Glitchy graphics? Random crashes? Shoes shows us why Power Supplies are the most important part of any computer, and how to shop for them.

If underpowered, the components of your system will stress themselves to the point of overheating and often times failure. If powered unstably, the overall stability of your system is drastically reduced, as is the life expectancy of the components themselves. For these reasons, it is my opinion that the Power Supply Unit (PSU) is the single most important part of any computer. Choosing an adequate power supply for their computers is a task that many gaming enthusiasts are not prepared to undertake. This is certainly the result of fallacious product descriptions, and incorrectly placed emphasis on certain specifications. All power supplies have different output levels and different efficiency, but one thing they all have in common is a similar (for the most part) architecture. Although not as important as choosing an appropriate model for your computer, understanding how a power supply supplies power is valuable knowledge for anyone getting ready to put together or upgrade their own custom system.

The Flow:
It all starts at the power plant where potential energy is converted into kinetic energy and exploited through various processes to create an electric potential energy in the form of current. These currents undergo a series of transformations and are sent out in an Alternating Current (AC) through power lines and eventually end up in your humble home, where they are eagerly waiting for you to flip a switch or plug something in. From here it flows through a surge protector (a normal wire that is regulated by MOVs to transfer excess voltages away from whatever you have plugged in), and, ideally, an Uninterruptible Power Supply that will provide constant and conditioned power to your computer even in the event of a brownout. From here it enters your computer, and things start to get complicated.

PC Power Supplies are of the switched-mode type as opposed to the less efficient, less compact, and less quiet linear type. They operate as follows:


Diodes are small semi-conductors that let current flow in only one direction.

Input Rectifier: When the current enters the power supply, it is immediately converted from AC into Direct Current (DC). This is achieved through the use of a circuit with Diodes, semiconductors that conduct current in only one direction. Depending on the manufacturer and relative age of the power supply model, the current might also be smoothed using large capacitors.


MOSFETS switch and regulate current with electrode-gates that inhibit current in patterns.

Frequency Inverter: The current then is converted back into AC at a much higher frequency than the standard 50 or 60 Hz that is present in your wall sockets. This is done with a collection of Metal-Oxide Semiconductor Field Effect Transistors (MOSFETs). MOSFETs are essentially “electrode-gates”, high power transistors that are ideal for switching a lot of current. The inverter MOSFETs basically pulse out the current at a high wave frequency by turning it off and on from 0 to full in a pattern that is recognized by the rest of the PSU as AC.

 


Transformers manipulate the voltage levels and produce the different rails of a power supply.

Voltage Conversion: Once the current has been manipulated into a high (20+ kHz) frequency, it is ready to be passed through a transformer, which vary in size depending on the input frequency (the higher the frequency, the smaller the transformer). A transformer works by manipulating the magnetic flux that flows through a ferro-magnetic core wrapped with a wire with the input voltage. Secondary coils tap the transformer at the desired voltages. In a computer PSU, these voltage lines (rails) are 5V and 12V (3.3V rails are usually regulated from the 5V rail, though some PSUs have their own 3.3V tap on the transformer).


Capacitors also allow a power supply to provide a short amount of power to your system so it can turn off in the event of a brownout.

Output Regulation: These rough 3.3V, 5V, and 12V rails are then rectified and regulated by another set of MOSFETs that bring them down to their actual specified voltages in the DC current. When the current leaves the MOSFETs, it is in a pulsed form, and is therefore sent through large inductors (often referred to as chokes) and capacitors, which build up charge on one plate before allowing it to discharge to the other plates. This process greatly reduces any ripple in the current by both smoothing the transition from no power to full power through the electromagnetic properties of an inductor, and supplementing each wave in the voltage with charge stored on the positive plate of the capacitor.


Heatsinks with large surface area dissipate energy lost as heat.

Cooling: None of the pieces in a power supply are totally efficient, not even the MOSFETs, and for this reason there are two large heat sinks present in most powers supplies. These heat sinks help transfer the wattage lost as heat from the components out of the power supply in an effort to prevent overheating.

 

So there it is. Now you are ready to learn what the numbers on a PSU’s label mean and how to use those numbers in determining which power supply is ideal for your particular system.
{mospagebreak heading=Intro, Components&title=Math}
As stated before, the difficulty in finding an adequate power supply for your system is a direct result of the emphasis placed on often superficial numbers that can be manipulated in several ways by manufacturers.

