AC:
(Alternating Current) Voltage or current which is periodically changing
polarity
AC-DC
Converter: Converts an alternating current power supply
eg: 230V mains into one or several DC voltages as required by particular
electronic circuits or in APC’s case mainly for the charging
of battery operated equipment.
Ambient
temperature: The still-air temperature of the immediate
environment measured a minimum of 100mm from the power supply
Amps:
(Amperes) Unit of measurement of electromotive current (A).
Basic
Insulation: According to international safety standards
(eg UL1950, EN60950) basic insulation provides basic protection
against electric shock. In contrast, operational insulation is needed
for the correct operation of the equipment, but does not protect
against electric shock. Quite frequently, safety standards call
for basic insulation between secondary circuits.
Breakdown
voltage: See: Isolation
Bridge:
Rectifier circuit incorporating four diodes (full-bridge)
or two diodes (half-bridge). Converter or chopper section of switching
power supplies incorporating four transistors (full-bridge) or two
transistors (half-bridge).
Brownout:
Condition during peak usage periods when electric utilities
reduce their nominal line voltage by 10% to 15%.
Brownout
protection: The ability of a power supply to continue operating
within specification through the duration of a brownout.
Burn
In:The period directly following the very first turn-on
of a given power supply. It is characterized by a relatively high
and declining failure rate.
Bus:
The system of conductors (wire, cable, copper bars, etc.) used to
transport power from the power supply to the load. A communications
structure used to control various instruments and subsystems.
Busbar:
A low impedance conductor which feeds power to various circuits
in, for example, an equipment room or within an equipment rack.
In the telecommunications industry the busbar voltage is often at
a standardised 48V, requiring dc-dc converters to convert the busbar
voltage to such voltages as are used by the electronics circuits
it is supplying
CB-report:
Document necessary for the mutual recognition of approvals between
different national test norms.
CEE
(International Commission on Rules for the Approval of
Electrical Equipment): A regional, European safety agency. NB: the
United States participates only as an observer.
Common-mode noise: The components of noise which is common to both
the DC output and return lines with respect to input ground
Constant current: A power supply that regulates current level regardless
of changes in load resistance.
Constant
current limiting circuit: Current-limiting circuit that
holds output current at some maximum value whenever an overload
of any magnitude is experienced.
Constant
voltage: A power supply that regulates voltage level regardless
of changes in load resistance.
Convection:
The transference of thermal energy in a gas or liquid by currents
resulting from unequal temperatures.
Convection-cooled
power supply is a PSU that is cooled from the natural motion
of an air over the surfaces of its components.
Crowbar:
A type of overvoltage protection circuit which rapidly places a
low resistance shunt across the power supply output terminals if
a predetermined voltage is exceeded.
CSA
(Canadian Standards Association): An independent Canadian organization
testing for public safety, similar to the function of Underwriters’
Laboratories in the United States.
Current:
Rate of transfer of electrical charge measured in amperes (A)
Current
limiting circuit: A circuit designed to prevent overload
of a constant-voltage power supply. It can take the form of constant,
foldback or cycle-by-cycle current limiting.
Cycle-by-cycle
current limiting circuit: Current-limiting circuit that
immediately reduces output current to some minimum level whenever
an overloaded of any magnitude is experienced.
DC:
Direct Current or undirectional voltage or current
Derating:
The specified reduction in an operating parameter to improve
reliability. Compensates for a change in one or more other parameters.
In power supplies, the output power rating is generally reduced
at elevated temperatures.
Differential
mode noise: That component of noise measured between the
DC and output return. It does not include common-mode noise.
DPA
(Distributed Power Architecture): A system where the supply
power is fed to each electronic unit on a busbar and then locally
converted at rack or PCB level to such voltages as the electronic
circuits require.
Drift:
See: Stability.
Dynamic
load: A load that rapidly changes from one level to another.
To be properly specified both the total change and the rate of change
must be stated.
Efficiency:
The ratio of total output power to total input power, expressed
as a percentage, under specified conditions.
EMC
(Electromagnetic compatibility): Any electromagnetic effect:
Emissions from elements within apparatus (motors, converters, choppers),
disturbance of elements and measures for improving the functionality.
EMI
(electromagnetic interference): Also called radio-frequency
interference (RFI), EMI is unwanted high frequency energy caused
by the switching transistors, output rectifiers, and zener diodes
in switching power supplies. EMI can be conducted through the input
or output lines or radiated through space.
