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Power Supply Specifications

In addition to power output, many other specifications and features go into making a high-quality power supply. I have had many systems over the years. My experience has been that if a brownout occurs in a room with several systems running, the systems with higher-quality power supplies and higher output ratings are far more likely to make it through the power disturbances unscathed, whereas others choke.

High-quality power supplies also help protect your systems. A power supply from a vendor such as Astec or PC Power and Cooling will not be damaged if any of the following conditions occur:

  • A 100% power outage of any duration

  • A brownout of any kind

  • A spike of up to 2,500v applied directly to the AC input (for example, a lightning strike or a lightning simulation test)

Decent power supplies have an extremely low current leakage to ground of less than 500 microamps. This safety feature is important if your outlet has a missing or an improperly wired ground line.

As you can see, these specifications are fairly tough and are certainly representative of a high-quality power supply. Make sure that your supply can meet these specifications.

You can also use many other criteria to evaluate a power supply. The power supply is a component many users ignore when shopping for a PC, and it is therefore one that some system vendors might choose to skimp on. After all, a dealer is far more likely to be able to increase the price of a computer by spending money on additional memory or a larger hard drive than by installing a better power supply.

When buying a computer (or a replacement power supply), you always should learn as much as possible about the power supply. However, many consumers are intimidated by the vocabulary and statistics found in a typical power supply specification. Here are some of the most common parameters found on power supply specification sheets, along with their meanings:

  • Mean Time Between Failures (MTBF) or Mean Time To Failure (MTTF). The (calculated) average interval, in hours, that the power supply is expected to operate before failing. Power supplies typically have MTBF ratings (such as 100,000 hours or more) that are clearly not the result of real-time empirical testing. In fact, manufacturers use published standards to calculate the results, based on the failure rates of the power supply's individual components. MTBF figures for power supplies often include the load to which the power supply was subjected (in the form of a percentage) and the temperature of the environment in which the tests were performed.

  • Input Range (or Operating Range). The range of voltages that the power supply is prepared to accept from the AC power source. For 110v AC current, an input range of 90v–135v is common; for 220v current, a 180v–270v range is typical.

  • Peak Inrush Current. The greatest amount of current drawn by the power supply at a given moment immediately after it is turned on, expressed in terms of amps at a particular voltage. The lower the current, the less thermal shock the system experiences.

  • Hold-Up Time. The amount of time (in milliseconds) that a power supply can maintain output within the specified voltage ranges after a loss of input power. This enables your PC to continue running without resetting or rebooting if a brief interruption in AC power occurs. Values of 15–30 milliseconds are common for today's power supplies, and the higher (longer), the better. The ATX12V specification calls for a minimum of 17ms hold-up time.

  • Transient Response. The amount of time (in microseconds) a power supply takes to bring its output back to the specified voltage ranges after a steep change in the output current. In other words, the amount of time it takes for the output power levels to stabilize after a device in the system starts or stops drawing power. Power supplies sample the current being used by the computer at regular intervals. When a device stops drawing power during one of these intervals (such as when a floppy drive stops spinning), the power supply might supply too high a voltage to the output for a brief time. This excess voltage is called overshoot, and the transient response is the time that it takes for the voltage to return to the specified level. This is seen as a spike in voltage by the system and can cause glitches and lockups. Once a major problem that came with switching power supplies, overshoot has been greatly reduced in recent years. Transient response values are sometimes expressed in time intervals, and at other times they are expressed in terms of a particular output change, such as "power output levels stay within regulation during output changes of up to 20 percent."

  • Overvoltage Protection. Defines the trip points for each output at which the power supply shuts down or squelches that output. Values can be expressed as a percentage (for example, 120% for +3.3 and +5v) or as raw voltages (for example, +4.6v for the +3.3v output and +7.0v for the +5v output).

