ABNORMAL FAILURE. An artificially induced failure of a component,
usually as a result of "abnormals" testing for regulatory agency safety
AMBIENT TEMPERATURE. The temperature of the environment, usually the
still air in the immediate proximity of the power supply.
APPARENT POWER. A value of power for AC circuits that is calculated as
the product of RMS current times RMS voltage, without taking the power factor
BANDWIDTH. A range of frequencies over which a certain phenomenon is
to be considered.
BIPOLAR TRANSISTOR. A transistor which operates by the action of
minority carriers across a P/N junction; and is a current controlled device as
opposed to a voltage controlled device.
BLEEDER RESISTOR. A resistor added to a circuit for the purpose of
providing a small current drain, usually to provide a load for improving output
voltage stability, or to assure discharge of capacitors.
BOBBIN. A device upon which the windings of a transformer or inductor
are wound, which provides a form for the coil and insulates the windings from
BODE PLOT. A graphic plot of gain versus frequency for a control loop,
typically used to verify control loop stability, including phase margin.
BREAKDOWN VOLTAGE. A voltage level at which dielectric insulation
fails by excessive leakage current or arcing. In reference to power supplies the
breakdown voltage is the maximum AC or DC voltage that can be applied from input
to output and/or chassis.
BRIDGE CONVERTER. A DC to DC converter topology (configuration)
employing four active switching components in a bridge configuration across a
BRIDGE RECTIFIER. A full wave rectifier circuit employing four
rectifiers in a bridge configuration.
BROWNOUT. A reduction of the AC mains' distribution voltage, usually
caused deliberately by the utility company to reduce power consumption when
demand exceeds generation or distribution capacity.
BROWNOUT PROTECTION. The ability of a power supply to continue
operating within specification through the duration of a brownout.
BURN-IN. Operating a newly manufactured power supply, usually at rated
load, for a period of time in order to force component infant mortality failures
or other latent defects before the unit is delivered to a customer.
CAPACITIVE COUPLING. Coupling of a signal between two circuits, due to
discrete or parasitic capacitance between the circuits.
CENTER TAP. An electrical connection made at the center of a
transformer or inductor winding, usually so as to result in an equal number of
turns on either side of the tap.
CENTERING. The act of setting the output voltage of a power supply
under specified load conditions, usually an auxiliary output of a multiple
output power supply with all outputs at half load.
COMMON MODE NOISE. Noise present equally on two conductors with
respect to some reference point; often used specifically to refer to noise
present on both the hot and neutral AC lines with respect to ground.
CONSTANT CURRENT POWER SUPPLY. A power supply designed to regulate the
output current for changes in line, load, ambient temperature, and drift
resulting from time.
CONSTANT VOLTAGE POWER SUPPLY. A power supply designed to regulate the
output voltage for changes in line, load, ambient temperature, and drift
resulting from time.
CONTROL CIRCUIT. A circuit in a closed-loop system, typically
containing an error amplifier, which controls the operation of the system to
CONVECTION. The transfer of thermal energy in a gas or liquid by
currents resulting from unequal temperatures.
CONVERTER. An electrical circuit which accepts a DC input and
generates a DC output of a different voltage, usually achieved by high frequency
switching action employing inductive and capacitive filter elements.
COOLING. Removal of heat, which, in a power supply, is generated by
transformation, rectification, regulation, and filtering. It can be accomplished
using radiation, convection, forced air, or liquid means.
CREST FACTOR. In an AC circuit, Crest Factor is the mathematical ratio
of the peak to RMS values of a waveform. Crest factor is sometimes used for
describing the current stress in AC mains supply wires, since for a given amount
of power transferred, the RMS value, and hence the losses, become greater with
increasing peak values. Crest Factor gives essentially the same information as
Power Factor, and is being replaced by Power Factor in power supply technology.
CROSS REGULATION. The effect of a load change on one output to the
regulation of another output. It usually only applies to non-postregulated
CROWBAR. An overvoltage protection method which shorts the power
supply output to ground in order to protect the load when an overvoltage fault
CURRENT LIMITING. An overload protection circuit that limits the
maximum output; current of a power supply in order to protect the load and/or
the power supply.
