Google Calls For Power Supply Design Changes

Raindance writes "The New York Times reports that Google is calling 'for a shift from multivoltage power supplies to a single 12-volt standard. Although voltage conversion would still take place on the PC motherboard, the simpler design of the new power supply would make it easier to achieve higher overall efficiencies ... The Google white paper argues that the opportunity for power savings is immense — by deploying the new power supplies in 100 million desktop PC's running eight hours a day, it will be possible to save 40 billion kilowatt-hours over three years, or more than $5 billion at California's energy rates.' This may have something to do with the electricity bill for Google's estimated 450,000 servers."

What in a modern computer actually uses 12V?

[Marxist Hacker 42 (638312)]

In the old days, disk drive motors and fans. But many of these now run on 5V, hence the cheap USB-powered drive cases out there. Chips at CMOS power levels run at 3.3v, TTL is 5v, but hardly anything runs at 12v anymore. It seems to me that if they'd just pick their hardware carfully, they could run their entire server rack off of 5v+- rails.

Re:What in a modern computer actually uses 12V?

[by Anonymous Coward]

Video cards use a ton of 12v power, enough that high-end cards get a dedicated connector featuring two wires of it.

Re:What in a modern computer actually uses 12V?

[ArcherB (796902)]

This is true, but Google is not throwing 7950's in their servers. These systems run with on-board video at best. Google has no need for a video card that can do anything more than text, as with all non-windows based servers. For that matter, after the first boot, there is no need for a video card at all.

Re:What in a modern computer actually uses 12V?

[Jahz (831343)]

Actually I would bet that Google servers DON'T have a video card, and that all of them have RJ-45 SOL support (or something like it). The reason being that Google has admitted that they fully embrace the commodity distributed server system. Google will periodically host talks at my university where they explain all this in [too much] detail.

Basically, when a machine fails, it is pulled from the rack and replaced with an identical machine with a cookie cutter image. Kinda like the Borg :)

When a box fails it is probably instantly detected by some machine monitor and taken offline (think: the 'crop' tenders in the Matrix I). The sysadmins arent going to waste time plugging a video cable into the rack... just pull it. Toss the box into a repair queue and let the tech's put a video card into it if needed. Remeber: 100's of machines fail for them every day. That's a fact from the Google talk in 05.

Re:What in a modern computer actually uses 12V?

[RedWizzard (192002)]

With 450,000 computers, couldn't Google afford to get their own custom designed power supplies built?

They can and do - read their white paper [google.com]. What they are saying is that if everyone else did what they do the power savings would be huge.

Re:What in a modern computer actually uses 12V?

[poot_rootbeer (188613)]

Google is not throwing 7950's in their servers. These systems run with on-board video at best. Google has no need for a video card that can do anything more than text, as with all non-windows based servers. For that matter, after the first boot, there is no need for a video card at all.

Seems to me Google doesn't want to fracture the commodity hardware market into server-class hardware using 5VDC power and desktop-class hardware using 12VDC. One standard, applied equally across the entire range of products.

Re:

[Bassman59 (519820)]

Video cards use a ton of 12v power, enough that high-end cards get a dedicated connector featuring two wires of it.

Video cards with a disk-drive-type power connector always use point-of-load switch-mode supplies to convert the +12V to whatever voltages are needed by the chips on the board. Nothing on the board uses the 12V directly, except maybe the fan (if that).

They use the disk-drive connector because:

• The GPUs and other devices on the board use more current than you can push through the PCI/AGP or P

Re:What in a modern computer actually uses 12V?

[by Anonymous Coward]

Almost nothing, but that is irrelevant. A modern PC uses so much power that it would be plain stupid to try and deliver it at lower voltages. The power is stepped down close to the actual load, because otherwise you'd need much heavier wires or lose much power to heated cables. That kinda is the point of the proposal: Every PC already has the necessary regulators because there's simply no other sane way to deliver the kind of power that graphics cards and CPUs consume. So what's the point in keeping the power supply complicated when the main consumers in a PC use the 12V line anyway?

Re:What in a modern computer actually uses 12V?

[Lonewolf666 (259450)]

Modern CPUs run on core voltages of 1.5 v or less, depending on model. DDR RAM is 2.5V IIRC.
So you will have to convert most of your power from 5 V to something else. And if you have to re-convert anyway, 5V as intermediate voltage is not optimal. When converting to 5V, the voltage drop in the power diodes and in the wires to the mainboard eats a much higher proportion of the power than with 12V as intermediate voltage.
24V or even 48V would be even better. The auto industry is currenly starting to introduce 48V systems BTW.

