Thermal runaway
Encyclopedia
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. It is a kind of uncontrolled positive feedback
.
In other words, the term "thermal runaway" is used whenever a process is accelerated by increased temperature, in turn releasing energy that further increases temperature. In chemistry
(and chemical engineering
), this risk is associated with strongly exothermic
reactions that are accelerated by temperature rise. In electrical engineering
, thermal runaway is typically associated with increased current flow and power dissipation, although exothermic chemical reactions can also occur under some conditions. Thermal runaway can occur in civil engineering
, notably when the heat released by large amounts of curing concrete
is not controlled. In the science of astrophysics
, thermal runaway of thermonuclear fusion in the cores of massive stars can cause Type Ia supernova
explosions.
, thermal runaway is a process by which an exothermic reaction
goes out of control, often resulting in an explosion
. It is also known as a runaway reaction in organic chemistry
.
Thermal runaway occurs when the reaction rate
increases due to an increase in temperature, causing a further increase in temperature and hence a further increase in the reaction rate. It has contributed to industrial chemical accidents
, most notably the 1947 Texas City disaster
from overheated ammonium nitrate in a ship's hold, and the disastrous release
of a large volume of methyl isocyanate
gas from a Union Carbide
plant in Bhopal, India in 1984.
Thermal runaway is also a concern in hydrocracking, an oil refinery
process.
Thermal runaway may result from unwanted exothermic side reaction(s) that begin at higher temperatures, following an initial accidental overheating of the reaction mixture. This scenario was behind the Seveso disaster
, where thermal runaway heated a reaction to temperatures such that in addition to the intended 2,4,5-trichlorophenol
, poisonous 2,3,7,8-tetrachlorodibenzo-p-dioxin was also produced, and was vented into the environment after the reactor's rupture disk burst.
Thermal runaway is most often caused by failure of the reactor
vessel's cooling
system. Failure of the mixer can result in localized heating, which initiates thermal runaway. Similarly, in flow reactors
, localized insufficient mixing causes hotspots to form, wherein thermal runaway conditions occur, which causes violent blowouts of reactor contents and catalysts. Incorrect equipment component installation is also a common cause. Many chemical production facilities are designed with high-volume emergency venting, a measure to limit the extent of injury and property damage when such accidents occur.
At large scale, it is unsafe to "charge all reagents and mix", as is done in laboratory scale. This is because the amount of reaction scales with the cube of the size of the vessel (V ∝ r³), but the heat transfer area scales with the square of the size (A ∝ r²), so that the heat production-to-area ratio scales with the size (V/A ∝ r). Consequently, reactions that easily cool fast enough in the laboratory can dangerously self-heat at ton scale. In 2007, this kind of a erroneous procedure caused an explosion of a 2400 gal-reactor used to metalate
methylcyclopentadiene
with metallic sodium
, causing the loss of four lives and parts of the reactor being flung 400 feet (121.9 m) away. Thus, industrial scale reactions prone to thermal runaway are preferably controlled by the addition of one reagent at a rate corresponding to the available cooling capacity.
Some laboratory reactions must be run under extreme cooling, because they are very prone to hazardous thermal runaway. For example, in Swern oxidation
, the formation of sulfonium chloride must be performed in a cooled system (–30° C), because at room temperature
the reaction undergoes explosive thermal runaway.
The UK Chemical Reaction Hazards Forum
publishes analysis of previously-unreported chemical accidents
to assist the education of the scientific and engineering community, with the aim of preventing similar occurrences elsewhere. Almost 150 such reports are available to view as of January 2009.
s are used for heating of various materials in cooking and various industrial processes. The rate of heating of the material depends on the energy absorption, which depends on the dielectric constant
of the material. The dependence of dielectric constant on temperature varies for different materials; some materials display significant increase with increasing temperature. This behavior, when the material gets exposed to microwaves, leads to selective local overheating, as the warmer areas are better able to accept further energy than the colder areas—potentially dangerous especially for thermal insulators, where the heat exchange between the hot spots and the rest of the material is slow. These materials are called thermal runaway materials. This phenomenon occurs in some ceramics.
or positive feedback
effect of thermal runaway can cause failure, sometimes in a spectacular fashion (e.g. electrical explosion or fire). To prevent these hazards, well-designed electronic systems typically incorporate current limiting protection, such as thermal fuses, circuit breakers, PTC
current limiters, or other such devices.
