Is it possible to use a Buck PFC converter ucc29910a for Pout=400W.
What output voltage?
If you have to comply with EN61000-3-2, then it's perhaps going to be a problem.
Page 10: http://www.ti.com/lit/ug/sluu505a/sluu505a.pdf
The harmonic content is very high for just 100W output at 84VDC = ~1,2A.
I's suggest a UCC28061 (interleaved boost PFC) or UCC28051 (BCM PFC).
I understand not a good idea to use a buck-PFC for 400W output power (84V &4.7A).
I built the UCC28061 for 800W and at 400W I had an efficiency of around 95-96% ! I used PQ3230 cores for 800W. Perhaps you can go down to PQ2620 in your design.
I only need 150 watts and I want to use 24V as an output voltage.
I think that will be better for harmonic content
Can the UCC29910 work as low as 24 V?