In the traditional rack-and-stack, if a power supply fan dies, the power supply will shut down -- which is why a server has two of them. (And indeed, in commodity servers, the fan in the power supply is what is most likely to fail.) All of this becomes a bit of a self-fulfilling prophesy: because there are so damned many of them, the power supplies in rack-and-stack servers are really not great -- which means they are more likely to fail. This is all sidestepped by having conversion consolidated in the rack, running high voltage (54V) DC up and down the busbar that is converted by each sled. (That converter throws off heat -- but can then leverage the high quality fans in the sled.)