  • Maximum Load Current. The largest amount of current (in amps) that safely can be delivered through a particular output. Values are expressed as individual amperages for each output voltage. With these figures, you can calculate not only the total amount of power the power supply can supply, but also how many devices using those various voltages it can support.

  • Minimum Load Current. The smallest amount of current (in amps) that must be drawn from a particular output for that output to function. If the current drawn from an output falls below the minimum, the power supply could be damaged or automatically shut down.

  • Load Regulation (or Voltage Load Regulation). When the current drawn from a particular output increases or decreases, the voltage changes slightly as well, usually increasing as the current rises. Load regulation is the change in the voltage for a particular output as it transitions from its minimum load to its maximum load (or vice versa). Values, expressed in terms of a +/– percentage, typically range from +/–1% to +/–5% for the +3.3, +5, and +12v outputs.

  • Line Regulation. The change in output voltage as the AC input voltage transitions from the lowest to the highest value of the input range. A power supply should be capable of handling any AC voltage in its input range with a change in its output of 1% or less.

  • Efficiency. The ratio of power input to power output, expressed in terms of a percentage. Values of 65%–85% are common for power supplies today. The remaining 15%–35% of the power input is converted to heat during the AC/DC conversion process. Although greater efficiency means less heat inside the computer (always a good thing) and lower electric bills, it should not be emphasized at the expense of precision, stability, and durability, as evidenced in the supply's load regulation and other parameters.

  • Ripple (or Ripple and Noise, or AC Ripple, or PARD [Periodic and Random Deviation]). The average voltage of all AC effects on the power supply outputs, typically measured in millivolts peak-to-peak or as a percentage of the nominal output voltage. The lower this figure is, the better. Higher-quality units normally are rated at 1% ripple (or less), which if expressed in volts would be 1% of the output. Consequently, for +5v that would be 0.05v or 50mv (millivolts). Ripple can be caused by internal switching transients, feed through of the rectified line frequency, and other random noise.

Power Factor Correction

Recently, the power line efficiency and harmonic distortion generation of PC power supplies have come under examination. This generally falls under the topic of the power factor of the supply. Interest in power factor is not only due to an improvement in power efficiency, but also because of a reduction in the generation of harmonics back on the power line. In particular, new standards are now mandatory in many European Union (EU) countries that require harmonics be reduced below a specific amount. The circuitry required to do this is called power factor correction (PFC).

The power factor measures how effectively electrical power is being used, and is expressed as a number between 0 and 1. A high power factor means that electrical power is being used effectively, whereas a low power factor indicates poor utilization of electrical power. To understand the power factor, you must understand how power is used.

Generally, two types of loads are placed on AC power lines:

  • Resistive. Power converted into heat, light, motion, or work

  • Inductive. Sustains an electromagnetic field, such as in a transformer or motor

A resistive load is often called working power and is measured in kilowatts (KW). An inductive load, on the other hand, is often called reactive power and is measured in kilovolt-amperes-reactive (KVAR). Working power and reactive power together make up apparent power, which is measured in kilovolt-amperes (KVA). The power factor is measured as the ratio of working power to apparent power, or working power/apparent power (KW/KVA). The ideal power factor is 1, where the working power and apparent power are the same.

The concept of a resistive load or working power is fairly easy to understand. For example, a light bulb that consumes 100W of power generates 100W worth of heat and light. That is a pure resistive load. An inductive load, on the other hand, is a little harder to understand. Think about a transformer, which has coil windings to generate an electromagnetic field and then induce current in another set of windings. A certain amount of power is required to saturate the windings and generate the magnetic field, even though no work is being done. A power transformer that is not connected to anything is a perfect example of a pure inductive load. An apparent power draw exists to generate the fields, but no working power exists because no actual work is being done.

