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Absolute Maximum Ratings

Absolute maximum ratings are limiting values that should not be exceeded even instantaneously. Exposure to conditions beyond these ratings may affect the lifetime and reliability of a device.

AC Filter

This is a filter circuit that removes unwanted frequency components (harmonics) from alternating current. There are some electromagnetic interference (EMI) filters that fall into this category.

Active High

Active High - Rising pulse of input will trigger the associated circuitry.

Active Low

Active Low - Falling pulse of input will trigger the associated circuitry.


The converter operates under controlled conditions for a predetermined period of time to allow troubleshooting.

Ambient Temperature

The temperature of still air surrounding a device or a circuit. A typical method of measuring ambient temperature is to record the temperature at a distance of half an inch from the component or circuit.


This refers to the amount by which a given parameter is relatively reduced in magnitude. Measurement attenuation is often encountered when measuring voltage, current, and power. Generally, the attenuation is expressed in decibels. For power, 1 dB = 10Log 10(P1/P2); for current, 1 dB = 20Log 10(I1/I2); for voltage, 1 dB = 20Log 10(V1/V2).


This is a system of wire sizes. Every increase of 3 wire gauges means that the cross-sectional area of the wire is reduced by half.

Balun Filter

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.


Refers to the frequency bandwidth occupied by the signal; when used to describe a channel, the bandwidth refers to the maximum frequency bandwidth of the signal that can effectively pass through the channel. For analog signals, the bandwidth is also called the bandwidth, and the unit is Hertz (Hz).

Base Plate

This is the substrate on which components are mounted in the circuit, or a metal plate on which the converter is mounted. It is usually used to remove the heat generated by the main circuit components.

Battery Backup

A subsystem of an electronic device that provides power to the electronic device once the input power source is lost. Backup batteries are common in DC-DC converters.


Communication industry standard proposed by Bellcore.

Breakdown Voltage

The maximum AC or DC voltage which may be applied from input to output and/or chassis of a power supply.

Burn In

Before the power supply products leave the factory, in order to eliminate the initial failure of components and other potential influences, they should usually be operated under rated load for a period of time. This process is called aging.


Power for such devices can be obtained from the main hub. Examples include USB hubs, keyboards, mouse, web cameras, etc.

Canadian Standards Association (CSA)

An independent organization that formulates safety standards for electronic components and systems for the Canadian market and conducts safety tests on electronic components and systems.


A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.

Case Temperature

The temperature of the case when the converter and the surrounding system are operating normally. Usually an indicator of DC-DC converters operating over a certain temperature range. Case temperature is sometimes referred to as the temperature of the base plate.


A unit of measurement of airflow in a system, cubic inches per minute.


This is an inductor used for filtering, filtering out unwanted signals.

Common Mode Filter

This is a commonly used EMI (Electromagnetic Interference) filter, which will present a high impedance to common mode current (or common mode noise) conductors and low impedance to the desired signal.

Common Mode Noise

The component of noise which is common to both the DC output and return lines with respect to input neutral.

Conduction Cooled

Cool the converter with solid material. Cool the power converter with a heat sink or mount the module in the system rack.


A power supply that regulates its output current for changes in line, load, ambient temperature, and time.


A power supply that regulates its output voltage for changes in line, load, ambient temperature, and time.

Convection Cooled

A cooling method that dissipates heat through the flow of gas on the surface of a heating element. Free air convection means that the natural flow of air (without the use of fans or blowers) is sufficient to maintain the converter within the specified temperature range.

Copper Loss

The power loss that occurs when current flows in a winding. It is equal to the square of the current (I) multiplied by the resistance (R) of the winding wire, ie I2*R. This power loss is converted into heat and dissipated.

Cross Regulation

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


An overvoltage protection circuit which rapidly places a low resistance shunt across the power supply output terminals if a predetermined voltage is exceeded.

Current Limit Knee

This is the point on the output voltage versus current curve where current limiting (or foldback) begins.

Current Limiting

An overcurrent protection circuit. In order to protect the load and power supply, the current limiting protection circuit can limit the maximum output current of the power supply.

Current Mode Control

A control method for switching converter circuits. A dual-loop control circuit regulates the PWM operation based on the measured output current.

Current Transformer

Typically used in current sensing devices, the primary winding of a current transformer usually has only one turn. The number of turns of the secondary winding depends on the sensitivity required, and the current transformer is usually made into a ring.

