Circuit protection devices are used to protect electronic components from unwanted transients such as current and voltage surges, electrostatic discharge (ESD), lightning, spikes and other signals that can imperil the operation or damage the component. They use two basic technologies: attenuation and diversion.
As next-generation transceivers and digital communications ICs scale to smaller geometries, the challenge for IC manufacturers to maintain reasonable levels of on-chip Electrostatic Discharge (ESD) circuit protection becomes greater. Proposed decreases in on-chip ESD protection mean that system designers must be more aware of building ESD protection into their designs by choosing the right circuit protection devices and following key design principles.
In the race to provide more and faster functionality, on-chip ESD protection is often sacrificed in favor of chip performance. According to the ESD Association, the ICs of tomorrow will not sustain the current levels (2kV) of on-chip ESD protection. In fact, there is a proposal to lower on-chip ESD stress target levels by more than half. At the system level, as on-chip ESD protection is reduced, ICs will be more sensitive to transients such as cable discharge events and ESD from the human body. With increased ESD sensitivity of current and future ICs, the need to protect systems with more robust off-chip transient voltage suppression (TVS) is greater than ever.
A surge protector (or surge suppressor) is an appliance designed to protect electronic components from voltage spikes. A surge protector attempts to limit the voltage supplied to an electric device by either blocking or by shorting to ground any unwanted voltages above a safe threshold. This article primarily discusses specifications and components relevant to the type of protector that diverts (shorts) a voltage spike to ground; however, there is some coverage of other methods.
The terms surge protection device (SPD), or the obsolescent term transient voltage surge suppressor (TVSS), are used to describe electrical devices typically installed in power distribution panels, process control systems, communications systems, and other heavy-duty industrial systems, for the purpose of protecting against electrical surges and spikes, including those caused by lightning. Scaled-down versions of these devices are sometimes installed in residential service entrance electrical panels, to protect equipment in a household from similar hazards
Types of Circuit Protection Devices
- Filters are relatively simple isolators that are installed in series.
- Isolation transformers usually have an electrostatic shield between their primary and secondary windings.
- Spark gaps and gas tubes are crowbar-style devices that serve as conductors during overvoltage conditions.
- Selenium transient suppressors use selenium rectifiers to permit reverse-breakdown currents at high energy levels. They have some self-healing characteristics, but lack the clamping ability of other circuit protection devices.
- Silicon carbide varistors are designed for high-power, high-voltage surge arrester application. However, their low impedance values require a series gap to block the normal voltage.
- Silicon avalanche diodes are often used in low-voltage applications because of their superior clamping characteristics.
- Metal oxide varistors (MOV) are non-linear circuit protection devices that maintain a relatively small voltage change across their terminals during large-surge current flow-through.
- Surge fuses are also commonly available.
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