There are some nodes with high frequency switching contributing to EMI in Flyback and Forward converters, which should have minimal copper area to minimize the EMI. Here we see the Forward high frequency nodes: enter image description here

What are the nodes that should have minimal copper area if I move the upper diode on the secondary side to the output negative rail (and also replace it with a FET but that doesn't matter this time I think)? Here is an image of a Flyback, with the mentioned change with the diode:

enter image description here

So the question is: which are the EMI contributing nodes in case of Flyback and Forward converter secondary sides assuming the diode (or synchronous FET) on the negative input rail?

  • \$\begingroup\$ The primary side switching will make more EMI than anything on the secondary side. \$\endgroup\$ – Andy aka Jun 12 '18 at 9:08
  • \$\begingroup\$ of course the red node is the biggest contributor to EMI, but I'm interested about the secondary in this changed topology \$\endgroup\$ – U.L. Jun 12 '18 at 9:09
  • \$\begingroup\$ In the sec. side flyback you have drawn, the fact that the sync. rectifier is installed in the low side of the transformer is a major source of radiated EMI. Most of designs that I have seen leave the transformer ground as it is and place the MOSFET in the upper rail with a means to properly bias it. Modern sync. rect. controllers now let you do that. With the flyback, you will have to minimize the area encompassing the rectifier and the capacitor and make connections as short as you can. \$\endgroup\$ – Verbal Kint Jun 12 '18 at 11:08
  • \$\begingroup\$ So you say that in this configuration I cant pour the secondary negative? \$\endgroup\$ – U.L. Jun 13 '18 at 7:13

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