How to: Choose the best Power Supply

Graph of two typical alternating currents.

One cycle consists of the voltage starting at zero, progressing to a peak positive voltage, falling back through zero again to a minimum negative voltage and then rising back to zero again. The number of cycles per an amount of time is called frequency. This frequency is measured in hertz (Hz) with 1Hz representing a single cycle per a second. Despite the fact that AC power presents negative, null and positive voltages, it will still appear to your power supply as a set positive voltage. This set voltage is called the VAC (Volts AC) which is obtained by finding its root mean square (RMS). For this article, it is not important to understand RMS other than to know that it has a relationship to the highest positive voltage and the fact that the voltage fluctuation occurs as a sine wave. Different countries have different utility frequencies and different VAC values. The two main utility frequencies are 50Hz and 60Hz. There is a far larger variety of VAC ratings around the world, though the two main values can be approximated to 115V and 230V. North America’s standard electricity is ~120V at 60Hz, while the European Union’s standard is 230V at 50Hz. Multiple standards can be confusing, but power supplies should be clearly marked as to what standard or standards they comply with.

Now that we understand AC power, it is important to know that your computer components do not run on AC power. Computer components run on direct current (DC), which is a constant flow of electrons through a circuit. This means that there is no utility frequency. Further still, your computer components run on much lower voltages than the power company is feeding into your house. The three main voltages which operate a computer are: 12V, 5V and 3.3V. The other voltages are -5V, 5sb and -12V.

The purpose of a computer’s power supply is to take the mains AC power from your wall socket and change it into DC power at the various different voltages that your computer components need to operate. The PC power supply is known as a switching mode power supply (SMPS), as opposed to a linear power supply. A linear power supply would take the AC power, transformer it directly to the necessary voltage and change it to DC power. Due to certain properties of electricity, these linear power supplies would simply be too large and inefficient. The amount of space required to transform voltages is inversely proportional to the frequency of the AC power. At 50Hz or 60Hz, the transformers would have to be very large. To overcome this obstacle, a SMPS takes the AC current, rectifies it to DC power and then uses devices known as switching transistors (from which the SMPS takes its name) to again convert this DC power into AC power, at a much higher frequency, hundreds or even thousands of times higher than the utility frequency from the wall. This high frequency current can be pushed through transformers to reduce the mains voltage to a lower computer compatible voltage. How all three voltages are obtained will be described later.

The guts of a PSU.

Everything up to this stage of the power supply is described as the primary stage. There are certain other circuits and portions of a power supply that I have omitted so far, some of which will not be included in all power supplies, or are not strictly necessary to the understanding of the very basics of a power supply’s function. From this point on the power supply’s secondary stage will be described. Once transformed into the necessary voltages, the power is then rectified back to DC power and filtered of most of the remnants of its AC frequency. There are several different designs for power supplies to create the various voltages needed by the different components in a computer. Rarely a power supply will provide outputs from the transformer for each voltage. Most power supplies however have an output for the 12V and an output for the 5V. The 3.3V can be obtained via a DC-DC converter from the 5V. On better designs, the 3.3V is obtained before the 5V is filtered and gets its own filtration. On lower end designs the 3.3V is obtained from the filtered 5V current and is not filtered any further. Some newer designs use DC-DC conversion to create both the 5V and 3.3V outputs from the 12V output. A power supply designed to give the three main voltages their own filtration is known as independently regulated. If any of these voltages have to share the filtration stage, the design is known as group regulated.

Specifications:

It is important to note that Intel has been kind enough to draw up specifications for all important aspects of a PC power supply. The main set of specifications for personal computers is known as ATX, although it is possible for a power supply to comply with ATX specifications and various other sets of specifications. These specifications outline such non-performance aspects as: PSU housing size, connector design and pin out, and AC power input compatibility. Important performance aspects that the specifications dictate are: efficiency, how far from nominal voltage outputs can waver, and maximum remnants of AC frequency, known as ripple. Two of the most important performance aspects are voltage regulation and ripple.

ATX specifications allow the three main voltages to deviate 5% over or under their nominal voltages. This means that the 12V output is allowed to be from 11.4V to 12.6V, the 5V output is allowed to be from 4.75V to 5.25V and the 3.3V output is allowed to be from 3.135V to 3.45V. Voltage regulation, however, means how steady a power supply can keep voltage outputs over a range of loads. While staying within the +/-5% deviation from nominal voltage, it is possible for a power supply to have poor voltage regulation. For example, if the 12V output starts out at 12.4V and drops to 11.7V, the voltage regulation is 5.8% (12.4-11.7 = 0.7; 0.7/12 = 0.058). While the voltage never deviates outside of the specification, any single voltage delta greater than 5% is unacceptable.

