Offline flyback: Connect primary FET heatsink to primary GND or Earth?

[treez]

In an offline Flyback SMPS, using a plastic case TO220 FET such as FSCQ1565, do you agree, its better to connect the FET heatsink to primary ground rather than earth? (Wouldn’t common mode EMC be worse if it was connected to Earth?)

Also: Same question as above but for the secondary TO220 diode?

FSCQ1565 datasheet

https://www.onsemi.com/pub/Collateral/FSCQ1565RT-D.pdf

 

[thunderdantheman]

Assuming the primary side here: Connecting the heatsink to the earth system can cause common mode issues with stray heatsink currents caused from capcitivly coupling between the switching devices and heatsink being added to the (unshielded) earth... whch will radiate. If your heatsink is isolated from your chassis and can meet all the other clearance requirments it can often be better (EMC Wise - Safety another issue) to connect it to the power supply neutral. Best to include prototyping options so they can be assessd in the test chamber. I'll look at the diode later..