I watched an interesting video in which the caster reverse engineers a modern 850W computer PSU. From this teardown one can see that 90% of the bulk of the PSU is devoted to synchronous conversion of the 120V input to the ultimate output which is a 12V flyback transformer. Once this 12V DC source is obtained, the outputs are generated using either direct output from the transformer or the output from two different synchronous buck regulators (one for 5.5V and 3.3V). In general, once the 12V is obtained, the only things downstream in the power supply are the following:
- 5V buck regulator
- 3.3 buck regulator
- voltage sensing circuit board (sends "power good" signal to motherboard)
- -12V inverter, which is another buck regulator (non-synchronous)
These components compose less than 10% of the mass of the PSU.
So, what I don't understand is, given the relatively small size of these components, why are DC-DC ATX converters so big? For example, PowerStream sells a 650W DC-DC ATX PSU which is just as big as the 850W PSU in the teardown video. But since the PowerStream is taking a 12V input, why does it need to be so big? Since it would only seem to need the 4 components listed above, couldn't it be much smaller?
One possible clue is that there is a reference to 1500V AC "isolation" in the specs. So, does that mean they are converting the input to 1500V AC, then downconverting again? If so, that would explain the bulk. If this "isolation" is indeed the reason, why would they need to do that? If the PSU is getting 12V power from a battery, the power will be smooth, so why go to all the trouble to take to 1500V AC and back again to 12V DC?