.

If it is in fact inductive coupling, you can try to shield with ferrite. But I had a similar situation once where I just had to put some copper tape on the SMPS and all worked OK.

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