Ripple is the remnants of an alternating current frequency in the DC output of the power supply. Ripple involves small changes in voltage, but is different than voltage regulation due to the high frequency at which these changes occur. The power supply uses capacitors and inductor coils to filter out the majority of ripple. ATX specification allows for a maximum of 120 milliVolts(mV) of ripple on the 12V output and 50mV of ripple on the 3.3V and 5V rails.

Efficiency:

High efficiency is essential but certification doesn’t make a PSU great.

The efficiency of a power supply describes how much DC power it outputs in relation to the AC power it pulls, determined by dividing DC output by AC input. For example, a power supply which is putting out 750W of DC power and pulling 904W AC from the wall is about 83% efficient. Efficiency will change at various power loads, and should be close to a bell curve with efficiency being lower at low loads, highest close to 50% of the power supply’s rated output and lower again towards the upper end of the power supply’s rated output. A power supply that is efficient will not only pull less power from your wall, but more importantly it will run cooler. Power that is pulled from the wall and does not make it to your components is mostly lost as heat and heat will reduce the life of your power supply and further reduce its efficiency. Note, that a power supply will not output more than the components are pulling from it. Without proper testing equipment, it is not possible to test these aspects of a power supply. Software is never an accurate way to test a power supply, and the hardware required to do proper tests is very expensive. Therefore, you will need to rely on review sites to help find the best performing power supplies. Remember, software is not a reliable method, so look for review sites that use equipment capable of testing power supplies at or near their rated output and use the proper hardware to measure the output characteristics of the power supply. The efficiency of many power supplies can be seen at 80plus.org. Initially only distinguishing between PSU’s above and below 80% efficiency, the newer qualifications of 80plus Bronze, Silver, and Gold further differenciate between high efficiency units. Since manufacturers have to pay for 80plus to certify their power supplies, not all PSUs that meet the requirements for 80plus certification will be listed here. However, if a company cannot afford to pay for such a publicly visible certification as 80plus, what other cost cutting moves might they be taking? Note that 80plus certification is not an absolute testification to the quality of units.

Manufacturer inconsistencies:

With an understanding of how a power supply works and how well regulated their outputs should be, it is important to know that not all power supplies are created equally. Some manufacturers blatantly ignore ATX specifications, and even with those manufactures who follow all specifications, quality can run a wide gamut. Component selection and overall design have the largest impact on quality. Choosing the right capacitors, transistors and even heat sink and fan design can have a huge impact on how well a PSU performs. Manufacturers have to strike a balance between unit cost and unit performance. Knowing there is such disparity in power supply quality is key to understanding one of the most important lessons in this article; wattage rating cannot be the only factor in choosing a power supply.

There are many reasons why one cannot rely solely on wattage rating when choosing a power supply. A power supply’s output will change as its components heat up. Different manufacturers rate their power supplies at different temperatures. Some manufactures will rate at an approximately normal room temperature of 25C. Other manufacturers will rate their power supplies at a normal PC operating temperature of 40C-50C. As heat increases the output capability of a power supply will decrease; precisely how much the output capability decreases per a certain rise in temperature is called a derating curve. Each power supply design can have its own derating curve. For example, if a power supply is rated for 750W at 25C and has a derating curve of 10W lost for every 1C increase in temperature, at a normal operating temperature of 45C, the PSU is only capable of outputting 550W. Furthermore, some manufactures will advertise their power supplies at peak power output. Peak power is usually only sustainable for a short period of time. Maximum continuous power is a more realistic rating of a power supply’s capability. Perhaps the most important factor to understand in power supply rating is the breakdown of the separate voltage outputs. Newer computers utilize more power from the 12V output rail than any other. Modern CPUs, graphics cards and the movement motors of hard drives and optical drives all use 12V power. It is important to choose a power supply with a majority of its output capabilities on the 12V rail.

