electrical engineering
Contenido
Algunos componentes electrónicos desarrollan resistencias más bajas o voltajes de activación más bajos (para resistencias no lineales) a medida que aumenta su temperatura interna. Si las condiciones del circuito provocan un flujo de corriente notablemente mayor en estas situaciones, el aumento de la disipación de potencia puede aumentar la temperatura aún más por el calentamiento de Joule. Un círculo vicioso o un efecto de retroalimentación positiva del escape térmico puede causar fallas, a veces de manera espectacular (por ejemplo, una explosión eléctrica o un incendio). Para evitar estos peligros, los sistemas electrónicos bien diseñados suelen incorporar protección de limitación de corriente, como fusibles térmicos, disyuntores o limitadores de corriente PTC.
Para manejar corrientes más grandes, los diseñadores de circuitos pueden conectar múltiples dispositivos de menor capacidad (por ejemplo, transistores, diodos o MOV) en paralelo. Esta técnica puede funcionar bien, pero es susceptible a un fenómeno llamado acaparamiento de corriente, en el cual la corriente no se comparte de manera equitativa en todos los dispositivos. Por lo general, un dispositivo puede tener una resistencia ligeramente inferior y, por lo tanto, consume más corriente, calentándola más que sus dispositivos hermanos, lo que hace que su resistencia disminuya aún más. La carga eléctrica termina canalizándose en un solo dispositivo, que luego falla rápidamente. Por lo tanto, una serie de dispositivos puede no ser más robusta que su componente más débil.
El efecto de acaparamiento de la corriente se puede reducir al combinar cuidadosamente las características de cada dispositivo en paralelo, o al utilizar otras técnicas de diseño para equilibrar la carga eléctrica. Sin embargo, mantener el equilibrio de la carga en condiciones extremas puede no ser sencillo. Los dispositivos con un coeficiente de temperatura positivo (PTC) intrínseco de resistencia eléctrica son menos propensos a la acumulación de corriente, pero aun así puede ocurrir un desbordamiento térmico debido a un pobre hundimiento de calor u otros problemas.
Muchos circuitos electrónicos contienen disposiciones especiales para evitar el desbordamiento térmico. Esto se observa con mayor frecuencia en las disposiciones de polarización de transistores para etapas de salida de alta potencia. Sin embargo, cuando el equipo se utiliza por encima de su temperatura ambiente diseñada, en algunos casos puede ocurrir un desbordamiento térmico. Esto puede ocasionar fallas en el equipo en ambientes calurosos o cuando se bloquean las salidas de aire.
Semiconductors
Silicon shows a peculiar profile in that its electrical resistance increases with temperature up to about 160°C, then begins to decrease, and decreases further when the melting point is reached. This can lead to runaway thermal phenomena within the internal regions of the semiconductor junction; Resistance decreases in regions that are heated above this threshold, allowing more current to flow through the overheated regions, which in turn causes even greater heating compared to the surrounding regions, leading to a further increase in temperature and a decrease in resistance. This leads to the phenomenon of "current saturation") and the formation of current filaments (similar to current pooling, but within a single device), and is one of the underlying causes of many semiconductor junction failures.
Bipolar Junction Transistors (BJTs)
"Leakage (electronic)") leakage current increases significantly in bipolar transistors (especially germanium-based bipolar transistors) as the temperature increases. Depending on the circuit design, this increase in leakage current can increase the current flowing through a transistor and therefore power dissipation, causing a further increase in leakage current between the collector and emitter. 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 bias is not compensated by temperature, then as the temperature increases both transistors will be increasingly biased, causing the current and power to increase even more. , and eventually destroying one or both devices.
A 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, which reduces the crossover bias voltage and therefore reduces 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.
In power transistors (which effectively consist of many small transistors in parallel), current buildup can occur between different parts of the transistor itself, with one part of the transistor becoming hotter than the others. This is called second breakdown and can lead to destruction of the transistor even when the average junction temperature appears to be at a safe level.
