Glossary of Power-Related Terms

/Glossary of Power-Related Terms
Glossary of Power-Related Terms2017-01-12T16:15:03-04:00


Absolute Maximum Ratings
Specifications that, if exceeded, could cause permanent damage to the converter. These are not continuous ratings, and proper operation is not implied.

AC Front-end
That part of a distributed power system that converts the AC line voltage to a semiregulated DC voltage level. Typically, the AC front-end will provide power factor correction and a universal (~85VAC to 265VAC) AC input. the output of an AC front-end is normally 350VDC to 400VDC. See Power Factor Correction.

Operating a converter under controlled conditions for a predetermined time in order to screen out failures. See Burn-in.

Ambient Air
An operating environment in which the air surrounding a power system has sufficient mass and flow to prevent heat regeneration and subsequent thermal runaway because of the heat dissipated by the power modules.

Apparent Power
The product of input RMS voltage times input RMS current. In AC input, switch-mode power systems, where the input current is distorted, high RMS values result in high apparent power.


(British Approvals Board for Telecommunications) An independent organization that has approval authority for telecommunications equipment sold into the UK market. BABT grants approvals and accredits testing laboratories.

Balun Filter
A type of input line filter often used within power systems/converters that includes a differential wound transformer. Balun filters present a low impedance to differential mode signals and a high impedance to common mode signals.

Base Plate
A substrate to which circuit components are mounted or, a metal plate to which the power converter is attached. Normally used to draw heat away from critical circuit components. See Heat Sink.

Base Plate Temperature
See Case Temperature.

Battery Backup
A subsystem for electronic equipment that provides power in the event of input power loss. Battery backed systems are a common application area for DC/DC converters and UPS (uninterruptible power supplies).

Bleeder Resistor
A resistor added to a circuit to provide a path for current drain. Typically used to insure capacitors discharge when a circuit is turned off. Bleeder resistors are often used in AC-input filter circuits.

A component used in the winding of transformers and inductors. The bobbin provides a physical frame that supports the transformer/inductor windings. Fabricated from nonconductive materials, the bobbin also keeps the windings isolated from the core.

Boost Regulator
A basic switching converter topology in which an input inductor is used to store energy. This energy is transferred to the output when the shunt switch is turned off. The boost regulator will take an unregulated input voltage, and produce a higher, regulated output voltage. See Buck Regulator, Bridge Converter, Flyback Converter, Forward Converter, Push-Pull Converter, and Resonant Converter.

Breakdown Voltage
The maximum AC or DC voltage that can be applied from the input to output (or chassis) or output to output of a power converter without causing damage.

Bridge Converter
A switching converter topology that employs four switching elements (full bridge) or two switching elements (half bridge). This topology is more often used in off-line supplies than DC/DC converters. Bridge converters provide high output power and low ripple, but are significantly more complex than other types of converter topologies and thus are more expensive and prone to failure. See Boost Regulator, Buck Regulator, Flyback Converter, Forward Converter, Push-Pull Converter, and Resonant Converter.

A reduction or sag in the AC power line. AC input power systems must be protected against input line sags (by extended input ranges or hold-up time) to avoid inadvertent shutdown.

(British Standards Institution) An organization that develops standards and test products/systems for compliance. An organization associated with B.S.I. called Technical Help to Exporters (T.H.E.) provides reference material on world wide standards.

Buck Regulator
A basic switching converter topology, in which a series switch chops the input voltage and applies the pulses to an averaging LC filter. The buck regulator will only produce an output voltage that is lower than the input voltage level. See Boost Regulator, Bridge Converter, Flyback Converter, Forward Converter, Push-Pull Converter, and Resonant Converter.

Buck-Boost Converter
See Flyback Converter.

Buck-Derived Converter
See Forward Converter.

The operation of newly manufactured power converters for a period of time prior to shipment. The intent is to stabilize the converter and eliminate infant mortality by aging the device. The time period and conditions (input power cycling, load switching, temperature, etc.) will vary from vendor to vendor. However, the less stringent the conditions, the less likely it is that potential problems will be caught by the vendor.


(Canadian Standards Association) An independent organization that establishes and tests safety standards for electronic components and systems for the Canadian marketplace.

Case Temperature
The temperature of the case when the converter (and surrounding system) are operating normally. Often used as a specification for DC/DC converters with extended temperature ranges, case temperature is at times referred to as base plate temperature.

(Comite pour European de Normalisation Electronic (European Commitee for Electrotechnical Standardization)) A technical committee that recommends standards for adoption by the European Community (EC). These standards (referred to in the applicable EC directive issued by the committee) cover EMI/RFI interference, intrinsic safety, immunity, etc.

