Thanks,
Our product is supplied by a cable with Live, neutral and earth, and so it is Class I.
If we do not connect the isolated secondary GND to earth, then it will be floating, which is going to be bad for emissions, would you agree?
[Also, i believe we would all agree that (though it wouldnt happen for safety reasons), if the primary and secondary of a offline flyback converter were connected together close to the transformer, then that provides a vast reduction in EMI problems.]
Its a well know fact that non isolated converters provide far less EMC problems in general, than isolated converters.
When its deemed that the secondary GND must be earthed, one then could look at how to go about doing that….
Would you agree, It needs to be done in such a way that there are no earth ground loops?, or at least, a minimal number of earth ground loops….as follows…..
..So Earth enters the product through the mains cable…the mains cable joins the mains input connector on the PCB at the offline SMPS’s AC input.….so we should be sure there are no other earth wires coming to the product other than that. This then means that earth has to be gotten from the primary side of the offline SMPS, over to the secondary side….
..the simplest way to do this is to simply have a thick, well insulated earth wire jumping over from primary side to secondary side.
The metal enclosure also needs to be earthed (for safety and EMI reasons). We don’t want the connection of the chassis to earth ground to form a big earth ground loop…..therefore, we have to make the chassis connection from the bit of earth at the offline SMPS input earth connection.
Does this sound like a plan?