To handle larger currents, circuit designers may have to resort to connecting multiple lower-capacity devices (e.g. transistors, diodes, or MOVs) in parallel
. This technique can work well, but is susceptible to a phenomenon called current hogging, in which the current is not shared equally across all devices. Typically, one device may have a slightly lower resistance, and thus draws more current, heating it more than its sibling devices, causing its resistance to drop further. The electrical load ends up funneling into a single device, which then rapidly fails. Thus, an array of devices may end up no more robust than its weakest component.
The current-hogging effect can be reduced by carefully matching the characteristics of each paralleled device, or by using other design techniques to balance the electrical load. However, maintaining load balance under extreme conditions may not be straightforward. Devices with an intrinsic positive temperature coefficient (PTC) of electrical resistance are less prone to current hogging, but thermal runaway can still occur because of poor heat sinking or other problems.
Many electronic circuits contain special provisions to prevent thermal runaway. This is most often seen in transistor biasing arrangements for high-power output stages. However when equipment is used above its designed ambient temperature, thermal runaway can still occur in some cases. This occasionally causes equipment failures in hot environments, or when air cooling vents are blocked.
shows a peculiar profile, in that its electrical resistance
increases with temperature up to about 160 °C, then starts decreasing, and drops further when the melting point is reached. This can lead to thermal runaway phenomena within internal regions of the semiconductor junction; the resistance decreases in the regions which become heated above this threshold, allowing more current to flow through the overheated regions, in turn causing yet more heating in comparison with the surrounding regions, which leads to further temperature increase and resistance decrease. This leads to the phenomenon of current crowding
and formation of current filaments (similar to current hogging, but within a single device), and is one of the underlying causes of many semiconductor junction failures.
increases significantly in bipolar transistors (especially germanium
-based bipolar transistors) as they increase in temperature. Depending on the design of the circuit, this increase in leakage current can increase the current flowing through a transistor and thus the power dissipation, causing a further increase in Collector-to-Emitter leakage current. This is frequently seen in a push–pull stage of a class AB amplifier. If the pull-up and pull-down transistors are biased
to have minimal crossover distortion
at room temperature
, and the biasing is not temperature-compensated, then as the temperature rises both transistors will be increasingly biased on, causing current and power to further increase, and eventually destroying one or both devices.
One rule of thumb to avoid thermal runaway is to keep the operating point of a BJT so that Vce ≤ 1/2Vcc
Another practice is to mount a thermal feedback sensing transistor or other device on the heat sink, to control the crossover bias voltage. As the output transistors heat up, so does the thermal feedback transistor. This in turn causes the thermal feedback transistor to turn on at a slightly lower voltage, reducing the crossover bias voltage, and so reducing the heat dissipated by the output transistors.
If multiple BJT transistors are connected in parallel (which is typical in high current applications), a current hogging problem can occur. Special measures must be taken to control this characteristic vulnerability of BJTs.
s typically increase their on-resistance with temperature. Under some circumstances, power dissipated in this resistance causes more heating of the junction, which further increases the junction temperature
, in a positive feedback
loop. However, the increase of on-resistance with temperature helps balance current across multiple MOSFETs connected in parallel, so current hogging does not occur. If a MOSFET transistor produces more heat than the heatsink can dissipate, then thermal runaway can still destroy the transistors. This problem can be alleviated to a degree by lowering the thermal resistance between the transistor die and the heatsink. See also Thermal Design Power
.
