[Behemot]

That's correct, ~230 V AC times square root of 2 equals to a bout 325 V. Where would DC appear from on the transformer? Since when can transformer transform direct current? A switch (usually transistor) cuts the DC into some kind of AC wave which can only than be transformed to higher or lower voltage.

The key value here is frequency. Low-frequency transformers (like the networks 50- or 60Hz) are huge and infficient for higher power passing through them. Switched-mode power supplies work usually within the 60-100 kHz range (but generaly can operate anywhere between 10 kHz and 10 MHz).