On Mon, 16 Jul 2018 06:13:18 +0100, alan_m wrote:

On 15/07/2018 21:48, Johnny B Good wrote:

If any of those power rails are eventually to become obsoleted, it's
more likely to be the +5 and +3,3 volt rails, That "Legacy +12v line"
will most likely become the 'defacto' standard for distributing DC
power to everything in PCs and IT kit in general simply because of the
ease by which the actual chip voltages required can be generated by
commodity on-
board dc-dc switching regulator chips.

The voltages required by digital chips will be lower and so a 3.3V power
supply will be more than adequate for all future semiconductors and
solid state storage devices. Today it's common for the core of many
large semiconductors to be running at 1V (or lower) and the interfaces
at 1.8V. The 3.3V is kept for interconnects for legacy purposes.

As technology moves on the required power supply voltages will be lower.

Well, you've just stated the case for standardising on a single 12v rail
voltage. The power demands of various devices can all be neatly serviced
by on board dc-dc converters whatever their actual voltage requirements.

It's already a long established technique for providing the dozens of
amps at circa one volt or so to modern CPU cores as well as the 1.8v for
dimms (and Gawd knows what else that happens to require a 'non-standard'
voltage).

So little of what's on board a modern MoBo these days uses the 5 volt
rail directly that it might as well be derived off the 12v rail and any
serious power demand on the 3.3v rail would be far better served by an on-
board 12v to 3.3v switching converter anyway.

As for the negative rail voltages, they were mainly used as low current
biassing voltages. In fact the -5v rail disappeared from the ATX spec a
decade or so back since virtually nothing used it even back then and any
modern adapters that still needed a -5v rail simply derived it from the
-12v rail using a low power analogue voltage regulator chip or a simple
zenner diode and resistor. Anything that needs non-trivial power levels
on a negative voltage rail can also be served by dc-dc switching voltage
inverting regulators powered from the +12v rail.

The +12v rail has always enjoyed a looser voltage tolerance spec (+/-10%
as opposed to the +/- 5% tolerance of the 5 and 3.3 volt rails). This
feature lends itself to the use of a 12v SLA as a simple battery backup
for a 12v only system (10.8 to 13.2v) where a slight increase of the
upper limit to a float charging voltage of 13.5v is unlikely to trouble
the only kit that uses it directly, the BLDC spindle motors in desktop
HDDs.

The simplified +12v only PC power supplies will be both cheaper to
implement and easier to design for the higher efficiencies required to
qualify for the Bronze 80 and higher certification standards of
efficiency.

Once a roadmap to a single rail PSU powered PC has been published and a
transition period agreed, we'll start seeing adapters advertised as being
"12 volt ready!", possibly even being described as "12 and 15 volt
ready!" to indicate that they can be used in the current multi-rail
powered PCs and transplantable into a modern 12v only system PC.

There's every good reason to eliminate the multiplicity of voltage rails
between the PSU and the MoBo so I wouldn't be at all surprised to see
such a transition wherein modern 12v only MoBos could still provide by
default the original +5 and +3.3 volts in their PCI and PCIe expansion
slots with a signalling system whereby a modern "12 volt only" adapter
could negotiate for additional 12v power via these otherwise unused
connections in the interface. This would allow otherwise expensive to
replace adapters with normal power requirements to continue being used.

Whether we'll actually see such a development in PC power standards
remains, to my knowledge at least, an unknown. It just seems the most
obvious next step for the manufacturers to take in improving reliability
and reduction of overall production costs whilst meeting an ever
increasing demand to reduce the energy consumption of desktop PCs.

--
Johnny B Good