Glossary


Ambient Temperature

The temperature of the still air surrounding a power supply, measured a minimum of 4 inches (10.2 cm) from the supply.

Baseplate temperature
the temperature at the hottest spot on the baseplate of the supply, usually at the center, but measurements of the hottest spots at the edges are valid approximations.

Brownout
The condition created during peak usuage periods when electric utility companies intentionally reduce their line voltage by approximately 10 - 15% to counter excessive demand.

CE Mark
A designation by the European community to signify compliance with applicable EMC and safety directives.

Common-mode noise
The component of noise in a power supply that is common to both the dc output and return lines with respect to the source power return point (input neutral).

Constant Current
As a prefix to the phrase power supply, the term describes a current stabilizer . (See Current Stabilization).

Constant Voltage
As a prefix to the phrase power supply, the term describes a voltage stabilizer. (See Voltage Stabilization).

Cooling
The process of removing heat, which in a power supply is generated by transformation, rectification, filtering, and the stabilization process that converts unwanted electrical energy to heat energy. Cooling means include convection and radiation, both "natural" and blower-aided, conduction to an external heat sink, and liquid cooling medium circulation.

Cross-regulation
In a multiple output power supply, the voltage change at one output caused by the load change on another, expressed as a percentage of the nominal voltage.

Crowbar
A type of overvoltage protection that rapidly places a low resistance shunt (typically an SCR) across the power supply output terminals when the output voltage exceeds a predetermined value.

CSA - Canadian Standards Association
In Canada, a body that issues standards and specifications prepared by various voluntary committees of government and industry. CSA harmonizes its standards with those of UL in the USA and VDE and TÜV in Germany to implement the recommendations of the IEC, International Electrotechnical Commission. Presently, Underwriters Lab (UL) is authorized to provide both UL and CSA certification. A new symbol, is in use.

Current Limiting
A bounding circuit designed to prevent overload of a voltage stabilizer in which, for load resistances smaller than the crossover resistance, the current is limited to a preset value, while the output voltage diminishes in proportion to the load's resistance.

Derating
Reducing one operating parameter to compensate for changes in other parameters to maintain reliability. For example, the reduction in output power at elevated temperatures.

Differential-mode noise
The component of noise in a power supply, excluding common-mode noise, that is measured between the dc output and output return.

Efficiency
The ratio of total output power to input power, expressed as a percentage. Efficiency must be specified at a specific combination of load and input voltage.

Electromagnetic interference (EMI)
Unwanted high-frequency energy that is conducted through the input or output lines or radiated into space by switching power supplies. Also known as radio-frequency interference (RFI).

Fault Tolerant
A system configuration to ensure the integrity of operation in the event of a single point failure. For power supplies, the requirement is usually to ensure the maintenance of system power despite the loss of any single power module. The usual technique is to provide redundant power modules on an N+1 basis, with sufficient isolation that the failure of any one power module does not cause system failure. Additional systems redundancy may require multiple source power inputs.

Feedback
The process of returning a part of the output of a system to its input. Negative feedback (out-of-phase return) is used to effect the corrective action that is basic to the process of stabilization.

Foldback current limiting

a type of power supply overload protection that decreases the output current as the overload increases, until the current reaches a minimal value at short-circuit. Foldback current limiting minimizes internal power dissipation under overload.

Forced Current Sharing
When voltage stabilizers are paralleled, they will self-arrange their operation so that only one unit controls the voltage. The other units in the parallel group are forced to either shut off or go into current limit mode. This produces a natural unbalance in the way they share the load current. To restore a balance, circuits are employed to force paralleled voltage stabilizers to share the load current. With parallel operation used for redundancy and N+1 combinations, the concept of forced current sharing is increasing in importance.

Forward converter
A power supply switching circuit that transfers energy to the transformer secondary when the switching transistor is on. Forward converter circuits store minimal energy in the transformer.

Ground loop
A condition that causes unwanted feedback when two or more circuits share a common electrical return or ground lines.