Power MOSFET
Power MOSFETs generally increase their on-resistance with temperature. In some circumstances, the power dissipated in this resistor causes increased junction heating, which further increases the junction temperature", in a positive feedback loop. As a consequence, power MOSFETs have stable and unstable regions of operation.[8] However, the increase in on-resistance with temperature helps balance the current across multiple MOSFETs connected in parallel, so current buildup does not occur. If a MOSFET transistor produces more heat than the heatsink, heat can be dissipated, thermal runaway can still destroy the transistors. This problem can be alleviated to some extent by reducing the thermal resistance between the transistor die and the heatsink. See also thermal design power.
• - Java demo applet For runaway thermal MOSFET.
Metal Oxide Varistors (MOV)
Metal oxide varistors typically develop lower resistance as they heat. If connected directly across an AC or DC power bus (a common use for electrical transient protection "Transient Regime (Electronics)"), an MOV that has developed a reduced pickup voltage can slip into catastrophic thermal runaway, possibly culminating in a small explosion or fire. To avoid this possibility, the fault current is usually limited by a thermal fuse, circuit breaker, or other current-limiting device.
Tantalum capacitors
Tantalum capacitors are, under certain conditions, prone to self-destruction by thermal runaway. The capacitor typically consists of a sintered tantalum sponge acting as an anode, a manganese dioxide cathode, and a dielectric layer of tantalum pentoxide created on the surface of the tantalum sponge through anodization. It may happen that the tantalum oxide layer has weak points that suffer dielectric breakdown during a voltage spike. The tantalum sponge then comes into direct contact with the manganese dioxide, and the increased leakage current causes localized heating; This typically causes 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 point of failure is high enough, a self-sustaining exothermic reaction can begin, similar to the thermite reaction, with metallic tantalum as fuel and manganese dioxide as oxidizer. This undesirable reaction will destroy the condenser, producing smoke and possibly flame.[10].
Therefore, tantalum capacitors can be deployed freely in small signal circuits, but application in high power circuits must be carefully designed to avoid runaway thermal failures.
Digital logic
The leakage current of logic switching transistors increases with temperature. In rare cases, this can lead to thermal runaway in digital circuits. This is not a common problem, as leakage currents typically make up a small portion of the overall power consumption, so the increase in power is quite modest: for an Athlon 64, power dissipation increases by about 10% for every 30 degrees Celsius. (kelvins per watt), which is about 6 times worse than the heat of the original Athlon 64. washbasin. (A stock Athlon 64 heatsink is rated at 0.34 K/W, although the actual thermal resistance to the environment is somewhat higher, due to the thermal boundary between the processor and the heatsink, rising case temperatures, and other thermal resistances.) , an inadequate heatsink with a thermal resistance of more than 0.5 to 1 K/W would result in the destruction of a 100W device, even without runaway thermal effects.
Batteries
When handled incorrectly or manufactured defectively, some rechargeable batteries can experience thermal runaway that can result in overheating. Sealed cells will sometimes explode violently if the safety vents are overwhelmed or inoperable.[12] Lithium-ion batteries, especially in lithium polymer battery form, are especially prone to thermal runaway. Reports of exploding cell phones occasionally appear in newspapers. In 2006, batteries from Apple, HP, Toshiba, Lenovo, Dell, and other laptop manufacturers were recalled due to fires and explosions.[13][14][15][16] The US Department of Transportation's Pipeline and Hazardous Materials Safety Administration (PHMSA) has established regulations regarding the transportation of certain types of batteries on airplanes due to their instability in certain situations. This action was partially inspired by a fire in a cargo hold on a UPS plane.[17]
One possible solution is to use safer and less reactive anode (lithium titanates) and cathode (lithium iron phosphate) materials, thus avoiding cobalt electrodes in many rechargeable lithium cells, along with non-flammable electrolytes based on ionic liquids.