PuppetMaster said:
But the first PS2 Slim models did have an external power brick.
The PS3's power requirements are still pretty high, even compared to the original PS2. So I think it will be a while before they can integrate it back into the system while keeping the size small.
I've just measured a (not terribly recent) 80W notebook power brick at 5.5x11x3cm. The actual PS3 PSU is probably smaller, even for the 110W+ models (only seen pics with no proper scale, so a grain of salt please). I just don't see how it's worth the trouble to externalise such a s small volume.
This isn't as drastic a shrink as PStwo. This is more of a half-step. There's still a good bit of space there.
Thermally, if your PSU efficiency is ~85%, keeping the PSU in the same case adds 1/0.85~=18% of the thermal load to the system. If this figure is a big deal, you'd have to cool your PSU anyway if you externalized it, or risk high failure rates (hello, early Xbox 360 power bricks). Better keep it connected to the same cooling system you already have for the main device. One fan and one heatsink is better (cheaper, quieter, more reliable) than two fans and two heatsinks. You only lose by separating them.
If the 18% are not a big deal anymore (30W or thereabouts and below) ... well, your PSU block is probably tiny anyway at that point.
I think the far bigger reason for external power bricks, at least for a while, were certain regulations that mandated any device with exposed conductive metal parts would need to be properly grounded, so that a circuit breaker would trip on insulation failure before someone gets a chance to hurt themselves. You could avoid the requirement by keeping the "high" mains voltage out of the main device, in a separate brick encased all around in plastic with no visible metal parts.
Do keep in mind that if you add in-between steps, the sum complexity of the PSU increases. You can step down to 12V in an external brick, but for any part that can't operate on 12V, you need to step to different voltages again, so why spend the parts and space in the first place? The PS3 chips and memories need different "non-standard" "low" voltages to run, you need 5V for USB, probably 12V for the fan and possibly some other voltage for the optical drive.
This is no PC where certain specs have to be followed to maintain interoperability between mixed-and-matched parts. There is no enforced "border line" between "PSU" and "motherboard". They can do whatever to get the most efficient whole.