On Mon, 2007-10-01 at 12:54 -0600, Karl Larsen wrote: > Kam Leo wrote: > > On 10/1/07, Karl Larsen <k5di@xxxxxxxxxx> wrote: > > > >> Bob Goodwin wrote: > >> > >>> Les wrote: > >>> > >>>> These supplies are switching supplies, not linear. You should check > >>>> some of the draws, I think you will be surprised about how little some > >>>> of them draw (admittedly there are various design strengths out there.) > >>>> > >>>> In general, a linear supply is about 20-35% efficient, meaning that a > >>>> 450 watt supply would draw about 90 watts all the time. But these new > >>>> switchers are about double that or more, meaning that the supply will > >>>> draw somewhere between 20 and 50 watts. Moreover that draw will > >>>> basically be independent of the size of the ultimate supply, simply > >>>> existing to control the switcher itself, and not the drive current. > >>>> Technology exists in some forms of "buck control" that could boost the > >>>> on line efficiency further reducing the drain. Power stuff is evolving > >>>> even faster than much of the other technology, but is less glamorous, so > >>>> doesn't get as much of the press. > >>>> > >>>> Regards, > >>>> Les H > >>>> > >>>> > >>>> > >>> Here's some more interesting information on power supplies. Note that > >>> best efficiency is attained at/near maximum load. So a 500 watt > >>> supply looks good but may not be what you want when considering power > >>> consumption and disposing of waste heat. > >>> > >>> http://services.google.com/blog_resources/PSU_white_paper.pdf > >>> > >>> Bob Goodwin > >>> > >>> > >> Hi Bob, I am interested in what Google has done but need a lot more > >> information. They say changing the PS to one making just plus 12 volts > >> will save energy. I do not see that as a given. In fact it makes me > >> think they are just passing on the voltage change to the mother-board > >> makers. I think the idea being the MB makers can make just the other > >> voltages their board needs. But converting 12 to 1.5 volts with economy > >> of power is complex and will jack up the MB cost somewhat, and will make > >> it hotter. > >> Snip sig. > > > > They're trying to get rid of distributing AC to all the racks and the > > losses resulting converting from AC to DC for each MB. If the MB > > design only requires 12 VDC input then it is more efficient to have > > only one AC to DC converter and buss the DC to all the boxes in a > > rack. By the way, people have been bussing 24 and 48 VDC in equipment > > racks for years. > > > > > The 12 volts can not be moved as 12 volts DC very far. The length of > a car is about it. So Google must have a lot of racks in close proximity. > And while moving this single voltage around has lots of advantages for racks, it is not such a clear advantage for single systems. I guess only time will tell us that story. What Karl has seen in working on Amatuer Radio, and my experience on a boat tells us the story on high current busing which is a very complex, very hazardous proposition. And yes I know that it is done all the time in racks, and along buses, however, check out the AWG specifications for 12v with 10% drop at 30' and think about the amount of copper needed for a data center. Up to about 5 racks is probably acceptable, but beyond that the copper cost and the copper losses begin to mount quite rapidly, especially for hard drives or other motor driven equipment. Anything that generates force, motors, relays, driver circuits for lasers, lamps etc. force the current requirements quite rapidly. In addition, as the voltage drops, the area of the conductor goes up by the square. This establishes a point of optimum economic return, and also establishes the acceptable connector loss and of course the resultant heat and fire potential, which are primarily current dependent (a 500Watt soldering gun actually uses only about 1.2V at its tip loop for example.) Then there is another whole subject that I touched on, that of power factor. Generally AC loads develop maximum power at 90degrees between current and voltage. But some components, such as totally passive ones, develop maximum power and efficiency at 0 degrees difference between current and voltage. The variance between 0 and 90 degrees determines the efficiency of power transfer. So a very efficient device at 0 degrees may only be 30% efficient at 90 degrees and vice-versa. However if the lines are loaded with a 0 degree phase, the copper losses mount and the wires carrying AC begin to heat up. This increases losses accelerating heating... a condition known as thermal runaway, and can result in catastrophic failure of a power delivery conduit. Not good. So as the world matures, and develops more and more electrical loads, we are all having to learn more and more about power and AC functionality in the engineering community. Lessons the RF guys have known for years are now beginning to affect the digital and power folks and the colleges are struggling to bring all this into the curriculum in a manner that reflects good design practices as well. I spent much of my life working on RF, so I find it interesting, but not too demanding to apply these principals to AC power. Those without a background in RF power electronics occasionally find themselves struggling to get a handle on the implications. Regards, Les H