Re:What in a modern computer actually uses 12V?

[Hirsto (601188)]

12V happens to be a sweet spot in terms of cost of the converter components as well as overall efficiency. Wire guage, voltage drop and capacitor size is significantly smaller than 5V or 3.3V primary supply. Think in terms of millions of units per month and compare the price of an NMOS FET and storage capacitor rated for 35V (safety margin in a 24V design) verses the cost of similar FETs and capacitors rated for 20V. In a synchronous buck design you can easily save $0.75 per converter section by using 12V rather than 24V and significantly increase conversion efficiency for free. Assuming a constant switch frequency the switching losses increase with the square of applied voltage, "I squared R" conduction losses in conductors will decrease with the square of current but the voltage dependant switching losses will dominate once the input voltage gets too high. For a given cost the overall converter efficiency is usually highest if your input voltage is relatively close to the output voltage. 12V to 3.3V conversion is significantly more efficient and less costly than 24V to 3.3V conversion.

Re:What in a modern computer actually uses 12V?

[NoMaster (142776)]

Actually, when I learned it, I was taught ~32V was the sweet spot between practicality, I^R losses, component size, and load type. Mind you, back then it was electrical / electromechanical loads (lights, motors, contactors, relays, etc) not electronic. And that was from a telco background (-48V), which made me wonder "well, why tell us that about 32V?" ;-)

But the point is, what the sweet spot is depends mostly upon the characteristics of the load - so it's wrong to come out with blanket statements like "12v happens to be a sweet spot in terms of cost of the converter components as well as overall efficiency". Yes, today, particularly with switchmode supplies and the actual maximum load V being 5v or less, it is. Tomorrow, when everything runs on 3.3v or less, it'll be closer to 5v~6v.

The other half of your argument only holds for certain types of power supplies too - but I'll give you a pass on that, seeing as you did explicitly state "synchronous buck" designs. It doesn't necessarily hold true, however, for other classes like linear, boost, buck-boost, etc. Your final assertation, however - that, for a given cost, the overall converter efficiency is usually highest if your input voltage is relatively close to the output voltage - is spot-on. Too far away from that, and the ol' V=I*R rule starts to bite you...

Re:What in a modern computer actually uses 12V?

[zootjeff (531575)]

If you look at just routing 12 volts everywhere, you just would have to put the regulators in the hard drives, and CDROMS so they don't need 3.3 and 5 volts. Then what do you do about +5 Stanby that allows you to hibernate? Do you still need a stand by voltage? It isn't and easy answer and will take the whole industry to adopt it. Checkout formfactors.org for ATX and BTX specifications that Intel is pushing. What's also interesting is the 600 and 700, etc Watt power supplies just keep their 3.3 and 5 volts at around 30 amps max, but keep adding +12V1 +12V2 +12V3, etc.. Looks like the industry is already going to mostly 12 volts for distribution anyway. But don't you still need PS_ON, PowerOK, etc.. You're just trying to phase out the +5 and +3.3, and -12 which hardly any motherboards use these days, and maybe the +5 Standby, then it's going to happen eventually anyway. Most of the power is going on the 12 volt lines anyway, so having inefficient +3.3 and +5 isn't really a big deal. I've studied this for a while as my big hobby is computers in cars, I built a power supply called DSX12V that takes a 8-16 volt input and makes a solid 12v output that I got over 97% efficiency on. This is good for people sticking computers in cars or running them off banks of batteries for solar power applications etc.

Re:What in a modern computer actually uses 12V?

[seanadams.com (463190)]

In the old days, disk drive motors and fans. But many of these now run on 5V, hence the cheap USB-powered drive cases out there. Chips at CMOS power levels run at 3.3v, TTL is 5v, but hardly anything runs at 12v anymore. It seems to me that if they'd just pick their hardware carfully, they could run their entire server rack off of 5v+- rails.

You are correct that hard drives generally use just 5V, but the rest of your points are not even close. Modern CPUs require lower voltages, higher current, and tighter regulation, which is why DC-DC power supplies are now on motherboards instead of running directly from an ATX supply.

Furthermore, running a rack of servers on 5V rails would be absolutely absurd. Do you have any idea what the amperage would be? The bus bars would have to be several inches thick, the transmission loss would be enormous, and if you accidentally shorted them.... forget it!

Something like 48VDC might work but then you lose out on all the economies of scale driven by the 110/240VAC standard.