s typically develop lower resistance as they heat up. If connected directly across an AC or DC power bus (a common usage for protection against electrical transients), a MOV which has developed a lowered trigger voltage can slide into catastrophic thermal runaway, possibly culminating in a small explosion or fire. To prevent this possibility, fault current is typically limited by a thermal fuse, circuit breaker, or other current limiting device.
s are under some conditions prone to self-destruction by thermal runaway. The capacitor typically consists of a sintered
tantalum
sponge acting as the anode
, a manganese dioxide cathode
, and a dielectric
layer of tantalum pentoxide created on the tantalum sponge surface by anodizing. It may happen that the tantalum oxide layer has weak spots that undergo dielectric breakdown during a voltage spike
. The tantalum sponge then comes into direct contact with the manganese dioxide, and increased leakage current causes localized heating; usually, this drives an endothermic
chemical reaction that produces manganese(III) oxide
and regenerates (self-heals
) the tantalum oxide dielectric layer.
However, if the energy dissipated at the failure point is high enough, a self-sustaining exothermic
reaction can start, similar to the thermite
reaction, with metallic tantalum as fuel and manganese dioxide as oxidizer. This undesirable reaction will destroy the capacitor, producing smoke
and possibly flame
.
Therefore, tantalum capacitors can be freely deployed in small-signal circuits, but application in high-power circuits must be carefully designed to avoid thermal runaway failures.
of logic switching transistors increases with temperature. In rare instances, this may lead to thermal runaway in digital circuits. This is not a common problem, since leakage currents usually make up a small portion of overall power consumption, so the increase in power is fairly modest — for an Athlon 64
, the power dissipation increases by about 10% for every 30 degrees Celsius. For a device with a TDP
of 100 W, for thermal runaway to occur, the heat sink would have to have a thermal resistivity of over 3 K/W (kelvins per watt), which is about 6 times worse than a stock Athlon 64 heat sink. (A stock Athlon 64 heat sink is rated at 0.34 K/W, although the actual thermal resistance to the environment is somewhat higher, due to the thermal boundary between processor and heatsink, rising temperatures in the case, and other thermal resistances..) Regardless, an inadequate heat sink with a thermal resistance of over 0.5 to 1 K/W would result in the destruction of a 100 W device even without thermal runaway effects.
. Reports of exploding cellphones occasionally appear in newspapers. In 2006, batteries from Apple, HP, Toshiba, , Dell and other notebook manufacturers were recalled because of fire and explosions.
The Pipeline and Hazardous Materials Safety Administration
(PHMSA) of the U.S. Department of Transportation has established regulations regarding the carrying of certain types of batteries on airplanes because of their instability in certain situations. This action was partially inspired by a cargo bay fire on a UPS
airplane.
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
.
In other words, the term "thermal runaway" is used whenever a process is accelerated by increased temperature, in turn releasing energy that further increases temperature. In chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
(and chemical engineering
Chemical engineering
Chemical engineering is the branch of engineering that deals with physical science , and life sciences with mathematics and economics, to the process of converting raw materials or chemicals into more useful or valuable forms...
), this risk is associated with strongly exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...
reactions that are accelerated by temperature rise. In electrical engineering
Electrical engineering
Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...
, thermal runaway is typically associated with increased current flow and power dissipation, although exothermic chemical reactions can also occur under some conditions. Thermal runaway can occur in civil engineering
Civil engineering
Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including works like roads, bridges, canals, dams, and buildings...
, notably when the heat released by large amounts of curing concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
is not controlled. In the science of astrophysics
Astrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
, thermal runaway of thermonuclear fusion in the cores of massive stars can cause Type Ia supernova
Type Ia supernova
A Type Ia supernova is a sub-category of supernovae, which in turn are a sub-category of cataclysmic variable stars, that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion...
explosions.