ESR
(Equivalent Series Resistance): The amount of resistance in series
with an ideal (lossless) capacitor, which duplicates the performance
of a real capacitor. In general, the lower the ESR, the higher the
quality of the capacitor and the more effective it is as a filtering
device. ESR is a prime determinant of ripple in switching power
supplies.
ETSI
(European Telecommunications Standards Institute): Non-profit making
organisation whose mission is to determine and produce the telecommunications
standards that will be used for decades to come.
Faraday
shield: An electrostatic shield wound on a transformer,
designed to reduce interwinding capacitance. The result is less
common- and differential-mode noise at the output of the power supply.
FCC
(Federal Communications Commission): United States federal regulating
body whose new EMI limitations are affecting the design and production
of digital electronics systems and their related subassemblies,
such as power supplies.
Ferroresonance:
The principle used open-loop (non-feedback) voltage stabilizing
power supply.
Filter:
A frequency-sensitive network that attenuates unwanted noise and
ripple components of a rectified output.
Flyback
converter: Switching power supply configuration using a
single transistor and a flyback diode.
Foldback
current limiting circuit: Current-limiting circuit that
gradually decreases the output current under overload conditions
until some minimum current level is reached under a direct short
circuit.
Forward
converter: Switching power supply configuration using a
single transistor.
Frequency
changer: Power-conversion equipment that transforms AC
electric power from one frequency to another without affecting its
other characteristics.
Full
Brick: An industry standard size and pin-out for DC-DC
Converters. The package size is 2.50” x 4.8” with the
pins on a 4.47” spacing. The height is usually 0.50”
without heatsink.
Full-bridge
converter: Four-transistor switching power supply configuration
used to handle high power levels.
Half-bridge
converters: Two-transistor switching power supply configuration
used in medium-power applications.
Half-wave
rectifier: Single-diode rectifier circuit that rectifies
only one-half the input AC wave.
Full-wave
rectifier:: Rectifier circuit that rectifies both halves
of an AC wave.
Ground
loop: An unwanted feedback problem caused by two or more
circuits sharing a common electrical line, usually a common ground
line.
Harmonic
distortion: AC current outputs with multiple harmonic frequencies
to AC line frequency provoked by the switching devices in a power
supply.
Head
room: In a linear regulator, the head room is the difference
between the secondary voltage supplied by the power transformer
at nominal input voltage and the regulated output voltage. Head
room is necessary to ensure proper regulation under full load and
low input voltage operation.
Heat
sink: Device used to conduct away and disperse the heat
generated by electronic components.
Hi-pot
(high potential voltage): See: Isolation test voltage.
Holdup
time: The time under worst case conditions during which
a power supply’s output voltage remains within specified limits
following the loss or removal of input power; also called “ride-through”.
Hybrid
supplies: A power supply that combines two or more different
regulation techniques, such as ferroresonant and linears or switching
and linear.
IEC
(International Electrotechanical Commissions): An international
safety agency headquartered in Geneva, Switzerland.
Inhibit:
The ability to electrically turn off the output of a power
supply from a remote location.
Input
voltage range: The range of source voltages for which the
power supply meets its specifications.
Inrush
current: A high surge of input current that occurs in switchers
and occasionally in linears upon initial turn on, caused by charging
of the input capacitors.
Isolating
power supply is a PSU that provides isolation between input
and output. Isolation is absence of DC current pass between two
circuits. Isolation in PSU is provided by transformers. Most of
commercially available AC-DC PSU are isolating. Low input voltage
DC-DC PSU are often non-isolating, such as a car phone charger.
Note that isolation does not mean a complete absence of AC current
path- some AC currents can still flow from input to output through
physical and/or stray capacitance.
Instantaneous
current limiting circuit: See: Cycle-by-cycle current limiting
circuit.
Insulation:
Material used to insulate a device by preventing or reducing the
transmission of electricity.
Inverter:
A power converter which changes DC Input power into AC output power.
Isolation:
The degree of electrical separation between two points.
It can be expressed in terms of voltage (breakdown), current (galvanic),
or resistance and/or capacitance (impedance). In power supplies,
it is important to maximize the input to output isolation.
Isolation
test voltage: Ability of a power supply to withstand a
high voltage potential placed either from the input terminals to
ground, from any of the output terminals to ground, or between any
pair of input and output terminals. This specification is important
for safety reasons and is partially dependent on the mechanical
design of the power supply.
Leakage
current: Current flowing between the output buses and chassis
ground due to imperfections in electronic components and designs.
It must be tightly controlled to satisfy safety regulations such
as UL and VDE.
Line
regulation: The variation of an output voltage due to a
change in the input voltage, with all other factors held constant.