Just match the power supply to the motherboard and be done with it. Standardizing on one voltage is impractical, and besides, how would it improve "efficiency"?

Re:What in a modern computer actually uses 12V?

[ottffssent (18387)]

Disk drive motors use 12V. Laptop drives (2.5" drives) use 5V exclusively, but standard desktop and server drives use 5V and 12V. SATA drives get 3.3V, 5V, and 12V. The VRMs that power your CPU and video card probably take their power off the 12V rail, as do many other components.

The reason you wouldn't want to power a machine off 5V is because you would need huge busses. Suppose you've got 40 svelte 1U servers in a rack, each drawing 100W. That's 4kW. Assuming that's a purely resistive load (hint: it's not), you'd need 800A at 5V for the whole rack. Are you familiar with the big connectors on car batteries? They're designed to pass less than half the 800A you'd need to run a rack off 5V, and your car battery only has to handle that for a few seconds while the engine is starting up; a rack would need to deal with that continuously. And that's for a pretty low-power rack.

Using 12V instead of 5V lets you get away with busses about 40% the size. Also, and probably more importantly, 12V DC is (IIRC - correct me if you're a PSU designer) easier to get efficiently than 5V DC. Once you split the 12V off into a few dozen servers, you can drop it down with small, fairly efficient CMOS regulators.

Re:What in a modern computer actually uses 12V?

[seanadams.com (463190)]

RS-232

Sorry no. Modern rs232 circuits, if it's not already built into the UART, use a chip like max232 that runs off 5V and has a built-in charge pump to generate (close to) RS232 output voltages.

Re:

[Craig Davison (37723)]

You've never seen a console port on a disk array, router, switch or UPS? That RJ-45 socket speaks RS-232 and will connect to your serial port with the right cable.

(Yes, some UPS's have USB)

Re:

[Marxist Hacker 42 (638312)]

And exactly what do you use such a server for? Not for internet, that's for damned sure, 115kbaud is far too slow to serve a 1.5TB line....

Consoles, of course.

[Junta (36770)]

Serial remains one of the most manageable approaches to console management. Video is, obviously, not loggable, not automatically monitorable, not greppable, and not amenable to low throughput, high latency remote access.. Serial devices and consequently drivers for them are so simple and straightforward, and the behavior so deterministic, that it is far preferable to something more complex (ethernet and usb) for a console. Ethernet certainly in questionable circumstances may suggest a driver unload/reload as a step to problem resolution, which is safer if not using as a console (though many times I have used ssh and chained the commands using semicolons). For example in that case, if your path contains an nfs mount, and you forget about it as you yank the network out, your chained command will hang as the shell tries to stat the nfs mount for the path. Part of the problem with relying solely upon the ethernet for console is the ethernet has more than one job to do, so it takes a fair amount more competent engineering to get to work right. Many newer systems offer to redirect textual serial traffic over IPMI, and that is admittedly decent *if* the vendor architects it robustly, which is difficult to ensure beyond hands-on experience with a brand and trusting in their consistancy. For example, e326 servers from IBM I wouldn't trust the net console, but an IBM x3455 I would be more confident in. USB, again, has similar complexity issues (it's multiplexed for keyboard/mouse/mass/storage/printing/scanning/etc etc). If you theoretically had bi-directional text console over some usb device, it's more difficult for a low level, simple piece of software to set up the usb controller and all requisite activities, then traverse the bus, identify the console devices, and then use it. Just like with an ethernet device where you may have cause to unload and reload a driver, a usb controller out to lunch with respect to a mass storage device would cause a similar issue. Enterprise distribution kernels tend to compile in the serial console and leave the usb controller modular, specifically with serial consoles in mind.

Serial console servers, in answer to your question, provide a scalable way for systems to access via the network serial consoles. By being dedicated, moderately simple systems with 40+ serial cables, they can provide access (via telnet generally) to a rack's worth of 1U servers, automatically log the content, or at the very least provide an administrator with remote console access at will to any given system.

Serial console is not obsolete in the least bit, just because it can't run your '31337' aero interface, or whatever nice and shiny interface that makes poser administrators and PHBs drool, doesn't mean good, serious systems administrators don't consider the technology to be a vital part of a robust management strategy.

good idea but...

