laptop power supplies
This term covers the 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 laptop power supply, linear
regulator, rectifier and inverter (electrical).
Chemical fuel cells and other forms of energy storage systems
Generators or alternators (particularly useful in vehicles of all shapes and
sizes, where the engine has torque to spare, or in semi-portable units
containing an internal combustion engine and a generator) (For large-scale
laptop power supplies, see electricity generation.) Low voltage, low power DC
laptop power supply units are commonly integrated with the devices they supply,
such as computers and household electronics.
Constraints that commonly affect laptop 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 laptop power supplies is done by incorporating circuitry to
tightly control the output voltage and/or current of the laptop power supply to
a specific value. The specific value is closely maintained despite variations in
the load presented to the laptop power supply's output, or any reasonable
voltage variation at the laptop power supply's input. This kind of regulation is
commonly categorised as a Stabilized laptop power supply.
laptop power supply types
laptop power supplies for electronic devices can be broadly divided into linear
and switching laptop power supplies. The linear supply is a relatively simple
design that becomes increasingly bulky and heavy for high current 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.
Linear laptop power supply
A home-made linear laptop power supply (used here to power amateur radio
equipment)An AC powered linear laptop 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 laptop 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 laptop power supply and attached circuit from overcurrent.
Adjustable linear laptop 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 laptop 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 laptop power supply circuit consists of a single diode and
resistor in series with the AC supply. This circuit is common in rechargeable
Main article: AC/DC (electricity)
In the past mains electricity was supplied as dc in some regions, ac in others.
Simple, cheap, linear laptop power supplies running directly from either ac or
dc mains, often without a transformer used. They used a rectifier and capacitor
filter; the rectifier was essentially a conductor, having no effect when
operating from dc.
Switched-mode laptop power supply
Main article: Switched-mode laptop power supply
A computers switched mode laptop power supply unit.A switched-mode laptop 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 laptop power supply. After the
transformer secondary, the AC is again rectified to DC. To keep output voltage
constant, the laptop power supply needs a sophisticated feedback controller to
monitor current draw by the load.
Modern switched-mode laptop power supplies often include additional safety
features such as the crowbar circuit to help protect the device and the user
from harm. 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. For decades PC computer laptop power supplies have
also provided a power good signal to the motherboard which prevents operation
when abnormal supply voltages are present.
Switched mode laptop power supplies have an absolute limit on their minimum
current output.  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 laptop power supplies have traditionally been a source of power
line harmonics and have a very poor power factor. Many computer laptop 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 laptop 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.
Uninterruptible laptop power supply
Main article: Uninterruptible laptop power supply
An Uninterruptible laptop 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
laptop power supply applications
Computer laptop power supply
Main article: Computer laptop power supply
A modern computer laptop 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, +3.3V and +5v StandBy. The first
computer laptop 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 current draw
requirements, which are difficult to all be supplied from the same switched-mode
source. Consequently most modern computer laptop 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 laptop power supplies are built to conform to
the ATX form factor. The power rating of a PC laptop power supply is not
officially certified and is self-claimed by each manufacturer.A common way to
reach the power figure for PC PSUs is by adding the power available on each
rail, which will not give a true power figure. The more reputable makers
advertise "True Wattage Rated" to give consumers the idea that they can trust
the power advertised.
Switched mode mobile phone chargerA linear or switched-mode laptop 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 pack", "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 laptop
power supplies must match the voltage of, and supply at least as much current
as, the original laptop power supply.
The least expensive AC units consist solely of a small transformer, 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 laptop 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 laptop power supply is popular among manufacturers of low cost
electrical items because
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.
The device itself doesn't need to be tested for compliance with electrical
safety regulations. Only the adapter needs to be tested.
Other advantages of using separate power adapters are: removing the transformer
as a source of heat, separating mains voltage, and making the devices smaller.
Diagram showing negative tip polarity on the right and positive tip polarity on
the leftAC-to-DC adapters have polarity; even if the plug fits into a device,
the positive and negative connections may be oriented the wrong way. It is
necessary to use an adapter with the correct polarity to avoid damage.
The term " laptop 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