Given an isolated DC/DC converter, such as Traco TMR4. The GND fill, and internal GND layer(s) would be referenced to the -Vout pin. The TMR4 has an application note showing a fairly typical EMI filter for this kind of components. (https://www.tracopower.com/sites/default/files/products/application_notes/tmr4_emc.pdf)

Should the section of the board that contains the EMI filter have a keepout for the GND fill (illustrated by option 1)? Or is is better to let the GND layer (and top layer fill - with appropriate clearance) cover the entire board (option 2)?

Blue layer is inner GND layer. (Illustration only, this is not the actual circuit)

EMI filter Option 1 Option 2

  • \$\begingroup\$ Please directly link to the application note pdf you mentioned above. If it's downloadable via the zip file download then I'm not risking it. \$\endgroup\$ – Andy aka Jan 12 at 12:46
  • \$\begingroup\$ Added the link tracopower.com/sites/default/files/products/application_notes/… \$\endgroup\$ – Arcatus Jan 12 at 12:50
  • \$\begingroup\$ Hmm there isn't enough information in that document to decide where they were connecting the earth/ground plane when they designed the filtering to meet the EMC standards. \$\endgroup\$ – Andy aka Jan 12 at 13:05

My take on this: You have three GNDs:

  • External GND with an unknown amout of external noise, that's the GND at the input clamps
  • Non-Isolated GND, the filtered Ground before your isolated power supply
  • Isolated GND, the Ground after your power supply.

Especially the non-isolated / isolated GND mustn't mix, look at your isolation requirements to decide on a suitable clearance. External GND and Non-Isolated GND can be made the same, but when you do the layout, think of them as separate. Don't use the "unfiltered" GND front of the filter for anything, and route it with a lot of clearance to other parts of the device, you may have bursts on there.


OP updated drawings. Option 1 is now a Fill only around secondary side and Option 2 is a Fill around the whole PCB area, with Input area cut out, but no additional circuitry who uses the GND anywhere.

Option 1 would be my approach. To decide if Option 2 has any Use, more information is needed. If your design includes some antenna for sending, it may be beneficial to have a bigger GND pour, for better antenna performance. But that is some complicated stuff I won't go into now. Pour your GND where you need it and leave it out where you don't need it.

  • \$\begingroup\$ Appolgies for the poor illustration, nothing is shorted out by the copper pour. Isolation clearances are offcource maintained. \$\endgroup\$ – Arcatus Jan 12 at 12:19
  • \$\begingroup\$ @Arcatus those clearances will never support 1600 volts isolation. \$\endgroup\$ – Andy aka Jan 12 at 12:39
  • \$\begingroup\$ @Andy aka Updated the illustration. The original was apparently too poor quality to illustrate my point. Clearances and functionality is fine, the question here is to provide a reference plane for EMI noise or not. \$\endgroup\$ – Arcatus Jan 12 at 12:43

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