CURRENT MODE. A control method for switch-mode converters where the
converter adjusts its regulating pulsewidth in response to measured output
current and output voltage, using a dual loop control circuit. Since output
current is measured, current mode control allows accurate sharing between power
CURRENT MONITOR. An analog power supply signal which is linearly
proportional to output current flow. Usually only feasible for single output
DERATING. A reduction in an operating specification to improve
reliability. For power supplies it is usually a specified reduction in output
power to facilitate operation at higher temperatures.
DESIGN LIFE. The expected lifetime of a power supply during which it
will operate to its published specifications.
DIFFERENTIAL MODE NOISE. Noise that is measured between two lines with
respect to a common reference point excluding common-mode noise. The resultant
measurement is the difference of the noise components of the two lines. The
noise between the DC output and DC return is usually measured in power supplies.
DRIFT. The change in an output voltage, after a warm-up period, as a
function of time when all other variables such a line, load, and operating
temperature are held constant.
DROPOUT. The lower limit of the AC input voltage where the power
supply just begins to experience insufficient input to maintain regulation. The
dropout voltage for linears is quite load dependent. For most switchers it is
largely design dependent, and to a smaller degree load dependent.
EFFICIENCY. The ratio of total output power to input power expressed
as a percentage. Normally specified at full load and nominal input voltage.
ELECTRONIC LOAD. An electronic device designed to provide a load to
the outputs of a power supply, usually capable of dynamic loading, and
frequently programmable or computer controlled.
EMI. Abbreviation for Electromagnetic Interference, which is the
generation of unwanted noise during the operation of a power supply or other
electrical or electronic equipment.
ESR. Equivalent Series Resistance. The value of resistance in series
with an ideal capacitor which duplicates the performance characteristics of a
FAULT MODE INPUT CURRENT. The input current to a power supply with a
short circuit on the output.
FERRORESONANT POWER SUPPLY. Power supply used at higher power levels
in fixed applications, since they are very heavy. Can only be used effectively
when the line frequency is very stable as they are sensitive to variations of
input AC frequencies.
FET. Field Effect Transistor, a majority carrier voltage controlled
FILTER. A frequency-sensitive network that attenuates unwanted noise
and ripple components of a rectified output.
FLOATING OUTPUT. An output of a power supply that is not connected or
referenced to any other output, usually denotes full galvanic isolation. They
generally can be used as either positive or negative outputs. Non-floating
outputs share a common return line, and are hence DC referenced to one another.
FLYBACK CONVERTER. The flyback converter is the simplest type of
switcher. In most cases, it uses one switch and only needs one magnetic element
- the transformer. Flybacks are limited to outputs of generally lower than 200
FOLDBACK CURRENT LIMITING. A type of protection circuit where the
output current decreases as the overload increases. The output current reaches a
minimum as the load approaches a short-circuit condition.
FORWARD CONVERTER. Similar to flyback converter but the forward
converter stores energy in the output inductor instead of the transformer.
FULL BRIDGE FORWARD CONVERTER. The full bridge is more complex than
other switcher topologies. It has the capability for very high performance. It
can product high power with four switchers and requires only two magnetic
GROUND. An electrical connection to earth or some other conductor that
is connected to earth. Sometimes the term "ground" is used in place of "common,"
but such usage is not correct unless the connection is also connected to earth.
GROUND LOOP. An unintentionally induced feedback loop caused by two or
more circuits sharing a common electrical ground.
HAVERSINE. A waveform that is sinusoidal in nature, but consists of a
portion of a sine wave superimposed on another waveform. The input current
waveform to a typical off-line power supply has the form of a haversine.
HEADROOM. Used in conjunction with series pass regulators, and is the
difference between the input and output voltages.
HEATSINK. Device used to conduct away and disperse the heat generated
by electronic components.
HIPOT. Abbreviation for High Potential, and generally refers to the
high voltages used to test dielectric withstand capability for regulatory agency
electrical safety requirements.
HOLD-UP TIME. The length of time a power supply can operate in
regulation after failure of the AC input. Linears have very short hold-up times
due to the CV squared energy storage product of their low voltage secondary side
output capacitors. Switchers have longer times due to their higher voltage
primary side energy storage capacitors.
