The still-air temperature in the immediate vicinity of a power supply,
measured a minimum of 4 inches (100 mm) from the supply.
A control technique which gives a power supply a constant current
characteristic above the rated output current. This permits direct paralleling of two or more supplies for increased
total output current.
A transformer which presents a high impedance to common-mode signals and a low impedance
to differential-mode signals. It is commonly used on the input of switching power supplies to suppress common-mode
noise. See Figure 1.
The maximum AC or DC voltage which may be applied from input to output
and/or chassis of a
power supply. See Figure 2.
Condition during peak usage periods when electric utilities reduce their nominal line voltage 10% to 15%.
The component of noise which is common to both the DC output and return lines with respect to input
The output voltage of a constant current power
CONSTANT CURRENT LIMITING CIRCUIT
Current-limiting circuit that holds output current at some maximum value whenever an overload of any magnitude
CONSTANT CURRENT POWER SUPPLY
A power supply that regulates its output
current, within specified limits, against changes in line, load,
ambient temperature, and time.
In a multiple output power supply, the percent voltage change at one output caused by the load change on
An overvoltage protection circuit which rapidly places a low resistance shunt across the power supply output
terminals if a predetermined voltage is exceeded. See Figure 3.
Canadian Standards Association. An independent Canadian organization concerned with testing for public
See Output Current Limiting.
CURRENT LIMITING CIRCUIT
A bounding 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.
The specified reduction in an operating parameter to improve
reliability. Generally for power supplies,
it is the reduction in output power at elevated temperatures. See Figure 4.
DIFFERENTIAL MODE NOISE
The component of noise measured between the DC output and output
return. See Ripple and Noise.
The change in output voltage of a power supply over a specified period of
time, following a warm-up period,
with all other operating parameters such as line, load, and ambient temperature held
DYNAMIC LOAD REGULATION
See Output Impedance.
The ratio of output power to input
power. It is generally measured at full-load and nominal line conditions.
In multiple output switching power supplies, efficiency can be a function of total output power and its division among the
ELECTROMAGNETIC INTERFERENCE (EMI)
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
Equivalent Series Resistance. The amount of resistance in series with an ideal capacitor which exactly
duplicates the performance of a real capacitor. In high frequency applications low ESR is very important.
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. See Figure 7.
An unwanted feedback condition caused by two or more circuits sharing a common electrical ground line.
HALF BRIDGE CONVERTER
A power switching circuit similar to the full bridge converter except that only two transistors are
the other two replaced by capacitors.
High Potential Test. A test to determine if the breakdown voltage of a transformer or power supply exceeds the
minimum requirement. It is performed by applying a high voltage between the two isolated test
See Hold-Up Time.
The time during which a power supplies output voltage remains within specification following the loss of input
The use of ceramic based substrates to support and interconnect components in power supplies. Resistors and
interconnections are screen printed on the substrate using various inks. These are then fixed (fired) in place by a high temperature process.
INPUT LINE FILTER
A low-pass or band-reject filter at the input of a power supply which reduces line noise fed to the
This filter may be external to the power supply.
INPUT PI FILTER
See Pi Filter.
INPUT VOLTAGE RANGE
The high and low input voltage limits within which a power supply or DC/DC converter meets its
The peak instantaneous input current drawn by a power supply at
INRUSH CURRENT LIMITING
A circuit which limits the inrush current during turn-on of a power
A power converter which changes DC input power into AC output
The electrical separation between input and output of a power supply by means of the power
The isolation resistance (normally in megaohms) and the isolation capacitance
(normally in picofarads) are generally specified and are a function
of materials and spacings employed throughout the power supply. See Figure 2.
The maximum AC or DC voltage which may be continuously applied from input to output
and/or chassis of a power supply.
No terms listed.
No terms listed.
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
LINE FREQUENCY REGULATION
The variation of an output voltage caused by a change in line input
frequency, with all other factors held constant.
This effect is negligible in switching and linear power supplies, but it is a critical specification of ferroresonant power
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
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.
Variation of the output voltage due to a change in the outputs load from no load to full
load, with all other
factors held constant. It is expressed as a percent of the nominal DC output
A magnetic device used to improve the cross regulation of multiple output AC/DC
Metal Oxide Semiconductor Field Effect
Transistor. The device of choice for the main switch in switched
mode power supplies having much better switching characteristics than Bipolar
Mean Time between Failure. The failure rate of a power
supply, expressed in hours, established by the
actual operation or calculation from a known standard such as MIL-HDBK-217.
Noise is the aperiodic, random component of undesired devitions in output
voltage. Usually specified in
combination with ripple. See PARD and Ripple.
The stated or objective value for a
quantity, such as output voltage, which may not be the actual
OFF-LINE POWER SUPPLY
A power supply which operates off the AC line
directly, without using a power transformer prior
to rectification and filtering.
OPERATING TEMPERATURE RANGE
See Temperature Range, Operating.
OPERATIONAL POWER SUPPLY
A power supply with a high open loop gain regulator which acts like an operational amplifier and
can be programmed with passive components.