Line regulation is expressed as the maximum percentage change in
output voltage as the input voltage is varied over its specified
range.
Linear
power supply is a PSU that regulates the output parameter
(usually output voltage) by varying voltage drop accross an electronic
component placed in series with the load which dissipates unused
power. This component may be a power semiconductor or a resistor.
The regulation is accomplished by changing its effective resistance
(if it is a power semiconductor) or by forcing extra current through
it (if it is a resistor).
Linear
regulator: A common voltage stabilization technique in
which the control device (usually a transistor) is placed in series
or parallel with the power source to regulate the voltage across
the load. The term “linear” is used because the voltage
drop across the control device is varied continuously to dissipate
unused power.
Load:
For voltage regulated power supplies, the load is the output current.
Load
regulation: Variation of the output voltage due to a change
in the output’s load within a specified range with all other
factors held constant. It is expressed as a percentage of the nominal
DC output voltage.
Logic
inhibit: The ability to turn a power supply off and on
with TTL signals. A logic low generally allows the power supply
to operate. A logic high turns off the power supply. See also: Logic
low.
Logic
low: A TTL voltage lower than 0.8 V. Also known as a “logic
0”.
Master/Slave
operation: In order to increase output power and provide
redundancy should one converter fail, several converters are often
connected in parallel. In most cases one type of converter is then
used as master, controlling the other (slave) converters
Modular:
A physically descriptive term used to describe a power supply made
up of a number of separate subsections, such as an input module,
power module, or filter module. Modular construction tends to lower
the MTBF.
MTBF
This measurement, expressed in hours, gives the relative
reliability, and can be based on actual operation or on a calculated
standard such as MIL
Multiple
output supply: A power supply that delivers two or more
different output voltages.
Noise:
Noise is a periodic, random component of undesired deviations
in output voltage. Usually specified in combination with ripple.
See: PARD and also: Ripple.
Nominal
output voltage: The intended, ideal voltage of any given
output.
Off-line
power supply is a PSU that takes its power directly from AC line
without using line frequency transformer. Such supply can still
be isolating if it uses high frequency transformer in one of its
power conversion stages. A typical off-line SMPS rectifies input
AC line voltage, converts it into high-frequency AC voltage by using
semiconductor power switches, steps that voltage up or down by using
inductors and/or transformers, then rectifies it again and filters
for DC output. An example of isolating off-line PSU is a computer
switching power supply.
Open-frame
construction: A construction technique common to OEM power
supplies where the supply is not provided with an enclosure. It
can be either a simple printed circuit board or circuit board mounted
on a metal chassis without a cover.
Operating
temperature: The range of temperatures within which a power
supply will perform within specified limits.
Opto-isolator:
Device that provides electrical isolation and a signal
path by making an electrical to optical to electrical signal transformation
from its input to output terminals. This is accomplished with a
light-emiting diode in close proximity to a phototransistor. Opto-isolators
are used in the feedback loop to maintain electrical isolation between
the input and output of the power supply. Ageing may provoke degraded
feedback response.
Output
impedance: The value of a fictional resistor in series
with an ideal voltage source that would give the same magnitude
of AC voltage across the supply terminals as observed for a particular
magnitude and frequency of alternating current.
Output
voltage: The voltage measured at the output terminals of
a power supply; A feature or device that senses and responds to
current or power overload conditions;
Overcurrent
protection: See: Current limiting circuit.
Overshoot:
The amount by which an output exceeds its final value in response
to a rapid change in load or input voltage, measured as a percentage
of the nominal. It is an important value at turn-on and following
a step change in load or line voltage.
OVP (overvoltage protection): A protection mechanism
for the load circuitry that does not allow the output voltage to
exceed a preset level. In most cases, the output voltage is reduced
to a low value, and the input power must be recycled to restore
the power supply output
Parallel
operation: The ability of power supplies to be connected
so that the current from corresponding outputs can be combined into
a single load.
PARD:
Acronym for “Periodic And Random Deviation” and used
as the specification term for ripple and noise. Ripple is the unwanted
portion of the output harmonically (periodically) related in frequency
to the input line and to any internally generated switching frequency.
Noise is the unwanted, a periodic output deviation.
Pass
element: The active circuit element, typically a transistor,
that forms the output power stage of a linear power supply.
Peak
charging: A rise in voltage across a capacitor caused by
the charging of the capacitor to the peak rather than rms value
of the input voltage. This generally occurs when a capacitor has
a high discharge resistance across it and large ripple and noise
or spikes on its input line. In a switcher, this parameter may affect
minimum load conditions (discharge resistance) on each output required
to maintain regulation.
