The most common switching supply topology in diy internet is terrible in ripple and emi sence
I`m not experienced in high grade PS and I have no ideas to do it
can You post some links about quiet ps shematics? 300mV ripple voltage at 350V@0.2A will be affordable
The two main problems are poor layout, and lack of filtering.
Layout is critical. A bad layout can affect the schematic, to the extent that the circuit is incapable of operating as intended (self interference)! A good layout reduces noise sources in the first place.
Filtering can always be done, and again, layout is important. Consider the case where all wires entering and exiting a module must pass through a shield: the shield provides a unipotential surface, and each wire passing through the shield must be filtered. By filtering all wires against a common (unipotential) shield, all unbalanced AC currents can be eliminated.
The same method still applies if you make compromises, like common mode filtering of input and output pairs (which allows you to save on the cost of inductors). Indeed, this is necessary for mains input, because mains is itself AC, so it can only be filtered so much before it ceases to be mains.
Here's an old circuit I made,
You can ignore the other outputs (like 6.3V, 2kV and -250V), and consider just the positive output (250V). Note the input and output filters.
This exact circuit never worked very well, due to its voltage mode design, and due to excessive recovery losses in the diodes, due in part to a poor transformer design. But the noise output was well within specification.
I later modified this design (rather aggressively; "modify" is something of a misnomer here, as more than half of the circuit board was replaced
) to a flyback type supply, which operates much more safely and efficiently.
Tim