(Cubic Feet per Minute) A measure of the volume of air flowing in a system. See LFM.

An isolated output section of a power system. A channel may consist of one or more outputs.

Chassis Ground
The voltage potential of the chassis or enclosure surrounding a power system.

Circulating Current
See Ground Loop.

Clearance Distance
The shortest distance (through air) separating two conductors or circuit components. See Creepage Distance.

Timing pulses used within a system or circuit to synchronize the operation of components. In a power converter, these pulses are generated by the pulse width modulation (PWM) chips.

A conductive path used as a return for two or more circuits. Common is often used interchangeably with ground, which is not technically correct unless it is connected to earth. See Ground.

Common Mode Noise
Noise component common to both power system output and return lines with respect to input common.

Compliance Voltage
The DC output voltage of a constant current power converter.

Conduction Cooling
The transfer of heat through a solid material. Used to cool a power converter by adding a heat sink or attaching the module to the system chassis. This effectively increases the module case surface area, lowering the case thermal resistance. Thermal resistance is proportional to the resistivity and length of a material and inversely proportional to its surface area. See Cooling, Convection Cooling, Free Convection and Forced Air Cooling.

Constant Current
A power converter that regulates its output current to within a specified range regardless of changes in output load, input line and ambient temperature.

Constant Voltage
A power converter that regulates its output voltage to within a specified range regardless of changes in output load, input line and ambient temperature.

Continuous Shield
See Six-Sided Shielding.

Convection Cooling
The transfer of heat via a fluid motion (typically air). In distributed power systems this is accomplished by the movement of air over the module or heatsink surface. See Cooling, Conduction Cooling, Free Convection and Forced Air Cooling.

The transfer of heat from a power system into the ambient air mass surrounding the system. See Convection Cooling, Conduction Cooling, Free Convection and Forced Air Cooling.

Creepage Distance
The shortest distance between two conductors (typically, one primary, one secondary).

Crest Factor
The ratio of peak to RMS value of an AC waveform. For a pure sinusoidal waveform, this value is 1.414. Crest factor was once used to approximate the current stress in an AC mains circuit. Today, the use of power factor is more common. See Power Factor.

Cross Regulation
For a multiple output power converter, the change in voltage on one output (expressed as a percent) caused by a load change on another output.

A circuit that crowbars or rapidly shuts down a power converter’s output if a preset voltage level is exceeded. The circuit places a low resistance shunt across the output when an overvoltage condition exists. (For non-isolated converters with a crowbar output over voltage protection circuit, a fuse must always be used in the input lines.)

Cuk Converter
A variation of the “buck-boost” converter that produces very low output ripple. Used primarily in applications that do not require input to output isolation. See Flyback Converter.

Current Foldback
See Foldback Current Limiting.

Current Limiting
A feature that protects the power converter (or load) from damage under overload conditions. The maximum power converter output current is automatically limited to a predetermined, safe value. If the converter is specified for auto restart, normal operation is automatically restored when the overload condition is removed. See Foldback Current Limiting and Current Limit Knee.

Current Mode Control
A control method used with switching converter topologies. A dual loop control circuit–a current loop within a voltage loop–adjusts the PWM operation in response to a measured output current and output voltage.

Current Sharing
Multiple power converters are often connected redundantly (to increase system reliability) or in parallel (to increase system power). When connected in this way, their outputs are strapped together and each power converter supplies approximately an equal “share” of the load current. Current sharing can be achieved with external passive circuits (by synchronizing multiple converters and trimming their outputs to within a tight error band) or active circuits (converters that feature internal circuits to monitor and adjust output load current). The most popular redundant topology is the “N+1” circuit. See Master/Slave Operation, N+1 and Dual-Redundant.


(Decibel) Logarithmic gain unit. Derived by the equation: dB = 20 Log (V2/V1)

For a power converter, the specified reduction in output power required for operation at elevated temperatures. The most common operating temperature range specified for commercial grade converters is 0°C to +70°C without derating. See Cooling, Convection Cooling, Conduction Cooling, Free Convection and Forced Air Cooling.

A material used to prevent two points in an electrical circuit from becoming conductively connected. Sometimes referred to as a dielectric barrier.

Dielectric Withstand
The maximum voltage an insulating material can withstand before breaking down (suffering punch through Voltage or arcing). See Breakdown Voltage and High Potential Test.

Differential Mode Noise
The noise component measured between two points with respect to a common point (minus common mode noise).

Differential Voltage
The difference in voltage levels measured at two points. The measurement is made with respect to a common reference point.

