Interesting question
The output is about 300 W
add about 10% over-design, 330 W
assuming 80% efficiency, the source, at input to the device, is about 413 W, or about 0.83 A at converter input
assuming a 10 AWG wire (2.588 mm diameter, nick-free soft annealed Cu wire having a tensile strength of 37000 pounds per square inch)
1 ohm per 1000 feet, or 3.28 ohm per km
at 100 km, that's 328 ohm for a total of 656 ohms of wire resistance
at 0.83 A, that's 542 V drop in the wire (1/2 from ground based source to device, and 1/2 back to ground based source)
so the actual source voltage has to be about 542 V (drop in wire) + 500 V (at converter input) at 0.83 A 860 W for 100 km
and about 54 V (drop in wire) + 500 V (at converter input) at 0.83 A 460 W for 10 km
sorry, don't know about "front end stage comprising of an input current regulated buck converter"
but I did build a current fed push pull one day, and made the mistake of having dead time
(as in a voltage fed push pull) instead of overlap time. it kept blowing up.
make sure you have a current path at all times (don't try to interrupt the current)