Also....

For this, you can use......

Two parallel Boost PFC stages…...make them share 10ms at a time, as this is easiest. So one has 10ms half sine, then the other, and so on.

Then put in a 3kW LLC converter for isolation. Vin = 400VDC, Vout = 400VDC.

Then have 3 Paralelled buckboost converters to go from the 400Vdc to either 1000v, or the 66V and below.

There are many ways to paralell the 3 buckboosts, eg

1...Parent child (parent sets vout, children copy parents output current)

2...Single error amp on vout , feeding each power stage’s pwm comparator (digitise the error voltage if you want, then DAC it back at each pwm comparator)

3….Active vout droop

4….Simply clamp each SMPS at its nominal max current (this is variable in your case, but for any specific case, you can clamp it)

5...Transconductance error amps in each buckboost, then connect their outputs together, so its like a single error amp.

6...UCC39002 based share bus.....or do a DIY version of this...ie tweak each error amp ref voltage till you get sharing....do this iteratively and continuously.

...make one power stage the parent, and just tweak all the others in accordance with this.

You could run the LLC in open loop, f(sw) at upper resonant frequency. Just have good current limiting. Put it in burst mode when at lower power levels.

But to start off, have the LLC in high frequency switching, as you charge up the output caps...have the load switched out for this startup period.

The go to f(sw) = upper res freq.

For open loop LLC, choose res components such that the gradient of Vout vs f(sw) for max load is not too steep.

And of course, ensure that upper res freq is sufficiently far from the peak of the Vout vs f(sw) [at max load] graph.

As you know, at the upper resonant frequency, an LLC converter just gives vout = vin * turns ratio...kind of like a mains transformer.