OK, but if I may guess at your situation…as this is common for people starting out in class d power supply…So, you want a power supply for class d, and you were going to buy off the shelf, but because the average power is so much lower than peak power, you think you can do a cheap power supply custom build yourself(?). Also, most off the shelf power supplies have fans in them and you don’t want the noise of a fan.
If it’s the fan, then maybe just remove it and use off the shelf…as long as you don’t overheat then its ok……or just pick a quiet fan and use that near it instead of the internal fan…….or just have the fan come on when loud music playing.
You cannot really massively downsize an smps if the average power is much less than the peak,……because you must respect the ferrite saturation etc etc…I suppose you can reduce heatsink size though.
Other point is that people often don’t like the no_load_to_full_load transient response of off the shelf power supplies for class d supply…as off the shelf offline PSU’s often have opto feedback and are very sluggish…….if that’s the problem then why not have offtheshelf for the mains connection, and then use your own (or bought in) DCDC after that with a faster feedback loop to give you your fast transient response.?
Attached is a document about offtheshelf psu's for class d guitar amp supply