Design engineers often give circuit protection only cursory attention. Fuses, diodes, and varistors are so well known that designers select them with little thought as to the nature of the overvoltage and overcurrent threats and the true value of circuit protection to their application. But designers should consider the costs of suboptimal device selection, many of which go far beyond protecting just the design.
Circuit-protection devices fulfill two primary purposes: safety and reliability. In terms of safety, disconnecting power in a circuit during overcurrent eliminates the possibility of electrocution as well as fire hazards. In addition, the right protection may be necessary to comply with agency standards for some end products. On the reliability side, dissipating transient voltages, lightning, electrostatic discharge (ESD), electrical fast transients (EFTs), and other dangers eliminates the risk of damage while ensuring the functionality of the application.
So, circuit-protection devices can strongly affect users’ perceptions of the end product. Warranty claims and replacement costs hit the bottom line when technicians go on overtime and/or when the maker starts issuing and shipping replacement units. In addition, these failures can affect a company’s top line if consumers lose faith in the brand. When a digital camera or MP3 player fails due to a user-generated ESD event, consumers may consider an alternative brand when they make their next purchase.
Fuses, the most common overcurrent devices, come in fastacting and Slo-Blo (time-lag) varieties. Slo-Blo fuses help minimize the nuisance of repeated replacements when a circuit experiences brief but recurring overcurrent spikes. But for personnel safety, designers should employ a current-interrupting fuse for critical ac equipment that requires frequent opening for maintenance operations.
A current-interrupting fuse clears within one-half of an ac cycle (0.00833 second). This limits let-through current to a fraction of the peak available current, preventing conductor and component overheating and reducing the severity of arc-flash events.
Resettable devices offer an alternative to fuses and circuit breakers. One good example is the positive temperature coefficient (PTC) thermistor. As current increases, self-heating increases PTC resistance and automatically limits current. Polymer-based (PPTC) materials are preferable as they exhibit a pronounced knee in their resistance versus temperature characteristics.
Usually, the trip current level is twice the holding current. At 10 times the holding current, a PPTC will typically trip in less than 1 second. Once the overload is gone, the PPTC cools, returning the circuit to normal operation. This prevents negative user perceptions that may result from frequent fuse replacements and circuit-breaker resets.
A wide range of overvoltage protection devices, each with unique capabilities and characteristics, is available (see the table). In keeping with the industry trend to minimize harmful effects to the environment, most of these devices are fully lead-free or comply comply with the European Union’s Restrictions on Hazardous Substances (RoHS).
CROWBAR VERSUS CLAMPING
The terms crowbar and clamping often are used to describe how overvoltage protection devices function during a transient event. A crowbar device reduces the voltage below the operating voltage of the system. When the transient is complete, the crowbar device resets and allows the circuit to operate normally. During a transient event, a clamping device holds the voltage just above the operating voltage of the system. It likewise resets after the transient event terminates.
Gas discharge tubes (GDTs) typically protect telecom and datacom lines, signal lines, and customer premise equipment from surge voltages. They are a good choice for reducing lightning-induced transients because they can handle surge currents up to 40,000 A. GDTs dissipate surge energy through a low-pressure gas plasma contained within a small sealed tube.
With energy ratings from 0.1 to 10,000 Joules, metal oxide varistors (MOVs) divert transient currents away from sensitive circuit components in a wide range of applications. They’re often used in surge suppressors (TVSS) made for home office equipment and are bundled with GDTs and protection thyristors to protect the ac input, cable TV (CATV)/satellite input, and tip/ring circuit inputs.
Protection thyristor devices are a special type of thyristor that suppresses overvoltage transients in a wide assortment of telecom and datacom equipment. They can divert currents up to 500 A within nanoseconds of reaching their break-over voltage. Additionally, they’re qualified to help customers meet various agency standards such as GR 1089, K.20/21, IEC 60950, and TIA-968-A for communication gear and telecom equipment.
Transient voltage suppressor (TVS) diodes form a general category of devices that also protect a wide variety of circuits and components from an assortment of threats, including repetitive pulses in electrical fast transients, inductive load switching, commutative transients, and lightning surges. Their p-n junctions have a much larger cross section than other diodes, Schottky, and Zener, allowing them to conduct large transient currents up to 10,000 A to ground without sustaining damage.
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ESD, which often originates f rom static buildup on the skin or clothing of equipment users and maintenance personnel, can destroy the processors, ASICs, interface chips, and other devices that are the brains behind today’s consumer electronics.
Multilayer-varistor (MLV) devices protect against low- to medium-energy transients in sensitive equipment operating at 0 to 120 V dc. They have peak current ratings from about 20 to 500 A and peak energy ratings from 0.05 to 2.5 Joules. Their small form factor (sizes down to 0402) makes them ideal for portable applications like cell phones and digital cameras.
Silicon protection arrays, which are multichannel devices designed to protect analog and digital signal lines from ESD and other overvoltage transients, typically come in array configurations so designers can protect multiple lines with a single device. Obviously, this approach reduces part counts and frees up board space. And, silicon protection arrays offer a very low clamping voltage, enabling them to protect even the most sensitive circuits.
Considered to be the newest protection technology on the block, polymeric ESD suppressors exhibit minimal parasitic capacitance values, in the realm of less than 0.2 pF. This suits them for use in highspeed digital circuits like HDMI 1.3 without degrading the signal.
TIME TO CHOOSE
While circuit protection usually is left until the very end of the design cycle, it is a good idea to consider it sooner. By doing so, designers can account for potential threats as well as the unique capabilities and characteristics of the available technologies. At the end of the day, both product and brand will be safe.