'Paul' wrote:

I'm not going to argue the merits of that statement, because I don't have
a copy of the appropriate spec that calls up the 240VA limit. I understand
it is traceable to something like 60950, but don't have a copy.
_____

See

"3.4.4. Over-current Protection
Overload currents applied to each tested output rail will cause the output
to trip before reaching
or exceeding 240 VA. For testing purposes, the overload currents should be
ramped at a
minimum rate of 10 A/s starting from full load."

on page 22 at
http://www.formfactors.org/developer...X12V_1_3dg.pdf .

Notice the phrase 'will cause the output to trip'.

and

8.1 North America - REQUIRED
The power supply must be certified by an NRTL (Nationally Recognized Testing
Laboratory) for use in the USA and Canada under the following conditions:
.. The power supply UL report "Conditions of Acceptability" shall meet in the
intended
application of the power supply in the end product.
.. The supply must be recognized for use in Information Technology Equipment
including Electrical Business Equipment per UL 60950-1 First Edition. The
certification must include external enclosure testing for the AC receptacle
side of
the power supply (see Appendices A, B, C, and D).
.. The supply must have a full complement of tests conducted as part of the
certification, such as input current, leakage current, hi-pot, temperature,
energy
discharge test, transformer output characterization test (open-circuit
voltage,
short-circuit performance), and abnormal testing (to include stalled-fan
tests and
voltage-select-switch mismatch).

on page 45 at

http://www.formfactors.org/developer..._Rev_1_2 .pdf .

And see

"If the selected power supply has any single output rated at more than 240VA
contact with circuits connected to the output must be prevented in user
access areas, protection may be achieved by insulation, guarding or
interlocks (refer to IEC 60950-1 First Edition, 2001 Clauses 0.2.2 and 2.1)"


at

http://www.intel.com/cd/channel/rese.../eng/35831.htm .


And, finally, see 'UL 60950-1 Information Technology', page 80

at

http://www.psui.com/1upower/pdf/901_ref.pdf .

Phil Weldon



"Paul" wrote in message ...
Phil Weldon wrote:
'Paul' wrote:
First of One wrote:
As you know, some power supplies made by Seasonic and PCP&C are really
single-rail, but were advertised as multi-rail at one time because it
was fashionable. Now single-rail supplies are trendy, you see all kinds
of monster PSUs re-labeled with 50A or even 70A ratings on the 12V.


I just wish the industry had some standards for this. It shouldn't boil
down to some review site opening the thing with a screwdriver, to figure
out how it works, and how it can be used
_____

There IS a standard; see
http://www.formfactors.org/developer...X12V_1_3dg.pdf
and
http://www.formfactors.org/developer..._Rev_1_2 .pdf )
.

The idea behind the separate current limited + 12 VDC rails is to avoid a
fault condition that can draw 500 or more Watts before safety circuits
kick in.

Phil Weldon

"Paul" wrote in message ...
First of One wrote:
As you know, some power supplies made by Seasonic and PCP&C are really
single-rail, but were advertised as multi-rail at one time because it
was fashionable. Now single-rail supplies are trendy, you see all kinds
of monster PSUs re-labeled with 50A or even 70A ratings on the 12V.


I just wish the industry had some standards for this. It shouldn't boil
down to some review site opening the thing with a screwdriver, to figure
out how it works, and how it can be used.

Paul


I've heard of the 240VA limit from IEC60950, but it appears to be
optional to support it.

http://www.pcpower.com/technology/myths/

"PC Power and Cooling is once again leading the industry. All of
our power supplies now feature a large, single 12-volt rail. The
design is favored by major processor and graphics companies,
complies with EPS12V specs (the 240VA limit is not a requirement)
and is approved by all major safety agencies such as UL and TUV.3"

I'm not going to argue the merits of that statement, because I don't have
a copy of the appropriate spec that calls up the 240VA limit. I understand
it is traceable to something like 60950, but don't have a copy.

I was referring more to the architecture of the supplies. While
formfactors
has a design document for dual rail supplies, I'm not aware of any
standard
for tri or quad rail supplies. (Maybe design guidance comes from some
server spec ?) And the assignment of connectors, seems to be largely left
to chance. On the plus side, I am finding more documentation than I used
to. A few years ago, it used to be really frustrating to find out
what the wiring plan was. (Took a review article, to get the story. The
manufacturer couldn't be bothered to list it.)

Paul