Distributed Power
A system level architecture in which power converters operating off an intermediate power bus (i.e. 24 or 48 VDC) provide localized power (and various voltage levels) to individual subassemblies and/or components. The type of power distribution system used is highly dependent upon the needs of a particular application.

The change in the output voltage of a power converter over a specified period of time. All other operating parameters (load, line, etc.) are assumed to be held constant. Often specified as starting after a warm-up period.

A topology that provides full power system redundancy. Sometimes referred to as a “100% redundant” system, the circuit consists of two complete power systems connected in parallel to the load. One system supplies all the load current, while the second power system runs “cool” (disconnected from the load via oring diodes). If the “hot” power system fails, the oring diodes forward bias and the second power system starts to supply full load current. While expensive, the dual redundant system allows separate input power sources to be used. See Master/Slave Operation and N+1.

Duty Cycle
The ratio of “on” time to “off” time for an electronic component or signal. In a power system duty cycle is typically used in reference to the semiconductor switch (in PWM controlled systems) or clock signal.

Dynamic Load
An output load that changes rapidly. Normally specified as a load change value as well as a rate of change.

Dynamic Response (Transient Response)
The output overshoot/undershoot that occurs when the output load of a power converter is turned on/off or abruptly changed. This overshoot/undershoot gives the high frequency output impedance of the converter. See Output Impedance.


The ratio of total output power to input power expressed as a percentage. Derived by the equation:
Efficiency(%) = (Output Power / Input Power) * 100
Efficiency is normally measured at full rated output power and nominal input line conditions at room temperature.

EMI – Conducted
(Electromagnetic interference) Noise generated by a power system (typically by the switching action of the more popular power converter topologies) and reflected back onto the input power bus. Acceptable limits for conducted EMI are set by various agencies (FCC, VDE, etc.). Switch-mode power converters typically include input filters to reduce noise to within agency limits.

EMI – Radiated
(Electromagnetic interference) Noise generated by a power system (typically by the switching action of the more popular power converter topologies) and emitted into the area surrounding a power system. Radiated EMI, consisting of broadband radio frequencies and narrow band emissions is set by various agencies (FCC, VDE, etc.) and is controlled by shielding.

Error Amplifier
An operational or differential amplifier used in the control feedback loop of a power converter. The amplifier produces an error voltage when the sensed output (tapped off a voltage divider network) differs from a reference voltage. This error voltage is used to adjust the operation of the PWM so as to correct the sensed output voltage. Sometimes called a reference amplifier.

(Electrostatic discharge) The current produced by two objects having a static charge when they are brought close enough to produce an arc or discharge.

(Equivalent Series Inductance) The inductance in series with an “ideal” capacitor. ESL sources could include terminals, electrodes, etc.

(Equivalent Series Resistance) The resistance in series with an “ideal” capacitor. ESR sources include lead resistance, terminal losses, etc. ESR is an important specification for high frequency applications.


Failure Mode
The reason for which a converter either does not meet or stops meeting its specified parameters.

Faraday Shield
An electrostatic shield that reduces coupling capacitance in transformers. The shield, which reduces output common mode noise, is placed between the primary and secondary windings of a transformer and connected to either input or output common.

Fault Mode Current
The input current drawn by a power converter when the output is shorted.

Fault Tolerant
A power system configuration optimized for continuing operation without shutdown. Typically involves the use of a redundant topology (i.e. N+1, etc.) and the ability to remove and replace power modules without disturbing system operation (hot-plug capability). Thus, the failure of any one power module will not cause a system failure or shutdown.

The process of returning a portion of the output signal of a system to its input.

Feed Forward
A method of improving line regulation by directly sensing the input voltage of the power converter. See Line Regulation.

A transformer in which part of the core is driven into saturation by a resonant tank circuit. The output of the transformer, taken from the saturated portion, is relatively immune to variations in input voltage. Used in ferroresonant power converters.

Flag Signal
An alarm or status signal generated by a power system. Typical, would be a front panel LED that indicates “Power Good.” “Power On,” “Overtemperature,” etc. or output logic signals (normally TTL/CMOS compatible).

Floating Output
A converter output that is ungrounded and not referenced to another output. Typically, floating outputs are fully isolated and may be referenced positive or negative by the user. Outputs that are not floating share a common return and as such, are referenced to one another.

Flyback Converter
The off time (flyback) during which power is transferred to the output of the primary switch. This technique is cost effective because of the minimum number of components required. See Boost Regulator, Buck Regulator, Bridge Converter, Forward Converter, Push-Pull Converter and Resonant Converter.

Foldback Current Limiting
A power converter protection technique. The circuit is protected under overload conditions by reducing the output current as the load approaches short circuit. This minimizes internal power dissipation under short circuit conditions. See Current Limiting and Current Limit Knee.