Chemical engineering
In chemical engineeringChemical engineering
Chemical engineering is the branch of engineering that deals with physical science , and life sciences with mathematics and economics, to the process of converting raw materials or chemicals into more useful or valuable forms...
, thermal runaway is a process by which an exothermic reaction
Exothermic reaction
An exothermic reaction is a chemical reaction that releases energy in the form of light or heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation:-Overview:...
goes out of control, often resulting in an explosion
Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...
. It is also known as a runaway reaction in organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
.
Thermal runaway occurs when the reaction rate
Reaction rate
The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
increases due to an increase in temperature, causing a further increase in temperature and hence a further increase in the reaction rate. It has contributed to industrial chemical accidents
Chemical accidents
The terms “chemical accident” or “chemical incident” refer to an event resulting in the release of a substance or substances hazardous to human health and/or the environment in the short or long term...
, most notably the 1947 Texas City disaster
Texas City Disaster
The Texas City Disaster was the deadliest industrial accident in U.S. history. The incident took place on April 16, 1947, and began with a mid-morning fire on board the French-registered vessel SS Grandcamp which was docked in the Port of Texas City...
from overheated ammonium nitrate in a ship's hold, and the disastrous release
Bhopal disaster
The Bhopal disaster also known as Bhopal Gas Tragedy was a gas leak incident in India, considered one of the world's worst industrial catastrophes. It occurred on the night of December 2–3, 1984 at the Union Carbide India Limited pesticide plant in Bhopal, Madhya Pradesh, India...
of a large volume of methyl isocyanate
Methyl isocyanate
Methyl isocyanate is an organic compound with the molecular formula CH3NCO. Synonyms are isocyanatomethane, methyl carbylamine, and MIC. Methyl isocyanate is an intermediate chemical in the production of carbamate pesticides . It has also been used in the production of rubbers and adhesives...
gas from a Union Carbide
Union Carbide
Union Carbide Corporation is a wholly owned subsidiary of The Dow Chemical Company. It currently employs more than 2,400 people. Union Carbide primarily produces chemicals and polymers that undergo one or more further conversions by customers before reaching consumers. Some are high-volume...
plant in Bhopal, India in 1984.
Thermal runaway is also a concern in hydrocracking, an oil refinery
Oil refinery
An oil refinery or petroleum refinery is an industrial process plant where crude oil is processed and refined into more useful petroleum products, such as gasoline, diesel fuel, asphalt base, heating oil, kerosene, and liquefied petroleum gas...
process.
Thermal runaway may result from unwanted exothermic side reaction(s) that begin at higher temperatures, following an initial accidental overheating of the reaction mixture. This scenario was behind the Seveso disaster
Seveso disaster
The Seveso disaster was an industrial accident that occurred around 12:37 pm July 10, 1976, in a small chemical manufacturing plant approximately north of Milan in the Lombardy region in Italy...
, where thermal runaway heated a reaction to temperatures such that in addition to the intended 2,4,5-trichlorophenol
Trichlorophenol
A trichlorophenol is any organochloride of phenol that contains three covalently bonded chlorine atoms. Trichlorophenols are produced by electrophilic halogenation of phenol with chlorine. Different isomers of trichlorophenol exist according to which ring positions on the phenol contain chlorine...
, poisonous 2,3,7,8-tetrachlorodibenzo-p-dioxin was also produced, and was vented into the environment after the reactor's rupture disk burst.
Thermal runaway is most often caused by failure of the reactor
Chemical reactor
In chemical engineering, chemical reactors are vessels designed to contain chemical reactions. The design of a chemical reactor deals with multiple aspects of chemical engineering. Chemical engineers design reactors to maximize net present value for the given reaction...
vessel's cooling
Coolant
A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...
system. Failure of the mixer can result in localized heating, which initiates thermal runaway. Similarly, in flow reactors
Flow chemistry
In flow chemistry, a chemical reaction is run in a continuously flowing stream rather than in batch production. In other words, pumps move fluid into a tube, and where tubes join one another, the fluids contact one another. If these fluids are reactive, a reaction takes place...
