PC Power Supplies: More Important than You Think

PC Power Supplies: More Important than You Think

Computers keep increasing their capabilities and their performance. These characteristics not only contribute to increases in their purchase costs, but also their costs of operation, particularly when it comes to power. Although AMD and Intel have curbed their high-flying ways - where CPUs with total power levels of up to 130 Watts were tamed using SpeedStep or Cool'n'Quiet - ATI/AMD and Nvidia's graphics cards continue to consume stratospheric amounts of wattage. As we reported in our German-language coverage Power-hungry Graphics Cards, power consumption levels at or over 200 Watts are not unusual. In dual-card configurations built around SLI or Crossfire technologies, graphics processing can add 500 watts or more to a system's total power consumption.

Such massive needs for power must be satisfied, and power supply manufacturers have reacted to meet them. At this year's Computex Taipei, numerous vendors introduced power supplies - also known as power supply units (PSUs) - rated as high as 2000 W. Gigabyte is one vendor that serves many global markets, and is perhaps best known for its motherboards and graphics cards. At that show, it introduced a new family of power supplies named Odin, after the one-eyed chief of the Norse pantheon, with capacities rated at 550, 680 and 800 Watts.

Power users and case modders alike have quested after the perfect PSU for some time now, driven as much by needs for high-end components as aesthetics and "bling". Thanks to the continuing debate on global climate change, this quest has begun to register for both OEM PC vendors and normal PC users as well. The following questions remain to be answered, however: "Are such monster power supplies really important?" and "Who really needs them, anyway?"

For years, power supplies have barely been discussed in the ongoing dialog on computing technologies. People tend to think first about motherboards, processors, hard disks and RAM. The PSU has traditionally been simply necessary but unappreciated component in a system build, usually included as an afterthought if not thrown in as part of a case purchase. These days, however, the PSU is an extremely important PC component, subject to the same kinds of strict specifications and requirements as a motherboard.

You can read about PSU specifications in a document entitled "ATX12V Power Supply Design Guide" at the formfactors.org Web site. Over time, this design guide has continued to track the latest developments in power conservation and delivery technology. This guide was most recently updated in March, 2005, as version 2.2. These latest specifications are comprehensive, in that they encompass not only the form factor and the dimensions for a typical 12-Volt power supply, but also state operating voltages and tolerances, and where cooling fans must be positioned to provide proper ventilation for such devices. These same specifications also require that the voltage sources that a PSU delivers be managed independently of one another. Such individual voltage sources are called rails in PSU-speak.

In certain technical dimensions, the ATX12V specifications are so thorough that the only decisions a power supply vendor has to make deal with which requirements a power supply actually meets. That said, the specifications shouldn't be interpreted as being an instrument for quality control. As always, the quality of a PSU depends on the engineering that the vendor puts into its design, and into the components that go into the device they build to implement that design.

Be Cautious About Fanless PSUs

The power consumption of a computer is as idiosyncratic as the user it serves. To meet all the various needs for energy consumption that a multitude of users presents, vendors typically offer PSUs rated at multiple different wattages. This is an area where vendors have a pretty free hand to anticipate and meet user demands. Most PSU models start at ratings of 300 Watts. For a long time, higher wattages came at increments of 50 watts to deliver higher levels of power. When ratings climb above 500 watts, though, the increments between models also tend to increase as well.

Other technical areas of freedom in PSU design permit the creation of actively and passively cooled models, whose selection and use depend on how a PC will be used. An actively cooled PSU includes at least one fan, and contributes to the ventilation of the entire computer case, as well as handling its own internal cooling needs. This situation plays a role in the evolution of the ATX standards, which requires the PSU to contribute to the overall ventilation of the PC's case. Older power supplies typically include fans that are no more than 80 mm in diameter; they're designed to suck warm air out of the case and blow it out of the back. Today, more and more PSUs include heftier 120 mm fans. Because of the increased diameter, more air gets moved at the same RPMs. Larger PSU fans can also run at lower rotational speeds to produce less noise. Most 120 mm fans are mounted at the bottom center of a PSU, whereas 80 mm models are mounted at the rear, which puts them right at the back of the PC case as well - a longer path from fan to outlet also means less audible noise from that outlet.

