Power supply

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Power supply is a reference to a source of electrical power. A device or system that supplies electrical or other types of energy to an output load or group of loads is called a power supply unit or PSU. The term is most commonly applied to electrical energy supplies, less often to mechanical ones, and rarely to others.

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[edit] Electrical power supplies

This term covers the power distribution system together with any other primary or secondary sources of energy such as:

Constraints that commonly affect power supplies are the amount of power they can supply, how long they can supply it without needing some kind of refueling or recharging, how stable their output voltage or current is under varying load conditions, and whether they provide continuous power or pulses.

The regulation of power supplies is done by incorporating circuitry to tightly control the output voltage and/or current of the power supply to a specific value. The specific value is closely maintained despite variations in the load presented to the power supply's output, or any reasonable voltage variation at the power supply's input. This kind of regulation is commonly categorised as a Stabilized power supply.

[edit] Power Supply Types

Power supplies for electronic devices can be broadly divided into linear and switching power supplies. The linear supply is a relatively simple design that becomes increasingly bulky and heavy for high amperage devices; voltage regulation in a linear supply can result in low efficiency. A switched-mode supply of the same rating as a linear supply will be smaller, is usually more efficient, but will be more complex.

[edit] Linear power supply

An AC powered linear power supply usually uses a transformer to convert the voltage from the wall outlet (mains) to a different, usually a lower voltage. If it is used to produce DC, a rectifier is used. A capacitor is used to smooth the pulsating current from the rectifier. Some small periodic deviations from smooth direct current will remain, which is known as ripple. these pulsations occur at a frequency related to the AC power frequency (for example, a multiple of 50 or 60 Hz).

The voltage produced by an unregulated power supply will vary depending on the load and on variations in the AC supply voltage. For critical electronics applications a linear regulator will be used to stabilize and adjust the voltage. This regulator will also greatly reduce the ripple and noise in the output DC current. Linear regulators often provide current limiting, protecting the power supply and attached circuit from overcurrent.

Adjustable linear power supplies are common laboratory and service shop test equipment, allowing the output voltage to be set over a wide range. For example, a bench power supply used by circuit designers may be adjustable up to 30 volts and up to 5 amperes output. Some can be driven by an external signal, for example, for applications requiring a pulsed output.

The simplest DC power supply circuit consists of a single diode and resistor in series with the AC supply. This circuit is common in rechargeable flashlights.

[edit] Switched-Mode power supply

A switched-mode power supply (SMPS) works on a different principle. AC mains input is directly rectified without the use of a transformer, to obtain a DC voltage. This voltage is then sliced into small pieces by a high-speed electronic switch. The size of these slices grows larger as power output requirements increase.

The input power slicing occurs at a very high speed (typically 10 kHz — 1 MHz). High frequency and high voltages in this first stage permit much smaller step down transformers than are in a linear power supply. After the transformer secondary, the AC is again rectified to DC. To keep output voltage constant, the power supply needs a sophisticated feedback controller to monitor current draw by the load.

Modern switched-mode power supplies often include additional safety features such as the crowbar circuit to help protect the device and the user from harm.[1] In the event that an abnormal high amperage power draw is detected, the switched-mode supply can assume this is a direct short and will shut itself down before damage is done. For decades PC computer power supplies have also provided a power good signal to the motherboard which prevents operation when abnormal supply voltages are present.

Switched mode power supplies have an absolute limit on their minimum amperage output. [2] They are only able to output above a certain wattage and cannot function below that point. In a no-load condition the frequency of the power slicing circuit increases to great speed, causing the isolation transformer to act as a tesla coil, causing damage due to the resulting very high voltage power spikes. Switched-mode supplies with protection circuits may briefly turn on but then shut down when no load has been detected. A very small low-wattage dummy load such as a ceramic power resistor or 10 watt light bulb can be attached to the supply to allow it to run with no primary load attached.

Power factor has become a recent issue of concern for computer manufacturers. Switched mode power supplies have traditionally been a source of power line harmonics and have a very poor power factor. Many computer power supplies built in the last few years now include power factor correction built right into the switched-mode supply, and may advertise the fact that they offer 1.0 power factor.

By slicing up the sinousoidal AC wave into very small discrete pieces, the portion of the AC current not used stays in the power line as very small spikes of power that cannot be utilized by AC motors and results in waste heating of power line transformers. Hundreds of switched mode power supplies in a building can result in poor power quality for other customers surrounding that building, and high electric bills for the company if they are billed according to their power factor in addition to the kilowatts used. Filtering capacitor banks may be needed on the building power mains to suppress and absorb these negative power factor effects.

[edit] Uninterruptible power supply

Main article: Uninterruptible power supply

An Uninterruptible Power Supply (UPS) takes its power from two or more sources simultaneously. It is usually powered directly from the AC mains, while simultaneously charging a storage battery. Should there be a dropout or failure of the mains, the battery instantly takes over so that the load never experiences an interruption. Such a scheme can supply power as long as the battery charge suffices, e.g., in a computer installation, giving the operator sufficient time to effect an orderly system shutdown without loss of data. Other UPS schemes may use an internal combustion engine or turbine to continuously supply power to a system in parallel with power coming from the AC mains. The engine-driven generators would normally be idling, but could come to full power in a matter of a few seconds in order to keep vital equipment running without interruption. Such a scheme might be found in hospitals or telephone central offices.