Forced Air Cooling
The use of a fan (or other air moving equipment) within a (sub)system to move air across heat producing components in order to reduce the ambient temperature. See Convection Cooling, Free Convection and Forced Convection.

Forced Convection
An operating environment in which air movement induced by a fan, blower, etc. is used to maintain power modules within operating limits. See Convection Cooling, Free Convection, and Forced Air Cooling.

Forward Converter
Also called a “Buck-Derived” converter, this topology, like the flyback converter, typically uses a single transistor switch. Unlike the flyback converter, energy is transferred to the transformer secondary while the transistor switch is “on,” and stored in an output inductor. See Boost Regulator, Buck Regulator, Bridge Converter, Flyback Converter, Push-Pull Converter and Resonant Converter.

Free Convection
An operating environment in which the natural movement of air (unassisted by fans or blowers) is sufficient to maintain the power module within its operating limits. Also called “natural convection.” See Convection Cooling, Forced Air Cooling and Forced Convection.

Full Bridge Converter
A converter topology that typically operates as a forward converter but uses a bridge circuit, consisting of four switching transistors, to drive the transformer primary. See Bridge Converter.

Full Load
The maximum value of output load specified for a converter under nominal operating conditions.

Full Wave Rectifier
A circuit (bridged or center tapped) that rectifies both halves of an AC waveform.


Galvanic Isolation
Two circuits which have very high resistance (several gigaohms) are considered to be “galvanically isolated” from each other. Galvanic isolation (separation) is achieved by using a transformer, opto-coupler, etc.

An electrical connection that is made to earth (or to some conductor that is connected to earth). A power converter “common” is not actually ground unless it is somehow connected to earth. See Common.

Ground Loop
A condition caused when two or more system components share a common electrical ground line. A ground loop is unintentionally induced, causing unwanted voltage levels.

Ground Plane
A conductive layer on the printed circuit board upon which a power converter is mounted that is at ground potential. Primarily used to shield system components from possible RFI noise generated by the power converter.


Half Bridge Converter
A converter topology that typically operates as a forward converter but uses a bridge circuit, consisting of two switching transistors, to drive the transformer primary. See Bridge Converter.

Half Wave Rectifier
A single diode circuit that rectifies one-half of an AC waveform.

Harmonic Distortion
For sinusoidal AC current waveforms, the distortion characterized by the presence of multiple harmonics of the fundamental frequency. This distortion is caused by the switching action of the power converter.

Heat Flux
The flow rate of heat across or through a material. Typically given in units of W/cm2.

Heat Rise
The increase in component/subassembly temperature caused by self-heating or heat absorption.

Heat Sink
A metal plate, thermally conductive potting material, extrusion, case, etc. that is used to transfer heat away from sensitive components and/or circuits. See Base Plate.

Heatsink Temperature
Average temperature of a heatsink attached to a power system module during normal operation. Typically, the heatsink temperature will be lower than the baseplate (or case) temperature.

Hiccup Mode
An operating mode triggered by an output fault condition (short-circuit) in which the converter cycles on and off. The duty cycle of on time to off time maintains the internal power dissipation at a safe level until the fault condition is corrected.

High Line
The maximum value of input line voltage specified for normal converter operation. See also Low Line and Input Voltage Range.

High Potential Test
(Hi-Pot Test) A test used to determine whether a converter passes its minimum breakdown isolation voltage specification. See Breakdown Voltage.

Hold-Up Capacitor
A capacitor added to the input of a distributed power system. This capacitor is intended to “hold-up” or maintain the input voltage to the power system in the event a fault causes a momentary loss of the input bus voltage.

Hold-Up Time
The period of time that a converter output will remain operating within specification following the loss of input power. This is a more common specification for AC/DC supplies.

The ability to insert or remove a power converter from a system while the system is powered and operating. Power converters must be specifically designed to allow this without disturbing other modules or subassemblies connected to the system power bus. Often used in redundant power systems to achieve fault tolerant operation.


(International Electrotechnical Commission) An organization based in Switzerland that sets standards for electronic products and components. IEC does not conduct any testing, however, their standards have been adopted by many of the national safety/standards agencies.

See Logic Inhibit/Enable.

Input Current
The current drawn from the input power bus by a power converter when operating under nominal conditions.

Input Line Filter
A low-pass or band-reject filter on the power converter input (internal or external) that attenuates noise introduced into the power bus from the power converter. See Balun Filter and Pi Filter.

Input Reflected Ripple Current
The AC component (typically generated by the switching circuit) measured at the input of a power converter. Given as a peak on the power bus. See Balun Filter and Pi Filter.

