26.1 Power Supply Characteristics

Here are the important characteristics of power supplies:

Form factor

As with cases, the primary characteristic of a power supply is its form factor, which specifies dimensions and mounting hole locations, which in turn determine which case form factor(s) the power supply fits. Form factor also specifies the type of motherboard power connectors the power supply provides, which in turn determines the type(s) of motherboards the power supply supports. Table 26-1 lists compatibility of power supplies with cases.

Table 26-1. Power supply compatibility with case form factors

	Accepts these power supplies
Case form factor	D/AT	T/AT	D/BAT	T/BAT	LPX	ATX	SFX	NLX	WTX
Desktop/AT (D/AT)
Tower/AT (T/AT)
Desktop/BAT (D/BAT)
Tower/BAT (T/BAT)
LPX
ATX
Mini-ATX
microATX
FlexATX
NLX
WTX

AT-variant case/power supply issues are confusing because of the lack of standards. For example, many current BAT desktop cases use Tower/BAT power supplies instead of Desktop/BAT power supplies! See the preceding chapter for details on AT-variant cases.

Rated wattage

The nominal wattage that the power supply can deliver. Nominal wattage is a composite figure, determined by multiplying the amperages available at each of the several voltages supplied by a PC power supply by those voltages. Nominal wattage is mainly useful for general comparison of power supplies. What really matters is the individual wattages available at different voltages, and those vary significantly between nominally similar power supplies, as detailed later in this chapter.

Nominal voltage

Nearly all PC power supplies can use either 110/115V or 220/230V nominal. Some detect input voltage and adjust themselves automatically. Many, however, must be set manually for 110V or 220V, usually via a red sliding switch on the rear panel. Be very careful if your power supply is not autosensing. If the switch is set for 220V and you connect it to a 110V receptacle, no damage is done, although the system will not boot. But if the power supply is set for 110V and you connect it to a 220V receptacle, catastrophic damage to your motherboard and other system components is likely to occur.

Operating voltage range

The highest and lowest AC voltages that the power supply can accept while continuing to supply DC output voltages and currents within specifications. Typical high-quality power supplies function properly if the input voltage is within about ±20% of the center of the range, i.e., 90V to 135V when set for nominal 110/115V input, and 180V to 270V when set for 220/230V nominal. Less expensive, but still name-brand, power supplies may have a range of only about ±10%, i.e., 100V to 125V when set for 110/115V nominal, or 200V to 250V when set for 220/230V nominal. Cheap, no-name power supplies often do not supply power to specification even when provided with nominal input voltages, if indeed they even list nominal output specifications. Having a broad operating voltage range is particularly important if you operate without a UPS or line conditioner to ensure that the voltage supplied to the power supply does not vary due to brownouts, sags, and surges. It is less important if you do have a line conditioner or line-interactive UPS, except as an indicator of overall quality of the power supply.

Input frequency range

The range of AC frequencies over which the power supply is designed to operate. Most power supplies function properly within the range of 47 Hz to 63 Hz, which is adequate for nominal 50 Hz or 60 Hz input. In practice, this means that the power supply will operate properly on any nominal 50 Hz input voltage so long as it does not drop below 47 Hz and any nominal 60 Hz input voltage so long as it does not rise above 63 Hz. This is seldom a problem, as utilities control the frequency of the power they supply very tightly. Inexpensive power supplies usually do not list input frequency range, although we have seen cheap Pacific Rim units that list their requirements as "50 Hz to 60 Hz AC," implying that they have no tolerance for frequency variations.