, localized insufficient mixing causes hotspots to form, wherein thermal runaway conditions occur, which causes violent blowouts of reactor contents and catalysts. Incorrect equipment component installation is also a common cause. Many chemical production facilities are designed with high-volume emergency venting, a measure to limit the extent of injury and property damage when such accidents occur.
At large scale, it is unsafe to "charge all reagents and mix", as is done in laboratory scale. This is because the amount of reaction scales with the cube of the size of the vessel (V ∝ r³), but the heat transfer area scales with the square of the size (A ∝ r²), so that the heat production-to-area ratio scales with the size (V/A ∝ r). Consequently, reactions that easily cool fast enough in the laboratory can dangerously self-heat at ton scale. In 2007, this kind of a erroneous procedure caused an explosion of a 2400 gal-reactor used to metalate
Metalation
Metalation is a chemical reaction which results in a metal atom beingattached to a molecule.Lithium metal in contact with an organohalide lithiates the organic molecule and gives the organolithium reagent and lithium halide...
methylcyclopentadiene
Methylcyclopentadiene
Methylcyclopentadiene describes three isomeric cyclic diolefins with the formula C5MeH5 . These isomers are the organic precursor to the methylcyclopentadienyl-ligand , commonly found in organometallic chemistry...
with metallic sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
, causing the loss of four lives and parts of the reactor being flung 400 feet (121.9 m) away. Thus, industrial scale reactions prone to thermal runaway are preferably controlled by the addition of one reagent at a rate corresponding to the available cooling capacity.
Some laboratory reactions must be run under extreme cooling, because they are very prone to hazardous thermal runaway. For example, in Swern oxidation
Swern oxidation
The Swern oxidation, named after Daniel Swern, is a chemical reaction whereby a primary or secondary alcohol is oxidized to an aldehyde or ketone using oxalyl chloride, dimethyl sulfoxide and an organic base, such as triethylamine...
, the formation of sulfonium chloride must be performed in a cooled system (–30° C), because at room temperature
Room temperature
-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...
the reaction undergoes explosive thermal runaway.
The UK Chemical Reaction Hazards Forum
UK Chemical Reaction Hazards Forum
The UK Chemical Reaction Hazards Forum is a group of process safety scientists, from different companies, who meet at regular intervals to share knowledge and experiences of recent chemical incidents, within their own companies, with the aim of preventing a repeat of the incident at a different...
publishes analysis of previously-unreported chemical accidents
Chemical accidents
The terms “chemical accident” or “chemical incident” refer to an event resulting in the release of a substance or substances hazardous to human health and/or the environment in the short or long term...
to assist the education of the scientific and engineering community, with the aim of preventing similar occurrences elsewhere. Almost 150 such reports are available to view as of January 2009.
Microwave heating
MicrowaveMicrowave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
s are used for heating of various materials in cooking and various industrial processes. The rate of heating of the material depends on the energy absorption, which depends on the dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
of the material. The dependence of dielectric constant on temperature varies for different materials; some materials display significant increase with increasing temperature. This behavior, when the material gets exposed to microwaves, leads to selective local overheating, as the warmer areas are better able to accept further energy than the colder areas—potentially dangerous especially for thermal insulators, where the heat exchange between the hot spots and the rest of the material is slow. These materials are called thermal runaway materials. This phenomenon occurs in some ceramics.
