PC power supply evolution
ATX update
Technical notes
Standards certification
Choosing the right power supply

PC Power Supply Evolution

The modern PC power supply is a direct descendant of IBM's PC XT design, as are many other parts of today's Personal Computer. XT power supplies established the four output voltages (+5V, -5V, +12V, -12V) and one status output, known as PG (Power Good), that all units, past and present, provide to PC computing systems. IBM's AT computer, successor to the XT, continued to influence industry designs and standards. AT power supplies changed shape to allow for expanded motherboard size and increased output power to around 200W, where it is today for desktop computing.

While IBM introduced early generations of PC microprocessor technology with their XT and AT computers, COMPAQ Computer produced the first widely available PC based on Intel's 80386 microprocessor. Circuit board technology also raced beyond IBM's control as computer makers began to build systems around an advanced version of the AT motherboard, dubbed the Baby AT. Baby AT motherboards improved performance, reduced size and became a widely available "off-the-shelf" part.

Computer manufacturers, filling the void left by IBM, spawned a number of power designs for their computing systems by adding special shapes, sizes and on/off switching schemes. The Mini PSU, also known as a Slim Line or PS2 style, eventually emerged as the de facto power supply standard because of its compact size and affordable price. Almost all of today's PCs use some version of a Mini PSU.

Note that power supply naming conventions are somewhat ambiguous, often relating to shapes rather than computing systems. For example, a XT Style PSU can operate a XT, AT, or Baby AT motherboard but will only fit into a XT system case. The real determining factor in choosing a power unit is the power consumption, motherboard size and system layout.

The new ATX form factor introduced in 1995 resulted in the first major internal design changes for PC power supplies since the XT. ATX is a exciting advance for power distribution and covered in the next section.



ATX Update

ATX power supplies and cases are becoming more prevalent as Intel's new ATX PC architecture gains marketplace acceptance. ATX and varients such as NLX and Micro ATX are open architectures, developed by Intel. They considerably upon existing designs by:
- relocating the processor and memory for easy removal/installation.
- increasing the number of full length expansion slots.
- integrating more I/O onto the baseboard, improving reliability.
- reducing cabling complexity and length, thus decreasing assembly errors and increasing disk drive speed.
- reducing system noise by using only one fan.
- including advanced power management features.
- providing standards for advancing PC performance.

Intel Pentium, PentiumPro and P6 motherboards are all now using the ATX form factor. Estimates had ATX capturing 18% of the motherboard market in 1996 and rising to 80% by the year 2000. Information on ATX vendors, products, news, updates and specification downloads can be found at http://www.teleport.com/~atx/ and Intel's ATX HomePage .

Information on NLX vendors, products, news, updates and specification downloads can be found at http://www.teleport.com/~nlx/ and Intel's NLX HomePage .

Information on Micro ATX vendors, products, news, updates and specification downloads can be found at http://www.teleport.com/~microatx/ .

Some of the best improvements of ATX are in the new power distribution system. Features include software shutdown, reduced cabling, 3.3V standard output, and increased system cooling.

ATX specifies on/off switching through the motherboard to the power supply instead of directly switching the supply on or off via a pushbutton switch. This allows software controlled system shutdown and reduces power supply cabling requirements. The standard power supply P8/P9 motherboard connectors have been combined into one 20 pin connector which includes previous voltages plus 3.3V, and system on/off switching.

The 3.3V output supports future processor technologies and the expected move to 3.3V PCI cards. No voltage regulator add-on is required with ATX. Finally, the power supply fan has increased in size, providing enough cooling for the CPU and system. Thus, case cooling and CPU fans are no longer required in most instances. The ATX power supply appearance is similar to a Mini-Style PSU with the exception of fan placement.



Technical Notes

Virtually all PC power supplies are of the same basic design. The technical name for this design is Constant Voltage Half-Bridge Forward Converting Switching power supplies, usually known simply as switching supplies. Constant voltage means that the output voltage is always the same as the current changes with changing power requirements. Half-Bridge Forward Converting is the type of switching design. Switching notes the regulation technique and gives a broad design definition. The main advantages of these power supplies over other designs are their high efficiency, low heat dissipation, small size, and affordable prices. Clone Power, a report detailing the theory of operation of a typical IBM desktop clone power supply is available for free with any order over $100. ATX operation is not covered in Clone Power at this time.

While all PC power supplies operate in a similar manner, there can be wide variations in quality and performance. Standards certification, the manufacturer's reputation, and the following basic technical parameters are useful when testing, evaluating, and specifying a computer power supply.

AC Ripple AC Ripple, also known as PARD (periodic and random deviation) measures the RMS (average) voltage of all AC components on the DC output. This average number can be deceptive because it does not provide information on the presence of periodic switching spikes. RMS AC Ripple should be accompanied by peak voltage measurements to assure switching spikes are not excessive (< 200mV).

Transient Response Time Imagine a computer using 150W (5VDC 30A) of power at time t 1 dropping to 75W (5VDC 15A) at time t 1 + t small as it completes an operation. The power supply samples the current drawn by the computer at regular intervals of t 1 , t 2 , t 3 , etc. At time t 1 + t small the supply is still delivering 150W of power even though the computer can only sink 15A of current. Knowing that P=VI at any point in time the voltage at time t 1 + t small can be calculated: V=P/I = 150/15 = 10VDC. What !?! Our constant 5VDC supply is delivering 10VDC to sensitive digital components ? Yes it is. The time for the power supply to correct itself is call the transient response time. The amount of the voltage increase is called overshoot. Overshoot was a problem of switching supplies many years ago, but modern designs have made great progress in its reduction.