Efficiency

The ratio of output power to input power expressed as a percentage. For example, a power supply that produces 350W output but requires 500W input is 70% efficient. In general, a good power supply is 70% efficient. However, calculating this figure is difficult because PC power supplies are switching power supplies rather than linear power supplies. The easiest way to think about this is to imagine the switching power supply drawing high current for a fraction of the time it is running and no current the remainder of the time. The percentage of the time it draws current is called the power factor, which is typically 70% for PC power supplies. In other words, a 350W PC power supply actually requires 500W input 70% of the time and 0W 30% of the time. Combining power factor with efficiency yields some interesting numbers. The power supply supplies 350W, but the 70% power factor means that it requires 500W 70% of the time. However, the 70% efficiency means that rather than actually drawing 500W, it must draw more, in the ratio 500W/0.7, or about 714W. If you examine the specifications plate for a 350W power supply, you may find that in order to supply 350W nominal, which is 350W/110V or about 3.18 amps, it must actually draw up to 714W/110V or about 6.5 amps. Other factors may increase that actual maximum amperage, so it's common to see 300W or 350W power supplies that actually draw as much as 8 or 10 amps maximum. That has planning implications, both for electrical circuits and for UPSes, which must be sized to accommodate the actual amperage draw rather than the rated output wattage.

One of the chief differences between premium power supplies and less expensive models is how well they are regulated. Ideally, a power supply accepts AC power, possibly noisy or outside specifications, and turns that AC power into smooth, stable DC power with no artifacts. In fact, no power supply meets the ideal, but good power supplies come much closer than cheap ones. Processors, memory, and other system components are designed to operate with pure, stable DC voltage. Any departure from that may reduce system stability and shorten component life. Here are the key regulation issues:

Ripple

A perfect power supply would accept the AC sine wave input and provide an utterly flat DC output. Real-world power supplies actually provide DC output with a small AC component superimposed upon it. That AC component is called ripple, and may be expressed as peak-to-peak voltage (p-p) in millivolts (mv) or as a percentage of the nominal output voltage. A high-quality power supply may have 1% ripple, which may be expressed as 1%, or as actual p-p voltage variation for each output voltage. For example, on a 5V output, a 1% ripple corresponds to ±0.05V, usually expressed as 50mV. A midrange power supply may limit ripple to 1% on some output voltages, but soar as high as 2.5% on others, typically -5V, +3.3V, and +5V_SB. We have seen cheap power supplies with ripple of 10% or more, which makes running a PC a crapshoot. Low ripple is most important on +5V and +3.3V outputs, although 1.5% or lower ripple is desirable on all outputs.

Load regulation

The load on a PC power supply can vary significantly during routine operations, for example as a CD burner's laser kicks in or a DVD-ROM drive spins up and spins down. Load regulation expresses the ability of the power supply to supply nominal output power at each voltage as the load varies from maximum to minimum, expressed as the variation in voltage experienced during the load change, either as a percentage or in p-p voltage differences. A power supply with tight load regulation delivers near-nominal voltage on all outputs regardless of load (within its range, of course). A high-quality power supply regulates +3.3V to within 1%, and the ±5V and ±12V outputs to within 5% or less. A midrange power supply might regulate +3.3V to within 3% or 4%, and the other voltages to within 5% or 10%. Regulation of +3.3V is critical and should never exceed 4%, although many inexpensive power supplies allow it to vary 5% or even more.

Load regulation on the +12V rail has become more important since Intel shipped the Pentium 4. In the past, +12V was used primarily to run drive motors. With the Pentium 4, Intel began using 12V VRMs on their motherboards to supply the higher currents that Pentium 4 processors require. ATX12V-compliant power supplies, typically advertised as "P4-compliant" or "P4-compatible," are designed with this requirement in mind. Older and/or inexpensive ATX power supplies, although they may be rated for sufficient amperage on the +12V rail to support a Pentium 4 motherboard, may not have adequate regulation to do so properly.

Line regulation

An ideal power supply would provide nominal output voltages while being fed any input AC voltage within its range. Real-world power supplies allow the DC output voltages to vary slightly as the AC input voltage changes. Just as load regulation describes the effects of internal loading, line regulation can be thought of as describing the effects of external loading, e.g., a sudden sag in delivered AC line voltage as an elevator motor kicks in. Line regulation is measured by holding all other variables constant and measuring the DC output voltages as the AC input voltage is varied across the input range. A power supply with tight line regulation delivers output voltages within specification as the input varies from maximum to minimum allowable. Line regulation is expressed in the same way as load regulation, and the acceptable percentages are the same.