DC Filter

A filter circuit that removes AC pulsation and ripple in DC current. It is usually composed of an inductor and a capacitor.

DC-DC Converter

A circuit or device that converts a DC input voltage (usually an unregulated DC voltage) into a regulated DC output voltage.

DCR (DC Resistance)

The resistance of an inductor to direct current (not alternating current). When designing an inductor, the DC resistance is often set as small as possible. The unit of DC resistance is ohms, and its maximum value is usually given.


The specified reduction in an operating parameter to improve reliability. Generally for power supplies, it is the reduction in output power at elevated temperatures.


A material used to prevent two points in an electrical circuit from becoming electrically connected. Sometimes called the dielectric barrier.

Dielectric Strength - Electric Strength - Hipot

Physically separating the source and secondary windings ensures that low voltage circuits are safely connected to circuit nodes where high potentials may be present, and that there is no conductive path. The rated voltage of the transformer is determined according to the dielectric strength, expressed in AC voltage effective value (VACRMS) or DC voltage (VDC). The transformer ensures that the primary and secondary windings are isolated from transient voltages that are strong but below the rated dielectric level. At certain possible voltages, leakage currents may flow through the protective insulation. When the magnitude of the leakage current exceeds a predetermined level, breakdown of the insulating layer, or breakdown of the dielectric layer, occurs.

Dielectric Withstand Voltage

The voltage that causes dielectric breakdown. When the dielectric breaks down, the conductors that are insulated from each other, or the conductor and the core can conduct electricity. The ability to withstand a breakdown voltage between the primary and secondary windings of a transformer.

Differential Mode

the noise source appears across power supply lines and is in series with the power supply line, and the noise current flows in the same direction as the power supply current. It is called “differential mode” because the outgoing and return currents are oppositely-directed.


The component of noise measured between the DC output and output return.

Distributed Capacitance

Capacitance that exists between the turns in a coil or choke, or between adjacent conductors or circuits, as distinguished from the capacitance concentrated in a capacitor.

Distributed Power

System-level architectures where converters power each subsystem/component locally (and possibly at different levels). As for which type of decentralized power supply system to use, it depends to a large extent on the needs of specific applications.


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 constant.

Duty Cycle

The product of the pulse duration and pulse repetition frequency of a pulse carrier, equal to the time per second that pulse power is applied. Also known as duty factor.

Dynamic Load Regulation

The rate of change of the output voltage when the output current changes rapidly.

Dynamic Response

During output load changes, the behavior of the output voltage shows the response of the overall feedback system.


The ratio of total output power to input power expressed in percent. This is normally specified at full load and nominal input voltage.

Electronic Load

An electronic device connected to the output terminal of the power supply as an analog load. This load can be adjusted dynamically and can also vary with frequency.

Electrostatic Discharge (ESD)

An arc or discharge of electrical current produced when two objects with a static charge come into close proximity.

EMI (Electromagnetic Interference)

Electromagnetic interference. Unwanted energy, generally emitted from switching power supplies, which may be conducted or radiated.


An enclosure or container used to enclose a converter. Generally speaking, the converter is packaged in a plastic or metal case.

Equivalent Series Inductance (ESL)

It is an effective inductance that is used to describe the inductive part of the impedance of certain electrical components.

Equivalent Series Resistance (ESR)

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.

Failure Mode

The reason why the converter did not meet the requirements or became out of specification.

Faraday Shiel Faraday

An electronic shield between input and output windings of a transformer. This can be used to reduce primary and secondary coupling capacitance, which in turn reduces output common mode noise.

Fault Mode Current

The input current drawn by the converter when the output is shorted.

Federal Communications Commission (FCC)

The U.S. government is responsible for formulating standards related to conduction and wireless emissions, and is responsible for managing the testing of conductive and wireless emissions. These are standards for the system in question, but are used to determine the converter's specifications.

Feed Forward

A method to improve regulation performance by directly sensing the input voltage of the converter.

Floating Output

The output of the converter is not grounded or referenced to other outputs. Typically, the floating output is fully isolated and can be specified as positive or negative by the user. Outputs that are not floating have a common return and are therefore referenced to each other.

