Breakdown voltage

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High voltage breakdown of an insulator string Lfa.JPG
High voltage breakdown of an insulator string

The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive.

Contents

For diodes, the breakdown voltage is the minimum reverse voltage that makes the diode conduct appreciably in reverse. Some devices (such as TRIACs) also have a forward breakdown voltage.

Electrical breakdown

Materials are often classified as electrical conductors or electrical insulators based on their resistivity. A conductor is a substance which contains many mobile charged particles called charge carriers which are free to move about inside the material. So if an electric field is created across a piece of the material by applying a voltage difference between electrical contacts on different sides of the material, the force of the field causes the charge carriers to move, creating an electric current through the material from the positive to the negative contact. For example, in metals one or more of the negatively charged electrons in each atom, called conduction electrons are free to move about the crystal lattice, An electric field causes a large current to flow, so metals have low resistivity, making them good conductors. In contrast in materials like plastics and ceramics all the electrons are tightly bound to atoms, so under normal conditions there are very few mobile charge carriers in the material. Applying a voltage causes only a very small current to flow, giving the material a very high resistivity, and these are classed as insulators.

However, if a strong enough electric field is applied to them, all insulators become conductors. If the voltage applied across a piece of insulator is increased, at a certain electric field the number of charge carriers in the material suddenly increases enormously and its resistivity drops, causing a strong current to flow through it. This is called electrical breakdown. Breakdown occurs when the electric field becomes strong enough to pull electrons from the molecules of the material, ionizing them. The released electrons are accelerated by the field and strike other atoms, creating more free electrons and ions in a chain reaction, flooding the material with charged particles. This occurs at a characteristic electric field strength in each material, measured in volts per centimeter, called its dielectric strength.

When a voltage is applied across a piece of insulator, the electric field at each point is equal to the gradient of the voltage. The voltage gradient may vary at different points across the object, due to its shape or local variations in composition. Electrical breakdown occurs when the field first exceeds the dielectric strength of the material in some region of the object. Once one area has broken down and become conductive, that area has almost no voltage drop and the full voltage is applied across the remaining length of the insulator, resulting in a higher gradient and electric field, causing additional areas in the insulator to break down. The breakdown quickly spreads in a conductive path through the insulator until it extends from the positive to the negative contact. The voltage at which this occurs is called the breakdown voltage of that object. Breakdown voltage varies with the material composition, shape of an object, and the length of material between the electrical contacts.

Solids

Breakdown voltage is a characteristic of an insulator that defines the maximum voltage difference that can be applied across the material before the insulator conducts. In solid insulating materials, this usually[ citation needed ] creates a weakened path within the material by creating permanent molecular or physical changes by the sudden current. Within rarefied gases found in certain types of lamps, breakdown voltage is also sometimes called the striking voltage. [1]

The breakdown voltage of a material is not a definite value because it is a form of failure and there is a statistical probability whether the material will fail at a given voltage. When a value is given it is usually the mean breakdown voltage of a large sample. Another term is also withstand voltage, where the probability of failure at a given voltage is so low it is considered, when designing insulation, that the material will not fail at this voltage.

Two different breakdown voltage measurements of a material are the AC and impulse breakdown voltages. The AC voltage is the line frequency of the mains. The impulse breakdown voltage is simulating lightning strikes, and usually uses a 1.2 microsecond rise for the wave to reach 90% amplitude, then drops back down to 50% amplitude after 50 microseconds. [2]

Two technical standards governing performing these tests are ASTM D1816 and ASTM D3300 published by ASTM. [3]

Gases and vacuum

In standard conditions at atmospheric pressure, air serves as an excellent insulator, requiring the application of a significant voltage of 3.0 kV/mm before breaking down (e.g., lightning, or sparking across plates of a capacitor, or the electrodes of a spark plug). In partial vacuum, this breakdown potential may decrease to an extent that two uninsulated surfaces with different potentials might induce the electrical breakdown of the surrounding gas. This may damage an apparatus, as a breakdown is analogous to a short circuit.

In a gas, the breakdown voltage can be determined by Paschen's law.

The breakdown voltage in a partial vacuum is represented as [4] [5] [6]

where is the breakdown potential in volts DC, and are constants that depend on the surrounding gas, represents the pressure of the surrounding gas, represents the distance in centimetres between the electrodes,[ clarification needed ] and represents the Secondary Electron Emission Coefficient.

A detailed derivation, and some background information, is given in the article about Paschen's law.