The most commonly used number when advertising power supplies is the Maximum Power output. Sadly, most consumers in the market for a power supply base their purchases off this number alone. There are so many problems with doing this that I am not even going to attempt to name them all, but I will point out the most prevalent ones.

First of all, a quick physics lesson:
The majority of numbers you see on a power supply label are followed by one of three letters; W, V, or A.

W = Watts. A watt is a measurement of power.
V = Voltage. Voltage is a measurement of the difference in Electric potential energy between two points in a wire (electromotive force).
A = Amperes. An amp is a measurement of current.

To relate them all metaphorically: voltage is the amount of water pressure in a hose, amperes is the rate of flow in the hose, and watts is the amount of work being done on whatever you’re spraying it at, per unit of time. They can be expressed in this equation (which is always true for DC currents):

V x A = W

Knowing this, you would think that due to the fixed voltage rails, strong currents on each rail will always mean a higher power rating and therefore a good product. This is true only to a certain extent. In the past, CPUs, video cards, hard drives, and other devices relied primarily on the 5V rail to deliver the high current they needed. This is why on older power supplies you see high amperage on the 5V rail, often times as much as 50A. However, modern PCs have swapped out the 5V rail in favor of the 12 and 3.3V rails to supply power to the high current components of a computer. Therefore, if you are using or purchasing an older power supply you will see a high power output, but not necessarily in the area you need it.

To illustrate the next reason why Maximum Power ratings are misleading, I have taken a picture of the label on my power supply to use as an example.


Here you can see 18A, 19A, and 16A on the 3.3V, 5V, and 12V rails respectively

Now do the math by plugging those numbers into the above equation, and you will see that the “Total output wattage” value of 250W is not the 346W that you get by using the equation. This is the result of “cross regulation”, where the combined power output of the 3.3V and 5V rails varies with the output of the 12V rail. Basically it means that each rail draws a different amount of power from the total output at different times. So when your video card and CPU are getting stressed when running a benchmark like 3DMark05, the 12V rail is drawing a lot more power, which means that the 3.3V and 5V rails are using less current than they would. Also note that although my power supply is rated at 250W, it is running my “recommended 300W minimum” ATi X800XT just fine, even when the card is overclocked.

A third and probably the most important reason is that different manufacturers make different quality products. You cannot tell whether or not a power supply is a good one just by looking at it, regardless of what it says on the label. One popular method of determining the quality of a power supply is its weight, which actually has virtually no correlation with performance. In a power supply the majority of the weight is coming from the transformer and the inductors, which physically are just long peices of wire wrapped in a coil. The manufacturer of the power supply chooses which components to use, and as such the quality of your power supply is very much at the discrimination of the company you buy it from. Suppose a manufacturer chooses to use a MOSFET that costs slightly less than a more expensive, yet smaller, lighter, and more efficient one. Not only does this mean the MOSFET weighs more, but it is also producing more heat (less efficient), which means that the heat sink attached to it is going to have to be bigger and bulkier to transfer the excess wattage lost as heat. To illustrate this point, take these two 550W power supplies:

1. Powmax PSAG550A
2. Fortron FSP550-60PLN

If you think for one second that I would put that POWMAX 550W in my system, you are dreaming. Although it costs more than 5 times as much, the Fortron supply is not only from a reputable manufacturer, but also offers dual +12V rails, the advantage of which will be discussed later. While the POWMAX might last you a month or two before frying, you could get several years of use out of the Fortron, and probably not damage any of your other components in the process. This is no attack on the POWMAX company, they make some fine higher-end models, but the quoted one is not up to this standard.

Furthermore, most of the testing done by power supply manufacturers is at low temperatures, often times as low as 25° C, this lowers resistance, which increases with temperature, and produces inflated performance ratings. Not only does such a power supply label mislead consumers about its actual performance, chances are that it will also fail with increased operating temperatures. Another way manufacturers deceive customers through labels is the VAC Tolerances. Poor power quality is a symptom plaguing many circuits these days. Power ran through too many electric devices starts coming in harsh bursts, resulting in large instances of instability. As I sit here typing this, the lights in my room dim momentarily as the air conditioning unit in my house switches itself on. Unfortunately we have relatively little control over power quality, even subtle differences caused by electromagnetic interference can wreak havoc on the integrity of the power available at your wall socket. Manufacturers realize this and often rate their power supplies within certain tolerances. We also see a trend here among reputable manufacturers as opposed to the lower quality ones. Supplies from companies such as PC P&C can operate through much higher range of voltages and frequencies than certain other companies. It all has to do with the input regulation stage explained above, but we can combat these power fluctuations using a combination of surge protectors and Uninterruptible Power Supplies.

