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Power supplies

The facebook announcement of specifications for their server farms has received interest from places like Tech.Blorge and The Register. Part of the reason is that it has ‘green tech’ tied to it and another is that it is presented as a open specification like in FOSS.

The upshot of the server design, according to Frank Frankovsky, director of hardware design and supply chain at Facebook, is that the Facebook servers cost about 20 per cent less than the boxes the company was previously using and have 22 per cent less metal and plastic in them. And when plugged into the Prineville data center, these servers consume 38 per cent less power.

The features provide a lot of points for consideration from the case design to peripheral support to rack design. Power is something that is often in the background but the facebook design illustrates a trend. Their servers run on 450 watt supplies that need a 277 VAC primary (a leg of 3 phase 480 volt to ground typical industrial power) with 48 VDC failover input to produce a 12.5 VDC output. The motherboards have to convert this 12.5 VDC to the various voltage levels needed for the various parts.

This 12 volt single power feed is becoming rather common. Many external hard drives and other devices use a wall wart providing 12 VDC. That is something that Digital DIY takes advantage of in its post on AC-DC Power Supplies – Using Wall Warts. That one has a good rundown on power supply design criteria and methods. The basics include the old line AC through a transformer, rectifier, filter, and regulator thing. All that used to be analog stuff and the DC regulation was a matter of voltage reduction via some resistance mechanism that wasted a lot of power. Nowadays, the DC regulation is done with switching circuits that break the DC up into something that can be fed through a transformer and then rectified and filtered for the desired output. This is often a single chip solution for the small loads levels. What that means is that wall warts used to have big 60 Hz transformers but are now smaller and lighter with high frequency transformers and less need to dissipate heat.

The Digital DIY item on power supplies is 9 pages – I hate that – and it appears that the site got swamped or something. I’ll have to try later to see if I can get to the remaining pages I haven’t read yet.

These wall warts using switching designs can also handle a wider variety of inputs. The Seagate Dockstar I have as a plug computer even has swappable plugs to facilitate its use in various countries that provide AC voltages running from 110 to 240 VAC at either 50 or 60 Hz.

The 12v output that is going to be further processed for the circuitry also has implications. A good design would allow for that 12 VDC to be rather flexible by isolating the circuit power from the input power. For instance, I have an EEE PC and a Vizio TV that both use wall warts producing 12 VDC. Both can run off an RV battery instead of the wall wart.

The output power has also changed quite a bit. The power supply for my Ten Tec Triton transceiver provided 13 volts at 20 amps (250 watt). That was a big deal thirty years ago and needed a big heavy transformer and some special, circuit board trace, resisters. Now-a-days, you can convert the power supply from an old PC to easily produce that much 13 volt power in a box a quarter the size and weight of the Ten Tec supply with better regulation, safety controls, efficiency and the cost is also only a quarter of the actual price from back then – inflation would increase that ratio to an effective cost to only ten percent or so.

The facebook open server and data center ideas illustrate how power distribution is changing. It is no longer one step between grid and circuit. Grid now has an option for backup and there is an intermediate localized power bus that circuits tap to adjust for their specific needs. This method is being facilitated by integrated circuits that take care of the common needs.

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