> When using a standard 5V, 3A (15W) USB-C power adapter with Raspberry Pi 5, by default we must limit downstream USB current to 600mA to ensure that we have sufficient margin to support these workloads.
Therefore 5V/3A can still boot RPi5, but it cannot provide enough power to its USB-A port. Therefore it cannot use something like 12V/2A, since RPi5 doesn't have transformer circuit on its board.
https://www.raspberrypi.com/news/introducing-raspberry-pi-5/
> When using a standard 5V, 3A (15W) USB-C power adapter with Raspberry Pi 5, by default we must limit downstream USB current to 600mA to ensure that we have sufficient margin to support these workloads.
Therefore 5V/3A can still boot RPi5, but it cannot provide enough power to its USB-A port. Therefore it cannot use something like 12V/2A, since RPi5 doesn't have transformer circuit on its board.