Peak
transient output current: The maximum peak current that
can be delivered to a load during transient loading conditions,
such as electric motor starts.
Phase
controlled modulation: A circuit used in switching regulators
where the operating frequency is held constant (typically 50 or
60 Hz line frequency) and the phase angle at which the control elements
are turned on its varied, controlling both line and load changes
with minimal dissipation.
Pin
fins: Type of heatsink that uses pins in place of conventional
extruded fins.
Planar
Magnetics: Use of high volume manufacturing processes and
technologies to replace conventional wire windings in magnetic components
with patterned conductors formed on a single or multi-layer substrates.
Planar magnetic offer advantages in terms of cost, reliability,
manufacturability and predictable electrical parameters.
Post
regulator: Usually a linear regulator used on the output
of a switching or ferro power supply to improve overall (load) regulation.
Power
factor: The ratio of actual power used in a circuit to
the apparent power. Power factor is the measure of the fraction
of current in phase with the voltage and contributing to average
power.
Power
fail detect: A circuit that senses the DC voltage across
the input capacitors of a switching power supply. Should the AC
input line fail, it senses an abnormally low DC level across the
capacitors and provides an isolated TTL output signal warning of
imminent loss of output power.
Power supply or Power Supply Unit (PSU) is a device
that transfers electric energy from the source to the load using
electronic circuits. A common application of power supplies is to
convert raw input power into a regulated voltage and/or current
required for an electronic equipment.
Pre-regulator:
A regulator circuit that provides a line-regulated output,
which is then processed by a second regulator, the post-regulator,
which provides load regulation.
Programming:
The capability of controlling the voltage of each output.
Push-pull
converter: Used in switching power supplies where the main
switching circuit uses two transistors operating in push-pull. The
main advantage is simplicity of design.
PWM
(Pulse width modulation): A circuit used in switching regulated
power supplies where the switching frequency is held constant and
the width of the power pulse is varied, controlling both line and
load changes with minimal dissipation.
Rated
output current: The maximum continuous load current a power
supply is designed to provide under specified operating conditions.
Recovery
time: The time required by a transient over or under shoot
in a stabilised output quantity to decay within specified limits
Redundancy:
The ability to connect power supplies in parallel so that if one
fails the other will provide continuous power to the load. This
mode is used in systems when power supply failure cannot be tolerated.
Reference:
A known stable voltage to which the output voltage is compared for
the purpose of stabilizing the output voltage.
Regulated
power supply is a PSU that maintains a given output parameter (usually
output voltage) to within specified limits under varying operating
conditions, such as input line, output load, ambient temperature.
PSU can be linear or switched-mode (switching) depending on the
method of regulation and mode of operation of power handling components.
Regulator:
The part of a power supply that controls the output voltage. In
most cases, the regulator acts to stabilize the output voltage at
a preset value.
Remote
on-off: See: Inhibit.
Remote
sensing: A method of moving the point of regulation from
the output terminals to the load. Compensates voltage drops in the
power distribution bus, but negative impact on dynamic load behaviour
must be tolerated.
Response
time: The time required (usually ms) for the output of
a power supply or circuit to reach a specified fraction of its new
value after a step change or disturbance.
Return:
An arbitrary name for the common terminal for all the outputs.
It carries the return current of all the outputs.
Reverse
voltage protection: The ability of a power supply to withstand
reverse voltage at the input terminals when hooked up in the reverse
polarity.
RFI
(radio frequency interference): See: EMI.
Ripple:
The periodic AC component at the power source output harmonically
related to source or switching frequencies.
Ripple
voltage: The periodic AC component of the DC output of
a power supply.
Schottky
diode: A device that exhibits a low forward voltage drop
(e.g.0.4 V) and a fast recovery time. This type of diode is especially
useful at high current, low voltage (typically 5VDC), where low
losses and high switching speed are important.
Semi-regulated
output: A secondary output on a multiple-output power supply
that receives line regulation only.
Sense
line: S+ and S- lines, complementary to the Vo+ and Vo-
lines, allowing the compensation of voltages drops due to line resistance.
Sequencing:
Controlling the time delay and order of output voltage appearance
and drop-out upon power supply turn on and turn off.
Series
regulator: A linear regulator in which the active control
element (transistor) is in series connection with the load.
Short-circuit
protection: See: Current limiting circuit.
Shunt
regulator: A linear power supply in which the active control
element (transistor) is in parallel with the load.
Slave:
The unit in a master-slave paralleling scheme that is controlled
by the master unit. See: Master/Slave operation.
