Primary and Secondary Protection

The use of Primary and Secondary Protection is very common. The concept is used for lightning and surge protection in the telecommunications industry and it is used for ESD protection in advanced integrated circuit technology. A primary protection element, closer to the source of stress, is intended to carry the bulk of the current stress while a secondary protection element, very close to the sensitive circuit element, provides protection where it is most needed.

Consider the case of a data line entering a building as shown in Figure 1 . The primary protection device is located at the entrance to the building. The data line may also have a current limiting element such as a fuse before being routed to the electrical systems inside the building. Secondary protection is located close to the sensitive circuit elements.

Figure 1 Primary and Secondary Protection for a simple data line entering a building

The need for two forms of protection arises from a variety of issues. The most important role of surge protection on a data line, such as a telephone line, is to prevent catastrophic loss such as fires or electrocution. This requires that there be a protection element at the entrance to a building, the primary protection. The primary protection element must therefore be very reliable and able to carry very large currents. The ability to prevent catastrophic damage does not guaranty that transient voltages, less than those that will cause fire or electrocution but large enough to damage sensitive electronics, will not get past the primary protection. The secondary protection, located at the sensitive equipment fills this role. The secondary protection needs to turn on fast and protect against transients passed by the primary protection. The secondary protection elements may also be customized for the protection requirements for the specific system being protected. Silicon protection products with their fast turn on times and low turn on voltages excel as secondary protection elements.

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