As described previously, an electrical overstress may be described as a transient voltage, spike, glitch, etc., but is in reality a short-term deviation from normal operating voltage or signal level. As transient voltages increase in amplitude, the risk of disrupting or damaging today's sophisticated electronic equipment increases.

Transient voltage surge suppression (TVSS) devices, also called surge protective devices (SPD), are available in many forms and protection levels.

A quality TVSS device will lower the threat level and "clamp" or "let through" only voltages that will not harm protected equipment.

Obviously, one must consider all paths to entry when planning protection against the TVSS threat.

Figure 4 shows a summary of TVSS device characteristics

Metal Oxide Varistor (MOV) - A voltage-dependent resistor made of metal oxide particles (usually zinc) compressed together. The contact portion of these particles acts like a semiconductor junction (P.N.). Millions of these junctions act like diodes that turn on at different voltages. As voltage increases more and more junctions conduct. The voltage (V) to current (I) relationship is very non-linear. Even manufacturer's curves, which are plotted on log graphs to flatten the curve, show a pronounced non-linear relationship.

The large number of semiconductor junctions allows a high current leakage rate, but also provides excellent power handling capability.

Key features are:

•  High device capacity - each PN junction has capacitance; e.g., 1500pF per MOV.

•  Response is fairly fast but non-linear (higher "let-through" voltage as higher current is applied).

•  High power handling capability, e.g. 6500 Amps @ 8x20 μs pulse for a 20 mm MOV.

•  A great deal of the transient energy is dissipated as heat by the MOV.

•  Follow-on current is low except when the device fails, then quite high.

•  Leakage is high, e.g., 5 mA at operating voltage for a 20 mm MOV.

•  MOV's performance degrades with exposure to transients. The effect of exposure is for the MOV actual operating voltage to become lower with each large transient until it equals the applied voltage at which time follow on current will destroy it. This phenomenon can be eliminated by careful test and selection of MOV's, then configuring them in parallel/redundant circuits. Life can be extended for over 15 years in real-world applications.

•  Failure mode - the MOV fails short when overstressed, then follow-on current normally causes catastrophic rupture and an open circuit. So much heat is generated that, unless protected, the PCB may carbonize and allow some leakage current, although the MOV has "opened". Therefore, proper fusing or circuit breaker selection is essential for MOV based TVSS devices which do not employ integral fusing.

Silicon Avalanche Diode (SAD) - A specialized semiconductor device that acts like a zener diode in turn on and current avalanche mode. However, the SAD utilizes a very large silicon chip sandwiched between large metal pellets giving it thousands of times more current carrying capability than a zener.

Key features are:

•  Fastest turn-on of any device available.

•  Response is essentially flat, that is as higher voltage is applied, more current will flow in a linear fashion up to the point of device failure.

•  Capacity is low. Capacitance is limited to a single PN junction capacitance; e.g., 100 pf for a 24V LCE SAD. Capacitance may be lowered by putting additional diodes in series, however lead inductance must then be accounted for in clamping or "let-through" performance.

•  Energy capability is low, devices are offered in 500, 1500, 5000 and 15000 watt sizes. High wattage devices are expensive.

•  Energy dissipation is low in conjunction with low wattage capability; e.g., 15kW devices often require heat sinking. Junction resistance at avalanche is low resulting in minimal heating during normal "within spec" pulses.

•  Leakage is extremely low in the order of µ amps.

•  Follow-on current is nil except should the device fail.

•  Failure mode - SAD devices fail short and normally remain "shorted" even with high current follow-on flow. The pellets simply weld together.

Zener Diode - The standard Zener device should never be used in transient suppression applications. The PN junction area and metal disc size are very small and incapable of handling significant transient current.

Gas Discharge Tube (GDT) - These devices function similar to air or carbon gap devices except they are hermetically sealed and charged with an argon/hydrogen mixture at about 0.1 Bar. Radioactive gases are often added to control spark-over. Construction is usually two large metal electrodes spaced at about 1 mm and sealed in a ceramic material.

Key features are:

•  Response is somewhat inconsistent and a bit non-linear.

•  Speed is slow; e.g., using the standard 8/20 µs pulse, a 90-volt gas tube will turn on or fire at about 400 volts (striking voltage).

•  Thus the overshoot or "let-through" voltage of a gas tube alone can exceed 400 volts for a low voltage tube and 800 volts for a 230-volt tube usually used in Telco applications.

•  Capacitance of a GDT is negligible.

•  Energy capability is quite high; e.g., 2, 5, 10, 20 and 40 kA GDT's @ 8 x 20 m s pulses are available.

•  Energy dissipation is high - in the presence of a transient, when the tube has fired or "spark-over" occurs, energy is dissipated as heat and light.

•  Follow-on current is high since after spark-over the ionized path has low resistance and small voltages can keep the tube "ON" - some method of extinguishing the "glow" is generally required in the form of parallel devices or a series resistor which must be large. Note that a series resistor adds significantly to "let-through" voltage.

•  Leakage current at operating voltage is negligible at 1 pf @ 60 Hz.

•  Failure Mode - The GDT generally fails open. The device will have its gas charge compromised or depleted. Under extreme lightning, the GDT may fail short.

Silicon Carbide Block - an air gap conductor designed years ago as a lightning arrestor.
Generally not used for suppression any longer.

Key features are:

•  Unpredictable turn on and response characteristics.

•  Very slow to fire or "spark-over".

•  Low to medium capacitance.

•  High-energy capability.

•  High-energy dissipation.

•  No follow on current.

•  Medium to low leakage.

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Air Gap - Construction is two conductive elements placed in close proximity with air (atmosphere) in between. Spark over occurs when the air is ionized by a sufficient voltage potential applied across the terminals - used in extremely high lightning risk areas as the primary protection in a multi-stage TVSS device.

Key features are:

•  Unpredictable turn-on and response characteristics.

•  Very slow to fire or "spark-over".

•  Low capacitance.

•  High-energy capability.

•  Extremely high-energy dissipation.

•  No follow-on current.

•  Low leakage.

Key features are:

•  Response is sharp, predictable turn-on and linear within specified power limits.

•  Speed is fast especially when turned on via DIAC drive.

•  Low capacitance.

•  High-energy capability.

•  Low energy dissipation due to "crow-bar" effect and very low device resistance after turn-on.

•  Follow-on current is high until device is turned off. Design must take into account the requirement for turn-off as low energy may keep device "ON".

•  Leakage is low.

•  Failure mode is a short circuit.

Hybrid Devices - These designs are multi-stage units utilizing a variety of the available TVSS discrete devices. The number of combinations possible is quite large although a few key designs typically dominate available commercial devices. When designed properly, these units will provide all of the best characteristics of each discrete device. The primary stages will absorb the brunt of the transient while later stages provide predictable low clamping or "let-through" voltage. Generally, discrete devices by themselves are inadequate because of either "too high let-through voltage" or insufficient power capability.

Components may be selected to cause the unit to fail short or open. If the TVSS fails open, indication that service protection has been lost is crucial.