[grapeape (137008)]

Its a nice idea and one that is probably a long time coming, but phasing something like that into place will take an incredibly long time. Look at the struggles of PCI express, its still not in 50% of the newer motherboards and systems though its benefits are more than apparent. Its just been in the past couple years that we have seen a shift to full usb and most machines still come with ps2, serial and parallel ports anyway. Dramatic changes to the PC standards are very difficult, there are millions of existing machines that still need support. Perhaps if it was tied to a new socket standard in the future it could slowly be phased in through upgrades, but I see the chances as very very slim.

Proposal spells doom for USB powered devices

[JavaManJim (946878)]

You can say goodbye to USB powered devices. An example would be the canned drink cooler.

Thanks,
Jim

Re:

[dgatwood (11270)]

Why? I can turn 12VDC into 5VDC (what USB uses) with nothing more than a voltage regulator (or if you want to waste a ton of power, a relatively trivial voltage divider).

I've wanted this for years.

[dgatwood (11270)]

The ability to have all my machines powered by a heavy cable carrying 12VDC would be pretty useful for several reasons.

• The UPS could be integrated into the power supply, avoiding lots of energy lost in converting it up to 110VAC and right back down again.
• The power supply would then be external, where it could be a fanless brick instead of being inside the case where it adds heat that must be dissipated.
• A switching power supply is theoretically more efficient than a wall wart. If everything were 12V, all those stupid little outboard devices could draw power off of the same supply source, resulting in better overall efficiency. More importantly, I would never let out the magic smoke when I accidentally plug a wall wart into the wrong device. :-)
• A 12V system can more easily be integrated with solar panels to reduce load on the power grid.

*sigh*

Re:

[fm6 (162816)]

You make a good point about wall warts, except you don't go far enough. If all portable devices accepted 12V power, somebody would come out with a single brick with multiple 12V plugs, which would be a godsend to travellers who currently schlep one wall wart for each device.

**big sigh**

Re:

[genericacct (692294)]

I'm all about the solar angle! Someday I'll wire my house with an off-grid 12-volt solar system, with 12-volt "car lighter" sockets and DC lighting (both LED and mini halogen). Laptop and WiFi router plug in to it.

And everything can plug into the car with the same cord. That's another awesome advantage, being able to put these same computers in cars and RVs.

I don't get it.

[pavon (30274)]

I agree that a standardized 12VDC connector on all electronic devices would be nice, like every other poster here has pointed out, but I don't think that is what google is talking about. You can already get power supplies that take 12VDC in, or even dual 48VDC (telecom standard), and I would be surprised if google isn't using something like that already.

What they are recommending is that the power supply only have 12V out, and all other DC-DC conversions take place on the mother board. Unfortunately, the ar

Why not -48?

[AaronW (33736)]

A lot of telco equipment is designed to run on -48 volts DC and PC and server power supplies are readily available at this voltage.

The advantage of -48 over 12 volts is that there will be less loss through resistance and smaller conductors can be used. Of course, there is a greater risk of electric shock, but I would think -48 would be pretty safe.

48 volts is also the standard for Power over Ethernet (IEEE 802.3af) [wikipedia.org]. This may not be compatible, though, since telcos run -48, not +48, though some equipment can operate with either (though some cannot).

Re:Why not -48?

[springbox (853816)]

48 volts is also the standard for Power over Ethernet (IEEE 802.3af) [wikipedia.org]. This may not be compatible, though, since telcos run -48, not +48

Sure they can! Just reverse the polarity and reroute all power to the main deflector dish!

Bad idea

[ErMaC (131019)]

Google's whitepaper is interesting but the fact is that DC in the Datacenter is already happening, and it's not gaining much momentum for multiple reasons.
Google's perspective is rather unique, they use super-cheap desktop systems that individually do not use a lot of power and thus running them off 12v DC might make sense. But in any other, more conventional datacenter, servers have multiple power supplies that can EACH pull 800w of power. Now when you're running 110v AC that means you're pulling ~7 amps through a single cable. You need datacenter grade power cables for this, but it's still sane. Now you can get datacenter equipment that runs 48v DC, but those cables end up running ~15 amps through them, so now you need substantially stronger cable - cable so thick that running it becomes a seriously difficult task due to the guage of the wire!
More likely the direction people are going (and have been for some time) is to 208v AC or 3 phase 220v AC. Now you've just halved the current draw, meaning that your PDUs don't need to be as hefty, your wire doesn't have to be as thick, your coils don't get as hot, etc.
Running 12v DC in any real data center would be ludicrous - the amount of current you'd have to draw through your cables would be way beyond a safe level.
Also AC/DC conversions are cheap these days. And remember, DC can kill you just as easily as AC when your DC Voltage is that low.

