Sanity check please: :)
I had some fun recently with a design that used a small isolated flyback module, with large EMI issues.
The device in question will pass conducted and radiated emissions at class B, if it is operated by itself with a load resistor, but when placed in a real board, emits all over the place, conducted and radiated, up through the 4000th harmonic (!) of the switching frequency.
I suggested to the vendor that they slow down the turn-on and turn-off of the FET if possible, re-select core material, and apply an inter- winding shield connected to the high side of the primary, but they don't like those options. They wanted to work with snubber values on primary and secondary, and on the rectifier diode, and they did try some shielding options, but nothing seemed to help much when the supply was attached to our system.
Boiling down a bunch of testing and experimenting, the supply has two capacitors in series, connected from the high side of the primary to the high side of the secondary. Adjusting these values seems to help a fair bit, but only in specific ranges, and of course this has implications for stability which limit the values that are usable.
One pair of observations were particularly interesting: When we put the supply in the system with the output leads disconnected and loaded with a resistor, everything was fine. When we attached ONLY the ground lead to the system, conducted and radiated emissions went WAY up. Also, attaching cables pushed up the conducted emissions.
I came up with a model for the problem that seems to work, considering the supply as a two-terminal noise source, with one terminal connected to the AC line cord, and the other connected to the rest of the system. I'm theorizing that the drain of the fet is pushing energy capacitavely through the transformer. The PCB, enclosure, and cables on the output end act as antennas at high frequencies, and some value of series capacitance to earth ground at lower frequencies. Adding any metal mass to the output end of the supply would then push up the conducted and radiated emissions, by lowering the impedance between the output terminals and earth ground.
Applying this idea, I suppressed the radiated emissions with a pair of
600 ohm ferrites in series with both output leads (one ferrite each), and the conducted by adding capacitance from the input terminals to the output using UL rated caps. Radiated emissions went WAY down, well below class B limits when the ferrites were applied, but of course they don't do much at