OUTPUT CURRENT LIMITING
An output protection feature which limits the output current to a predetermined value in order
to prevent damage to the power supply or the load under overload conditions. The supply is automatically restored to normal operation
following removal of the overload. See Figure 7.
The ratio of change in output voltage to change in load
The nominal value of the DC voltage at the output terminals of a power
OUTPUT VOLTAGE ACCURACY
For a fixed output supply, the tolerance in percent of the output voltage with respect to its
nominal value under all minimum or maximum conditions.
An output protection feature which limits the output current of a power supply under overload
conditions so that it will not be damaged.
A transient change in output voltage, in excess of specified output accuracy limits, which can
occur when a power supply is turned on or off, or when there is a step change in line or
load. See Figure 8.
A power supply feature which shuts down the supply, or crowbars or clamps the output,
when its voltage exceeds a preset level.
The connection of the outputs of two or more power supplies of the same output voltage to obtain a
higher output current than from either supply alone. This requires power supplies specifically designed to share the load.
Periodic and Random Deviation. A term used for the sum of all ripple and noise components measured
over a specified band width and stated in either peak-to-peak or RMS values. See Figure 9.
P.F.C. POWER FACTOR CORRECTION
Standard AC/DC converters draw line current in pulses around the peaks in line voltage. This may be
undesirable for several reasons. PFC circuits ensure that the line current is drawn sinusoidally and in phase with the sinusoidal line voltage.
A commonly used filter at the input of a switching supply or DC/DC converter to reduce reflected ripple current.
The filter usually consists of two parallel capacitors and a series inductance and is generally built into the
supply. See Figure 10.
POWER FAIL DETECTION
A power supply option which monitors the input voltage and provides an isolated logic output signal when there is
loss of line voltage.
A linear regulator used on the output of a switching power supply to improve line and load regulation
and reduce output ripple voltage.
A power supply feature whereby the input power is reduced to a low value under output overload conditions.
The regulation at the front-end of a power supply, generally by a type of switching regulator; this is
followed by output regulation, usually by a linear type regulator.
PROGRAMMABLE POWER SUPPLY
A power supply with an output controlled by an external resistor, voltage, or digital code.
A method of voltage regulation used in switching supplies whereby the output is controlled by varying the width,
but not the height, of a train of pulses which drive a power switch.
A power switching circuit which uses a center-tapped transformer and two power switches which are driven on and
off alternatively. This circuit does not provide regulation by itself.
No terms listed.
RATED OUTPUT CURRENT
The maximum load current which a power supply was designed to provide at a specified ambient temperature.
The stable voltage, generally a Zener diode, from which the output voltage of a regulated supply is controlled.
REFLECTED RIPPLE CURRENT
The AC current generated at the input of a power supply or DC/DC converter by the switching operation of
the converter, stated as peak-to-peak or RMS. See Figure 11.
A technique of regulating the output voltage of a power supply at the load by means of sensing leads which
go from the load back to the regulator. This compensates for voltage drops in the load
leads. See Figure 12.
For an adjustable supply, the smallest change in output voltage that can be realized by the adjustment.
A class of power converter topologies which reduce the level of switching losses by forcing either zero
voltage across, or zero current through the switching device when it is turned on or off.
The name for the common terminal of the output of a power supply; it carries the return current for the
REVERSE VOLTAGE PROTECTION
A feature which protects a power supply against a reverse voltage applied at the input or output terminals.
RIPPLE AND NOISE
The magnitude of AC voltage on the output of a power supply, expressed in millivolts peak-to-peak or RMS,
at a specified band width. This is the result of feed through of the rectified line frequency, internal switching transients and other random
noise. See Figure 9.
No terms listed.
The average percent change in output voltage per degree centigrade change in ambient temperature over a specified
TEMPERATURE RANGE, OPERATING
The range of ambient or case temperatures within which a power supply may be safely operated and meet
An internal safeguard circuit in a power supply which shuts down the unit in the event of excess internal
A characteristic of a dual or other multiple output power supply whereby one or more outputs follow another
output with changes in line, load and temperature, so that each maintains the same proportional output voltage, within specified tracking tolerance,
with respect to common.
TRANSIENT RECOVERY TIME
The time required for the output voltage of a power supply to settle within specified output accuracy limits
following a step change in output load current or a step change in input
voltage. See Figure 8.
Underwriters Laboratories Incorporated. An independent, non-profit U.S. organization that tests products for
A transient change in output voltage, below output accuracy limits, which can occur when a power supply is
turned on or off, or when there is a step change in line or load. See Figure 8.
Uninterruptible Power Supply. A power supply which continues to supply power during a loss of AC input power.
This is accomplished by means of a back-up battery and a DC/AC inverter or DC/DC converter.
Verband Deutscher Elektrotechniker. A German organization which tests equipment for public safety and emitted
The difference in magnitude, in percent, between the two output voltages of a dual output power supply where
the voltages have equal nominal values with opposite polarities.
The initial change in output voltage of a power supply from turn-on until it reaches thermal equilibrium at nominal line,
full load, 25C ambient temperature.
The time required, after initial turn-on, for a power supply to meet its performance specifications.
No terms listed.
No terms listed.
No terms listed.