Glossary of Terms



Air Clearance Distance

The shortest path separating two conductors or two circuit components.

Alternating Current (ac)

Current which changes the direction in which it flows.

Agency Approval

UL, CSA and TUV are safety agencies that test specifications such as component spacing, hi-pot isolation, leakage currents, circuit board flammability and temperature rating. Hi-pot (high-potential) isolation is the ability to accept voltage surges with safety.

Ambient Temperature

Average temperature of the environment directly surrounding the power supply.


Unit of measurement for electrical current or rate of flow of electrons (Coulombs/sec) through a wire. The symbol for electrical current is “I”.

Apparent Power

Power Value obtained in an AC circuit as the product of RMS current x RMS voltage


Advanced technology Extended (Term introduced by Intel) is a form factor. A full size ATX board is 305 x 244mm (12 x 9.6"). An ATX power supply does not directly connect to the system power button, allowing the computer to be turned off via software. Many ATX power supplies have a manual switch on the back to ensure the computer is truly off and no power is being sent to the components. ATX used one large, keyed connector instead, making a reversed connection very difficult.

AWG (American Wire Gauge aka. Browne & Sharpe wire Gauge)

Standard for sizing cross sectional areas of wire and measure sheet-metal thickness. The cross-sectional area of each gauge is an important factor for determining its current carrying capacity.  The numbers are retrogressive where a larger number denotes a smaller wire.



Mounting platform for a power supply (PSU) (either ac/dc or dc/dc) component.

Baseplate Temperature

The temperature at the hottest point on the power supply’s mounting platform.


Boost Regulator

A topology for switching power supplies where energy from the input is stored in an inductor during the pulse then released to the output after the pulse. This renders the output to always be greater than the input voltage.

Bridge Rectifier

Full-wave rectifier circuit employing multiple rectifiers in a bridge configuration to rectify single-phase or multi-phase AC power.

Buck Regulator

A topology for switching power supplies where a series of pulses control a switching device that switches the input on/off, and an averaging L-C filter that conveys the energy to a load. This renders output voltage to be less than the input voltage.

Buck-Boost Regulator

A topology for switching power supplies where a series of pulses control a switching device that delivers energy to an inductor that stores the energy and later delivers it to a load. This renders the output voltage to be less than the input voltage.

Bulk Capacitor

Energy storage capacitor at the front or the load end of a regulator.



Device that stores an electrical charge. A simple capacitor consists of two conducting surfaces separated by a dielectric. Capacitance is measure with Farads (F).


Inherent property of an electric circuit or device that opposes change in voltage.

Common mode noise

The component of noise voltage that appears equally and in phase on conductors relative to a common reference.


Agency certification that a product meets its standard.

Conformal Coating

An insulating layer that can be applied by spraying, dipping or vapor deposition that covers and protects the components on a circuit board.

Constant Current Power Supply

Power supply that regulates its output current, within specified limits, against changes in line voltage, load, ambient temperature and time.

Constant Voltage Power Supply

Power supply that regulates its output voltage within specified limits, against changes in line voltage, load, ambient temperature and time.

Continuous Short Circuit Protection

Provides an extra level of protection against the catastrophic failures resulting from the shorting of the positive rail to ground. A critical safety feature to protect the load circuit and the converter from dangerously high current levels.  The converter will immediately shut its output off when excessive load current is detected, with an auto-recovery restart feature the converter will periodically attempt to restart its output automatically.

Convection cooled power supply

Power supply that is cooled only from the natural motion of a gas or liquid over the surface of heat dissipating elements.

Creepage Distance

The shortest distance separating two conductors as measured along an insulating surface touching both conductors.

Cross Regulation

In a multiple output power supply, the percent voltage change at one output caused by the load change on another output.


The rate of transfer of electrical charge measured in Amperes (A). The sign for current is “I”.



Direct Current, Electrical current that flows in a single direction.

DC/DC Converter

A circuit or device that changes a DC input to a DC output of a different voltage (or in some cases the same).

Demonstrated MTBF

The most accurate type of MTBF rating is “demonstrated MTBF”, where MTBF stands for Mean Time Between Failures. Demonstrated MTBF ratings are developed by analyzing the failures for a group of units that has been operating in the field over an extended period (typically several years). Consequentially, demonstrated MTBF is a statement of historical fact.