Input Transient
A spike or step change in the input line to a power converter. Input transient protection circuits are used to shield sensitive components (such as the MOV, high-voltage zener diode, etc.) from possible damage due to transients.

Input Voltage Range
The minimum and maximum input voltage limits within which a power converter will operate to specifications. Often given as a ratio of high line to low line (i.e. a range of 9VDC to 18VDC is 2:1).

Inrush Current
The maximum, instantaneous input current drawn by a power converter at turn on. Also called input surge current. (Typically the charging current of the input capacitance.)

Inrush Current Limiter
A protection circuit that limits the current a power system draws at turn on.

A non-conductive material used to protect and separate electronic components or circuits.

Insulation Resistance
The resistance offered by an insulating material to current flow.

Internal Power
The power dissipated (as heat) within the power converter during normal operation. Primarily a function of the power handling capability and efficiency of the power converter. Internal power dissipation is normally given as a maximum specification that cannot be exceeded without risking damage to the power converter.

A power conversion circuit that converts DC power to AC power.

The electrical separation between the input and output of a power converter. Normally determined by transformer characteristics and component spacing, isolation is specified in values of resistance (typically megohms) and capacitance (typically picofarads).

Isolation Voltage
The maximum voltage (AC or DC) that can be continuously applied between isolated circuits without a break-down occurring. On converters, this is normally specified as input-output or input-case isolation. Minimum isolation voltage levels must be maintained to meet most safety regulations. See Breakdown Voltage, High Potential Test and Isolation.


L-C Filter
A low-pass input filter consisting of a series inductor and a parallel capacitor (series and parallel being referenced to the input). Used in power converters to reduce input reflected ripple current.

Leakage Current
The current flowing from input to output or input to case of an isolated power converter at a specified voltage level.

(Linear Feet per Minute) A measure of the velocity of air. Used in distributed power systems to give the air flow over a baseplate or heatsink surface area. LFM is equal to m/s multiplied by 196.8.

Life Test
A reliability test in which a power system is operated (typically under accelerated conditions) over some period of time in order to approximate its life expectancy.

The bus used to deliver power to the input (terminals, pins, etc.) of a power converter. See Bus, High Line, Mains and Low Line.

Line Regulation
The percentage change in output voltage caused by varying the input voltage over a specified range (with output load, temperature, etc. remaining constant).

Line Voltage
The AC input voltage (off the mains) to a power subsystem.

Linear Regulation
A power supply regulation technique in which the regulating device (typically a transistor) is placed in series or parallel with the load. Voltage variations across the load are controlled by changing the effective resistance of the regulating device to dissipate unused power. See Series Regulator, Shunt Regulator and Post Regulation.

The electronic components or circuitry connected to the outputs pins of a power system. The characteristics (resistance, reactance, etc.) of the load determine the amount of power drawn from the power system.

Load Decoupling
The placement of filter components (typically µF capacitors) at the power terminals of the load in order to reduce noise.

Load Regulation
The change in the output voltage of a power converter (expressed as a percentage) caused by varying the output load over a specified range (with input line, temperature, etc. remaining constant).

Load Sharing
See Current Sharing.

Local Sensing
Using the output terminals of the power converter to provide feedback to voltage regulation circuits. See Remote Sensing.

Logic Inhibit/Enable
A signal (typically TTL/CMOS compatible) used to turn a power converter output on/off. Also called a remote on/off.

Long Term Stability
The change in output voltage of a power system over time with all other factors (line, load, temp, etc.) remaining constant. Expressed as a percent, the output change is primarily due to component aging.

Low Line
The minimum value of input line voltage specified for normal converter operation. See High Line and Input Voltage Range.


Adjusting the output of a power converter up and down (typically by 5%) to test system performance. Generally used to verify the resilience of a system to fluctuations in supply voltage.

Maximum Load
The highest amount of output load allowable under the continuous operating specifications of a power converter.

Maximum Operating Temperature
The maximum temperature at which a power converter will start and operate to within specified operating parameters. Typically specified as either ambient or case.

Standards published by the US government that specify operating, test and environmental standards for equipment to be used in military/aerospace applications.

Minimum Load
The minimum amount of output load required by a power converter to maintain normal continuous operating specifications. Usually associated with PWM (Pulse Width Modulation) controlled power converters.

Minimum Operating Temperature
The minimum temperature at which a power converter will start and operate to within specified operating parameters. Typically specified as either ambient or case.

An encapsulated power converter.

(Mean Time Between Failure (MTBF)) A unit of measure, expressed in hours, that gives the relative reliability of a converter. MTBF data is based upon actual operating data (demonstrated) or derived per the conditions of MIL-HDBK-217F (calculated).