INDUCED NOISE. Noise generated in a circuit by a varying magnetic
field produced by another circuit.
INHIBIT. The ability to electrically turn off the output of a power
supply from a remote location.
INPUT LINE FILTER. An internally or externally mounted low-pass or
band-reject filter at the power supply input which reduces the noise fed into
the power supply.
INRUSH CURRENT. The peak current flowing into a power supply the
instant AC power is applied. This peak is usually much higher than the steady
state input current due to the charging of the input filter capacitors.
INRUSH CURRENT LIMITING. A circuit which limits the amount of inrush
current when a power supply is turned on.
INVERTER. A power supply which produces an AC output, usually from a
ISOLATION. Two circuits that are completely electrically separated
with respect to DC potentials, and almost always also AC potentials. In power
supplies, it is defined as the electrical separation of the input and output via
ISOLATION VOLTAGE. The maximum AC or DC voltage which maybe
continuously applied from input to output and/or chassis of a power supply.
LAYER WINDING. A transformer winding technique where the primary and
secondary windings are wound over each other and separated by an insulation
LEAKAGE CURRENT. A term relating to current flowing between the AC
supply wires and earth ground. The term does not necessarily denote a fault
condition. In power supplies, leakage current usually refers to the 60 Hertz
current which flows through the EMI filter capacitors which are connected
between the AC lines and ground (Y caps).
LINE REGULATION. The change in output voltage when the AC input
voltage is changed from minimum to maximum specified. It is usually a small
value, and may be near zero with current mode control.
LINEAR REGULATOR. A regulating technique where a dissipative active
device such as a transistor is placed in series with a power supply output to
regulate the output voltage.
LOAD REGULATION. The change in output voltage when the load on the
output is changed.
LOCAL SENSING. Using the voltage output terminals of the power supply
as sense points for voltage regulation.
LOGIC ENABLE. The ability to turn a power supply on and off with a TTL
signal. A logic low generally turns the supply off; a logic high turns it on.
LONG TERM STABILITY. Power supply output voltage change due to time
with all other factors held constant. This is expressed in percent and is a
function of component aging.
MAGNETIC AMPLIFIER. Sometimes abbreviated "Mag Amp," a saturating
inductor which is placed in series with a power supply output for regulation
MAINS. The utility AC power distribution wires.
MARGINING. Adjusting a power supply output voltage up or down from its
minimal setting in order to verify system performance margin with respect to
supply voltage. This is usually done electrically by a system-generated control
MINIMUM LOAD. The minimum load current/power that must be drawn from
the power supply in order for the supply to meet its performance specifications.
Less frequently, a minimum load is required to prevent the power from failing.
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. (Mean Time Between Failures) may be calculated or demonstrated.
The usual calculation is per Mil-Std 217 rev E. Demonstrated reliability is
usually determined by temperature accelerated life testing. Demonstrated MTBF is
almost always greater than calculated MTBF.
NOISE. Noise is the aperiodic, random component of undesired
deviations in output voltage. Usually specified in combination with
See: PARD and also: Ripple.
NORMAL VALUE. A usual, average, normal, or expected operating
condition. This stated value will probably not be equal to the value actually
OFF LINE. A power supply which receives its input power from the AC
line, without using a 50/60 Hz power transformer prior to rectification and
filtering, hence the term "off line" power supply.
OPEN FRAME. A power supply where there is no external metal chassis;
the power supply is provided to the end user essentially as a printed circuit
board which provides mechanical support as well as supporting the components and
making electrical connections.
OPTOISOLATOR. An electro-optical device which transmits a signal
across a DC isolation boundary.
OUTPUT GOOD. A power supply status signal which indicates that the
output voltage is within a certain tolerance. An output which is either too high
or too low will deactivate the Output Good signal.
OUTPUT IMPEDANCE. The ratio of change in output voltage to change in
OUTPUT NOISE. The AC component that may be present on the DC output of
a power supply. Switch-mode power supply output noise has two components: a
lower frequency component at the switching frequency of the converter and a high
frequency component due to fast edges of the converter switching transitions.
Noise should always be measured directly at the output terminals with a scope
probe having an extremely short grounding lead.