[edit] Power Supply Applications

[edit] Computer power supply

Main article: Computer power supply

A modern computer power supply is a special type of switched-mode supply designed to convert 110-240 V AC power from the mains, to several low-voltage DC power outputs known as +12v, -12v, +5v and -5v. The first computer power supplies were linear devices, but as power requirements increased, the transformer size became too large and heavy and a change to a more efficient but more complex switch-mode supply was needed.

The diverse collection of output voltages also have widely varying amperage draw requirements, which are difficult to all be supplied from the same switched-mode source. Consequently most modern computer power supplies actually consist of several different switched mode supplies, each producing just one voltage component and each able to vary its output based on component power requirements, and all are linked together to shut down as a group in the event of a fault condition.

The most common modern computer power supplies are built to conform to the ATX form factor. The power rating of a PC power supply is not officially certified and is self-claimed by each manufacturer.[3] The more reputable makers advertise "True Wattage Rated" to give consumers the idea that they can trust the power advertised.

[edit] AC Adapter

A linear or (rarely) switched-mode power supply (or in some cases just a transformer) that is built into the top of a plug is known as a "wall wart", "power brick", "plug-in adapter", "adapter block", "domestic mains adapter" or just "power adapter". They are even more diverse than their names; often with either the same kind of DC plug offering different voltage or polarity, or a different plug offering the same voltage. "Universal" adapters attempt to replace missing or damaged ones, using multiple plugs and selectors for different voltages and polarities. Replacement power supplies must match the voltage of, and supply at least as much current as, the original power supply.

The least expensive AC units consist solely of a small transformer, and while DC adapters include a few additional diodes. Whether or not a load is connected to the power adapter, the transformer has a magnetic field continuously present and normally cannot be completely turned off unless unplugged.

Because they consume standby power, they are sometimes known as "electricity vampires" and may be plugged into a power strip to allow turning them off. Expensive switched-mode power supplies can cut off leaky electrolyte-capacitors, use powerless MOSFETs, and reduce their working frequency to get a gulp of energy once in a while to power for example a clock, which would otherwise need a battery.

This type of power supply is popular among manufacturers of low cost electrical items because

  1. Devices sold in the global marketplace don't need to be individually configured for 120 volt or 230 volt operation, just sold with the appropriate AC adapter.
  2. The device itself doesn't need to be tested for compliance with electrical safety regulations. Only the adapter needs to be tested.

[edit] Polarity

Diagram explaining standard symbols for polarity.
Diagram explaining standard symbols for polarity.

AC-to-DC adaptors have polarity (positive or negative). It is necessary to use an adaptor with the correct polarity to avoid damage.

[edit] Alarm Power Supply Unit

Main article: Alarm Power Supply Unit

The Alarm Power Supply Unit (APSU) takes its power from the AC mains and converts this to a DC supply while simultaneously charging a 12-24 volt storage battery. During a mains power dropout or failure, the battery instantly takes over so that the load never experiences an interruption. The APSU's main function is to power Network Access Devices (Modems, Switches, Routers) and includes an IP stack that supports either SNMP, UDP, TCP/IP Alarm monitoring protocols or SMTP, POP email clients. The APSU can report a mains failure, battery low etc and most importantly has the ability to reboot Network Access Devices to ensure the communication gateway is always available. Designed to provide increase reliability of broadband circiuts where details of such failures are able to be easily reported to any conventional Alarm monitoring service provider and/or the APSU owner.

[edit] Power conversion

The term "power supply" is sometimes restricted to those devices that convert some other form of energy into electricity (such as solar power and fuel cells and generators). A more accurate term for devices that convert one form of electric power into another form (such as transformers and linear regulators) is power converter. The most common conversion is AC-DC. This is a conversion from the household current AC, to the DC current that is used in your car, and most electronics.

[edit] Mechanical power supplies

[edit] Terminology

  • SCP:Short circuit protection
  • OPP:Overpower (overload) protection
  • OCP:Overcurrent protection
  • OTP:Overtemperature protection
  • OVP:Overvoltage protection
  • UVP:Undervoltage protection
  • UPS:Uninterruptable Power Supply
  • PSU:Power Supply Unit
  • APSU:Alarm Power Supply Unit

[edit] References

  1. ^ Quoting US patent #4937722, High efficiency direct coupled switched mode power supply: The power supply can also include crowbar circuit protecting it against damage by clamping the output to ground if it exceeds a particular voltage. http://www.patentstorm.us/patents/4937722-description.html
  2. ^ Quoting US Patent #5402059: A problem can occur when loads on the output of a switching power supply become disconnected from the supply. When this occurs, the output current from the power supply becomes reduced (or eliminated if all loads become disconnected). If the output current becomes small enough, the output voltage of the power supply can reach the peak value of the secondary voltage of the transformer of the power supply. This occurs because with a very small output current, the inductor in the L-C low-pass filter does not drop much voltage (if any at all). The capacitor in the L-C low-pass filter therefore charges up to the peak voltage of the secondary of the transformer. This peak voltage is generally considerably higher than the average voltage of the secondary of the transformer. The higher voltage which occurs across the capacitor, and therefore also at the output of the power supply, can damage components within the power supply. The higher voltage can also damage any remaining electrical loads connected to the power supply. http://www.patentstorm.us/patents/5402059-description.html
  3. ^ http://www.silentpcreview.com/article28-page3.html SPCR: Power Supply Fundamentals: POWER SHMOWER or How PSU Power Ratings Mean Almost Nothing
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[edit] See also

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