Electrical engineering
Some electronic components develop lower resistances or lower triggering voltages (for nonlinear resistances) as their internal temperature increases. If circuit conditions cause markedly increased current flow in these situations, increased power dissipation raises the temperature further. A vicious circleVicious Circle
Vicious Circle is an album released in 1994 by L.A. Guns. Most of the songs have Phil Lewis on lead vocals, but the track "Nothing Better to Do" features Kelly Nickels on lead vocals, and "Tarantula" is instrumental. MC Bones drums on several songs. Lewis and Bones also played together in the band...
or positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
effect of thermal runaway can cause failure, sometimes in a spectacular fashion (e.g. electrical explosion or fire). To prevent these hazards, well-designed electronic systems typically incorporate current limiting protection, such as thermal fuses, circuit breakers, PTC
Temperature coefficient
The temperature coefficient is the relative change of a physical property when the temperature is changed by 1 K.In the following formula, let R be the physical property to be measured and T be the temperature at which the property is measured. T0 is the reference temperature, and ΔT is the...
current limiters, or other such devices.
To handle larger currents, circuit designers may have to resort to connecting multiple lower-capacity devices (e.g. transistors, diodes, or MOVs) in parallel
Parallel
-Mathematics and science:* Parallel , an imaginary east-west line circling a globe* Parallel circuits, as opposed to series* Parallel * Parallel evolution* Parallel transport* Parallel of declination, used in astronomy-Computing:...
. This technique can work well, but is susceptible to a phenomenon called current hogging, in which the current is not shared equally across all devices. Typically, one device may have a slightly lower resistance, and thus draws more current, heating it more than its sibling devices, causing its resistance to drop further. The electrical load ends up funneling into a single device, which then rapidly fails. Thus, an array of devices may end up no more robust than its weakest component.
The current-hogging effect can be reduced by carefully matching the characteristics of each paralleled device, or by using other design techniques to balance the electrical load. However, maintaining load balance under extreme conditions may not be straightforward. Devices with an intrinsic positive temperature coefficient (PTC) of electrical resistance are less prone to current hogging, but thermal runaway can still occur because of poor heat sinking or other problems.
Many electronic circuits contain special provisions to prevent thermal runaway. This is most often seen in transistor biasing arrangements for high-power output stages. However when equipment is used above its designed ambient temperature, thermal runaway can still occur in some cases. This occasionally causes equipment failures in hot environments, or when air cooling vents are blocked.
Semiconductors
SiliconSilicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
shows a peculiar profile, in that its electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
increases with temperature up to about 160 °C, then starts decreasing, and drops further when the melting point is reached. This can lead to thermal runaway phenomena within internal regions of the semiconductor junction; the resistance decreases in the regions which become heated above this threshold, allowing more current to flow through the overheated regions, in turn causing yet more heating in comparison with the surrounding regions, which leads to further temperature increase and resistance decrease. This leads to the phenomenon of current crowding
Current crowding
Current crowding is a nonhomogenous distribution of current density through a conductor or semiconductor, especially at the vicinity of the contacts and over the PN junctions....
and formation of current filaments (similar to current hogging, but within a single device), and is one of the underlying causes of many semiconductor junction failures.
Bipolar junction transistors (BJTs)
Leakage currentLeakage (semiconductors)
In semiconductor devices, leakage is a quantum phenomenon where mobile charge carriers tunnel through an insulating region. Leakage increases exponentially as the thickness of the insulating region decreases. Tunneling leakage can also occur across semiconductor junctions between heavily doped...
increases significantly in bipolar transistors (especially germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
-based bipolar transistors) as they increase in temperature. Depending on the design of the circuit, this increase in leakage current can increase the current flowing through a transistor and thus the power dissipation, causing a further increase in Collector-to-Emitter leakage current. This is frequently seen in a push–pull stage of a class AB amplifier. If the pull-up and pull-down transistors are biased
Bipolar transistor biasing
Bipolar transistor amplifiers must be properly biased to operate correctly. In circuits made with individual devices , biasing networks consisting of resistors are commonly employed...
to have minimal crossover distortion
Crossover distortion
Crossover distortion is a type of distortion which is caused by switching between devices driving a load, most often when the devices are matched...
at room temperature
Room temperature
-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...
, and the biasing is not temperature-compensated, then as the temperature rises both transistors will be increasingly biased on, causing current and power to further increase, and eventually destroying one or both devices.