What to look for:

Now that power supply function and rating have been discussed, it is time to detail what one should look for when buying a PC power supply. First make sure the power supply you are buying accepts your AC input. Rarely a power supply will be rated only for a certain AC input, but if you are buying from a store or site specifically for your region, this should not be a problem. This information should be located on the PSU’s output label along with the outputs for the various voltage rails. Remember to look for a PSU with a majority of its power on the 12V rail; 80% of the units rated output or higher should be a starting point. When you are reading a label, the various outputs may be listed in amperes. Power (measured in watts) is equal to electrical potential (measured in volts) times current (measured in amps), so to obtain a wattage rating for a particular rail, multiply its voltage by its current rating. Often the 12V rating will be broken up into multiple ratings. Rarely can these ratings be added to find the combined rating for 12V power. Instead a proper PSU label will show the correct combined rating as watts or amps, either in the table or in a sentence below the table. The splitting of 12V ratings will be described later in this article.

The PSU label contains a plethora of information (click the image for item descriptions).

Before shopping for a power supply, you should have at least a general idea of what PC components you want to run with it, especially what CPU and graphics card(s). A multiple CPU or graphics card setup will have the biggest impact on what power supply you choose. Overclocking your components can also greatly increase their power draw and should thus be factored into your power supply buying decision. Other components to consider when buying a PSU are: number of drives, number of fans and whether you plan to use water cooling with a pump or multiple pumps. Other components that can pull massive amounts of 12V power if you choose to use them are thermo-electric coolers.

Beyond power draw, you should also choose a power supply which has enough of the correct connectors for your components. Look for enough PCI-express connectors to power your graphics card or cards, also make sure they are the correct type(s) of PCI-express connectors, whether 6 pin or 8 pin (6+2 pin connectors can serve as either 6 pin or 8 pin). Check the motherboard you plan to use and see whether it takes a 4 pin or 8 pin CPU connector and a 20 pin or 24 pin ATX connector. Most PSUs will come with an ATX 20+4 pin connector that can be used as either a 20 pin or 24 pin connector, and will come with a 4 pin CPU connector and/or an 4+4 pin connector which can be used as either an 8 pin or a 4 pin connector. If you end up with a PSU that has only a 20 pin ATX connector or a 4 pin CPU connector, it may possible to use these even in motherboards that accept 8 pin CPU and 24 pin ATX connectors. Doing this can possibly limit some of your motherboard’s functionality. Be sure to check your motherboard’s documentation. PSUs with only a 20 pin ATX connector are also based off of an older ATX design and may not have enough 12V power for a modern gaming computer. Also look for enough SATA power connectors to run your SATA drives, or 4 pin peripheral connectors to run your IDE drives and fans. If you plan to run a floppy drive make sure your PSU will have a floppy connector. It is often possible to use adapters to create connectors that did not come on your PSU, but this can cause unwanted clutter in your case. One component that can be picked before or after the PSU is the case. Make sure your power supply has cables long enough to reach all the components in your case. In a case with a PSU at the bottom the CPU connector may not stretch and in a case with the PSU at the top, the PCI-E connectors may not stretch to lower video cards. Also make sure your PSU will fit in the case. Although Intel provided specifications for housing dimensions, it is not always possible for manufacturers to fit all components in those dimensions.

With copious varieties of components on the market, it would be impossible to give a breakdown of power supply outputs required. There are various resources on the web that allow one to plug in the various components they plan to use to find a suggested power supply output range. Many of these calculators are incomplete (for example lacking 12V power requirements), or give gross exaggerations of the power a configuration will require. Often you will be able to find the individual power usage for your various components on the internet. When choosing a power supply, remember that it is best to operate below 65%-70% of your power supply’s rated output and that highest efficiency will fall around 50% of a power supply’s rated output. Remember efficiency is one factor in keeping a PSU cool, a bigger factor however is the fan or fans. Power supply fan configurations generally come in two flavors, a small high RPM fan or a large low RPM fan. Sometimes there will be several small fans or a small fan to supplement the large fan. If high quality fans and the proper temperature dependent fan controls are used, either configuration should give sufficient cooling. Smaller high RPM fans are generally louder, but again if the right fan and temperature control configuration is used, these setups will be no louder than some large fan setups.

Don’t get tangled up:

Another factor to consider when selecting a power supply is cabling. With so many connectors needed for a modern high power computer, the various wires can make for an awful mess and an eyesore in your case. Poor wiring can potentially affect airflow in your case or even lead to exposed wires due to chaffing. Textile sleaving on cables can make your computer builds more visually appealing and improve airflow in your case. Modular cables can help you eliminate unused cables all together. Some modular power supplies are fully modular, while others leave some more essential cables hardwired into the circuit board. If modularity is something that interests you, be sure to research which wires are modular and which are not on the power supplies you are considering.

