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Choosing the right PC power supply

When building or upgrading a PC, you need an adequate power supply. In this article, you'll learn how to decode power supply specs so you can choose a power supply that best maintains system stability.

When you build or upgrade a PC system, it’s very important to choose a quality power supply that will provide enough power and help maintain system stability. By doing so, you can prevent frustrating power-related problems. You cannot, however, determine the quality of a PC power supply based on its maximum power rating alone. To choose an appropriate power supply, you should examine its specifications and determine how they can affect your system. In this Daily Feature, I’ll show you how.

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Power supply basics
To understand power supply specifications, it is helpful to first have an understanding of power supplies in general. The basic function of any line-input power supply is to provide power by converting AC power (from an outlet) into DC power usable by electronic devices. Power supplies used in electronics must also be capable of holding an output voltage steady despite any load changes within specifications (regulation). For example, if a CD-ROM spins up and requires more electrical current, the power supply should accommodate the current change with negligible effect on the output voltage. Tight regulation is critical for devices that use low-voltage digital logic, because any significant voltage fluctuations can cause data errors.

Linear power supplies use large iron core transformers to step down (reduce) the line voltage before converting it to DC. They also provide regulation by varying the resistance of a voltage control component. Both of these methods produce heat and therefore waste energy. PCs use switchmode power supplies (SMPS or switchers) instead of linear supplies. Switchers eliminate the need for large transformers and provide regulation through a rapid on/off switching action. These characteristics make a switcher much more efficient than a standard linear supply because less power is wasted as heat. In fact, some linear power supplies waste more than half of their input power, whereas switchers can have efficiency ratings of more than 80 percent. Switching power supplies also run cooler; use smaller, lighter, less expensive transformers; and are less susceptible to input voltage variations than traditional linear supplies.

Output specifications
Let’s begin by looking at specifications that express the output characteristics of a power supply. Figure A shows typical specifications for a 300-watt (W) supply as you might see them on a specifications sheet.

Figure A
This table shows the typical output specifications for a quality 300-W power supply.

  • Maximum power: This is the most familiar rating and should be used only as a general measure of the supply’s power capability. It indicates the maximum continuous amount of power that can be provided by the supply under full-load conditions. This is a nominal value, and the actual output power differs slightly. Power is measured in watts.
  • Maximum load: This specifies the maximum current, in amps (A), available at each output voltage. It is sometimes included in a table that also contains other specs related to each of the supply’s output voltages. The maximum loads for the –12V and –5V rails are not very important and are usually provided only for backwards compatibility. The most important maximum loads are those given for the +12V, +5V, and +3.3V output voltages. The +12V rail is used to power drive motors and fans. The maximum load for this voltage is especially important during system startup because hard drives use more power than normal when they spin up. With this in mind, supplies are designed to exceed maximum power specifications for short periods of time. The +12V maximum load is also important on PCs that have several drives and fans. The +5V rail is used mainly for the electronics of the PC components, and the +3.3V rail is used to power processors and memory. Generally, the higher the maximum load at each of the critical voltage outputs, the better. A maximum load multiplied by its output voltage is the output power for that rail.
  • Minimum load: This is the minimum amount of current that is needed at a specific output in order for the supply to function properly. Unlike linear supplies, switchers normally need a load in order to function properly.
  • Load regulation: This is how well a supply can keep a voltage from changing when the load across that voltage changes. It’s usually expressed as a percentage change for a given voltage. It can also be expressed as an actual peak-to-peak voltage change. Good power supplies will regulate the +5V and +12V outputs to within ±5 percent and the 3.3V output to ±1 percent. Excellent supplies will keep all three of these outputs to within ±1 percent.
  • Ripple: Switchers are inherently electrically noisy due to the rapid switching regulation technique they use. This is one of the main causes of small fluctuations on the output voltages. These fluctuations should be kept as small as possible, especially on the +5V and +3.3V rails. Excellent power supplies will limit ripple to 1 percent peak-to-peak on all outputs, but a ripple of 1.5 percent is common on good supplies.
  • +5 and +3.3V combined output: Although the maximum load at the individual +5V and +3.3V rails is important, a supply cannot provide the maximum loads to both of these outputs simultaneously. Therefore, a combined output power is usually specified. This combined output specification is a very important consideration when using a power-intensive CPU. Athlon processors, in particular, use a tremendous amount of power, and this specification is vital when selecting a supply for an Athlon-based system. You may notice that the sum of all of the voltage rail powers is greater than the maximum power rating for the supply. This is the result of using the combined output power value, instead of the individual +5V and +3.3V powers, when calculating actual power.
  • Hold-up time: Hold-up time is a measure of how long a supply will hold output voltages to within specifications after input power has been lost. A supply with sufficient hold-up time can keep your computer from locking up when extremely short power outages occur. Also, sufficient hold-up time will give a standby power supply (SPS) enough time to switch over to battery power. A quality supply will have a hold-up time of 20 or 30 milliseconds (ms).
  • Power good delay: When a supply is turned on, it sends a power good signal to the motherboard after it has stabilized and is ready to provide power. Some motherboards are picky as to when they will receive this signal. A quality supply will provide power good within 300 ms.

Input specifications
Figure B shows specifications that relate to the input parameters of a power supply. Typical specs for a 300-W supply are used as an example.

Figure B
This table shows the typical input specifications for a 300-W PC power supply. Electromagnetic Interference (EMI) may be provided by various entities.

  • Operating range: This specifies the AC input voltage range at which the supply will operate reliably. A wide range may help to keep your PC operating properly during brownouts and when voltage surges occur. Usually, two ranges are given that correspond to the 115 and 230 volts alternating current (VAC) input voltage settings. These settings are manually set through a switch on the back of the supply or are automatically set by some supplies (autoswitched) to the proper input voltage. Typical operating voltage values for high-quality supplies are 90–135 VAC and 180–270 VAC.
  • Input frequency range: This is the frequency range at which the supply operates properly. This spec is not very critical since electric companies tightly control power frequencies. In the United States, AC power is provided at 60 hertz, or cycles per second (Hz), while in some countries it is supplied at 50 Hz. Typical ratings for this spec are 47 to 63 Hz for quality supplies.
  • Inrush current (or Input Surge Current): When you turn on a PC power supply, it initially appears as a short circuit to the input voltage source, and a very large current flows for a short instance. This inrush current can be very damaging to components and can trip circuit breakers. However, quality switchers are designed to limit this current. The inrush current is given as the maximum current (in amps) that can appear for an input voltage setting.
  • Input current: This is the maximum input current that the supply may use for a given input voltage setting.
  • Efficiency: Efficiency is calculated as the output power divided by input power. As I mentioned, switchers are very efficient, and most quality supplies generally provide 70 percent efficiency or better. A higher efficiency supply will produce less heat, thereby wasting less power and keeping your PC cooler.

General specifications
These are two important general specifications that you might also see listed.
  • Safety features: A well-designed supply will provide overvoltage (excessive voltage at the outputs) and overcurrent (overload or short circuit) protection.
  • Transient response: This is a measure of how quickly a supply’s voltage can return to specification after a load has changed by a certain percentage. A low-quality supply with a poor transient response can cause problems with disk-drive operations that are hard to detect. Quicker times for greater load changes are better.

Although PC power supply specifications may be intimidating at first, understanding them will allow you to compare power supplies more effectively, choose the right one for your needs, and avoid power-related problems specific to your electrical setup.

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