I am designing an EMC filter of a two wire (no earth connection) flyback SMPS.

In the EMC filter, AC line usually has two Y capacitors after common mode choke connecting earth (PE). Since this SMPS has no earth connection, can I connect them to the primary side circuit ground?

  • \$\begingroup\$ Yes, the Y-capacitors will offer you a HF virtual ground. But EMC starts with board layout and design, not adding filters. How does the spectrum look like without any filters? \$\endgroup\$
    – winny
    Commented Sep 5, 2023 at 13:40

1 Answer 1


No you can't. It will make the output to capacitively float at half mains, giving tingles to people and possibly destroying equipment when hot-plugging them together.

That is why you are not supposed to plug in a device that requires earth for mains filter into a non-earthed socket.

Devices with two pronged inlets just have a filter without Y caps to earth.

  • \$\begingroup\$ Thank you, please tell me how can I design EMC filter for two wire flyback SMPS. \$\endgroup\$ Commented Sep 5, 2023 at 15:06
  • 2
    \$\begingroup\$ There are plenty of designs out there that use a Y capacitor from output side to the primary side. There are plenty of questions on this site on the subject of getting little tingles and, there are plenty of answers that seek to explain there is nothing dangerous about this. I'm happy to be told differently of course. \$\endgroup\$
    – Andy aka
    Commented Sep 5, 2023 at 15:07
  • \$\begingroup\$ @user3521355 Since you only have two mains wires, live and neutral, you use a CM choke with an X cap, as there can be no Y caps to GND as there is no GND on mains inlet. And BTW, having a single Y cap between mains side and low voltage side is another matter, and you did not ask about that, only about how to do a mains inlet filter without ground. Getting tingles is annoying, but it can destroy things that are plugged together due to potential difference and discharge current from the Y caps. \$\endgroup\$
    – Justme
    Commented Sep 5, 2023 at 22:52
  • \$\begingroup\$ @Justme, If I use CM choke with an X cap is it become a DM filter? \$\endgroup\$ Commented Sep 6, 2023 at 11:20

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