What is the usage of theses negative voltages? Are they there only for backward compatibility?
In nowadays PC power supplies, we have:
But the current rating of the negative rails are much smaller than the positive ones.
If we were back to the 80' where op-amps were always powered symmetrically at +12V -12V: Okay.. But nowadays, almost everything you may find on a motherboard is digital logic only powered by positive voltages.
Except for the RS232, which is an almost obsolete bus, I don't see any reason for having negative rails distributed by the power supply.
Because it's very high volume, I suppose that cost drives everything here. Thus, why each PSU has to deliver those voltages if they are barely used ? (the very low current rating of the negative rails of PSUs let me suppose this).
Wouldn't it be less expensive to let every hardware provider to add their own embedded SMPS when a negative voltage is required?