The Gigabyte Odin series uses quiet and efficient 120 mm fans.

Ironically, those who decide to use passively-cooled PSUs in an ATX case must usually install a system fan somewhere else in their PCs. Otherwise, they risk overheating key components to the point where they might be damaged or destroyed.

In some situations, noise output from a PC is a critical factor. For example, Media Center PCs or Home Theater PCs usually reside in a living or family room, where their primary function is entertainment. These kinds of PCs are often equipped with passively-cooled power supplies to keep noise levels to an absolute minimum. If these PCs also omit active cooling entirely, the remaining system components must be carefully chosen to produce less heat than typical desktop system components, to avoid potential overheating problems.

Selection Of PC Components

At some level, the components chosen for specific PCs follow from standard interfaces, which gradually evolve over time to accommodate ever-changing technologies. Thus, in February 2003, the primary power connector for PC motherboards was extended by 4 pins, from 20 to 24. This was necessary to provide sufficient power for PCIe graphics cards, which can draw as much as 75 W through the motherboard. In addition, more powerful PCIe graphics cards can draw still more power directly from the power supply through a secondary 6-pin cable. An everyday example occurs in high-end graphics cards such as the Nvidia 8800 or ATI/AMD 2900 series.

This 24-pin connector delivers power to modern PC motherboards.

This six-pin connector delivers additional power to PCIe cards.

These soldered pins on the right-hand side attach to leads for power delivered directly from the PSU.

Thanks to the proliferation of Serial ATA (SATA) hard disks, the number of Molex connectors in modern PSUs has been reduced. These connectors are normally designated as part number 0015244048, but are also identified in the ATX12V Power Design Guide as 8981-04P. These wide, four-pin Molex connectors do continue to still be used to deliver power to UltraATA hard disks and other drives as well (primarily, DVD and CD burners or players).

Power connectors for ATA devices such as CD players and hard disks.

A SATA power connector

The good old floppy drive power connector persists unchanged since the 1980s.

Modular Cables And Connectors

Another PSU trend is to improve cable management, in an attempt to undo the rat's nest of cables that so often unwinds inside PCs. Cheaper PSUs hard-wire the necessary cable bundles directly to their internal components. This leads to the unfortunate consequence that even unused cables must be accommodated inside the PC enclosure - in a worst case situation, this can interfere with air circulation.

For a few dollars more, you can purchase a power supply with a reduced cable bundle that services only the most important components. Additional cables may be plugged into modular outlets as needed. This not only improves air circulation, it also makes the PC's innards much tidier and easier on the eyes.

Modular cable management is available from plenty of other vendors, as well as in Gigabyte's Odin PSUs...

... including a little pocket to do away with unneeded cables.

The complete retail package and contents.

Where Power Supplies Fail

Sometimes the specifications are at odds with vendor technical preferences. This explains the situation typical in all cheap PSUs - whose production is more about quantity than quality - that consume more energy than is really necessary, or that fail when used in certain system configurations. On the other hand, high-quality PSUs tend to function flawlessly, in our experience. This also reflects the last endurance testing in which five out of nine models rated at up to 550 W failed before the 24-hour testing period expired.

In part, some vendors tend to exaggerate PSU power outputs. This leads to official ratings that are simply unreachable in actual use. Alas, this can lead to underpowered system components, and contribute to intermittent instability problems. In addition, the components inside a PSU consume energy themselves, which can lead to greater or lesser power consumption and heat production, depending on the quality of those components. As with motherboards, defective condensers in a power supply can bring an entire system crashing down.