A power system topology used to achieve high reliability levels through system redundancy. The circuit consists of a number of power converters connected in parallel, sharing the power drawn by the system load. One more power converter than is necessary to provide full load current is used (i.e. for a 400W load, three 200W converters are used). Thus, if any single power converter fails, the remaining modules will continue to supply current to the load. See Master/Slave Operation and Dual- Redundant.

An “N+1” redundant circuit in which more than one extra power converter is used (i.e. N+2, N+3, …N+x).

No Load Voltage
The voltage level present at the output pins of a power converter when 0% load is applied.

Unwanted variations in the power converter output that are unrelated to the switching frequency. Normally called “Ripple and Noise” and given as peak to peak value with a specified bandwidth. See EMI – Conducted, EMI – Radiated, Ripple and Noise and Periodic and Random Deviation.

Nominal Value
An ideal value that is used as a reference point.


Off Line Power Supply
A power supply (linear or switching) that operates directly off the AC line. The input voltage is rectified and filtered prior to any isolation transformer.

Operating Temperature
The range of temperatures over which a power system can be operated safely to within specified limits. Range normally specified as ambient, however, at times case or baseplate temperature are also sued.

Oring Diodes
Diodes that isolate a faulty power converter from the load and other power converters. Typically, these diodes are used external of a power converter.

Output Current Limiting
See Current Limiting and Foldback Current Limiting.

Output Filter
A low-pass filter (typically an L-C filter) inserted between the output rectifiers and output pins that attenuates output noise.

Output Impedance
The ratio of change in output voltage to a change in output load current.

Output Ripple & Noise
See Noise and Periodic and Random Deviation.

Output Voltage Accuracy
The maximum allowable deviation the DC output of a converter from its ideal or nominal value. Expressed as a percentage of output voltage. Often called output voltage tolerance.

Output Voltage Range
The minimum and maximum output voltage limits within which a power converter will meet its operating specifications.

Overload Protection
See Current Limiting and Foldback Current Limiting.

A transient change in output voltage that exceeds specified accuracy limits. Typically occurs on converter turn on/off or with a step change in output load or input line.

(Overvoltage Protection) A feature on select power converters in which an output monitoring circuit is activated if a preset voltage level is exceeded. Depending on the type of circuit used, the OVP will shut the converter down, “crowbar” the faulty output or switch the converter to a different operating mode.


Parallel Operation
A power system topology used to achieve high output levels. In a parallel circuit, the output currents of two or more power converters are summed together into a single load, providing a higher level of output power than that available from a single converter. Parallel operation requires power converters specifically designed to share loads. Although parallel operation is an integral part of a redundant circuit; power system redundancy is not necessarily achieved through a simple parallel connection. See Redundant Operation.

(Periodic And Random Deviation) The noise and ripple voltage that is superimposed on the DC output of a power converter. Typically specified at full load, it is expressed in peak-to-peak or RMS volts over a given bandwidth.

Pi Filter
An input filter consisting of two capacitors connected in parallel with a series inductance. Often used in power converters to reduce input reflected ripple current. A Pi Filter resembles the Greek letter p (Pi) where the horizontal line is the inductor and the two vertical lines are the capacitors.

The ability of a converter to produce an output that is positive or negative as referenced to ground. See Floating Output.

Post Regulation
An output circuit that sues a linear regulator to improve line/load regulation and reduce ripple and noise. In PWM-controlled converters, post regulation adds expense and degrades converter supply efficiency.

Power Density
The ration of converter output power to converter volume.

Power Factor
In an AC input power converter, the ratio of true input power to apparent input power. In these circuits, power factor is a measure of the input current that is in phase with the input voltage (and thus contributing to the average power).

Power Factor Correction
(PFC) In a AC power converter, a circuit that forces the AC input current to be approximately in phase with the input voltage. This reduces the harmonic distortion of the power system and increases the useful power drawn from the mains. PFC circuits can be active or passive. See Power Factor.

Power Fail
A signal (typically a TTL output) that indicates the DC input to the power converter has failed. Gives some warning of an input power bus failure, so the application can execute an orderly shutdown procedure during the hold-up time of the power converter.

Power Good
A signal (typically a visible LED) that indicates the DC output of the primary channel of a power converter is still present.

Power Rating
The specified power available at the converter output pins.

Primary Circuit
The input side of an isolated DC/DC converter. See Secondary Circuit.

Pulse Width Modulation
(PWM) A circuit used in converters to regulate output voltage. Regulation is achieved by varying the conduction time of the transistor switches.