Flyback Converter

Also known as a "buck-boost" converter, this circuit uses a single transistor switch, eliminating the need for an output inductor. During the first half of the switching cycle, when the transistor is on, energy is stored in the primary winding of the transformer. During the second half of the cycle, the transistor is turned off and the energy stored in the primary winding is transferred to the secondary winding of the transformer and to the load.

Foldback Current Limiting

A power supply output protection circuit whereby the output current decreases with increasing overload, reaching a minimum at short circuit.

Forced Air Cooling

In one system, a fan (or other device that moves air) blows air over the heating element to reduce the ambient temperature. Also known as forced cooling.

Forward Converter

A power supply switching circuit in which energy is transferred to the transformer secondary when the switching transistor is on. In this circuit minimal energy is stored in the transformer.


A power switching circuit in which four transistors are connected in a bridge configuration to drive a transformer primary.

Full Load

In the case of continuous operation, the maximum output load.


An electrical connection to earth (or to some conductor connected to earth).


Return currents or magnetic fields from relatively high-powered circuits or components which generate unwanted noisy signals in the common return of relatively low-level signal circuits.


A power switching circuit similar to the full bridge converter except that only two transistors are used, with the other two replaced by capacitors.

Hall Effect Transducer

This is a device that generates a voltage in response to the applied DC voltage and the magnetic field to which the device is exposed. The amplitude of the output is a function of the magnetic field and the angle of the Hall device relative to the magnetic field.

Heat Sink

A piece of thermally conductive metal attached to a semiconductor or other electronic device and designed to prevent it from overheating by conducting heat away from it and radiating it to the environment. Heat sinks often have fins to increase their surface area. They occasionally have fans attached. Heat sink compound can be smeared between the device and the heat sink to improve thermal conduction.

Hiccup Mode

The working state caused by the output fault (short circuit), at this time, the switching converter maintains the duty cycle of each cycle from on to off and from off to on to keep the internal power consumption within a safe range until the fault is eliminated .

High Line

The continuous maximum voltage allowed to be input to the converter while maintaining the normal operation of the converter.

High Potential Test (Hi-Pot Test)

Hight Potential test. A test to determine if the break-down voltage of a transformer or power supply exceeds the minimum requirement. It is performed by applying a high voltage between the two isolated test points.

Hold -Up Time

The time during which a power supply's output voltage remains within specification following the loss of input power.

Hot Plug-In

common requirement in distributed power systems. The power strip can be plugged in and out without damage. Components on the power board must be protected from damage caused by the resulting inrush current.


The total resistance experienced by an electric current that includes alternating current (AC) and direct current (DC) components. DC resistance is the resistance experienced by the DC component of electric current. The AC component of the impedance includes the reactance of the inductor, denoted by XL, XL = 2πfL.


This is a characteristic of a circuit element, and it is the resistance against the current flowing in the element. For an inductor, its inductance is related to the material of the core, the shape and size of the core, the number of turns of the winding, the shape of the winding, etc. The inductance of an inductor is usually expressed in microhenry (μH) or millihenry (mH).

Input Current

The current value measured by the power supply or power converter can be measured by an ammeter or an oscilloscope.

Input Line Filter

A low-pass or band-reject filter at the input of a power supply which reduces line noise fed to the supply. This filter may be external to the power supply.

Input Reflected Ripple Current

The AC component (usually produced by switching circuits) measured at the input of a converter. Expressed in peak-to-peak or effective value (RMS).

Input transient

A spike or step change applied to a converter input. Input transient protection circuitry is used to prevent sensitive components such as semiconductor switches from seeing these spikes or step changes.

Input Voltage Range

The range of input voltages within which a power supply or power converter can operate normally.

Inrush Current

The peak instantaneous input current drawn by a power supply at turn-on.

Inrush Current Limit

A circuit which limits the inrush current during turn-on of a power supply.


A well-insulated transformer can withstand very harsh environmental conditions and last for many years. The operating temperature and the voltage the dielectric can withstand determine which type of insulation should be used and the level of insulation.

Insulation Resistance

The insulating material has the property of insulating, and its unit of measurement is ohm.

Internal Power Dissipation

During normal operation, the power dissipation generated inside the converter (which is converted to heat). Basically it is related to the power and efficiency of the converter. Usually the internal power consumption index refers to the maximum value of power consumption. Do not exceed this value, otherwise the converter will be damaged.

International Electrotechnical Commission (IEC)

The agency is based in Tuas, which develops standards for electronic products and components. IEC does not conduct any testing work. However, its standards have been adopted by security inspection agencies or standard-setting agencies in many countries.