Power Supply Rating Myths

In theory, the power that a power supply delivers can only be as great as the power the device itself consumes. In reality, this represents 100% efficiency, a performance level that power supplies can never attain. The transformation of 110 V or 220 V A/C power into various D/C voltage levels inside a PSU involves some waste, with the majority of such waste energy making itself felt in the form of heat produced inside the PC's case. This means that the rated wattage that a power supply can deliver must be strictly less than the energy it consumes before it starts the voltage transformation process.

By calculating the ratio between energy consumption and energy production, we produce a number somewhere between zero and one. Thus for example, net energy production of 450 W divided by gross consumption of 550 W at maximum load produces a value of 0.818. This number represents the efficiency of the power supply. Commonly this efficiency index is represented as a percentage value, which may be calculated by multiplying the previous ratio by 100, to produce in our example a value of 76.4%.

Vendor wattage ratings on PSUs always represent the maximum output that the device can deliver. A 350 W PSU with an efficiency rating of 70% must therefore consume a maximum of 500 W, though this occurs only when the components that the power supply drives actually consume the entire 350 watts. The real efficiency of a PSU is not a constant value either; rather, it changes with the amount of power that the device delivers at any given moment. The ATX12V Power Supply Design Guide requires that PSUs deliver minimum efficiency of 65% under light load, 72% under normal load, and 70% at peak load. There is also a recommended efficiency regimen that ups these levels to 75% for light loads, 80% for normal loads, and 77% at peak loads. Here, the term "load" must be understood as the power consumption of the system, as measured in amperes.

Why Are Efficiency Measures So Important?

Two years ago, we conducted a live stress test between an AMD and an Intel system, which measured the power consumption of both systems (among other things). At full or peak load, we measured average gross power consumption at 342 Watts on the Intel system, which included a dual core Intel Pentium Extreme Edition 840, a Gigabyte GA-8N-SLI Royal motherboard, OCZ DDR2 DIMMs, two GeForce 6800GT graphics cards in an SLI configuration, and two 160 GB 7,200 RPM Western Digital hard disks. Following the recommendations of the Power Supply Design Guide that the PSU be 77% efficient at peak load, the average power output from the device was around 263 Watts. One of the consequences of this test was that nearly 80 watts of energy was transformed into waste heat, adding significantly to operating costs for energy.

Let's restate this in plain terms: when it comes to paying for power, we have to cover the costs of gross consumption. In addition, we must also get rid of the waste heat that results from lower efficiency levels, which increases the need for cooling (itself an overhead energy consumer) and increases noise levels as fan speeds go up. If the waste heat isn't expelled from the PC case, this has a negative impact on PSU lifetime, because the lifetimes of its individual components sink as temperatures rise.

Now, let's look at our power supply selection from a different perspective. Power users are less interested in lower efficiency ratings at lower loads than they are in the PSU's ability to deliver sufficient power on demand. A 1,000 W PSU that is used to deliver only 200 W of actual power consumed works best to meet demand, though users must then pay for lower efficiency levels and rising costs of electricity.

With its Odin family of PSUs, Gigabyte seeks to deliver at least 80% efficiency over the entire load range. Over time, this means that when such a device is compared to the total cost of ownership for a cheaper power supply (purchase cost plus energy costs, in other words) it produces net savings. This helps to offset the higher costs of initial purchase, but only if the vendor ratings for the device are accurate. That's why we put the Odin PSUs to the test in our labs.

Higher efficiency across the whole load range promises cost savings.

Proper Sizing For Power Supplies

The total wattage that a power supply must deliver depends on the actual components that go into any given computer. If you start with the requirement that each PCIe x16 slot be allotted up to 75 watts, and then add two or four PCI Express graphics cards into the equation, it's easy to understand how such a system simply wouldn't work at full load with a 300 watt power supply. It's also not hard to conceive that high-end CPUs will always require more power than mainstream models need.

