Electronic Design

USE GDTs For Surge Protection In Broadband Digital Comm

Gas discharge tubes (GDTs) have evolved to a level of providing very reliable and effective surge protection in telecommunications systems and equipment, safeguarding against lightning and power-fault conditions.

Due to their robust nature and superior electrical characteristics, GDTs have already become the preferred replacement for carbon blocks in traditional telephone-service applications. Because of their ultra-low capacitance plus low insertion and return loss performance, they’re also replacing solid-state semiconductor solutions in broadband-communication designs.

General GDT Operation
Finding extensive employment, GDTs protect against overvoltages caused by lightning, power switching, and fault conditions. When a voltage disturbance reaches a GDT’s spark-over value, it will switch into a virtual short, known as the arc mode. The GDT virtually shorts the line, diverting the surge current through the GDT to ground, and removes the voltage surge.

At normal operating voltages below the GDT’s rated dc breakdown (DCBD) voltage, measured at a rise rate between 100 to 2000 V/s, the GDT remains in a high-impedance OFF state. With an increase in voltage across its conductors, it will enter into the glow-voltage region, which is where the gas in the tube starts to ionize due to the charge developing across it.

In the glow region, the increase of current flow will create an avalanche effect in the gas ionization process that will transition the GDT into a virtual short-circuit mode. The current, which depends on the impedance of the voltage source, will pass between the two conductors. We refer to the voltage across the GDT during the short-circuit mode as the arc voltage.

The transition time between the glow and arc region depends on the available current of the impulse, distance and shape of the electrodes, gas composition, gas pressure, and the proprietary emission coatings. The active emission coating allows the tubes to transition into arc mode at currents lower than 500 mA with arc voltage specified at less than 10 V at 1 A.

The GDT will switch back or reset into a high-impedance state once there is not enough voltage and/or current to keep it in the arc condition. This is known as the extinguishing voltage, holdover voltage, or impulse reset voltage.

Due to the typical arc voltage of less than 10 V, a GDT is ideal for protecting against high-energy impulses and acpower cross conditions. Under ac, the power dissipated in the device needs careful monitoring. A switch-grade, fail-short mechanism can protect against thermal overload under these ac conditions.

At elevated GDT temperatures, a spring-loaded clip operates like a switch to short the TIP/RING conductors to ground. Importantly, it is not a good practice to hold a GDT in its glow region as this will significantly reduce its life expectancy. In this condition, significant heat can develop on the electrodes that can damage the emission coatings and cause premature failure of the tube.

Also avoid using a variable ac source such as a curve tracer or equivalent to vary the voltage and power across the GDT’s DCBD voltage. It is highly unlikely, however, that a condition could exist in the field that would maintain a GDT in the glow mode (Fig. 1).

Dynamic Performance With Telecom Surges
Characterization of a GDT’s impulse spark-over specifications is via an impulse voltage waveform such as 100 V/µs or 1 kV/µs. This ramp voltage is not an accurate representation of a real-world scenario, i.e., disturbances entering a twisted-pair cable from a lightning strike.

Based on rise and decay times, a lightning strike is viewable as a charge dissipating through the impedance of the line. There is a relationship between the DCBD, ramp-impulse, and surge-impulse voltages, which translates into similar GDT performance between the 100-V/µs impulse breakdown voltage and the 100-A, 10/1000-µs surge-impulse voltage (Fig. 2).

Field studies show that lightning-strike energy is similar to a 10/250-µs waveform for a positive stroke. Primary-protection telecom standards such as Telcordia GR-974-CORE additionally specify end-of-life mode tests using a series of surge generators delivering 10/1000-µs, 10/250-µs, and 8/20-µs type wave shapes. The dynamic performance of the GDT with these wave shapes is less known.

As secondary protection is often required to protect voltagesensitive equipment from the primary protector let-through, it is important to know how the GDT performs under these types of impulses. Standards such as Telcordia GR-1089-CORE for secondary protection still rely on worst-case, carbon-block primaryprotection technology to cover legacy equipment while the GR- 1089-CORE, issue 4 release allows secondary protection to depend more on the primary-protector technology used in the field.