One rule of thumb to avoid thermal runaway is to keep the operating point of a BJT so that Vce ≤ 1/2Vcc
Another practice is to mount a thermal feedback sensing transistor or other device on the heat sink, to control the crossover bias voltage. As the output transistors heat up, so does the thermal feedback transistor. This in turn causes the thermal feedback transistor to turn on at a slightly lower voltage, reducing the crossover bias voltage, and so reducing the heat dissipated by the output transistors.
If multiple BJT transistors are connected in parallel (which is typical in high current applications), a current hogging problem can occur. Special measures must be taken to control this characteristic vulnerability of BJTs.
Power MOSFETs
Power MOSFETMOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
s typically increase their on-resistance with temperature. Under some circumstances, power dissipated in this resistance causes more heating of the junction, which further increases the junction temperature
Junction temperature
Junction temperature is the highest temperature of the actual semiconductor in an electronic device. In operation it is higher than case temperature and the temperature of the part's exterior...
, in a positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
loop. However, the increase of on-resistance with temperature helps balance current across multiple MOSFETs connected in parallel, so current hogging does not occur. If a MOSFET transistor produces more heat than the heatsink can dissipate, then thermal runaway can still destroy the transistors. This problem can be alleviated to a degree by lowering the thermal resistance between the transistor die and the heatsink. See also Thermal Design Power
Thermal Design Power
The thermal design power , sometimes called thermal design point, refers to the maximum amount of power the cooling system in a computer is required to dissipate. For example, a laptop's CPU cooling system may be designed for a 20 watt TDP, which means that it can dissipate up to 20 watts of heat...
.
Metal oxide varistors (MOVs)
Metal oxide varistorVaristor
A varistor is an electronic component with a "diode-like" nonlinear current–voltage characteristic. The name is a portmanteau of variable resistor...
s typically develop lower resistance as they heat up. If connected directly across an AC or DC power bus (a common usage for protection against electrical transients), a MOV which has developed a lowered trigger voltage can slide into catastrophic thermal runaway, possibly culminating in a small explosion or fire. To prevent this possibility, fault current is typically limited by a thermal fuse, circuit breaker, or other current limiting device.
Tantalum capacitors
Tantalum capacitorTantalum capacitor
The tantalum capacitor is a highly reliable type of electrolytic capacitor, available in both solid-bodied and separately encased forms. The encased "wet" variant is not used often in modern designs...
s are under some conditions prone to self-destruction by thermal runaway. The capacitor typically consists of a sintered
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
tantalum
Tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
sponge acting as the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
, a manganese dioxide cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
, and a dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
layer of tantalum pentoxide created on the tantalum sponge surface by anodizing. It may happen that the tantalum oxide layer has weak spots that undergo dielectric breakdown during a voltage spike
Voltage spike
In electrical engineering, spikes are fast, short duration electrical transients in voltage , current , or transferred energy in an electrical circuit....
. The tantalum sponge then comes into direct contact with the manganese dioxide, and increased leakage current causes localized heating; usually, this drives an endothermic
Endothermic
In thermodynamics, the word endothermic describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat. Its etymology stems from the prefix endo- and the Greek word thermasi,...
chemical reaction that produces manganese(III) oxide
Manganese(III) oxide
Manganese oxide is the chemical compound of formula Mn2O3.-Preparation and chemistry:Heating MnO2 in air at below 800°C α-Mn2O3 is produced . γ-Mn2O3 can be produced by oxidation followed by dehydration of manganese hydroxide...
and regenerates (self-heals
Self-healing
Self-healing is a phrase applied to the process of recovery , motivated by and directed by the patient, guided often only by instinct. Such a process encounters mixed fortunes due to its amateur nature, although self-motivation is a major asset...
) the tantalum oxide dielectric layer.