OVERLOAD PROTECTION. A power supply protection circuit that limits the
output current under overload conditions.
OVERSHOOT. A transient output voltage change which exceeds the high
limit of the voltage accuracy specification and is caused by turning the power
supply on or off, or abruptly changing line or load conditions.
OVERTEMP WARNING. A TTL compatible signal which indicates that an
overtemperature condition exists in the power supply. Most commercial power
supplies are designed to shut down if an overtemperature condition exists.
OVERVOLTAGE PROTECTION. A circuit which either shuts down the power
supply or crowbars the output in the event of an overvoltage condition.
PARALLEL OPERATION. Connecting the outputs of two or more power
supplies with the same output voltage for the purpose of obtaining a higher
output current. This requires power supplies specially designed for load
PARD. Periodic and random deviation, referring to the sum of all
ripple and noise components on the DC output of a power supply, regardless of
nature or source.
PEAK POWER. The absolute maximum output power that a power supply can
produce without immediate damage. Peak power capability is typically well beyond
the continuous reliable output power capability and should only be used
POST REGULATOR. A secondary regulating circuit on an auxiliary output
of a power supply to provide full regulation on that output.
POWER FACTOR. The ratio of true power to apparent power in an AC
circuit. In power conversion technology, power factor is used in conjunction
with describing the AC input current to the power supply.
POWER FAIL. A power supply interface signal interface signal which
gives a warning that the input voltage will no longer sustain full power
PRELOAD. A small amount of current drawn from a power supply to
stabilize its operation. Preloads are usually provided by a bleeder resistor.
PRIMARY. The input section of an isolated power supply which is
connected to the AC mains and hence has dangerous voltage levels present.
PULSE WIDTH MODULATION (PWM). A switching power conversion technique
where the on-line (or width) of a duty cycle is modulated to control power
transfer for regulating power supply outputs.
PUSH-PULL CONVERTER. A switch mode power supply topology which
utilizes a center-tapped transformer and two power switches. The two switches
are alternately driven on and off.
QUASI REGULATED OUTPUT. The regulation of an auxiliary output that is
accomplished by regulation of the main output. A transformer turns ratio,
commensurate with the desired auxiliary output voltage, is used in conjunction
with the output around which the main control loop is closed. Quasi regulated
outputs can be reasonably well regulated, but are signifiCan'tly affected by
second order effects in the converter.
RATED OUTPUT CURRENT. The maximum load current that a power supply can
provide at a specified ambient temperature.
REFLECTED RIPPLE CURRENT. The RMS or peak-to-peak AC current present
at the input of the power supply which is a result of the switching frequency of
REGULATION. The ability of a power supply to maintain an output
voltage within a specified tolerance as referenced to changing conditions of
input voltage and/or load.
REGULATION BAND. The total error band allowable for an output voltage.
This includes the effects of all of the types of regulation: line, load, and
REMOTE INHIBIT. A power supply interface signal, usually TTL
compatible, which commands the power supply to shut down one or all outputs.
REMOTE SENSE. Wires connected in parallel with power supply output
cables such that the power supply can sense the actual voltage at the load to
compensate for voltage drops in the output cables and/or isolation devices.
RETURN. The designation of the common terminal for the power supply
outputs. It carries the return current for the outputs.
REVERSE VOLTAGE PROTECTION. A protection circuit that prevents the
power supply from being damaged in the event that a reverse voltage is applied
at the input or output terminals.
RFI. An abbreviation for Radio Frequency Interference, which is
undesirable noise produced by a power supply or other electrical or electronic
device during its operation. In power supply technology, RFI is usually taken to
mean the same thing as EMI.
RIPPLE AND NOISE. The amplitude of the AC component on the DC output
of a power supply usually expressed in millivolts peak-to-peak or RMS. For a
linear power supply it is usually the frequency of the AC mains. For a switching
power supply, it is usually the switching frequency of the converter stage.
SAFETY GROUND. A conductive path to earth that is designed to protect
persons from electrical shock by shunting away any dangerous currents that might
occur due to malfunction or accident.
SECONDARY. The output section of an isolated power supply which is
isolated from the AC mains and specially designed for safety of personnel who
might be working with power on the system.