Transient response

If the load on the power supply varies momentarily from the baseline and then returns to baseline, it takes a certain period for the output voltages to return to nominal. Transient response is characterized in three ways, all of which are interrelated: by the percent load change, by the amount of time required for output voltages to return to within a specified percentage of nominal, and by what that percentage is. These figures are difficult to compare because different manufacturers use different parameters that are not directly comparable. For example, a high-quality power supply may state that after an instantaneous 50% load change, the power supply requires 1 millisecond (ms) to return to within 1% of nominal on all outputs. A midrange power supply may specify the load change as only 20% and state that the ±5V and ±12V outputs return to within 5% of nominal within 1 ms. If the load change were 50% instead of 20%, that same midrange power supply might require 2 or 3 ms to return to within 5% of nominal and 10 ms to return to 1% of nominal (if in fact it could even control voltages to within 1% under normal conditions, which it probably couldn't). In general, a power supply with excellent transient response will specify (a) a load change of 50% or thereabouts, (b) a return to at or near its standard regulation range, and (c) a time of 1 or 2 ms. A decrease in the first figure or an increase in either or both of the second two is indicative of relatively poorer transient response. The major benefit of good transient response is increased reliability in disk operations, both read and write. A power supply with poor transient response may cause frequent disk retries, which are visible to the user only as degraded disk performance. Many users who upgrade to a better power supply are surprised to find that their disk drives run faster. Hard to believe, but true.

Hold-up time

That period for which, during a loss of input power, the power supply continues to provide output voltages within specification. Hold-up time may be specified in milliseconds or in cycles, where one cycle is 1/60 second, or about 16.7 ms. High-quality power supplies have hold-up times of 20 ms or higher (> 1.25 cycles). Lower quality power supplies often have hold-up times of 10 ms or less, sometimes much less. There are two issues here. First, if you are running a standby power supply (commonly, if erroneously, called a UPS) that has a switchover time, hold-up keeps the PC running until the UPS has time to kick in. This is less a problem with modern SPSes/UPSes, which commonly have transfer times of ~1 ms, compared to the 5 ms to 10 ms transfer times common with UPSes a few years ago. Hold-up time is even more important if you are not using a UPS, because about 99% of all power outages are of one cycle or less, many so short that you aren't even aware they occurred because the lights don't have time to flicker. With such outages, a power supply with a long hold-up time will allow the PC to continue running normally, while one with a short hold-up time will cause the PC to lock up for no apparent reason. The first comment most people make who do not have a UPS and upgrade to a better power supply is that their systems don't lock up nearly as often. That's why.

Power Good signal

A power supply requires time to stabilize when power is first applied to it. When it stabilizes, the power supply asserts the Power Good (AT) or PWR_OK (ATX) signal to inform the motherboard that suitable power is now available, and continues to assert that signal so long as suitable power remains available. The time a power supply requires before asserting Power Good varies between models, between examples of the same model, and even between boots with the same power supply. Some motherboards are sensitive to Power Good timing, and may refuse entirely to boot or experience sporadic boot failures when used with a power supply that has lengthy or unpredictable Power Good timing. A superior power supply may raise Power Good within 300 ms plus or minus a few ms of receiving power. A midrange power supply may require from 100 to 500 ms before asserting Power Good. Another aspect of Power Good that is seldom specified is how long the power supply continues to supply good power after dropping the Power Good signal. A good power supply should continue to provide clean power for at least one ms after deasserting Power Good.

Noise and fan air flow rating

The power supply fan produces air flow that cools both the power supply itself and other PC components such as processors and drives. In general, doubling the air flow reduces system operating temperature by about 50%, which in turn increases the life of system components. The old chem lab rule says that increasing the temperature by 10°C (18°F) doubles the rate of reaction, and reducing it by 10°C halves the rate. That ratio holds roughly true for component life as well. A processor with a design operating temperature of 50°C, for example, will last twice as long if run at 40°C. But in the process of moving air, the fan generates noise. The amount and nature of that noise depend upon the number, design, size, pitch, and rotation speed of the fan blades; the size, design, and bearing type of the hub; the internal layout of power supply components; the depth and configuration of the venturi (air path); and other factors. In general, high cooling efficiency power supplies are noisier than those that move less air, and power supplies that use sleeve bearings are quieter (albeit less durable) than those that use ball bearings. Noise is measured on the logarithmic dB(A) scale at a distance of 1 meter from the fan. On the dB(A) scale, each 3 dB change indicates a doubling or halving of sound energy. A very quiet power supply may be rated at 34 to 36 dB(A), which is almost inaudible in a typical work environment, and provide 20 to 30 cubic feet per minute air flow. A typical power supply may generate 40 to 44 dB(A), which is audible but not overly intrusive in most work environments, and provide 25 to 35 CFM. A high-performance power supply may generate 44 to 48 dB(A), which is distinctly noticeable, and provide 35 to 50 CFM.