Reduction in the power output of a power supply due to external environmental factors such as ambient temperature to insure proper functioning of the part.

DIN Rail

Standardized mounting scheme for electrical and electronic components.


Dual Inline Package

Duty Cycle

Time interval occupied by a device on intermittent duty in starting, running, stopping and idling.


An amplifier used to control another device or circuit.



Ratio of total output power to total input power, expressed as percentage, under specified conditions, normally at full load under nominal input voltage.

Electromagnetic Interference (EMI)

Noise generated by the switching action of the power supply and other system components. Conducted EMI is radiation reflected back into the power line, which is normally controlled with a line filter. Radiated EMI is that portion that would radiate into free space, but is suppressed by enclosing a power supply's circuitry in a metal case. The FCC governs conducted and radiated emission levels in the U.S.


Electromagnetic compatibility

EMI Filter

Circuit composed of reactive and resistive components for the reduction of electromagnetic interference being emitted from a power supply.


Electromagnetic susceptibility


Fall Time

Time required for a pulse to decrease from 90% to 10% of its peak.

Fan Rating

Airflow rated in cubic feet per minute. A 100% increase in airflow will reduce system operating temperature by 50% relative to ambient temperature. For each 18 degrees (Fahrenheit) of reduction, the life of the system is doubled (Arrhenius equation).

Farad (F)

Unit of measurement for capacitance


One or more discrete components positioned in a circuit to diminish signal energy in a specified band of frequencies.


Number of periods per second.

Forced Air Cooling

Heat transfer by moving a cooling medium with the help of a fan or pump.

Full Brick

Industry standard footprint for dc-dc converters. Dimensions are 116.8 x 61mm (4.6 x 2.4in).

Full Load

The maximum specified output load for a converter under continuous operating conditions.




Good Manufacturing Practices (Quality System for Manufacturing)


Conducting connection to earth (or virtual earth; note virtual ground)


Half Brick

Industry standard footprint for dc-dc converters. Dimensions are 61 x 57.9mm (2.4 x 2.28in).

Heat Sink

Mechanical structure through which thermal energy is transferred between a device and an external cooling medium.

Henry (H)

Unit of measurement for Inductance. A coil has one Henry of inductance if an EMF of one volt is induced when the current through an inductor is changing at a rate of 1 Ampere per Second.

Hertz (Hz)

Unit of measurement for frequency; equivalent to 1 cycle per Second.

Hiccup Mode

An operating mode of a power converter in which the converters cycles on/off after being triggered by an output fault condition.

Hi-pot Test

A dielectric test performed by applying a high voltage for a specified time of two isolated points in a device to determine adequacy of insulating materials.

Hold-up Time

Time that a power supply's output will remain within specified limits, following power disturbances or a loss of input power. Adequate hold-up time keeps the computer running until a standby UPS takes over within a few milliseconds.



International Electrotechnical Commission

Inductance (L)

Property of an electric circuit by which a varying current induces an electromotive force in that circuit or a neighboring one.

Input Current

Current that a device draws from the input power bus under nominal operating conditions.

Input Voltage Range

The range of input voltage values over which a power supply (PSU) or device will operate within its specified limits. This provides a greater flexibility when used with loosely regulated input voltages (unstable), or with battery powered applications where the input voltage varies greatly with the battery charge.

I/O (Input/Output) Isolation

 Circuit techniques that provide DC voltage isolation between input and output circuitry of a power supply.

Inrush Current

Peak instantaneous input current drawn by a circuit at turn-on or application of input power.


Non-conductive materials used to separate electrical circuits and to prevent electric shock.

Integrated Circuit (IC)

Combination of several active/passive circuit elements contained on a single semiconductor substrate.


Power supply that produce AC output from a DC input.

Isolation Voltage

The maximum ac or dc specified voltage that may be continuously applied between

isolated circuits.


LC Filter

Also known as an averaging filter. A low pass filter consisting of a capacitor and an inductor.

Leakage Current

The AC or DC current flowing from input to output of an isolated device at a specified voltage.


Light Emitting Diode which is a semiconductor light source.

Line Regulation

Change in output voltage due to varying input voltage. Expressed as a percent of the normal output voltage, a power supply with tight line regulation delivers optimum voltages throughout the operating range. This is tested by measuring the difference in output voltages while varying the input voltage from minimum to maximum, i.e., from 85 to 135V.

