UC2843: Emission Issue

Nothing inside the UC2843 is operating anywhere near 55MHz. The likely cause is coming from the power stage or are there any other circuits active in tghe system such as high-speed digital clocks, crystal oscillators, etc. You should check:

1. EMI ambient environment to assure the 55MHz spike is not present when the DUT is not operating
2. When the DUT is operating, check: 
   1. Transformer emissions (sniff with near field probe) and add shielding if necessary. You can try adding a copper band around the transformer.
   2. Main MOSFET VDS. Check for high frequency ringing and possibly add/adjust snubber value to dampen the ringing. Does the ringing change with varying input voltage?
   3. If operating the converter in CCM, check the secondary side rectifier reverse recovery ringing and add snubber to dampen, if necessary?
   4. Make sure you have an input CM/DM EMI filter that closely resembles below. Your provided schematic does not show any EMI filter:

Regards,

Steve