RTFA

[Wesley Felter (138342)]

Google is proposing 12V between the server's internal power supply and the motherboard. Everything outside the server would still be 208V.

This is about voltage to the boards, not the box

[Animats (122034)]

Most of the postings so far have it all wrong. Google is not proposing 12VDC into a desktop PC or 12VDC distribution within the data center. What they're proposing is that the only DC voltage distributed around a computer case should be 12VDC. Any other voltages needed would be converted on the board that needed it.

This is called "point of load conversion", and involves small switching regulators near each load. Here's a tutorial on point of load power conversion. [elecdesign.com]

It's been a long time since CPUs ran directly from the +5 supply. There's already point of load conversion on the motherboard near the CPU. Google just wants to make that work off the +12 supply, and get rid of the current +5/-5/+12/-12 output set.

Re:This is about voltage to the boards, not the bo

[Bruce Perens (3872)]

Oh. So, we have lots of switching power supplies and tantalum capacitors (because we have to supply lots of current at low-voltage) on the MB. Thus moving work from a cheap part of the computer to an expensive part. Not sure I want more power-supply electronics on the MB than is already there.

FWIW..

[jcr (53032)]

The Mac Mini will boot and run on a 12-volt supply. It only takes 17v so that it can provide Firewire power.

-jcr

No...

[rsilvergun (571051)]

google hires experts on Electrical Engineering to figure out how to reduce the power bill on those 450,000 servers. Hell, I'm all for it. Less power means less heat means quieter fans (w/o spending an arm/leg on an Antec Sonata or whathaveyou).

Re:No...

[TheSHAD0W (258774)]

Considering how many machines Google has to maintain, I'm surprised they just don't order motherboards and power supplies to their own spec, and then allow the mfrs to distribute the design to others who request it. They're big enough and have enough whuffie that they can start a trend all by their lonesomes.

Re:No...

[TheSHAD0W (258774)]

After reading Google's blog entry [blogspot.com] on the subject, I'm left puzzled by their call for a new standard with no further details, especially since it seems they're already using the technology. A power supply is simple enough, but I'd like to see what sort of strategy they're using for voltage conversion on their motherboards. What connectors are they using for power?

The funny thing is, this idea is relatively old, though AC was used instead of DC. Remember the Imsai 8080? The S-100 bus used an 18V AC supply, and each card had its own DC conversion and voltage regulator(s).

Re:No...

[sillivalley (411349)]

The early S-100 systems (Altair, Imsai, Poly, Northstar) used linear supplies and ran unregulated DC on the S-100 bus. Most designers aimed for +8 to +9 volts to feed the onboard +5 volt regulators (and the3 volt or so headroom for 7805 regulators). Can't think of any that ran high current AC on the bus. Some systems, such as the Poly, ran a squared-up 60 HZ signal for real-time clocks.

The heat losses in S-100 on-card linear regulators were immense! That and the weight of the (linear) transformers helped make the Apple ][, with its switching power supply, so popular (I still have an old Poly power transformer; makes a great doorstop).

Some mainframe computers used the scheme mentioned by others -- polyphase high-frequency AC distribution. High frequency (think 800 Hz) power transformers are small and efficient; that's why switching supplies run at high frequencies (in the hundreds of KHz range).

Efficiency is not only about wasting less power, it's about generating less heat!

Re:No...

[kasperd (592156)]

Efficiency is not only about wasting less power, it's about generating less heat!Which is of course exactly the same. In the end all the energy you put into a computer turns into heat. The energy wasted in the power supply turns into heat in the power supply, and all the heating of the power supply is energy wasted rather than used to supply the computer.

Re:No...

[alienw (585907)]

If you can cut power consumption by 10 watts a machine (quite realistic) and you have 100,000 machines (Google has more) you just saved 1 megawatt of power, or about a million dollars per year in electricity (without even taking into account the electricity required for cooling). That's quite a chunk of cash.

Re:

[MoxFulder (159829)]

Yeah, Google hires experts on anything pretty much, I'm told.

Apparently they hired expert ergonomic and industrial designers to figure out how many servers and workstations they could cram into a mobile semi-trailer lab, while still making it comfortable to work in. Kind of a neat optimization problem I think.

Re:

[stunt_penguin (906223)]

"how many servers and workstations they could cram into a mobile semi-trailer lab"

I'm guessing the answer was lots and lots...... there are quite a few technical challenges as you say, power, cooling, and making sure that the machines survive the journey, too.