Linear Regulator

An active device connected in series or shunt with the load of a power supply to maintain a constant output voltage or current.


Any combination of resistance, capacitance, inductance which is connected at the output terminals of a power source and thus determines power used.

Load regulation

Change in output voltage due to a varying load. Expressed as a percent of the normal output voltage, a power supply with tight load regulation delivers optimum voltages regardless of system configuration. This is tested by measuring the difference in output voltage when applying a light load and a heavy load.


Maximum Load

The highest allowable power output specified for the outputs of a power converter under certain conditions.

Maximum Operating Temperature

Highest Ambient Temperature at which a power supply will operate safely within technical parameters.

Mean Time Between Failure (MTBF)

The mean length of time between device failures, this does not include infant mortality and related end of life.

Minimum Load

Lowest current to be drawn on a constant voltage power supply for the voltage to be in specified range (if constant current supply then it is max value of load resistance).

Minimum Operating Temperature

Lowest ambient temperature where a power supply will turn on and operate properly.


Multi-Layer Ceramic Capacitor


Power converter made to standard dimensions and capable of being integrated directly with other units in a larger system.


Moisture Sensitivity Level


No Load Current

When the DC/DC converter is switched “ON” using the control pin (internal oscillator is working) without any load connected. Normally this current is in milliamps.

No Load Voltage

Voltage at the output when no current is flowing in the external circuit.


Unwanted electronic signal superimposed on the output.

Nominal value

Specified normal value of a quantity

Nominal Voltage

The stated value of a voltage, this may well be different to the measured value (for many reasons, including setting accuracy, etc.).


Ohm (Ω)

Unit of measurement of Resistance and Reactance

Off Idle Current

When the DC/DC converter is switched “OFF” using the control pin, the input current is usually in micro-amps, and is independent of whether there is no load or full load. This is usually caused by the leakage current of the input circuit.

Open-Frame Construction

A construction technique where the power supply (PSU) is not provided with

an enclosure. Care needs to be exercised when installing an open frame power supply to ensure that adequate spacing and shock protection is maintained. Additional care needs to be taken to ensure that the unit is installed to meet the EMC standards and thermal performance.

Operating Temperature Range

Range of ambient or case temperatures where power supplies are specified to operate safely and perform per specification.

Output current

Maximum current that can be continuously drawn from the output of a power supply. PC motherboards and expansion cards draw 5V current. Drive motors draw 12V.

Output Filter

One or more discrete components used to attenuate output ripple & noise.

Overcurrent Protection

Feature that interrupts a current to the load when a specified maximum current is reached.

Overload protection

Feature that senses and responds to a current or power overload condition.


A voltage that exceeds specified limits.

Overvoltage protection

Feature that senses and responds to a voltage exceeding a certain limit.

Over temperature protection

Feature that senses and responds to temperatures exceeding the safety zone.



Maximum value of a waveform reached during a cycle or operating time.

Pin out

The pin assignment of a device


Adjustable Resistor


Insulating material for encapsulating circuit elements


Rate of using energy measure in watts(W). 1W=1Joule/second

Power density

Ratio of power available from a power source to its size.

Power Factor

Ratio of total active power to total apparent power in Volt Amperes in an AC circuit. The voltage and current are RMS values that include harmonic effects and phase displacements.

Power Rating

Power available at the output terminals of a power supply based on the manufacturer’s specifications.

Power Supply

A device which supplies electrical energy to a load. Typical application of power supplies includes converting raw input power (can be either ac or dc) to a controlled or stabilized voltage and/or current for the operation of electronic equipment. Often used to convert between hazardous voltages available from wall sockets (usually 110Vac or 230Vac) to voltages which can be used by electronic equipment (CPU's, motors, telephones, etc.).

Printed Circuit Board (PCB)

An insulating substrate with conductive (usually copper) tracks connecting

components. The PCB is used to both support and interconnect components. PCBs are made of many materials including resin impregnated paper, fiberglass (FR-4), various plastics, ceramic or metal (usually aluminum).

PCBs can have one or more layers of copper tracks and the different layers are connected with Vias.

Power Factor Correction (PFC)

Technique of changing the power factor so that the power factor approaches unity.


