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You can see this site for the core material and design. **broken link removed** . you can double the core (if it is E+E or E+I) for higher power. Also see ETD59, EC70 core size from MAGNETICS, EPCOS, FERROXCUBE etc. Different materials are available for losses at higher frequency, so select the appropriate material from the availability.
250KHz at that high voltage is asking for trouble with switching losses.
cuzz remember at 2KW you will be looking to slow down the switch on/off of those fets, in order to avoid to much EMC noise problems.
so i reckon you are looking at 100KHz max switching frequency.
of course, resonant methods are often used to reduce switching losses....but i do not believe you will be using a resonant topology, since once again, 420V input is too high for a resonant topology....because resonant topologies incur vast excursions of voltage and current , and 420 Volts is too high to embark on resonant topology because your voltage excursions will be too much.
I know a company that do LLC resonant full bridge converters at 1KW.....but they rectify the 230VAC mains...and then BUCK it down to about 50 to 100V so that they can use a resonant stage for the bridge.
.....which means they loose out on all the efficiency benefits of resonant topology...becasue they have the buck in there too....however, its for radar and it (resonant topology) reduces noise from messing up the RF signal
I understand, that you wonder, if the said 420V/2kW/250kHz are a profound design, or just a concept, without considering details as switching losses. I also don't know in this case, cause the originator is only asking for the transformer design without mentioning any details.
But I'm sure, that it can be designed, as a hard switcher and as a resonant converter as well. All these reservations in your post, that you can't do this or that, are basically application specific design parameters, without general validity. You can find contradicting design examples for each of it.
It can be meaningful to opt for 100 kHz switching frequency rather than 250 after calculating the losses for an existing power stage, of course.
Regarding the original question, it's rather easy to find a suitable core, either for 250 or 100 kHz. I think, the winding design is actually more critical, and it' necessary to know the available manufacturing options, e.g. flat wire, foil, litz wire and also to specify winding isolation, possibly shielding, acceptable leak inductance.
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