Radiated EMI can be generated in a couple of ways. Switching currents in large loop areas that create magnetic fields and high voltage switching on large surface areas that radiate electric fields. The rule of thumb is to keep the primary and secondary switched currents paths as short as possible and keep the loop area small. For example on the primary side, the current flows from the bulk capacitor through the primary winding into the drain and out of the source back to the bulk capacitor. In an ideal case, the current from the source to the bulk capacitor would be routed on the PCB next to the trace from the bulk cap to the transformer and drain. An example for radiated emissions: If the tab (drain connection) of a TOPSwitch (Y package) is attached directly to a heat sink. The heat sink acts like a antenna. For emi considerations, it is better to ground the heat sink (keep the displacement current loop small) and insulate the TOPSwitch from the heatsink. Another area to minimize EMI is to design the drain clamp to eliminate any high frequency ringing when the drain turns off. Also a snubber across the output rectifier will help. Sometimes connecting the transformer core to primary ground may reduce EMI. I would recommend reading AN-15 on our website. It addresses conduct EMI but poor conducted EMI performance can cause radiated issues. Z. Cochrane