View A Video on Power Supplies - A Quick Introduction
Power supplies can be integrated into a device or externally attached, portable modules, depending on their operating temperature and risk of overheating. Power supplies are necessary to the operation of just about every electrical device, including desktop and laptop computers, cell phones, lasers, telecommunications like radio, phone lines and the internet, medical equipment, lamps, appliances and industrial machinery. They provide either AC power supply, which is an alternating current, or DC power supplies, which offer a direct current. Today, most electronic devices in the home and office are powered by 12v power supply, while industrial applications employ high voltage power supply. Different devices and electronics require certain types of current, frequency and voltage. In these cases, AC to DC converters are used to switch the type of current, which are also called power inverters. They use rectifier, which contain diodes that alter and regulate the electrical current. Frequency converters and switching power supplies are integrated into the power supply unit in order to give off the appropriate output. Some variable power Supplies are able to adjust the output voltage to specific requirements for product testing and design. Most electronic products today require regulated power supply, a type that produces stable and constant output at a certain, unwavering voltage regardless of power outages, brown outs or surges. Most power supplies are protected by a backup battery. These are called uninterruptible power supplies, and are reliable even when there is no power available.
There are two main types of electrical current that are regulated, controlled and altered by power supplies-alternating current and direct current. Both are used to power different kinds of electrical products, but the input into a power supply from a battery or other power source is almost always AC. Alternating current exhibits electrical charge that consistently and periodically reverses direction. It moves forward then backwards over and over. This form of current is used in commercial businesses and residential buildings. The alteration of the current's direction is measured in Hertz. For example, 60 Hertz refers to the number of alternative directions the current takes in a second. Direct current, on the other hand, refers to electric charge flow that runs in a single, linear direction. It flows in metal conductors like wires, semiconductors, insulators or even a vacuum. Cell phones and laptop computers use DC, as well as medical equipment, video technology and process control systems. Direct current units are usually external from the electronic device and held within a protective casing.
Power supply manufacturers offer many different designs and configurations of power supply units, which range depending on their application, type of current, frequency and voltage level. Some of the unit designs and styles are external, meaning they are separate components to the electronic device. These include board, cabinet, desktop, module, open frame, enclosed, rack mount and wall mount. If their operating temperature is low enough, many power supply units are integral parts of the device, located inside them. Some of the display choices available, which provide information about the voltage and current that are the result of measuring and monitoring, are digital numerical displays, analog visual indicators and graphic or video displays. Some of the various features include adjustable voltage, which can be increased or decreased by a dial or knob, adjustable frequency, computer interface, fan cooling systems, as well as integral heatsink and overcurrent protection. Many also work in conjunction with a back up battery that is employed in case of a power outage. Other features include overvoltage protection, power factor correction, pure sine output, remote on and off switch, short circuit protection and water cooling. When looking at power supply units, consider some of these specifications: the number of outputs, DC output voltage, DC output power, AC output voltage, AC output frequency, operating temperature and apparent power.
One of the most common applications for power supply manufacturers is electrical power for computers. This vital component is a smaller, black metal box typically located on the back of the computer in a corner of the case. The power supply unit also contains the power-cord receptacle and the cooling fan and is usually visible from the back of a system. Power supply units use switcher technology to convert AC input to lower DC voltages, so they are commonly referred to as switching power supplies. The voltages commonly supplied are 3.3 and 5 volts, which are used by digital circuits, and 12 volts, which run motors in disk drives and fans. In order for the power button to work when the PC is off, the power supplies have a circuit that supplies 5 volts, called "standby voltage" or VSB.
There are three types of uninterruptible power supplies (UPS). An offline UPS is basically a standby system that provides battery power to equipment when the main power supplies fall below a set limit. These power supplies do not cost much and are recommended for home office use. A line interactive UPS is similar to an offline UPS in that it switches to battery mode during a blackout. However, this UPS actually boosts the main power supply when it falls, using a regulator. These power supplies are ideal for business applications. The highest level of protection for an electrical device is an online UPS unit that converts AC to DC and then back to AC to supply critical power loads. These UPS units, often referred to as double conversions, contain an automatic bypass to ensure continuous power supplies during a short-term overload or UPS failure. On-line UPS systems are perfect for critical loads and sensitive equipment, such as medical technology.
Power Supply Types
converts power from an AC input, such as a wall outlet, into DC current.
are units that provide power
to an electronic device by converting AC current, such as that which
comes from a wall outlet, to DC current at the proper voltage.
control the output current for alterations
in load, line and ambient temperature and time within particular limits.
control the output voltage in load,
line, ambient temperature and drift resulting from changes over time
are used to increase or decrease the voltage level
of DC electrical power, because, unlike AC, DC cannot be changed using
such as linear power supplies, switching power supplies, DC/DC converters
and high voltage power supplies,
an input power and output the required form of DC power.
are utilized at higher levels in static
applications, due to their weight. Ferroresonant power supplies are
effective only when the line frequency is extremely stable, as they
are sensitive to changes of input AC frequencies.