A power converter which changes DC input power into AC output power.


The electrical separation between input and output of a power supply means of the power transformer.

Isolation Voltage

The maximum AC or DC voltage which may be continuously applied from input to output and/or chassis of a power supply.

Layer Winding

The method of winding a transformer whereby the primary and secondary are wound in layers over one another, separated by an insulation layer.

Leakage Current

The AC or DC current flowing from input to output and/or chassis of an isolated power supply at a specified voltage.


A unit of measure for the velocity of air used to cool power converters.

Life test

Reliability testing of converters. The converter is operated (under a stressful condition) for a period of time in order to estimate the life expectancy.

Line Regulation

The change in output voltage in percent as the input voltage is varied over its specified limits, with load and temperature constant.

Linear regulation

A popular stabilization circuit in which a control device is placed in series(or parallel)with the load to give a constant voltage across the load.

Linear regulator

a linear regulator is a system used to maintain a steady voltage. The resistance of the regulator varies in accordance with both the input voltage and the load, resulting in a constant voltage output.


An electronic component/circuit connected to the output of a converter. The characteristics of the load (resistance, reactance) determine the amount of power drawn from the converter.

Load Decoupling

Connect the filter element (usually a μF capacitor) to both ends of the load to reduce noise.

Load Regulation

The percent change in output voltage as the load is changed from minimum to maximum, at constant line and constant temperature. The load change may be specified for other than no load to full load, such as 20% load to full load.

Long Term Stability

The change in converter output voltage over time with all other factors (input voltage, load, temperature, etc.) held constant. Expressed as a percentage, the output voltage change is mainly caused by component aging.

Low Line

Maintain the lowest input voltage at which the converter operates normally.

Maximum Ambient Operating Temperature

The maximum ambient temperature at which the circuit is expected to operate normally.

Maximum Load

The maximum output load allowed by the converter in continuous operation.

Mean Time Between Failure (MTBF)

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.

Minimum Load

In order to meet the technical requirements, the power supply equipment should have the minimum load current or minimum power. In order to prevent power supply equipment from malfunctioning, the frequency of the power supply cannot be too low, and the load current cannot be too small.

Minimum Operating Temperature

This is the lowest temperature at which the converter will start and operate within specifications.

No Load Voltage

The voltage at the output pin when there is no load connected to the output of the converter.

Nominal Value

The stated or objective value for a quantity, such as output voltage, which may not be the actual value measure

Normal Operating Current

Under normal operating conditions, the maximum current flowing through a circuit.


English abbreviation for outer diameter

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 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.

Operating Temperature Range

The ambient temperature range within which a component can operate safely. The operating temperature range is different from the storage temperature range. The operating temperature range is related to the temperature rise of the component itself caused by operating losses in the component.

Output Current Limit

A 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.

Output filter

A low-pass filter placed at the output of a rectified power converter smooths the converter's square or sine wave output. Filters include LC, RC and π-type filters.

Output Impedance

The ratio of change in output voltage to change in current load.

Output voltage

The nominal value of DC voltage at the output terminals of a power supply.

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.

Overcurrent Protection

When the output current and power exceed the rated value, the power supply or power converter implements a measure to stop the output or reduce the overall power loss to avoid damage to it.

Overload Protection

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.

Overtemperature Protection

The function used to protect in the case of exceeding the operating temperature range for which it is designed.

Overvoltage Lockout , OVLO

A design feature designed to protect the power supply and its downstream surroundings from the maximum supply voltage exceeding the operating voltage limit.

Overvoltage Protection (OVP)

A power supply feature which shuts down the supply, or crowbars or clamps the output, when its voltage exceeds a present level.

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 from either supply alone. This requires power supplies specifically designed to share the laod.


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.

Peak On-state Surge Current

The current flowing through an element in the conduction state.

Peak Pulse Current

The rated maximum value of the peak pulse current of the rated amplitude and waveform.

Periodic and random Deviation (PARD)

It is the sum of all ripple and noise components at the DC output of the power supply, and this offset has nothing to do with the original state or the power supply. It is usually represented by peak-peak value or effective value (RMS) in a certain frequency band.


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.

Positive Temperature Coefficient , PTC

A term used to describe a material whose electrical resistivity increases as its temperature increases.