Load Regulation Load regulation, more accurately called voltage load regulation, measures the output voltage change from minimum load (minimum current sourced) to maximum load (maximum current sourced). In general, the voltage tends to drop off as the current rises and Load Regulation is a negative number. In actual practice, power supply manufacturers rarely indicate whether regulation is positive or negative.

Load Regulation = [(min. load voltage - max. load voltage)/min. load voltage] X 100

Line Regulation Line regulation, more accurately called voltage line regulation, measures the output voltage change from minimum AC input to maximum AC input. The typical 115VAC power supply is designed to accept an AC input ranging from 90VAC to 135VAC with little change in the DC outputs.

Line Regulation = [(max. output volt. - min. output volt.)/output volt at nominal line] X 100

Mean Time To Failure The Mean Time To Failure (MTTF) is a useful parameter to specify the quality of a system. The MTTF is the expected time that a system will operate before the first failure occurs. The MTTF is calculated by applying a mathematical formula to the individual component failure rates. More detailed explanation can be found in MIL-HDBK-217. A MTTF of 30K hours is fairly common for PC power supplies, equating to around 3.5 years of continuous use.

Efficiency Efficiency is simply the ratio of output power to input power. For a computer power supply this ratio is usually between 65-85%; the other 15-35% dissipated as heat during the AC to DC conversion. Efficiency is important for a couple of reasons. First, the less heat dissipated, the better. Excessive heat can cause shortened lifecycles and induce poor system performance. Second, greater efficiency means money saved on electric bills. Often efficiency is a design choice; increased efficiency may sacrifice load regulation and other parameters.

A current and future consideration is the direction of EPA's Energy Star Program. Presently, there are no specifications for power supplies, only for systems. The computer, not including monitor, must not exceed 30W in the low-power (inactive) state. Efficient power supplies help computer makers meet 'Green' certification.



Standards Certification

Safety and Quality. UL, CSA, TUV, VDE, and NEMKO are five major agencies which certify a product's safety and quality. UL certifies for the United States, CSA for Canada, TUV & VDE for Germany, and NEMKO for Norway. TUV, VDE, and NEMKO represent the European continent with TUV the leading agency. Large manufacturers who sell products internationally usually have UL, CSA, and TUV listings. Power supplies will vary from no certification to certification from these five agencies and more. In general, a UL listing should be considered a good baseline for product safety and quality. Here's a saying to remember about supplies with no certification - "All power supplies not UL listed are not necessarily bad power supplies but all bad supplies are not UL listed".

EMI/RFI. EMI/RFI refers to electromagnetic and radio frequency interference. The FCC has rules to limit the potential for harmful interference being caused by computers and other products. Power supply EMI/RFI is an interesting topic, especially if the power supply manufacturer claims FCC Class B certification. The FCC does not certify computer power supplies. Quoting Title 47 of the Code of Federal Regulations, Part 15, Section 15.101(c): 

      "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."

The base unit of FCC certification is a motherboard, case and power supply together. So if a manufacturer claims FCC Class B, they mean that a base unit with their power supply has passed certification. The next question is which motherboard(s) and case(s) were used in the testing. Most manufacturers claiming FCC Class B will test with a wide variety of cases and motherboards.

Peripherals such as video cards, printers, monitors, and keyboards are added to the base unit for individual Class B certification. Further information can be obtained from the Federal Communications Commission, Office of Engineering and Technology, Washington DC, 20554. Ask for OET Bulletin 62.

Green. The EPA's Energy Star Program is a voluntary partnership with the computer industry to promote the introduction of energy-efficient personal computers, monitors, and printers in an effort to reduce air pollution caused by power generation. The Energy Star specification requires a low power state to reduce consumption and applies only to computers, monitors, printers, and controlling devices (products which can shut off a computer, monitor or printer after a period of inactivity). Note that Energy Star does not apply directly to power supplies. "Green" power supplies are power supplies used in an Energy Star (Green) computer. As with FCC Class B, it's important to realize the computer is the certified item, not the power supply, even though efficient supplies aid in obtaining "Green" status.

EPA Energy Star Specification Summary
Computers and Monitors Monitors Personal Computers All-in-One Units
Maximum Watts in Low Power State 30 30 60

Power supply manufacturers may join the EPA's "Ally" agreement which basically requests the manufacturer to promote energy efficiency. A power supply with a Energy Star logo does not necessarily mean the product is any better or more efficient than a supply without it. More detailed information may be obtained by calling the EPA's Energy Star Hotline at (202) 233-9114.



Choosing the Right Power Supply

As mentioned earlier, a power supply's technical specifications, standards certification, and general vendor reputation are factors that should be studied prior to buying. However, a more basic decision is selecting the unit's correct output power. A power supply's output power, measured in Watts, is the most common metric for defining cost and performance.

Power Rating. Power ratings indicate a power supply's ability to do work without generating excessive heat. Excessive heat is a cause of early failure and poor performance. A unit's heat generation is proportional to the percentage of rated power actually used; e.g., a 200W PSU operating a computer consuming 200W will generate considerably more heat than a 200W unit running a computer using only 60W. Most computer systems use less than 100 watts.

Like the rest of us, computer manufacturers trade cost against performance and need when deciding on a power rating. When choosing a rating, consider the computer's present power consumption, expected lifespan, future upgrades and desired reliability. In general, 200W and 230W supplies are sufficient for most desktop applications while servers, high-end PCs, and mission critical computers should consider 250W and higher ratings.

Newer, more efficient designs, such as the ATX computers, do not require as much power as older systems. Most ATX desktop computers are equipped with power supplies ranging from 150-200 Watts.


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