Power correction:

Another possible factor in choosing a power supply is power factor correction (PFC). Power factor is the difference between how much AC power your system is using and how much power the AC mains system thinks your power supply is using. Due to the nature of AC power and how your power supply uses this power, it introduces unusual waves back into the AC line. This will not increase the bill for a home user, but for a large company with many appliances affecting power factor, the electric company may charge extra since they are “dirtying” the grid power. The European Union has enacted a law requiring all power supplies to contain PFC. There are two types of PFC, active and passive. Passive PFC uses a capacitor to filter the current while active PFC uses circuitry to correct power factor. APFC circuitry costs the manufacturer money, and again if costs are cut here, there may be other cost cutting measures utilized in the power supply; however, the lack of APFC cannot speak for the quality of a power supply. A PSU that lacks APFC will usually have a red switch on the back of the power supply facing outside of the computer. It is necessary for the user to choose the correct voltage setting for their power grid (115V or 230V). If you are on a 230V grid and set the switch to 115V, prepare for fireworks, because setting the switch to 115V sends the power through a voltage doubler. For the most part, PFC has no discernable affect for the end user.

Finally you should consider the company you are buying from. What is their product warranty? How long is it? What does it cover? What is the company’s reputation for customer service? How easily will it be for you to return your power supply for warranty service? How long will you be without a power supply if you have to send it in for warranty service and what will it cost you to send in the unit?

Understand the marketing hype:

Armed with new knowledge, you will easily narrow down your options.

Now that the characteristics to look for in a power supply have been discussed, it is important to point out some aspects which some marketers claim you should be concerned about. Remember companies are out to sell their products and will often exaggerate the benefits of certain features that their products have, while implying that those lacking these features will somehow be less desirable.

One claim is that modular cables will detrimentally affect your power supply’s performance due to added resistance. A modular interface adds no more resistance than the interface between the power supply’s connectors and your components power slots. In fact a properly designed modular interface will add less resistance than the length of the power supply’s cables.

Another claim is that multiple 12V outputs will starve certain components while trapping available power on outputs dedicated to components that do not need as much power. This can certainly be true for poorly designed set ups. Fortunately most manufacturers have learned from the mistakes of those who failed with multiple 12V outputs, namely those who are now marketing against multiple 12V outputs. Simply put, the reason for having separate 12V outputs (or rails) is safety. If a single 12V wire is allowed to pull too much current (due to a short circuit for example) then the heat generated can burn through the wire’s insulation and possibly cause a fire, and certainly catastrophic consequences for your power supply. At one point Intel’s ATX specification even mandated splitting the 12V outputs beyond a certain current draw. A properly designed 12V multi-rail design will provide ample power for components allocated to the various 12V rails. Generally most multi-rail power supplies worth considering will be designed with plenty of power available to all components. This is another area where a proper review site can help a prospective buyer determine the quality of a PSU. Most multi-rail PSUs utilize a single 12V rail and use a form of circuitry called over current protection (OCP) to separate that single rail in to multiple virtual rails. OCP limits are usually set higher than the overall 12V capability divided by the number of rails. This is done to prevent power from being trapped away from the components that need it. Often the OCP is even set higher than that stated on the power supply’s output label. Once an OCP limit is reached, the PSU will simply shut down, meaning instability problems cannot be blamed on OCP limits. The one area where a single rail setup may be needed is when many power sucking thermo-electric coolers are used. Another misconception floating around on forums is that a higher 12V rail voltage will allow higher overclocks. Some manufacturers even provide user adjustable potentiometers for adjusting voltages (often at the expense of the warranty). Since the 12V power goes through a voltage regulator on the motherboard before it reaches the processor, higher 12V outputs will not facilitate overclocking.

Conclusion:

This is a lot of information to digest, and truly the only way to know if a particular power supply is a quality unit is to find reviews done by competent reviewers. Posters in certain forums can also help lead you in the correct general direction, but you should always be wary when dealing with either forum posters or user generated reviews on retail websites. Often you will come across opinions that are uninformed or posters who have ulterior motives. Since components and their power draw change so rapidly, and such a disparity exists in quality amongst various power supplies, choosing the right power supply is not as simple as picking by wattage ratings. Ideally you will understand some of this article and at least have enough information to know what to look for in a PSU and narrow down your search. If you are interested in building a PC, have a look at our build guides for the $1000 and $500 budgets.