Transient operation of transistor with inductive load

A transistor operating with an inductive load may develop a collector-emitter short circuit when the transistor is suddenly turned off. The secondary breakdown of the collector characteristics determines the susceptibility to this type of failure. The secondary breakdown is greatly influenced by the reverse base current. The reverse base bias voltage and impedance affect the reverse base current in a predictable manner. The failure mechanism can be explained in terms of the characteristics of a four-layer device. Transistor requirements and design considerations are examined. For safe operation, the secondary breakdown current should be greater than the maximum operating current. When the reverse base current is minimized, the transistor is also protected. Several circuits which prevent the flow of reverse base current are presented.