Diodes and other semiconductors

Diode I-V diagram Diode-IV-Curve.svg
Diode I-V diagram

Breakdown voltage is a parameter of a diode that defines the largest reverse voltage that can be applied without causing an exponential increase in the leakage current in the diode. Exceeding the breakdown voltage of a diode, per se, is not destructive; although, exceeding its current capacity will be. In fact, Zener diodes are essentially just heavily doped normal diodes that exploit the breakdown voltage of a diode to provide regulation of voltage levels.

Rectifier diodes (semiconductor or tube/valve) may have several voltage ratings, such as the peak inverse voltage (PIV) across the diode, and the maximum RMS input voltage to the rectifier circuit (which will be much less).

Many small-signal transistors need to have any breakdown currents limited to much lower values to avoid excessive heating. To avoid damage to the device, and to limit the effects excessive leakage current may have on the surrounding circuit, the following bipolar transistor maximum ratings are often specified:

VCEO (sometimes written BVCEO or V(BR)CEO
The maximum voltage between collector and emitter that can be safely applied (and with no more than some specified leakage current, often) when no circuit at the base of the transistor is there to remove collector-base leakage. Typical values: 20 volts to as high as 700 volts; very early Germanium point-contact transistors such as the OC10 had values around 5 volts or less.
VCBO
The maximum collector-to-base voltage, with emitter open-circuit. Typical values 25 to 1200 volts.
VCER
The maximum voltage rating between collector and emitter with some specified resistance (or less) between base and emitter. A more realistic rating for real-world circuits than the open-base or open-emitter scenarios above.
VEBO
The maximum reverse voltage on the base with respect to the emitter.
VCES
Collector to emitter rating when base is shorted to emitter; equivalent to VCER when R = 0;

Field-effect transistors have similar maximum ratings, the most important one for junction FETs is the gate-drain rating.

Some devices may also have a maximum rate of change of voltage specified.

Electrical apparatus

Power transformers, circuit breakers, switchgear and other electrical apparatus connected to overhead transmission lines are exposed to transient lightning surge voltages induced on the power circuit. Electrical apparatus will have a basic lightning impulse level (BIL) specified. This is the crest value of an impulse waveform with a standardized wave shape, intended to simulate the electrical stress of a lightning surge or a surge induced by circuit switching. The BIL is coordinated with the typical operating voltage of the apparatus. For high-voltage transmission lines, the impulse level is related to the clearance to ground of energized components. As an example, a transmission line rated 138 kV would be designed for a BIL of 650 kV. A higher BIL may be specified than the minimum, where the exposure to lightning is severe. [7]

See also

Related Research Articles

Diode abstract electronic component with two terminals that allows current to flow in one direction

A diode is a two-terminal electronic component that conducts current primarily in one direction ; it has low resistance in one direction, and high resistance in the other. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other materials such as gallium arsenide and germanium are also used.

Insulator (electricity) Material which does not conduct an electric current

An electrical insulator is a material whose internal electric charges do not flow freely; very little electric current will flow through it under the influence of an electric field. This contrasts with other materials, semiconductors and conductors, which conduct electric current more easily. The property that distinguishes an insulator is its resistivity; insulators have higher resistivity than semiconductors or conductors. The most common examples are non-metals.

Transistor Basic electronics component

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

Vacuum tube Device that controls electric current between electrodes in an evacuated container

In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

JFET type of field-effect transistor

The junction gate field-effect transistor is one of the simplest types of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically-controlled switches, amplifiers, or voltage-controlled resistors.

In physics, the term dielectric strength has the following meanings:

Bipolar junction transistor transistor that uses both electrons and holes as charge carriers

A bipolar junction transistor is a type of transistor that uses both electrons and holes as charge carriers.

Rectifier AC-DC conversion device; electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction.

Electromotive force scalar physical quantity

Electromotive force, is the electrical action produced by a non-electrical source. A device that converts other forms of energy into electrical energy, such as a battery or generator, provides an emf as its output. Sometimes an analogy to water "pressure" is used to describe electromotive force.

Zener diode diode that allows current to flow in the reverse direction at a specific voltage

A Zener diode is a type of diode that allows current to flow in the conventional manner - from its anode to its cathode i.e. when the anode is positive with respect to the cathode. When the voltage across the terminals is reversed and the potential reaches the Zener voltage, the junction will breakdown and current will flow in the reverse direction - a desired characteristic. This effect is known as the Zener effect, after Clarence Zener, who first described the phenomenon. Zener diodes are manufactured with a great variety of Zener voltages (Vz) and some are even variable.

