Every power supply must obtain the energy it supplies to its load, as well as any energy it consumes while performing that task, from an energy source. Depending on its design, a power supply may obtain energy from:
- Electrical energy transmission systems. Common examples of this include power supplies that convert AC line voltage to DC voltage.
- Energy storage devices such as batteries and fuel cells.
- Electromechanical systems such as generators and alternators.
- Solar power.
Commonly specified power supply attributes include:
- The amount of voltage and current it can supply to its load.
- How stable its output voltage or current is under varying line and load conditions.
- How long it can supply energy without refueling or recharging (applies to power supplies that employ portable energy sources).
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[edit] Power supplies types
Power supplies for electronic devices can be broadly divided into line-frequency (or "conventional") and switching power supplies. The line-frequency supply is usually a relatively simple design, but it becomes increasingly bulky and heavy for high-current equipment due to the need for large mains-frequency transformers and heat-sinked electronic regulation circuitry. Conventional line-frequency power supplies are sometimes called "linear," but that is a misnomer because the conversion from AC voltage to DC is inherently non-linear when the rectifiers feed into capacitive reservoirs. Linear voltage regulators produce regulated output voltage by means of an active voltage divider that consumes energy, thus making efficiency low. A switched-mode supply of the same rating as a line-frequency supply will be smaller, is usually more efficient, but will be more complex.[edit] Battery
Main article: Battery (electricity)
There are two types of batteries: primary batteries (disposable batteries), which are designed to be used once and discarded, and secondary batteries (rechargeable batteries), which are designed to be recharged and used multiple times. Batteries come in many sizes, from miniature cells used in hearing aids and wristwatches to room-size battery banks that serve as backup power supplies in telephone exchanges and computer data centers.
[edit] DC power supply
For purposes such as charging batteries the ripple is not a problem, and the simplest unregulated mains-powered DC power supply circuit consists of a transformer driving a single diode in series with a resistor.
Before the introduction of solid-state electronics, equipment used valves (vacuum tubes) which required high voltages; power supplies used step-up transformers, rectifiers, and filters to generate one or more direct voltages of some hundreds of volts, and a low alternating voltage for filaments. Only the most advanced equipment used expensive and bulky regulated power supplies.
[edit] AC power supply
An AC power supply typically takes the voltage from a wall outlet (mains supply) and lowers it to the desired voltage (e.g. 9 VAC), some filtering may take place as well.[edit] Linear regulated power supply
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 may be used to set the voltage to a precise value, stabilized against fluctuations in input voltage and load. The regulator also greatly reduces the ripple and noise in the output direct 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 adjusted over a 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.
[edit] AC/DC supply
Main article: AC/DC (electricity)
In the past, mains electricity was supplied as DC in some regions, AC
in others. Transformers cannot be used for DC, but a simple, cheap
unregulated power supply could run directly from either AC or DC mains
without using a transformer. The power supply consisted of a rectifier
and a filter capacitor.
When operating from DC, the rectifier was essentially a conductor,
having no effect; it was included to allow operation from AC or DC
without modification.[edit] Switched-mode power supply
Main article: Switched-mode power supply
Switched-mode power supplies are always regulated.[citation needed] To keep the output voltage constant, the power supply employs a feedback controller that monitors current drawn by the load. The switching duty cycle increases as power output requirements increase.
SMPSs often include safety features such as current limiting or a crowbar circuit to help protect the device and the user from harm.[1] In the event that an abnormal high-current power draw is detected, the switched-mode supply can assume this is a direct short and will shut itself down before damage is done. PC power supplies often provide a power good signal to the motherboard; the absence of this signal prevents operation when abnormal supply voltages are present.
SMPSs have an absolute limit on their minimum current output.[2] They are only able to output above a certain power level and cannot function below that point. In a no-load condition the frequency of the power slicing circuit increases to great speed, causing the isolated 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-power 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 sinusoidal AC wave into very small discrete pieces, a portion of unused alternating current 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 actual power used. Filtering capacitor banks may be needed on the building power mains to suppress and absorb these negative power factor effects[citation needed].
Some switch-mode power supplies use L-C resonance in the primary circuit to convert what would otherwise be a square wave into a sinusoidal waveform. This can decrease the losses in the switching devices and reduce RF harmonics of the switching frequency, but it adds to the circuit complexity and places higher demands on design tolerances.
[edit] Programmable power supply
Programmable power supplies can furnish DC, AC, or AC with a DC offset. The AC output can be either single-phase or three-phase. Single-phase is generally used for low-voltage, while three-phase is more common for high-voltage power supplies.
Programmable power supplies are now used in many applications. Some examples include automated equipment testing, crystal growth monitoring, and differential thermal analysis.[3]
[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 . 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] High-voltage power supply
High voltage refers to an output on the order of hundreds or thousands of volts. High-voltage supplies use a linear setup to produce an output voltage in this range.Additional features available on high-voltage supplies can include the ability to reverse the output polarity along with the use of circuit breakers and special connectors intended to minimize arcing and accidental contact with human hands. Some supplies provide analog inputs (i.e. 0-10V) that can be used to control the output voltage, effectively turning them into high-voltage amplifiers albeit with very limited bandwidth.
[edit] Voltage multipliers
Main article: Voltage multiplier
A voltage multiplier is an electrical circuit that converts AC
electrical power from a lower voltage to a higher DC voltage, typically
by means of a network of capacitors and diodes. The input voltage may be
doubled (voltage doubler), tripled (voltage tripler),
quadrupled (voltage quadrupler), and so on. These circuits allow high
voltages to be obtained using a much lower voltage AC source.Typically, voltage multipliers are composed of half-wave rectifiers, capacitors, and diodes. For example, a voltage tripler consists of three half-wave rectifiers, three capacitors, and three diodes (as in the Cockcroft Walton multiplier). Full-wave rectifiers may be used in a different configuration to achieve even higher voltages. Also, both parallel and series configurations are available. For parallel multipliers, a higher voltage rating is required at each consecutive multiplication stage, but less capacitance is required. The voltage rating of the capacitors determines the maximum output voltage.
Voltage multipliers have many applications. For example, voltage multipliers can be found in everyday items like televisions and photocopiers. Other applications can be found in the laboratory, such as cathode ray tubes, oscilloscopes, and photomultiplier tubes.[4][5]
[edit] Power supply applications
[edit] Computer power supply
Main article: Power supply unit (computer)
A modern computer power supply is a switch-mode power supply that
converts AC power from the mains supply, to several DC voltages.
Switch-mode supplies replaced linear suplies due to cost, weight, and
size improvement. The diverse collection of output voltages also have
widely varying current draw requirements.[edit] Welding power supply
Main article: Welding power supply
Arc welding uses electricity to melt the surfaces of the metals in order to join them together through coalescence. The electricity is provided by a welding power supply, and can either be AC or DC. Arc welding typically requires high currents typically between 100 and 350 amps. Some types of welding can use as few as 10 amps, while some applications of spot welding employ currents as high as 60,000 amps for an extremely short time. Older welding power supplies consisted of transformers or engines driving generators. More recent supplies use semiconductors and microprocessors reducing their size and weight.