Power Supply Unit

Pulse width modulation (PWM)

Method of regulating the output of a switch mode power supply by varying the duration of the duty cycle (not the frequency) of the pulses.


Quarter Brick

Industry standard footprint for dc-dc converters. Dimensions are 57.9 x 36.8mm (2.3 x 1.5in).



Quiescent Current

This is the standing current that flows in a circuit when the signal is not applied (no load). The quiescent current is usually very low or lower than when processing a signal. This is also known as an idle current.



Rated Output Current

Maximum load current on a power supply

Recovery Time

The time required for the measured characteristic to return within specified limits following an abnormal event.

Resistance (R)

A measure of how much a component (or other object) opposes the flow of current. The unit of resistance is the ohm (Ω).


Alternation current component in the output of a power supply arising from sources within the power supply.

Rise time

Time required for a pulse to rise from 10% to 90% of its maximum amplitude.


Reduction of Hazardous Substances, refers to government program implemented to eliminate use of substances deemed to be hazardous.


Safe Operating Area

Manufacturer specified zone of operation that must be respected to prevent damage to power semiconductors.

Safety Compliance

Certification listing, licensing, recognition or approval by safety agencies.

Sense Line

Conductor which routes output voltage to the control loop.

Short Circuit

Initial value of the current obtained from a power source in a circuit of negligible resistance.

Short-Circuit Current (SCC)

The initial value of the current obtained from a power supply in a circuit of

negligible resistance.

Short-Circuit Protection

A protective feature that limits the output current of a power supply (PSU) to prevent damage to the power supply. Care must be taken to ensure that connecting leads and any PCB tracks are designed to allow for the maximum current possible from the power supply (or supply additional protection circuitry / fusing for protection).



Switched Mode Power Supply

Soft Start

Controlled turn-on to limit inrush current and control the rate of rise of output voltage.

Storage Temperature Range

The range of temperatures through which an unpowered power supply

(PSU) can remain in storage without degrading its operation.

Surface Mount Device (SMD)

A type of component which is intended to be mounted directly upon the

surface of a printed circuit board.


Abnormally high transient voltage, current, or power

Switching Frequency

Rate at which the input dc voltage is switched in a power supply.

Switching Regulator

A voltage regulator which operates by rapidly switching the current into the load to stabilize the output voltage. They are inherently more efficient (over 90%), smaller and lighter than linear regulators but are more complex.



Temperature Coefficient

The average percent change in output voltage per degree change in ambient temperature over a specified range of temperatures.

Transient Recovery Time

The time required for the output voltage of a power supply (PSU) to settle

within specified output accuracy limits following a transient.

Transient Response

Time required for the output voltage to return within the regulation envelope following a 50% load change. A power supply with quick transient response will reduce the risk of read/write errors.

Turn on Time

The time from the application of input power to the point where the output voltage is in regulation and meets the specifications.

Turn off Time

The time from removing of input power to the point where the output voltage is in regulation and meets the specifications.



Underwriter’s Laboratory which is an independent organization that writes specs for and provides testing services with regards to product safety.

Undervoltage Protection

Circuit that inhibits the power supply when the output voltage is below a specified minimum.



Acronym for volts of alternating current

Volt (V)

Unit of measurement for electromotive force or potential difference. A current of 1 A passing through a resistance of 1 ohm will produce a potential difference of 1V.


A derivative electrical quantity (E) measured in volts and defined in terms of the independently obtained ampere (I) and the unit or Resistance, ohm (R) by Ohm’s Law E=IR.

Voltage Regulation

The process of holding voltage constant between selected parameters.


Warm-up Time

The time required after a power supply (PSU) is initially turned on for it to operate per specified performance limits.

Watt (W)

Unit of measurement for Power.



Waste Electrical and Electronic Equipment Directive. The directive imposes the responsibility for the disposal of electrical and electronic equipment waste on the manufacturers of such equipment. Those companies should establish an infrastructure for collecting WEEE, in such a way that "Users of electrical and electronic equipment from private households should have the possibility of returning WEEE at least free of charge". Also, the companies are compelled to use the collected waste in an ecologically-friendly manner, either by ecological disposal or by reuse/refurbishment of the collected WEEE.



Zero-current switching


Zero-voltage switching