However, if the energy dissipated at the failure point is high enough, a self-sustaining exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...
reaction can start, similar to the thermite
Thermite
Thermite is a pyrotechnic composition of a metal powder and a metal oxide that produces an exothermic oxidation-reduction reaction known as a thermite reaction. If aluminium is the reducing agent it is called an aluminothermic reaction...
reaction, with metallic tantalum as fuel and manganese dioxide as oxidizer. This undesirable reaction will destroy the capacitor, producing smoke
Smoke
Smoke is a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product of fires , but may also be used for pest...
and possibly flame
Flame
A flame is the visible , gaseous part of a fire. It is caused by a highly exothermic reaction taking place in a thin zone...
.
Therefore, tantalum capacitors can be freely deployed in small-signal circuits, but application in high-power circuits must be carefully designed to avoid thermal runaway failures.
Digital logic
The leakage currentLeakage (semiconductors)
In semiconductor devices, leakage is a quantum phenomenon where mobile charge carriers tunnel through an insulating region. Leakage increases exponentially as the thickness of the insulating region decreases. Tunneling leakage can also occur across semiconductor junctions between heavily doped...
of logic switching transistors increases with temperature. In rare instances, this may lead to thermal runaway in digital circuits. This is not a common problem, since leakage currents usually make up a small portion of overall power consumption, so the increase in power is fairly modest — for an Athlon 64
Athlon 64
The Athlon 64 is an eighth-generation, AMD64-architecture microprocessor produced by AMD, released on September 23, 2003. It is the third processor to bear the name Athlon, and the immediate successor to the Athlon XP...
, the power dissipation increases by about 10% for every 30 degrees Celsius. For a device with a TDP
Thermal Design Power
The thermal design power , sometimes called thermal design point, refers to the maximum amount of power the cooling system in a computer is required to dissipate. For example, a laptop's CPU cooling system may be designed for a 20 watt TDP, which means that it can dissipate up to 20 watts of heat...
of 100 W, for thermal runaway to occur, the heat sink would have to have a thermal resistivity of over 3 K/W (kelvins per watt), which is about 6 times worse than a stock Athlon 64 heat sink. (A stock Athlon 64 heat sink is rated at 0.34 K/W, although the actual thermal resistance to the environment is somewhat higher, due to the thermal boundary between processor and heatsink, rising temperatures in the case, and other thermal resistances..) Regardless, an inadequate heat sink with a thermal resistance of over 0.5 to 1 K/W would result in the destruction of a 100 W device even without thermal runaway effects.
Batteries
When handled improperly, or if manufactured defectively, some rechargeable batteries can experience thermal runaway resulting in overheating. Sealed cells will sometimes explode violently if safety vents are overwhelmed or nonfunctional. Especially prone to thermal runaway are lithium-ion batteriesLithium ion battery
A lithium-ion battery is a family of rechargeable battery types in which lithium ions move from the negative electrode to the positive electrode during discharge, and back when charging. Chemistry, performance, cost, and safety characteristics vary across LIB types...
. Reports of exploding cellphones occasionally appear in newspapers. In 2006, batteries from Apple, HP, Toshiba, , Dell and other notebook manufacturers were recalled because of fire and explosions.
The Pipeline and Hazardous Materials Safety Administration
Pipeline and Hazardous Materials Safety Administration
The Pipeline and Hazardous Materials Safety Administration was created under the Norman Y. Mineta Research and Special Programs Improvement Act of 2004. United States president George W. Bush signed the legislation into law on November 30, 2004...
(PHMSA) of the U.S. Department of Transportation has established regulations regarding the carrying of certain types of batteries on airplanes because of their instability in certain situations. This action was partially inspired by a cargo bay fire on a UPS
United Parcel Service
United Parcel Service, Inc. , typically referred to by the acronym UPS, is a package delivery company. Headquartered in Sandy Springs, Georgia, United States, UPS delivers more than 15 million packages a day to 6.1 million customers in more than 220 countries and territories around the...
airplane.