Holdup time
The time during which a power supply's output voltage remains within specified limits following the loss or removal of input power. Holdup time is normally measured at full load and nominal line conditions.

IEC
International Electrotechnical Commission. A standards writing body headquartered in Geneva which produces safety recommendations that are incorporated by the various national standards organizations. Currently, power supplies are governed by IEC 60950 which has been widely adopted by national bodies.

Input voltage range
The range of input voltage values for which a power supply or dc-ac converter operates within specified limits.

Inrush current
The peak instantaneous input current drawn by a power supply when it is initially turned on.

Inrush circuit limiting

a circuit that limits the inrush current when a power supply is turned on.

Insulation resistance
the dc resistance between two defined points at a specific voltage in a controlled environment (25°C temperature and less than 50% relative humidity).

Inverter
A device that changes dc power at its input into ac power at its output. Also called a power converter.

Isolation
The electrical separation between the input and output of a power supply due primarily to the power transformer. The isolation is a function of materials and spacings throughout the supply.

Isolation voltage
The maximum ac or dc voltage that may be continuously applied from input to output and/or chasis of a power supply.

Line regulation
The maximum change in output voltage, expressed as a percentage, that occurs as the input voltage varies over its specified limits, with load and temperature constant.

Load regulation
the change in output voltage, expressed as a percentage of nominal voltage, that occurs as the load changes from minimum to maximum, at constant line and constant temperature. Load change may be specified for other than no load to full load as, for example, 50% load to full load.

Local sensing
Using the power supply output voltage terminals as the error-sensing points to provide feedback to a voltage regulator.

Low Voltage Directive (LVD)
Safety standards promulgated by the European community as one of the requirements for the CE mark. Presently the applicable LVD for instrumentation power supplies is EN61010. For component (modular) power supplies it is EN60950.

Mean time between failure (MTBF)
The failure rate of a power supply, expressed in hours, either predicted as prescribed by Military Standard MIL-HDBK-217 or measured as prescribed by Military Standard MIL-STD-781C.

N+1
A system configuration by which power supplies are wired in parallel, with isolation (OR-ing) diodes, and sized so that the total system power can be supported by N modules. If there are N+1 modules, then one module may fail without interrupting operations of the system. A variant will allow X modules to fail by paralleling N+X modules in the system.

Operating temperature range
The range of ambient or case temperatures through which a power supply may operate safely and perform within specified limits. For Abbott military power supplies, this range refers to baseplate temperature.

Output current limiting
A protective feature that keeps the output current of a power supply within predetermined limits during overload to prevent damage to the supply or the load. The supply automatically returns to normal operation following the removal of the overload.

Overload protection
A protective feature that limits the output current of a power supply under overload conditions so that it will not be damaged.

Overvoltage protection (OVP)
A protective feature that shuts down a power supply (reduces the output voltage to a minimal level) to prevent damage to the load when the output voltage exceeds a predetermined limit.

Parallel operation
The connection of the outputs of two or more power supplies of the same output voltage to obtain a higher output current than either supply can provide alone. Parallel operation requires power supplies that are specifically designed to share the load.

Periodic and random deviation (PARD)
The unwanted periodic (ripple) or aperiodic (noise) deviation of the power supply output voltage from its nominal value. Ripple is a function of the input line and switching components. PARD is expressed in minivolts peak-to-peak or rms, at a specified bandwidth.

Power Factor
The ratio of real to reactive power. In sinusoidal circuits, it is the measure of the fraction of current in phase with the voltage and contributing to the average power.

PFC - Power Factor Correction
Better defined, however, as a technique for harmonic reduction in a power supply's source current. Off line rectifiers, un-corrected, draw large peaks of current from the source (mains) near the peak of the input sinusoid. The flattening, caused by the non-zero source impedance of the utility mains, causes harmonic distortion. Power Factor Correction may be of two types: Passive correction uses a choke input filter with the choke sized so that the rectifiers conduct continuously (critical choke value). Active correction uses a separate converter in front of the rectifier having the effect of causing conduction to occur over nearly the whole mains cycle. This produces near sinusoidal source current reducing harmonic generation and improving source power utilization. (See waveforms showing clipping distortion and power factor corrected a-c line current.)