It would be a neat side business if Google went into providing server farms and data centers for other businesses; as other people have mentioned they have a lot of smart people working on the associated problems.

Hey, it could save their asses

Re:No...

[JahToasted (517101)]

Why do that when they can just rent out space in one of their super massive server farms. Think about it... you get some good bandwidth, your data will be mirrored on geographically and topographically separate systems. You don't have to worry about hardware failure or anything like that and you'll be able to get all the bandwidth you could ever want. You don't have to worry about database replication or syncing up data or anything like that, its all taken care of for you. Depending on your needs, you can have gmail, google maps, google office, adsense all integrated with whatever it is you're setting up... web app, file server, database system, whatever it is you're setting up you'll be able to get it from Google along with some nice cross platform tools to make it as easy as possible.

And because of economies of scale the price will be very reasonable, ie. cheaper than rolling your own solution. Hell, I'd consider it, wouldn't you?

Re:No...

[stunt_penguin (906223)]

Actually, you're spot on- why sell the physical machines (a fixed profit per machine sold) when you can sell a service (a steady income) .... that said a lot of companies would rather have 'their' people with their hands on their machines, but I'm sure a lot of organisations would benefit. They could even call the service Google Farm ;)

Come to think of it, given the recent story on slack govornmental data security, maybe they could have Google serve their data for them. They already know how hard it is to get data out of Google, right? :p

Re:No...

[x2A (858210)]

Better than that guy they spent $50,000 who said moving the plant from the window and installing a water feature would allow the energy would flow much better...

If google come out with a "can save energy this way...", and gets the world to follow, the marketing value speaks for itself. That kind of reputation doesn't come easily.

Cargo container server room

[yppiz (574466)]

The Internet Archive addressed a similar problem: can you build an Internet Archive in a storage container and ship it? They came up with a design for this around standard racks of low-power, low-heat, high-storage nodes.

Their answer is the Petabox [archive.org]. It's a server setup designed to be "shipping-contained friendly", meaning they can build out a container stuffed with these racks, and have it operational on site with connections for power, cooling, and bandwidth. With this design, they can deploy a mirror of t

Re:

[MarcoAtWork (28889)]

given that the article says

Although Google does not plan to enter the personal computer market, the company is a large purchaser of microprocessors and has evolved a highly energy-efficient power supply system for its data centers.

I assume Google is employing some smart electrical engineers, which are more than qualified to make this kind of recommendations I would think...

Re:Big ego department

[purpledinoz (573045)]

I would bet a lot of the employees at Google have Electrical Engineering degrees. Don't underestimate the brain power Google has in its employee base. But the power supply issue they're trying to address isn't a technical challenge, but a political challenge.

Re:Big ego department

[ve3id (601924)]

I have been saying this for years. We lose 10-20 % of energy charging a battery in a UPS with 117V, we lose another 20-30% in the inverter to get it back to 117V, and then we lose another 10% getting the 117V back to usable voltages for the PC.

It does not take an expert in electrical engineering, just common sense.

Can I sue google for stealing my idea?

Re:Big ego department

[Skapare (16644)]

There are different kinds of UPSes that do this in different ways. The two major types used for PCs are called "line interactive offline" and "dual conversion online". The first just passes the AC power straight through to the output. If the AC power coming in goes out of range, then it flips a switch internally (relay, contactor, thyristor, etc) to supply the power from an inverter driven by the battery. The second converts the AC coming in to DC all the time, and converts that DC back to AC for output. It then does the switching in DC, or parallels the DC with the battery directly. These variations are classified as "topology" by many manufacturers.

Both of these kinds can have inverters that produce square waves, pseudo-sine waves, or very nice clean sine waves. The dual conversion type can also isolate a poor power factor (the deviation of the current wavefrom from being a sine wave in sync with the voltage) of the PC power supply from the power source. A poor power factor means the product of the average current times the average voltage (apparent power) exceeds the actual real power (average of all the products of the voltage and current and each point in time) being used, which results in reduced efficiency and other problems.

• Uninterruptible power supply [wikipedia.org]
• Power factor [wikipedia.org]
• Switched-mode power supply [wikipedia.org]
• AC power [wikipedia.org]

here it is

[TheSHAD0W (258774)]

n/m found it.

http://services.google.com/blog_resources/PSU_whit e_paper.pdf [google.com]

What lower voltage power supply?

[argent (18001)]

They're not talking about reducing the voltage the PS uses, they're talking about not having the PS produce things like +5 and -5 as well as +12, INSIDE the computer.