, a special type of transducer,
are simply electrical currents that convert periodic signals into their
equivalents. The most common frequency converters are frequency-to-digital
and frequency-to-DC converters.
are power switching circuits consisting of
two transistors and two capacitors. Half bridge converters function
similar fashion to full bridge converters.
are capable of providing hundreds or
thousands of volts from one or more DC outputs, using linear technology.
high voltage power supplies have adjustable local or computer interface
outputs and are used in specialized applications, including telecommunications,
video technology and medical equipment.
change DC current to AC current and may be mechanical
(e.g. motor), ferroresonant and solid state.
have a bulky steel or iron laminate transformer
that acts as a safety barrier for the low voltage output from the
AC input and reduces that input to a much lower voltage. The AC
output is then rectified by two or four diodes, and electrical converters
change the output into low voltage DC, which is regulated into
required output voltage by dropping the difference in voltage across
the shunt regulator.
are comprised of a number of separate subsections,
such as power, input and filter modules.
operate directly off the AC line. Off-line
power supplies do not use a power transformer before the process
of rectification and filtering.
have a high open loop gain regulator,
for which passive components can be used to program. The
like an operational amplifier.
- Power inverter converts DC current to AC current.
are electrical components containing sets of diodes that
change AC into DC.
rectify and smooth AC voltage using diodes
and capacitors, resulting in a high voltage DC, which
in turn is converted by a small ferrite transformer and FETs or transistors
into a safe,
low voltage, high frequency voltage. Another set of diodes,
capacitors and inductors convert that DC voltage into the required
the corrections of which are done by adjusting the pulse
of the high
are power supply units that continue
to provide power during the loss of AC input power,
which is achieved through a back up battery and a DC/AC inverter or
A stand alone UPS unit is external to the equipment
while a battery back up UPS is implanted in the equipment.
Power Supply Terms
- The highest AC or DC voltage that may be applied
from the input, output and/or chassis of a power supply.
- Operating a newly
manufactured power supply, usually at rated load, for a period of time
in order to force early failures or
other latent defects of the component before the unit is delivered to
- Noise that is typical of DC output and return
lines with respect to input ground.
- The voltage output from a constant current
- The removal of heat in a power supply by convection,
forced air, radiation or liquid. Heat comes from regulation, transformation,
filtering and rectification.
- The percent of voltage change at one output
of a multiple output power supply resulting from the load change on another
overload protection circuit, which controls the highest output current
of a power supply to safeguard the
power supply or the load.
- The projected lifetime of a power supply during which
it will run at its stated specifications.
- Also referred to as "ripples," it
is the noise measured between the DC output and the output return.
- With operating parameters including line, load and ambient
temperature held constant, it is the change in output voltage, following
a warm-up period, over a certain period of time.
- The ratio of power in terms of the input power against
the output power. Efficiency is measured at full load and nominal line
- Also known as "radio-frequency
interference (RFI)," it is unwanted high frequency energy conducted
through the input or output lines of switching power supplies or radiated
through space. EMI is caused by the switching transistors, output rectifiers
and zener diodes.
- A current limiting circuit
that, when under overload conditions, will gradually decrease the output
current to a specified minimum current level under a direct short circuit.
- An electrical connection to earth that has a zero voltage
or another conductor connected to earth.
- The capability of remotely switching off the output power
of a power supply.
- A low-pass or band-reject filter used to decrease
the noise fed to the supply. Input line filters are located at the input
of a power supply and may be external.
- The highest AC or DC voltage that can be continuously
run from a power supply chassis or from input to output.
- Altering a power supply output voltage, either higher
or lower from its minimal setting, in order to confirm the system performance
margin in respect to the supply voltage. Margining is typically done
electrically via a system generated control signal.
- The least amount of load current or power that needs
to be drawn from the power supply in order for the supply to meet its
- A feature of a converter
such that it continues to provide voltage to a load at a set upper limit
without turning off and without necessitating a reset when the overvoltage
- A circuit that either shuts down the power
supply or shorts the power supply to ground if an overvoltage condition
- The connection of the outputs of multiple
power supplies with the same output voltage that are designed to share
a load. The parallel operation generates a higher output current than
would be available from a single supply.
- The absolute highest output power that a power supply
can create without immediate damage. Typically, peak power is much higher
than the continuous reliable output capacity and ought to be utilized
- A signal from the power supply interface that relays
a warning that the input voltage is not sustaining full power regulated
- A protection circuit that prevents
damage to the power supply if a reverse voltage is applied at either
the output or input terminals.
- A conductive path to earth intended to safeguard
people from electrical shock by shunting away any dangerous currents
that could happen from accident or malfunction.
- Also known as "warm up time," it is
the time a converter needs to start running within specification after
proper power has been applied.
- A sensing circuit for the
input voltage located within the power supply that automatically switches
to the necessary voltage range.