Push-Pull Converter
A converter topology that typically is configured as a forward converter but uses two transistor switches and a center tapped transformer. The transistor switches turn on and off alternately. See Boost Regulator, Buck Regulator, Bridge Converter, Flyback Converter and Resonant Converter.


Multiple output power converters often have auxiliary outputs that are regulated via the primary output (the primary output is controlled by a direct feedback loop). The output voltage of the auxiliary is set by the turns ratio of the isolation transformer. Sometimes referred to as semi-regulation, quasi-regulated outputs are significantly affected by changes in the primary output.


Radiation Cooling
The transfer of heat between two materials at different temperature levels. Radiant heat does not play a significant role in the cooling of distributed power systems.

Redundant Operation
A parallel configuration of converters used in distributed power system to increase system reliability. Converters are typically configured in an “N+1,” dual redundant or master/master architecture. See Master/Slave Operation, Dual-Redundant and N+1.

The ability of a power converter to maintain an output voltage to within specified limits under varying conditions of input line and output load. See Linear Regulation.

Regulatory Agencies
Organizations (both independent and government supported) that develop specifications and/or test power converters to specifications that define product performance or intrinsic safety.

Remote Sensing
Using sense leads connected at the output load to provide feedback to voltage regulation circuits within a power converter. This arrangement is used to compensate for voltage losses caused by long leads to a load.

Resonant Converter
A switching power converter technology in which a resonant tank circuit operating at high frequencies is used to transfer energy to the output. Typically found in higher power (>100W) DC/DC applications.

The common terminal on the output of a power converter. It is the return current path for the output. See Common.

Reverse Current
Maximum input/output current a power converter will allow to pass. If the input/output polarity is reversed, a catastrophic failure may occur unless the converter is specifically designed for such conditions. See Reverse Voltage.

Reverse Voltage
A power converter feature that prevents damage to internal components if a reverse voltage is inadvertently applied to the input or output terminals.

Ripple and Noise
See Periodic and Random Deviation.

Ripple Voltage
The periodic AC component imposed on the output voltage of a power converter. Normally expressed as part of “Ripple & Noise” and given as a peak to peak value over a specified bandwidth. See Periodic And Random Deviation.

Royer Converter
A self-oscillating push-pull switching circuit configuration that is commonly used in low-cost, low-power DC/DC converter designs. Also called the Classic Converter.


Secondary Circuit
The output side of an isolated power converter. See Primary Circuit.

Secondary Outputs
In a multiple output power converter, the outputs that are not directly controlled by the output feedback loop. These outputs are essentially line regulated, with the output voltage being set by the turns ratio of the isolation transformer. Also called auxiliary outputs.

(Safety Extra Low Voltage) A term used by safety regulatory agencies (UL, CSA, etc.) to describe the highest voltage level that can be contacted by a person without causing injury. It is usually defined as 60VDC.

Sense Line
An output line used in a “remote sensing” connection to route the output voltage (at the load) back to the control feedback loop. See Remote Sensing and Local Sensing.

Series Operation
A configuration in which two or more isolated power converters are connected to obtain a higher output voltage level (converter inputs connected in parallel) or wider input voltage range (converter inputs connected in series) than that obtainable from one module.

Series Regulator
A linear regulator (internal or external to the power converter) that is placed in series with the load to achieve a constant voltage across the load. This is the most popular method of linear regulation. See Linear Regulation, Post Regulation and Shunt Regulator.

Set Point Accuracy
The ratio of the actual output voltage of a power converter to its specified output voltage.

Shunt Regulator
A linear regulator (internal or external to the power converter) that is placed in parallel with the load to achieve a constant voltage across the load. See Linear Regulation, Post Regulation and Series Regulator.

Six-Sided Shielding
A power converter packaging technique in which the unit is placed into a metal case. This metal shielding minimizes any noise radiation from the converter components. A continuous shielded case has the base (or header) welded on, further reducing potential noise leakage.

Also called a “booster,” a slave is a power converter that has no internal feedback loop. The output regulation of a slave unit is controlled by a master power converter to which its operation is synchronized. See Master.

(Surface Mount Technology) A manufacturing technique in which components that are designed for mounting on the surface of a substrate or PC board are used. Often used in conjunction with assembly automation equipment.

Soft Start
An input circuit that allows the output to reach its final value by controlling the on time of the input switching transistors at turn on.

The device (battery, generator, utility mains, etc.) that provides input power to a power converter.

Standby Current
The current drawn by a power converter when it has no load and/or has been shut down by a logic inhibit signal.

Start-Up Delay
The time delay between applying input power to a power system and having output voltages within regulation. Would also apply to the period between the application of a “remote on” signal to a power system and having the outputs achieve regulation. May also include soft start time.