One last thing I would like to mention before moving on is what is called the Power Factor and Power Factor Correction. PF and PFC are figures you may or may not see when shopping for a power supply. A power factor is basically the ratio of how much power a device is actually using to how much power the device is supposed to be using. This ratio varies on a scale from 0 to 1.0, 1.0 being the best. Most power supplies have PF ratings of around .6 to .7. Adding “passive” PFC, usually either a capacitor (for inductive loads) or a an inductor (for capacitive loads), can increase a supply’s power factor by roughly one and a half tenths. Adding an “active” PFC, such as a secondary switching circuit to condition the input power, has the potential to raise a power factor to almost perfect. This does not mean anything in terms of efficiency however, because the power is still being used and wasted by something in one way or another.
{mospagebreak title=Terminology}
So now that we know how a power supply works and how manufacturers try to deceive us, it is time to look at what qualities are desirable when selecting a power supply for your new gaming systems.

Standards: There have always been “standards” in the power supply industry. Early power supplies were on the AT standard. Then came the ATX v1.0-2.0 standards, followed by the current ATX 12V v2.01. The standard versions are usually regulated by Intel and other contributors. Standards are either really important or not that important at all depending on how you look at them. They lay out some basic defaults like fluctuation allowances and amperage on certain rails, as well as interference isolation and minimum efficiency ratings under maximum load. More importantly, they adjust power connector sizes to allow for technologies and support for motherboards. Technically any ATX power supply is backwards and forwards compatible with any other ATX power supply, but it is highly advisable to stick with the latest standard, as it is the best suited for current systems and you will not have to use any adapters.

The most important development in power supplies as far as future technology is concerned is the division of the 12V rail. ATX 12V v2.0 saw that introduction of dual 12V rails, which was a development meant to benefit the new SLi technology from nVidia. Any and every SLi certified power supply that you see will be on the v2.0 or later standard, not that you need an SLi certified power supply to run an SLi system. Power supplies compliant with the ATX 12V v2.0 standard have two 12V taps on the main transformer, which means two 12V rails. The main advantage of the dual 12V rail as I see it is to isolate mechanical interference in the rail. This means that you have one rail dedicated for the most part to mechanical devices such as hard drives, case fans, lights, optical drives, compressors, pumps, and other things that “make noise” when they are operating or starting up. This leaves the other rail for purely electronic devices such as a video card, motherboard, and CPU, which are much more susceptible to damage by fluctuations and interference than are other components.

You don’t “need” to have dual 12V rails. In fact, if you are running a system with just one video card, and no huge RAID arrays or a lot case fans and fancy lights, chances are you will see no benefit at all from having dual rails. However, in very noise ridden systems it has proven invaluable. For more on these standards, read up.

The 12V Rail: I mentioned earlier that manufacturers are now relying, almost exclusively, on the 12V rail to provide the high current components of a PC with power. In modern systems, the 12V rail powers all of the most important components. It does not power everything however, 3.3V and 5V rails are not just factors to be disregarded when considering a power supply, they are just of much less importance than the 12V rail. Many of the high-end power supplies featuring just one 12V rail boast tremendous currents, as much as 36A. This is more than enough for any configuration you can throw at it, provided the other rails are also up to scratch. Numbers you should look for on the 12V rail when purchasing a power supply are: >24A on one rail, >16A on each rail in a dual rail setup. These numbers are not necessarily required, but they will likely be adequate for your system now, and allow for at least a little bit of upgradability in the future.

Voltage Stability: There are loads of programs and utilities on the internet that allow you to monitor the voltages of each rail in your power supply. The problem with these utilities is that they are woefully inaccurate. Even the BIOS reports faulty readings most of the time. The only way to truly know the stability of the rails of your power supply is to use a multimeter and test each rail at startup, idle, and load. You should be looking to stay within the tolerance of plus or minus %5 on each rail. You should not test these voltages by yourself without knowing exactly what you are doing. The specifics of this test will be covered in a later article.

In the meantime, you are probably reading this article because you want to buy a power supply, not have one that you want to test. The best way to determine whether or not you will have stable voltages before you buy is by buying from a good manufacturer. It really is as simple as that.

