A power supply (sometimes known as a power supply unit or PSU) is a device or system that supplies electrical or other types of energy to an output load or group of loads. The term is most commonly applied to electrical energy supplies.
General description
The complete range of power supplies is very broad, and could be considered to include all forms of energy conversion from one form into another. Conventionally though, the term is usually confined to electrical or mechanical energy supplies. Constraints that commonly affect power supplies are the amount of power they can supply, how long they can supply it for 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.
Electrical power supplies
This term covers the mains power distribution system together with any other primary or secondary sources of energy such as:
- Conversion of one form of electrical power to another desired form and voltage. This typically involves converting 120 or 240 volt AC supplied by a utility company (see electricity generation) to a well-regulated lower voltage DC for electronic devices. For examples, see switched-mode power supply, linear regulator, rectifier and inverter (electrical).
- Batteries
- Chemical fuel cells and other forms of energy storage systems
- Solar power
- Generators or alternators (particularly useful in vehicles of all shapes and sizes, where the engine has rotational power to spare, or in semi-portable units containing an internal combustion engine and a generator) (For large-scale power supplies, see electricity generation.) Low voltage, low power DC power supply units are commonly integrated with the devices they supply, such as computers and household electronics.
Computer power supply
Main article: Computer power supply
A computer power supply typically is designed to convert 110 V or 230 V AC power from the mains to usable low-voltage DC power for the internal components of the computer. The most common computer power supplies are built to conform with the ATX form factor.
Domestic mains adaptors
A 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 or just power adapter. Because they consume Standby power, they are sometimes known as electricity vampires.
Linear power supply
A simple 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 circuit is employed either as a single chip, an array of diodes sometimes called a diode bridge both for fullwave rectification or a single diode yielding a half wave (pulsating) output. More elaborate configurations rectify the AC voltage at first to pulsating DC. Then a capacitor smooths out part of the pulses giving a type of DC voltage. The smaller pulses remaining are known as ripple. Because of a fullwave rectification they occur at twice the mains frequency (in
In the simplest case a single diode is connected directly to the mains and uses a resistor in series with a more or less fixed load to recharge a battery. This circuit is common in rechargeable flashlights.
Switched-mode power supply
In a switched-mode power supply the incoming power is passed through a transistor and transformer network that switches on and off at typical rates of 10 kHz to 1 MHz. The original design was invented by Dr Gary Bocock in 1965 often referred to as the father of power conversion. This means that a smaller, less expensive, lighter transformer can be used, because the voltage is being made to alternate faster, and thus a smaller magnetic core can be used. The switch mode PSU is also significantly more efficient than the linear design. Typically 80 - 90% compared to 50 - 60%. The downside to this design is that the switching on and off at high speeds introduces electrical noise into the system which can cause interference with the power supply load or other connected systems.
Switching power supplies can be used as DC to DC converters. In this application, the power supply is designed to accept a limited range DC input and then output a different DC voltage. This is particularly useful in portable devices, as well as power distribution in large electronic equipment. A transformerless switching power supply that outputs a voltage higher than its input voltage is typically called a boost converter. A transformerless switching power supply that outputs a voltage lower than its input voltage is typically called a buck converter. These transformerless switching power supplies use an inductor as the primary circuit element in converting the voltage. Circuitry is used to pass current through the inductor to store a certain amount of electrical energy as a magnetic field. The current flow is then stopped, and the magnetic field collapses causing the stored energy to be released as current again. This is done rapidly (up to millions of times per second). By carefully metering the amount of energy stored in the inductor, the current released by the inductor can be regulated thus allowing the output voltage to be tightly regulated. A switching power supply incorporating a transformer can provide many output voltages simultaneously, and is typically called a flyback converter. Switching power supplies are typically very efficient if well designed, and therefore waste very little power as heat. Because of these efficiencies, they are typically much smaller and lighter than an equivalently rated linear supply.
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 of electric power (such as transformers and linear regulators) is power converter.
Uses in aviation
The most exotic power supplies are used in aviation to enable reliable restarting of stalled engines.
In jet transports, an engine is restarted using power produced by 400 Hz three-phase AC generator(s) attached to the shaft(s) of the other engine(s). Most of the starting torque generated by the engine's motor/generator is provided by the current at the peaks of the AC waveform.
If the aircraft electronics used simple rectifying power supplies, they would use current only from these peaks, since the diodes conduct only during voltage peaks where input voltage is higher than output voltage. This could prevent the pilot from restarting an engine in an emergency.
Therefore, aircraft power supplies take energy evenly from all parts of the AC waveform. This uses a switching power supply technique called "power factor correction," which creates a balanced current draw over the entire AC waveform.
Mechanical power supplies
- Flywheels coupled to electrical generators or alternators
- Compulsators
- Explosively pumped flux compression generators
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