PC Power Supplies: More Important than You Think
Marcel Binder Tomshardware.com
August 9, 2007 12:04
Sensible Or Stupid? High-End PSU Buyers
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?"
Gigabyte answers these questions with its Odin PSUs
From Necessary Evil To High-Tech Product
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 (by our German staff
from February 2007) 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 (for more details, see our article How to Fix Your Motherboard for $15).
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
Functional Differences Among PSUs
By continuing to incorporate better-quality components and optimizing their designs, it's becoming hard to
distinguish among power supply manufacturers on the basis of quality. These days, the overriding goal among
vendors is to maximize efficiency, be it for 300 or 800 W units, even though it's understood that 100%
efficiency remains unattainable, especially given today's technologies.
The number of players in the power supply game continues to shrink, so those vendors who remain in the game
must continue to innovate and add new features to their products so as to stand out in the field. Gigabyte has
added such a feature to its Odin GT series: these power supplies include a USB connection, in addition to their
more conventional power cables and connectors. Once a user installs Gigabyte's P-Tuner software on a PC
(which is included with the PSU), he or she can employ the USB connection to monitor the inner workings of
the power supply, including peak power consumption levels and current wattage input and output values. This
display also includes values for the 3.3 V, 5 V, and 12 V rails, and flashes an alarm if any of these slip outside
required tolerance ranges.
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.
Tomshardware.com