Because determining the effective power consumption of a PC build involves more heavy lifting (and math) than most people are willing to undertake, the process of selecting the proper power supply for a PC should always derive from analysis of the worst case scenario. Most components in a PC operate at 12 Volts, so we simplify matters by assuming that the whole computer is serviced only with a 12 V rail, then use this to analyze the amperage ratings for power supplies.

Starting with the 263 watts that our test rig consumed and taking our assumption that this is delivered over a single 12 V rail into account, this leads to power levels of about 22 Amps (263 W / 12 V = 21,916 mA). Because delivering the total power budget for the system on a single 12 V rail is just a theoretical construct, and doesn't really happen in practice, our assumption leads us to the conclusion that a PSU that can deliver 22 amps on the 12 V rail will indeed suffice to meet the needs of our test system.

Combined Power

Those who take the time to read the ratings on a power supply carefully will immediately notice that they include numerous different values. Among them, 12 volt values appear multiple times, where wattage ratings apply only to the 3.3 V and 5 V rails. Voltages are divided among multiple rails, so we can infer the power levels that these rails carry as a fraction of the total power levels for the device. Moreover, the values for the 3.3 V and 5 V rails are typically presented as a single value, known as combined power. Those who have followed the fortunes of combined power values on power supplies for any length of time can't help but notice the trend toward increasing use of 12 V power in computers. Whereas older 300 W PSUs typically claimed combined power values of 180 to 190 W, current 300 W models are more likely to claim 120 W.

Vital stats on a power supply label show amperage for each rail

Gibabyte Odin GT Series With Software Control

Indicators for current, voltage, and performance in the Gigabyte Odin GT.

The P-Tuner software also enables its users to manage fan behavior in the power supply. Users can choose from among three profiles: performance mode, normal mode, and whisper mode. They can also manage fan voltage in conjunction with temperature values from one of four optional temperature sensors included with the unit, according to their own customized settings.

On systems where graphics cards produce lots of heat, users can manage the Odin's fans to help compensate, using the P-Tuner software and its sensors to detect and react to that situation.

In addition, the P-Tuner software also permits users to set alarms based on device performance, voltage, current, fan speed and temperature.

When values exceed user-set thresholds, an alarm goes off.

Summary And Conclusions

The well-known market principle often stated as "faster, better, cheaper" also applies to power supplies, though added capacity often cancels out cost savings to deliver more functionality today for the same costs paid for less functionality yesterday, instead of driving absolute costs down. Small but potent PSUs can crank out up to two kilowatts of power nowadays. As a consequence, efficiency ratings for PSUs also continue to improve, so that we can use more of the juice we must pay for to actually get something done.

That's also why it's a good idea to draw up a general power budget for a PC build before purchasing a power supply, rather than relying on the sometimes misleading ratings that vendors assign to their PSUs. You can do this by adding up the total energy draw from each of your system's components. CPUs typically fall in a range of 35 to 130 watts, the motherboard from 25-50 watts sans RAM, drives usually fall between 15-20 Watts apiece, and graphics cards may require anywhere from 30 to 200 watts depending on the specific make and model in use. Add 30 percent to this total when you're finished just to be on the safe side. If you want to make room for future components or upgrades, bump this fudge factor even higher, but don't forget that power supplies tend to be somewhat less efficient as loads increase.

Heavy-duty power supplies get expensive pretty quickly, and in view of quad core CPUs that will impose widely different power draws depending on their energy saving regimes (a la SpeedStep and Cool'n'Quiet) switching individual cores on and off, we can only recommend extreme models when they are really necessary.

The big impetus for power supply makers going forward should not be to build ever-bigger and -beefier units, but rather to keep improving efficiency ratings. To be sure, power supplies rated at 600 watts and up have legitimate uses, but the total population of users who need that much power is miniscule comparison to the legions of average users. With a little knowledge and some considered calculations, savvy buyers can save money on both power supply purchase and operating costs.