When the transformer is connected to a load, it uses both working power and reactive power. In other words, power is consumed to do work (for example, if the transformer is powering a light bulb), and apparent power is used to maintain the electromagnetic field in the transformer windings. In an AC circuit, these loads can become out of sync or phase, meaning they don't peak at the same time, which can generate harmonic distortions back down the power line. I've seen examples where electric motors have caused distortions in television sets plugged into the same power circuit.

PFC usually involves adding capacitance to the circuit to maintain the inductive load without drawing additional power from the line. This makes the working power and apparent power the same, which results in a power factor of 1. It usually isn't just as simple as adding some capacitors to a circuit, although that can be done and is called passive power factor correction. Active power factor correction involves a more intelligent circuit designed to match the resistive and inductive loads so they are seen as the same by the electrical outlet.

A power supply with active power factor correction draws low distortion current from the AC source and has a power factor rating of 0.9 or greater. A non–power factor corrected supply draws highly distorted current and is sometimes referred to as a nonlinear load. The power factor of a noncorrected supply is typically 0.6–0.8. Therefore, only 60% of the apparent power consumed is actually doing real work!

Having a power supply with active PFC might or might not lower your electric bill (it depends on how your power is measured), but it will definitely reduce the load on the building wiring. With PFC, all the power going into the supply is converted into actual work and the wiring is less overworked. For example, if you ran a number of computers on a single breaker-controlled circuit and found that you were blowing the breaker periodically, you could switch to systems with active PFC power supplies and reduce the load on the wiring by up to 40%, meaning you would be less likely to blow the breaker.

The International Electrical Committee (IEC) has released standards dealing with the low-frequency public supply system. The initial standards were 555.2 (Harmonics) and 555.3 (Flicker), but they have since been refined and are now available as IEC 1000-3-2 and IEC 1000-3-3, respectively. As governed by the EMC directive, most electrical devices sold within the member countries of the EU must meet the IEC standards. The IEC1000-3-2/3 standards became mandatory in 1997 and 1998.

Even if you don't live in a country where PFC is required, I highly recommend specifying PC power supplies with active PFC. The main benefits of PFC supplies is that they do not overheat building wiring or distort the AC source waveform, which causes less interference on the line for other devices.

Power Supply Safety Certifications

Many agencies around the world certify electric and electronic components for safety and quality. The most commonly known agency in the United States is Underwriters Laboratories, Inc. (UL). UL standard #1950—the Standard for Safety of Information Technology Equipment, Including Electrical Business Equipment, Third Edition—covers power supplies and other PC components. You should always purchase power supplies and other devices that are UL-certified. It has often been said that, although not every good product is UL-certified, no bad products are.

In Canada, electric and electronic products are certified by the Canadian Standards Agency (CSA). The German equivalents are TÜV Rheinland and VDE, and NEMKO operates in Norway. These agencies are responsible for certification of products throughout Europe. Power supply manufacturers that sell to an international market should have products that are certified at least by UL, the CSA, and TÜV—if not by all the agencies listed, and more.

Apart from UL-type certifications, many power supply manufacturers, even the most reputable ones, claim that their products have a Class B certification from the Federal Communications Commission, meaning that they meet FCC standards for electromagnetic and radio frequency interference (EMI/RFI). This is a contentious point, however, because the FCC does not certify power supplies as individual components. Title 47 of the Code of Federal Regulations, Part 15, Section 15.101(c) states as follows:

"The FCC does NOT currently authorize motherboards, cases, and internal power supplies. Vendor claims that they are selling 'FCC-certified cases,' 'FCC-certified motherboards,' or 'FCC-certified internal power supplies' are false."

In fact, an FCC certification can be issued collectively only to a base unit consisting of a computer case, motherboard, and power supply. Thus, a power supply purported to be FCC-certified was actually certified along with a particular case and motherboard—not necessarily the same case and motherboard you are using in your system. This does not mean, however, that the manufacturer is being deceitful or that the power supply is inferior. If anything, this means that when evaluating power supplies, you should place less weight on the FCC certification than on other factors, such as UL certification.

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