Pulse-width modulation (PWM)
A method of regulating the output voltage of a switching power supply by varying the width, but not the height, of a train of pulses that drives a power switch.

Rated output current
The maximum load current that a power supply is designed to provide at a specified ambient temperature.

Recovery Time
The time required by a transient overshoot in a stabilized output quantity to decay to within specified limits (usually within the individual effect band of the influence quantity whose step change initiated the transient).

Regulator
The power supply circuit that controls or stabilizes the output voltage at a preset value.

Remote sensing
A technique for regulating the output voltage of a power supply at the load by connecting the regulator error-sensing leads directly to the load. Remote sensing compensates for voltage drops in the load leads.

Resolution
For an adjustabe supply, the smallest change in output voltage can be realized by an adjustment.

Response Time
The response time for a transient disturbance with an exponential decay is the time corresponding to a single time constant. Response time is thus distinguished from recovery time for which the decay is timed to a specific error limit.

Reverse voltage protection
A feature that protects a power supply from damage caused by a voltage of reverse polarity applied at the input or output terminals.

Series regulation
A popular method of linear regulation in which the control device is connected in series with the raw dc and the load to achieve constant voltage across the load.

Short-circuit protection
a protective feature that limits the output current of a power supply to prevent damage to the supply caused by short circuits.

Slew Rate
The maximum rate of change that a power supply output can respond to when controlled (programmed) by an overdriving or forcing control quantity.

Soft start
A feature that ensures the smooth, controlled rise of the output voltage. This feature protects the switching transistors from transients when the power supply is turned on.

Step Load Change
An abrupt and sustained change in one of the influence or control quantities. When employed as a test means to observe transient behavior, a step change shall be complete in less than one-tenth the transient's response time.

Storage temperature
The range of ambient temperatures through which an inoperative power supply can remain in storage without degrading is subsequent operation.

Switching frequency
The rate at which the dc voltage is switched in a dc-dc converter or switching power supply.

Switching regulator

A high-efficiency switching circuit that operates in a closed loop system to regulate the voltage across a load, generally by means of a pulse-width modulator.

Temperature, Ambient
The environmental temperature which exists unmodified by the operation of dissipative apparatus.

Temperature coefficient

The average change in output voltage per change in degree of a baseplate temperature, expressed as a percentage of nomimal output voltage, over a specified temperature range.

Thermal protection
A protective feature that shuts down a power supply if its internal temperature exceeds a predetermined limit.

Tracking
A characteristic of a multiple-output power supply that describes the changes in the voltage of one output with respect to changes in the voltage or load of another.

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 input voltage.

TÜV - Technische Uberwachungs-Verein (Rhineland)

A laboratory licensed by the German government to test products to international standards.

UL - Underwriters Laboratory.
A laboratory established in the United States to test electrical apparatus mainly for fire safety. In recent years, UL has written standards that are (nearly) harmonized with the equivalent IEC standard. UL examines power supplies for recognition as a "listed component."

Under Voltage Protection

A circuit to detect and react to a prolonged output voltage that is below a threshold value. It is principally used to protect against prolonged short circuit faults.

Universal Input
Refers to a power supply's ability to accept a-c mains voltage through a wide range of voltage (typ: 85-264V a-c) without manual selection. This may be accomplished in a variety of ways. In low powered models, it is achieved by simply sizing the components appropriately and requiring the regulator's control range to accommodate the voltages. In higher power models that use PFC, power factor correction, the preregulator that causes the input current to conduct over the whole of the mains cycle also accommodates the range of a-c input voltage.

Voltage Limiting
A bounding circuit designed to prevent overload of a current stabilizer. For load resistances larger than the crossover resistance, the voltage is limited to a preset value, while the output current diminishes in proportion to the load's resistance.

Warm-up time
The time required after a power supply is initially turned on before it operates according to specified performance limits.


 

 


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