Maximum Power Output: I know I thoroughly trashed this specification earlier, but the fact remains that it is still important. The best way to approach this number is by using a wattage calculator, such as this one. You choose all of your components and at the bottom there will be a nice figure for what the minimum wattage is for your system. This should only be used as a rough guideline for the selection of your power supply. A good thing to do with this number is multiply it by 1.25. This will adjust for any errors that you might have made in configuring the calculator. Then, realize that the produced number is a minimum. Getting an 800W power supply when your system only “needs” 250W is not going to hurt your components. Further, it is not going to make your computer use any more power. It is definately in your best interests to invest in a power supply that is significantly well over the figure produced by that calculator. This will allow you to overclock your current system if you choose to, as well as upgrade it in the future without having to update your power supply.

Value: I will recommend power supplies that are the absolute best. Doing this would mean you are paying a premium for the ultra-high quality. However, once you are at a certain level of quality, it is very difficult to improve upon it, and a lot of different manufacturers fall into this category. This allows for competition across the board, and lowers prices. Looking for the best quality power supply for the lowest price will provide you with the best value, which for most budget-conscience gamers is the way to go.

Noise: Power supplies can be the noisiest component of your system. They are getting better these days with lower speed fans in better configurations and so on and so forth. There are also completely “silent” PSUs out there that are either fanless or fanless up until a certain temperature. This is purely preference, and if sound is not an issue for you then there is no need in even considering how loud your power supply is. Although if it is loud all the time, chances are the components are running at low effeciency producing a lot of heat and keeping the fan at full speed, one sign of a low quality supply.
{mospagebreak title=Recommendations (OLD!!!!!!)}
My Recommendations: First of all let me make a short list of the manufacturers that most PSU specialists recommend:
Antec
OCZ
Fortron (FSP Group)
Zippy
Enermax
Seasonic
Raidmax
Vantec
Silverstone
Thermaltake
PC Power & Cooling

As far as specific power supplies:

Entry
-Fortron AX400-PN $39.99
18A and 16A on dual +12V rails. 40 Dollars. Need I say more? For any gamer on a budget, this is the way to go.
-Antec SmartPower 2.0 SP-500 $68.99
Strong dual +12V rails and wide availability make this power supply very nice, and for $70 it cannot be beat.
-Antec TRUEPOWERII TPII-430 $69.99
My local PC shop uses these religiously, and for good reason. An excellent supply for even a middle range system, the TruePower series from Antec is a very solid choice.
-ENERMAX EG425P-VE SFMA 2.0 $69.99
A nice looking PSU with plenty of ability to power basic systems

Mid-Range
-Fortron AX450-PN $49.99
The 450W version of the aforementioned Fortron power supply. This one has greatly improved amperage on the +3.3V and +5V rails, making it more suitable for older (AGP based) computers. It also adds an extra 2A to the 2nd +12V, fully capable of powering your gaming machine, and for only $50.
-ENERMAX EG651P-VE FM(24P) $108.99
The only thing I can even imagine being able to stress this power supply is a high-end system running heavily overclocked, or a SLI system with 2 7800GTX’s or something insane like that. 36A on the +12V, come on. For people looking at this type of power supply (one strong +12V), also consider the OCZ PowerStream OCZ520ADJSLI.
-Antec TruePower 2.0 TP2-550 $99.99
A step up in the excellent TruePower 2.0 series.

High-End
-PC Power & Cooling 510 SLI-PFC $219.99
Absolutely the best quality power supply available. SLi certified for your insane gaming setups, and top-notch customer support in the unlikely event of a problem.
-ENERMAX EG851AX-VH(W) $235.99
Enermax’s top of the line. 3 +12V rails, 2 with 18A, and one left over for anything else after SLI. This power supply will do well with any system.

Ultra High-End
-PC Power & Cooling 850 SSI SLI $499.99
When money is not an object, this power supply is simply the best. Very expensive, very powerful, and most people that have used/reviewed it say it is worth every penny.

( SEVERAL RECOMMENDATIONS ADDED AND PRICES UPDATED BY SHOES ON 1/14/2006)

CREDITS:
Although all of the thoughts and opinions expressed in this article are genuinely mine, I would be remiss to even hint that I put together this article unassisted. Complementary information was gathered from

Powering Your PC: A Guide to Power Supplies (May 18, 2005) Jason Rabel. ExtremeOverclocking.com
Switched-mode power supply (August 29, 2005) Wikipedia.org

Thanks to David Hammock, A. John Mallinckrodt, Christian “ZeGermans” Koebel, and The PC Guys in Corona, CA for helping, in one way or another, with this article.

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