Snubber:
A network containing a resistor, capacitor, and diode used in the
switching power supplies to trap high-energy transients and to protect
sensitive components.
Soft
start: Input surge-current limiting in a switching power
supply where the switching drive is slowly ramped on.
Stability:
The change in output voltage of a power supply over a specific period
of time, following a warm up period, with all other operating parameters
such as line, load and ambient temperature held constant.
Standby
current: The input current drawn by any power supply under
minimum load conditions.
Static
load: A load that remains constant over a given time period.
It is usually specified as a percentage of full load.
Step
change: An abrupt and sustained change in one of the influence
or control quantities (e.g. load current).
Stress-ageing:
The process of subjecting a completed power supply to a variety
of stresses to force the occurrence of all burn-in-failures.
Surface
Mount Technology (SMT): A space saving technique whereby
special leadless components are soldered onto the surface of a PCB
rather than into holes in a PCB. The parts are smaller than their
leaded versions and PCB area is thus saved.
SMPS
or Switched-mode power supply is a PSU that incorporates
power handling electronic components that are continuosly switching
on and off with high frequency in order to provide the transfer
of electric energy. By varying duty cycle, frequency or a phase
of these transitions an output parameter is controlled. Typical
frequency range of SMPS is from 20 kHz to several MHz. The actual
choice of operating frequency is usually the trade off between size
and efficiency.
Switching
frequency: The rate at which the source voltage is switched
in a DC to DC converter.
Switching
regulator: A high-efficiency non-isolated DC to DC converter
consisting of inductors and capacitors to store energy and switching
elements (typically transistors or SCRs), which open and close as
necessary to regulate voltage across a load. The switching duty
cycle is generally controlled by a feed-back loop to stabilize the
output voltage.
Synchronous
Rectification: A circuit arrangement where the output rectifier
diodes of a power supply are replaced with active switches such
as MOSFETs. The switches are turned on and off under control and
act as rectifiers. This results in considerably lower losses in
the output stage and subsequently much higher efficiency. They are
particularly useful with low voltage outputs.
Temperature
coefficient: The average percentage of change in output
voltage per degree change in temperature with load and input voltage
held constant.
Thermal
protection: A protective feature that shuts down a power
supply if its internal temperature exceeds a predetermined limit.
Thermal
regulation: See: Temperature coefficient.
Thermistor:
A device with relatively high electrical resistance when cold and
almost no resistance when at operating temperature. Thermistors
are sometimes used to limit inrush current in off-line switchers.
Topology:
Topology is the fundamental circuit design of a clearly identifiable
and characteristic type. DC-DC converters can be designed along
several different topologies (using different fundamental design
principles). A patent for a particular topology can be very powerful
in that it can encompass any circuit solution regardless of power
output, falling within the design principles of the topology in
question.
Transformer:
A magnetic device that converts AC voltages to AC voltages at any
level. An ideal transformer is a lossless device in which no energy
is lost and that requires no magnetising current.
Transient:
A temporary and brief change in a given parameter. Typically associated
with input voltage or output loading parameters.
Transient
response time: The amount of time taken for an output to
settle within some tolerance band, normally following a step change
in load.
UL
(Underwriters’ Laboratories): An independent, non-profit organization
testing for public safety in the United States.
Undershoot:
The amount by which an output falls below its final value in response
to a rapid load change.
UPS
or Uninterruptible power supply is a PSU that continues
to supply electric power to the load for specified periods of time
during a loss of input power or when the input line varies outside
normal limits. UPS is implemented with a backup battery and an additional
DC-AC inverter.
VDE
(Verband Deutscher Elektrotechniker): A German organization
testing for public safety.
Volt:
Unit of measurement of electromotive force, electric potential or
potential difference (V).
Voltage
converter technically has the same definiton as the switching
power supply, except the term converter is usually used for a single-stage
DC-DC SMPS. A typical
off-line switching power supply may consist of several cascaded
voltage converters. The term converter may also imply an incomplete
PSU, which may need additional components (filters, protective devices,
housing, etc.) to make a complete PSU.
Warm
up drift: The change in output voltage that occurs during
warm-up from turn on of a cold supply until about 30 minutes after
turn on. Warm-up drift is measured at constant load, input line,
and ambient temperature and is primarily due to internal components
reaching thermal equilibrium.
Warm
up time: The time needed, after turn on, for the power
supply to reach thermal equilibrium with a constant load. Usually
estimated to be about 30 minutes.
Watt:
Unit of measurement of power equal to 1 joule/sec (W). DC power
can be calculated by multiplying voltage and current.
