Hi Amund:
Being a safety guy and not an emc guy... (disclaimer)
> Ground connections is usually nice to have when dealing with EMC. But what
> about EMC and a class 2 (double insulation) power supply (shall comply with
> MIL-STD-461E) ..... I'm thinking on, what to do with the regularly Y-caps,
> does it really have any meaning with these caps when ground is absent?
How do Y caps suppress EMC?
I believe that they provide a low-Z return path for
signals that are coupled into the ground due to stray
capacitances in the construction of the power supply.
If we think of the switching circuits as an emc
generator, and some of the emc voltage is coupled to
the ground, then the Y caps provide a return path for
the emc current to return to the switching circuits.
In a 2-wire product, there is no ground, so there is
no stray capacitance.
However, there is stray capacitance to the secondary.
So, it is customary to put a Y-cap between the common
side of the secondary back to the primary circuit,
usually the dc common point of the switching circuits.
This provides a low-Z path for emc voltage in the
secondary to return to the primary.
EMC experts will want to correct this thinking on how
Y caps suppress emc.
Best regards,
Rich
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