Variable Dynamic Parameters
Designers often question the inconsistent electrical measurements of a GDT. These inconsistencies are the result of contaminants introduced into the gas from normal operation. Contaminants in the gas change the electrical characteristics of the GDT, causing increases in DCBD voltage values.

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To describe this effect, emission coatings in the tube attract the contaminants to the electrodes. As the impurities decrease in the gas atmosphere, the DCBD voltage also decreases. This can result in variations seen in the measurements during repeated tests on the same device.

Calculating Expected Peak Voltage
Lightning strikes usually range from 30 to 250 kA with a very short duration, but the energy seen by a primary protector at the end of a twisted pair is more like a 500-A, 10/250-µs surge. There are also ground-potential rises (GPRs), with surges entering the ground plane and damaging equipment through the backplane.

Furthermore, there is the belief that a faster rising voltage will provide a higher impulse voltage. While this belief is correct, some components can see a decline of the surge and ramp impulse voltage as the rate of rise of surge voltages increases (Fig. 3).

The ramp or surge impulse voltage is calculated using VSURGE = (1.2 × VDCBD) + 280 to find the expected peak voltage with a known DCBD voltage option. This is fairly simple when the only impact on the surge-impulse voltage rating comes from the shape of the surge rising-edge voltage waveform.

A Bourns GDT requires a surge voltage of about 15% above the dc spark-over rating for a 10/1000-µs surge. This increases to 50% under a 2/10-µs impulse to ensure the GDT operates. This identifies the stress applied to the secondary protection when the GDT is about to switch into arc mode by just knowing the impulsevoltage value of the GDT primary protector.

A common practice is to utilize the 1-kV/µs rating, which is often 150% above the DCBD rating in high-lightning conditions. Using the 1-kV/µs figure will provide a worst-case scenario or maximum tolerance, though at the expense of higher secondary protection requirements.

As per Figure 4, worst-stress conditions occur in secondary protection or the equipment when subjecting the GDT to a 2-kV, 2/10-µs or 8/20-µs surge. Increasing the generator voltage past this point will reduce the surge impulse voltage, and the higher current will only be testing the ground return current path. Determining the surge-impulse-voltage rating of the GDT with a 2-kV, 2-O, 8/20-µs surge will help to ensure coordination with the secondary protection components.

This test can also be useful for equipment in high-lightning, remote-access applications. The GR-1089-CORE, issue 4, section 4.7 specification covers lightning protection tests for equipment in high-exposure premises or online service provider (OSP) facilities and uses a 10/250-µs generator delivering up to 4 kV at 500 A. Further investigation of this waveform is necessary to see if 4 kV provides maximum stress on the equipment.

Primary protectors have to meet Telcordia GR-974-CORE and GR-1361-CORE general requirements for telecom-line protection in the U.S. and ITU-T K.12 for most other countries. In the U.S., secondary protection needs to meet GR-1089-CORE or TIA-968-A general requirements and ITU-T K.20/K.21 for other countries.

Previously, the design of primary and secondary protectors in telecom systems was totally exclusive to each. Now, standards such as Telcordia GR-1089-CORE, issue 4 present the first opportunity for considering the benefits of a GDT during secondaryprotection design work.

Selecting the Correct GDT
Though considered slow operating overvoltage protectors, GDTs can handle impulse currents many times higher than faster technologies like solid-state devices. The components have to develop enough voltage across their conductors to create an arc while the gas between the contacts controls the electrical parameters and makes them repeatable under a variety of conditions.

Additionally, evolving GDT technology now enables fasterswitching devices that reduce the impulse sparkover rating, but often at the expense of other key parameters such as tube life and high glow-to-arc transition currents or high arc voltages. Addressing budget limits, a component may also employ lower-quality materials such as the electrode material or electrode coating material, though at the expense of the GDT life in the field.

Essentially, to select the proper GDT for the job, establishing the tradeoffs of dynamic parameters, longevity, and price is key to designing the correct product for the application. For example, it may not be necessary to design a GDT with a 20-year lifespan for an end product with a five-year service life.

Tim Ardley is the senior telecom field applications engineer with the Bourns Inc. circuit protection division. He has more than 20 years of experience in the semiconductor industry and holds a BSc (Hons) from Luton University in England.

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