Post Regulation

An output circuit that uses a linear regulator to stabilize the input voltage or load changes and reduce ripple voltage and noise. In a PWM-controlled converter, the use of a back-end regulator adds cost and reduces the efficiency of the converter's power supply.

Power Density

The ratio of the output power of a converter to its volume.

Power Dissipation 功率散逸 /

Power dissipation is caused by the current flowing through the element and the voltage across the two ends of the element when the element is in operation.

Power Factor Correction

A design technique used at the input of a grid-powered converter that improves the converter's power factor and reduces the harmonics that the converter sends to the grid.

Power Fail Signal

A power supply option which monitors the input voltage and provides an isolated logic output signal when there is a loss in line voltage.


A power supply feature whereby the input power is reduced to a low value under output overload conditions.

Power Rating

The power that the converter can output.


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.


A term used to describe a material whose electrical resistivity increases as its temperature increases.

Pulse Transformers

The excitation current in this type of transformer is a series of pulses with a short duration and repeated at a certain speed.

Pulse Width Modulation (PWM)

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.

Push-Pull Converter

A power switching circuit which uses a center-tapped transformer and two power switches which are driven on and off alternately. This circuit does not provide regulation by itself.

Rated Output Current

The maximum load current which a power supply was designed to provide at a specified ambient temperature.


A device that only allows current to flow in one direction, such as a diode.

Reduntant Operation

In the decentralized power supply system, the converters are connected in parallel to improve the reliability of the system. Converters can be used in an "N+1" fashion.


The reference point is usually regarded as the reference point of the voltage level or 0V.


Under various conditions of input voltage or output load changes, the converter has the ability to maintain the output voltage within the specified range of technical indicators. See "Linear Regulators"

Relected 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.

Remote Sensing

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.


The name for the common terminal of the output of a power supply; it carries the return current for the outputs.

Reverse Current

Small value of direct current that flows when a semiconductor diode has reverse bias.

Reverse Voltage Protection

A feature which protects a power supply against reverse voltage applied at the input or output terminals.


English abbreviation for Radio Frequency Interference. This older, more restrictive term is used interchangeably with EMI (electromagnetic interference).

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 a result of feed through of the rectified line frequency, internal switching transients and other random noise.

Ripple voltage

Periodic alternating voltage superimposed on the output of a switching voltage converter. Ripple voltage is usually expressed in peak-to-peak value.

Secondary Winding

This is the winding in the transformer that supplies power to the load, also known as the secondary winding. The electrical energy it transmits to the load is converted from the magnetic energy in the core.

Self Resonant Frequency

The frequency at which the distributed capacitance in the inductor forms a resonance with the inductance. Over frequency, the inductive reactance of an inductor equals the capacitive reactance of a capacitor and cancels each other out. At this point the inductor becomes a pure resistance. At the natural frequency, the inductor presents a very high impedance. Distributed capacitance is formed by stacking each layer of coils and winding them on the magnetic core. Distributed capacitance and inductance are connected in parallel. Above the natural frequency, the parallel distributed capacitance plays a major role. At the natural frequency, since the inductive reactance is equal to zero, the Q value of the inductor is equal to zero. The natural frequency is expressed in MHz, and its minimum value is given in the product manual. See "Distributed Capacitance"

Series Operation

A way to connect the outputs of two or more power converters to increase the output voltage and the overall output power.

Series Regulator

The most popular method of linear regulation in which the control device is in series with the raw DC and the load to achieve constant voltage across the load.


A method of blocking electromagnetic interference to protect components, equipment, and devices that are sensitive to electromagnetic interference. In an inductor, it's placed inside a very thin sheet of metal. The winding and the core itself have a shielding effect.

Short Circuit Protection

A feature which limits the output current of a power supply under short-circuit conditions so that the supply will not be damaged.

Shunt Regulator

A linear regulator in parallel with the load (whether inside or outside the converter) can be used to get the desired voltage across the load.

Six-sided Shielding

Converter packaging technology. The converter is placed in a metal case. This metallic shielding reduces any noise emitted from the converter components. The shield shell is welded with a bottom plate, which further reduces the noise that may leak out

Soft Start

A feature which limits the start-up switching currents of a switching power supply and causes the output voltage to rise gradually to its final value.


The power line that brings power to the DC-DC converter.


Under the change of time, load and environment, the output voltage and various performance indicators can remain unchanged or change very little.