Static Load
A load, typically specified as a percentage of full load, that remains constant.

Step Change
A sudden change in a power converter parameter. Typically used when referring to changes in output load or input line during power system testing.

Still Air
An operating environment in which the air surrounding the power system is restricted in small enclosures (often sealed) where it cannot move freely. Because it does not have sufficient mass or flow, still air is prone to thermal runaway caused by the thermal output of the power system.

Storage Temperature
The range of ambient temperatures over which a power converter can be safely stored. See Operating Temperature.

Switching Frequency
The rate at which the input voltage is switched or “chopped” in a power converter. Sometimes referred to as frequency of operation.

Switching Regulator
A circuit (typically a pulse width modulator) that uses a closed loop design to regulate the output voltage.


Temperature Coefficient
The average change in output voltage per degree centigrade. Expressed as a percentage of the output voltage over a specified temperature range.

Thermal Adhesive
A type of thermal joint compound that is epoxy (rather than oil) based. Once cured, thermal adhesives provide a mechanical bond between the power module baseplate and heatsink, as well as, insuring high thermal conductivity across the junction. See Thermal Joint Compound.

Thermal Gasket
A flexible pad or water with a very low thermal resistance that is put between a power module baseplate and heatsink to insure high thermal conductivity across the junction. Using a ‘dry’ gasket eliminates some problems associated with thermal joint compounds (inconsistent coating, contaminants, etc.) Gaskets are available in a number of standard precut forms.

Thermal Joint Compound
A fluid or paste that is spread between the mating surfaces of a power module baseplate and a heatsink (or system chassis). Typically it consists of a silicon or synthetic oil that carries a low thermal resistance filler (e.g. aluminum). The compound fills the voids caused by irregularities in the mating surfaces of the power module baseplate and heatsink, insuring high thermal conductivity across the junction.

Thermal Resistance
(q) A measure of the opposition a given material will have to the flow of heat. Used to calculate the temperature drop that occurs when power flows through a material or across the junction of two materials. Given in units of degrees Celsius per watt (°C/W).

Thermal Resistivity
(r) A measure of the ability of a material to impede the flow of heat, Typically given in units of (°C)(T)/W, where T equals the material thickness and W equals the power flowing through the material in watts.

Thermal Runaway
A circuit condition in which an increase in the ambient temperature surrounding a power converter causes an increase in the internal power dissipated within the power converter, increasing the case temperature, which increases the ambient temperature of the surrounding air. The spiraling effect of these temperature increases will eventually lead to failure of the power system. This condition occurs when inadequate measures (air flow, system venting, power derating, etc.) are taken to get heat away from critical components.

Thermal Protection
A feature that shuts the power converter down if the internal temperature exceeds preset limits. Also called Thermal Shutdown.

Three Terminal Regulator
A linear regulator packaged in a standard 3-terminal transistor package. These devices can be configured as either a shunt or series regulator.

For a multiple output power converter, the parameter that gives the change in one output voltage caused by a change in the voltage level or load on another output.

A spike or step change in a power converter parameter. Commonly used in describing input line and output load characteristics.

Transient Recovery Time
The time required for a power converter output to return to within specified limits following a step change in output load current. Expressed as a percentage of rated value.

True Power
The actual power consumed by an AC circuit.

(Technischer Uberwachungs-Verein) An organization approved for testing products to VDE standards. US-based companies often use TUV in place of VDE because they have established facilities in the US.


(Underwriters Laboratories) An independent organization that conducts safety testing of products to established standards.

A transient change in a converter output voltage that does not meet the lower limit of the voltage accuracy specification. Typically occurs at converter turn on/off or with some step change in output load or input line. See Voltage Accuracy.

Universal Input
An AC input to a power system that accepts all the standard voltage levels available from AC mains. Typically specified at 85VAC to 265VAC.

(Uninterruptible Power Supply) A power supply that will continue to operate after the loss of AC input power. A UPS normally uses some type of automatic battery backup system.


(Verband Deutsher Electrotechniker) A German organization that sets standards for product safety and noise emissions; VDE tests and certifies products to those standards.

Voltage Accuracy
The deviation from a nominal value. Expressed as a percentage (%). See Nominal Value.

Voltage Balance
For a multiple output power converter, the percentage difference in voltage level of two outputs with opposite polarities and equal nominal values.

Voltage Regulation
See Regulation.


Warmup Time
The time required for a power converter to operate to within specifications after turn-on. This time normally precedes a long-term drift specification.

Withstand Voltage
The maximum voltage level that can be applied between circuits or components without causing a breakdown. See Breakdown Voltage and Isolation.