En los circuitos que utilizan transistores de potencia que operan cerca de sus límites, se deben tomar varias precauciones para que no se superen y con esto ocurra su quema. Un límite poco conocido, y no siempre observado es el de la llamada segunda ruptura. En este artículo abordaremos el tema, explicando cuál es la segunda ruptura y cómo evitar.

Cuando polarizamos una juntura de semiconductores en la dirección inversa, por ejemplo, un diodo, el instante llega cuando ya no logra aislar la tensión aplicada y, por lo tanto, se produce una ruptura. La juntura pierde sus propiedades aislantes y se convierte en conductora, fluyendo una corriente intensa que normalmente causa la quema del componente.

Otros componentes, como los diodos Zener, aprovechan esta tensión de ruptura para mantener la tensión de sus terminales, operando con intensidades que no causan su quema. Sin embargo, para los transistores de potencia, hay un fenómeno que ocurre y que se muestra en algunas curvas SOA, llamadas TU o Segunda Ruptura o en inglés " second breakdown ".

Este fenómeno ocurre en la polarización directa, y si no se observa puede hacer que el componente se queme. En la figura 1 tenemos una curva SOA en la que se muestra esta segunda ruptura.

 

Figura 1 – Segunda ruptura o “second breakdown”
Figura 1 – Segunda ruptura o “second breakdown” | Clique na imagem para ampliar |

 

 

Es un fenómeno que se produce en una juntura de un transistor de potencia (y otros componentes también) cuando el tensión, corriente y La disipación de potencia es alta, pero aún por debajo de los límites permitidos para un funcionamiento seguro. Lo que ocurre es que, en un transistor ideal, cuando en la conducción se espera que la corriente del colector se distribuya uniformemente en el área correspondiente del inserto de silicio, y con esto la potencia generada también se distribuye uniformemente.

 

Sin embargo, en la práctica esto no es lo que ocurre. Puede haber pequeñas áreas donde la corriente es más alta formando puntos calientes o " hot spots” si adoptamos el término en inglés. Cuando se conmuta el componente, ambos en el momento en que se enciende y se apaga, estos puntos calientes que se forman pueden causar su quema,

 

El fenómeno se debe al hecho de que los portadores minoritarios de la carga material semiconductora tienen un coeficiente negativo de resistencia en relación con la temperatura, es decir, su resistencia disminuye cuando aumenta la temperatura.

 

Para evitar la segunda ruptura hay un cuidado importante que se debe observar en el uso de transistores, y otros componentes que pueden manifestar el problema.

 

Las precauciones principales son:

a) Mantenga la disipación dentro de los límites determinados por las características del componente

b) Utilice un snubber para evitar la disipación excesiva a veces cuando se conmuta el componente

c) Tenga cuidado de que el componente opere dentro del área de operación segura.

d) Observe en la calidad del transistor si se fabrica con una tecnología que permita una distribución uniforme de la corriente para evitar el problema.

e) Trabaje en el proyecto con una polarización base que ayude a reducir rápidamente la corriente en el componente al apagarse.

 

 

Conclusión

Dos transistores, incluso si son del mismo tipo no son iguales. Por esta razón es importante consultar sus límites de funcionamiento dados por el fabricante para asegurar que, dentro de las tolerancias permitidas, siempre se detienen en condiciones de funcionamiento seguro.

 

 

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