Mean Time Between Failures (MTBF)

MTBF is a much-misunderstood way of specifying component reliability. MTBF for power supplies is a projected estimate based on a combination of operating data and calculated data as specified in MIL-HDBK-217. The MTBF projected failure curve for a particular model of power supply takes the form of a skewed bell curve, with a few power supplies of that model failing very early, the vast majority failing from a year to a few years out, and (at least in theory) a tiny number surviving for decades, with that number tailing off as time passes to almost (but never quite) zero. A good power supply has an MTBF of approximately 30,000 to 100,000 hours; a midrange power supply may have an MTBF of perhaps 15,000 to 35,000 hours; and a cheap power supply may have an MTBF of 10,000 hours or less. A 100,000 hour MTBF does not mean, however, that you can expect your power supply to last 100,000 hours, nor does it mean that that unit is "twice as reliable" as a unit with a 50,000 hour MTBF. Use MTBF only as a rough basis for comparison. It is safe to say that a unit with a 100,000 hour MTBF is probably more reliable than a unit with a 50,000 hour MTBF, which in turn is probably more reliable than a unit with a 10,000 hour MTBF, but don't attribute much more to it than that.

Another important characteristic of power supplies is the emissions and safety standards with which they comply. This information is useful both as it pertains specifically to the item being regulated and generally in the sense that power supplies that meet more and/or tighter regulatory approvals tend to be better built and more reliable.

Overvoltage protection, overcurrent protection, and leakage current

Properly designed power supplies include overvoltage protection circuitry that shuts down the power supply if output voltage exceeds specified limits, and overcurrent protection circuitry that protects the power supply (and the PC) from excessive current. At minimum, overvoltage protection should be provided for +3.3V (if present) and +5V and should cause the power supply to trip to reset if either of these voltages exceed nominal by 25% or more. Better power supplies also provide similar protection for +12V. Overcurrent protection should prevent any level of overcurrent, including a dead short, from damaging the power supply or PC. A good power supply might provide latching protection (a level-sensitive cutout) for +3.3V at 60 Amperes (A), +5V at 50A, and +12V at 20A. Leakage current specifies the maximum current that can leak to ground during normal operation, and should be less than one milliampere (ma) at 220/240V.

Emissions approvals

Electromagnetic interference (EMI) is noise generated by the switching action of the power supply, and comes in two varieties. Conducted interference is noise of any frequency that the power supply places on the AC source line. Conducted interference may cause problems for other devices connected to the same circuit, and is controlled by means of capacitive and/or inductive line filters to isolate the power supply from the AC source. Radiated interference is radio frequency interference (RFI) that may affect nearby electronic devices even if they are not connected to the same AC circuit (or any AC circuit at all). Radiated interference is controlled by physical shielding of the power supply, both by the power supply enclosure itself and by the shielding provided by the PC chassis. Both types of interference are regulated in the United States by the Federal Communications Commission (FCC), and in other countries by various regulatory agencies. A power supply should have FCC Class B approval (and/or the roughly equivalent CISPR22), although many inexpensive units have only the less restrictive FCC Class A.

Safety approvals

Various safety standards are promulgated by standards organizations in the U.S. and elsewhere. Any power supply you use should have at least UL certification (UL 1950). Other standards to look for include: CSA Std. C22.2, TUV EN60950, IEC950, KS, SEMKO, NEMKO, DEMKO, SETI, and CCIB.