The output voltage change of a power supply, in percent, due to time only, with all other factors held constant, Long-term stability is a function of component aging.

Standby Current

The input current drawn by a power supply under no load or when shut down by a control input.

Step Change

An instantaneous change in a quantity from one value to another.

Storage Temperature Range

Ambient temperature range in which it can be safely stored

Switch Mode Power Supply

This is a power conversion technology, which converts the input power into high-frequency pulses through the closing and cutting actions of the switch, and converts the pulses into DC power again at the output end. Using this technique, an unregulated input voltage can be converted to a more regulated output voltage with higher conversion efficiency

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 which uses a closed loop system to regulate the output voltage, generally by means of a pulse-width modulator.

Technischer Uberwachnge-Verin (TUV)

An institution authorized to test products according to VDE standards. Companies in the United States often pass TUV certification according to VDE standards, because TUV has a security inspection agency in the United States.

Temperature Coefficient

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


The range of ambient or case temperature within which a power supply may be safely operated and meet its specifications.


The range of ambient temperatures within which a power supply may be safely stored, non-operating, with no degradation in its subsequent operation.

Temperature, Maximum Ambient Operating

The maximum ambient temperature at which the circuit is expected to operate normally

Thermal Conducttivity

The ability of a material to conduct heat. It is defined as the amount of heat that passes through a material with a thickness of one meter when the temperature difference between the two sides of the material is 1 °C per unit time.

Thermal Gasket

A flexible gasket with very low thermal resistance is placed between the power module and the heat sink, so that the connecting parts of the two have excellent thermal conductivity.

Thermal Joint Compound

A liquid or paste that is applied between the baseboard surface of a power device and the heat sink or system cabinet.

Thermal Protection

An internal safeguard circuit in a power supply which shuts down the unit in the event of excess internal temperature.

Thermal Resistance

The resistance of a material to the flow of heat. Its unit of measurement is °C/W.

Thermal Resistivity

A measure of the resistance of a material to the flow of heat. Its unit is ℃T/W, where T is the thickness of the material and W is the power flowing through the material in watts.

Three-Terminal Regulator

A voltage regulator packaged in a standard transistor case. This device can be a switching regulator, or a linear regulator.


A transformer is a passive component used to change voltage, current or impedance to a desired value. It is usually formed by winding more than two windings on a soft magnetic core. Apply a voltage across the primary winding and it will generate a magnetic field in the core and induce a voltage in the secondary winding.


Spike or step changes in converter parameters. It is generally used to describe the characteristic parameters of input voltage and output load.

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 current load or a step change in input voltage.

Transient Suppression

Special devices are used to reduce the effects of transient voltages in electronic circuits. Transient voltage suppression devices include metal oxide varistors (MOV), semiconductor transient suppressors (TVS) and gas discharge tubes.

Turn Ratio

The ratio of the primary voltage (or primary winding turns) to the secondary voltage (or secondary winding turns).

Twisted Pair

A cable consisting of two AWG 18 or 24 copper wires twisted together. Twisting together provides protection against the effects of EMI and RFI.


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.

Undervoltage lockout

The UVLO function provides that when the input voltage is lower than the input range, the output of the power supply or power converter is turned off or stopped to avoid unnecessary loss at the input end or damage that may be caused by a large current at the input end.

Underwriters Laboratory (UL)

Underwriters Laboratories, Inc. An independent, non-profit U.S. organization that tests products for safety.

Uninterruptible Power supply (UPS)

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.


The UVLO function provides that when the input voltage is lower than the input range, the output of the power supply or power converter is turned off or stopped to avoid unnecessary loss at the input end or damage that may be caused by a large current at the input end.

Verband Deutscher Electrontechniker (VDE)

Verband Deutscher Elektrotechniker. A West German organization which tests equipment for public safety and emitted noise.

Voltage Balance

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.

Warmup Drift

The initial change in output voltage of a power supply from turn-on until it reaches thermal equilibrium at nominal line full load, 25 degree C ambient temperature.

Warmup Time

The time required, after initial turn-on, for a power supply to meet its performance specifications.

Withstand Voltage

The maximum voltage that can be applied between circuits or between components without causing breakdown.

Zener Diode

A diode whose voltage across it maintains a constant voltage when the reverse voltage across it increases beyond a certain point. Such diodes are called Zener diodes, or Zener diodes.

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