In electronics, an avalanche diode is a diode that is designed to experience avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current concentration and resulting hot spots, so that the diode is undamaged by the breakdown. The avalanche breakdown is due to minority carriers accelerated enough to create ionization in the crystal lattice, producing more carriers which in turn create more ionization. Because the avalanche breakdown is uniform across the whole junction, the breakdown voltage is nearly constant with changing current when compared to a non-avalanche diode.

Spark gap arrangement of two conducting electrodes separated by a gap

A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conductors exceeds the breakdown voltage of the gas within the gap, a spark forms, ionizing the gas and drastically reducing its electrical resistance. An electric current then flows until the path of ionized gas is broken or the current reduces below a minimum value called the "holding current". This usually happens when the voltage drops, but in some cases occurs when the heated gas rises, stretching out and then breaking the filament of ionized gas. Usually, the action of ionizing the gas is violent and disruptive, often leading to sound, light and heat.

p–n junction semiconductor–semiconductor junction, formed at the boundary between a p-type and n-type semiconductor

A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contains an excess of electrons in the outer shells of the electrically neutral atoms there. This allows electrical current to pass through the junction only in one direction. The p-n junction is created by doping, for example by ion implantation, diffusion of dopants, or by epitaxy. If two separate pieces of material were used, this would introduce a grain boundary between the semiconductors that would severely inhibit its utility by scattering the electrons and holes.

Paschens law

Paschen's law is an equation that gives the breakdown voltage, that is, the voltage necessary to start a discharge or electric arc, between two electrodes in a gas as a function of pressure and gap length. It is named after Friedrich Paschen who discovered it empirically in 1889.

Electrical breakdown when current flows through an electrical insulator when the voltage applied across it exceeds the breakdown voltage

Electrical breakdown or dielectric breakdown is when current flows through an electrical insulator when the voltage applied across it exceeds the breakdown voltage. This results in the insulator becoming electrically conductive. Electrical breakdown may be a momentary event, or may lead to a continuous arc if protective devices fail to interrupt the current in a power circuit.

High voltage electrical energy at voltages high enough to inflict harm on living organisms (numerical definition depends on context)

The term high voltage usually means electrical energy at voltages high enough to inflict harm on living organisms. Equipment and conductors that carry high voltage warrant particular safety requirements and procedures. In certain industries, high voltage means voltage above a particular threshold (see below). High voltage is used in electrical power distribution, in cathode ray tubes, to generate X-rays and particle beams, to demonstrate arcing, for ignition, in photomultiplier tubes, and in high power amplifier vacuum tubes and other industrial, military and scientific applications.

An electron avalanche is a process in which a number of free electrons in a transmission medium are subjected to strong acceleration by an electric field and subsequently collide with other atoms of the medium, thereby ionizing them. This releases additional electrons which accelerate and collide with further atoms, releasing more electrons—a chain reaction. In a gas, this causes the affected region to become an electrically conductive plasma.

Avalanche breakdown phenomenon that can occur in both insulating and semiconducting materials

Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. It is a type of electron avalanche. The avalanche process occurs when carriers in the transition region are accelerated by the electric field to energies sufficient to create mobile or free electron-hole pairs via collisions with bound electrons.

Capacitor Passive two-terminal electronic component that stores electrical energy in an electric field

A capacitor is a device that stores electrical energy in an electric field. It is a passive electronic component with two terminals.

Field effect (semiconductor)

In physics, the field effect refers to the modulation of the electrical conductivity of a material by the application of an external electric field.

References

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  2. Emelyanov, A.A., Izv. Vyssh. Uchebn. Zaved., Fiz., 1989, no. 4, p. 103.
  3. Kalyatskii, I.I., Kassirov, G.M., and Smirnov, G.V., Prib. Tekh. Eksp., 1974, no. 4, p. 84.
  4. G. Cuttone, C. Marchetta, L. Torrisi, G. Della Mea, A. Quaranta, V. Rigato and S. Zandolin, Surface Treatment of HV Electrodes for Superconducting Cyclotron Beam Extraction, IEEE. Trans. DEI, Vol. 4, pp. 218<223, 1997.
  5. H. Moscicka-Grzesiak, H. Gruszka and M. Stroinski, ‘‘Influence of Electrode Curvature on Predischarge Phenomena and Electric Strength at 50 Hz of a Vacuum
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  7. D. G. Fink, H. W. Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition, McGraw-Hill, 1978, ISBN   007020974X, page 17-20 ff