The electronics industry's evolution toward small-geometry CMOS as the dominant design medium has bolstered signal-processing and computational performance, energy efficiency, economy, and compactness. At the same time, however, it has reduced ICs' native robustness in the face of common and unavoidable electrical transient hazards.

In broad strokes, transient sources segment into lightning, switching, EMP (electromagnetic pulse), and ESD (electrostatic discharge). Of the four, EMPs are thankfully rare, deriving most famously from nuclear events. But, like EMP, virtually all sources of electrical transients derive from a sudden release of stored energy.
Lightning and ESD result from the sudden release of static charge; switching transients typically originate from the collapse of an electrostatic or electromagnetic field due to a sudden change in current or voltage in the presence of explicit or parasitic reactances.

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For any given node that is susceptible to a hazardous transient, a protection method must simultaneously meet several requirements. The method must be able to clamp the protected node to a safe potential. An appropriate protection device, therefore, varies according to the type of circuit it is protecting. The protection device must respond fast enough to the transient's rising edge to keep the node voltage below the destructive threshold.
Low-inductance shunt devices and low-capacitance series elements help meet this requirement but only if the pc board's design exploits good high-speed-layout techniques for the protection network. Lastly, the protection method must either be able to absorb the energy that a transient event delivers or cause the dissipation to occur in the transient source impedance. For this reason, OEM designers cannot always depend on the semiconductor's on-chip protection cells and often must add pc-board-level protection elements.
A prudent first step in determining a protection method for a given transient type, therefore, is to calculate the total strike energy that your circuit must absorb. Also consider the likely repetition rates and thermal time constants to ensure that clamping elements don't overheat in the heat of the moment.

The most commonly susceptible nodes are a system's exposed ports. These include the power entry and signal I/Os. They can also include internal nodes that are near an insulating surface as is the case in keyboards and displays. Transients needn't originate on a particular lead to damage its associated circuitry.
Transients occurring on leads connected to one subsystem can capacitively couple to leads connected to other subsystems before breakdown or can inductively couple during the current ramp that immediately follows breakdown. In such cases, protecting the primary victim may not suffice. For example, if an installation bundles signal leads with power feeds, then a power-line transient induced by, say, a motor could couple to a signal node, albeit at reduced amplitude.

High-speed circuits are challenging to protect because they can be sensitive to the additional shunt-capacitance loading that clamping schemes add to the protected node. This reason is one of several that fiber-optic feeds are attractive in high-speed communications, even for short-haul applications, such as rack-to-rack links (references 1 and 2).


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