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Telecom GDT Training. Training Agenda GDT Definition and Telecom Circuit Protection GDT Characteristics and Device Physics GDT Telecom Applications Protection Example GDT Telecom Applications Product Selection Littelfuse GDT Product Road Map Telecom GDT Technology Challenges.

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Telecom gdt training
Telecom GDT Training

Training Agenda

  • GDT Definition and Telecom Circuit Protection

  • GDT Characteristics and Device Physics

  • GDT Telecom Applications Protection Example

  • GDT Telecom Applications Product Selection

  • Littelfuse GDT Product Road Map

  • Telecom GDT Technology Challenges

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Telecom GDT Training

Section 1 GDT Definition and Telecom Circuit Protection

GDT Definition

  • A Gas Discharge Tube (GDT) is a gas discharge plasma device that provides a crowbar current path to protect electronic components from transient threat.

    Circuit Protection Concepts

  • Potential safety threats that requires circuit protection

    • Protect against environment threats such as Lightning, EFT Surges, and ESD

  • Regulatory requirements related to circuit protection

    • IEC Regulations

    • UL Regulations

    • Other Regulations

  • Method of circuit protection

    • Over voltage

    • Voltage transient

  • Added value of circuit protection

    • Prevent disaster such as fire

    • Avoid law suit and warranty issues

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Telecom GDT Training


Ignition Voltage (Overshoot)




Gas Discharge Tubes

  • Advantages

  • Inherently bidirectional

  • Low capacitance

  • Ability to handle high surgecurrents (>20kA)

  • Very low impedance followingspark over

  • Disadvantages

  • Poor initial “let through voltage” (overshoot)

  • Slow response (compared to SIDACtor or PGB)

  • May require series resistance to preventfollow-on current

  • Trigger voltage varies

  • Degrades with repetitive transients

  • Fails open circuit

Gdt definition and telecom circuit protection
GDT Definition and Telecom Circuit Protection

Circuit Protection Needs in Telecom Systems

  • Thunderstorms around the world deliver 8 million lightning flashes every day. Peak current in lightning discharges range from a few KA to many hundreds of KA. Induced currents from indirect strikes range from 10A to 20KA.

  • ESD results from the build up of electrical charge, when two non-conductive materials are brought together then separated. The potential between a human body & an object can exceed 35,000 volts. An ESD event can occur to the telecom system or portable devices through human contact and usage of the telecom devices.

  • Inductive Load Switching is caused when an inductive load is interrupted. It occurs in factory/industrial environments where motors and relays (inductive loads) are turned on and off.

  • Short Circuit or Power Cross events can occur due to human error (such cutting a phone and power line simultaneously during construction) or natural disaster such as hurricane, thunderstorm.

  • One or a combination of the above threats can have obvious adverse effects on semiconductor/IC devices, electro-mechanical contacts, wiring insulation, etc., to cause interruption of telecom equipment operation, telephone service, and even fire.

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GDT Definition and Telecom Circuit Protection

GDT Technology for Telecom Overvoltage Circuit Protection

Telecom equipment should be protected from overvoltage conditions using GDTs, MOVs, or silicon devices such as SIDACtors or TVS diodes.

Gas Discharge Tubes / Surge Arresters are mostly used for Primary Protection against transients caused by lightning.

Secondary Protection applications traditionally have used Silicon solutions but due to the increased bandwidth now required by telephone lines the GDT has the benefit of having significantly lower capacitance levels and are now competing with Silicon solutions due to their relatively high capacitance.

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Telecom GDT Training

Section 2 GDT Characteristics and Device Physics

  • Basic GDT Characteristics

    • Electrical Characteristics

      • V-I curve characteristics

      • Trigger voltage

      • Clamping voltage

      • Response time

      • Leakage current

  • GDT Construction and how it affects the related GDT characteristics

    • GDT structure vs. electrical characteristics

    • GDT structure vs. maximum ratings

    • GDT de-rating

Gdt characteristics and device physics
GDT Characteristics and Device Physics

Electrical GDT Characteristics

VH (holdover voltage) is the key parameter. With the exception of 75V and 90V devices that have a holdover voltage of 50V, all other devices have a holdover voltage of 135V which will not cause latch up for the telecom system.

VSDC (DC breakover voltage) is measured at a slow rate of rise, usually 100V/sec. This is to make sure that the lower DC breakover voltage is above the system voltage.

VS (dynamic breakover voltage) determines the level of protection clamping voltage.

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GDT Characteristics and Device Physics

GDT Construction

The gas plasma arrester operates as a voltage dependent switch. When the spark over voltage applies across the device greater than its breakdown voltage, an arc discharge takes place within the tube.

A gas plasma device can typically handle 5kA to 20kA current.

The performance of the GDT can be determined by the following factors:

- Electrode material, shape and surface finish.

- Electrode coatings/activation, electron work function, and Electrode spacing.

- Gas volume and pressure, temperature Type of gas, and ionization energy of the gas.

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Telecom GDT Training

Section 3 GDT Telecom Applications Protection Examples

  • Exchange Equipment

  • Main Distribution Frame

  • Subscriber Terminal

  • Telephones

  • Fax Machines

  • Modems

  • Base Stations for Cell Phones

  • Multiplexers

  • LAN Network Terminals

  • Amplifiers

  • ADSL/VDSL/HDSL Equipment

Gdt telecom applications protection examples
GDT Telecom Applications Protection Examples

Telecom 3 Point Protection Solution (typically employed in outside plant protectors)

All outside plant protectors perform a Primary Protection function, meaning they act as the first line of defense. The Gas Plasma device, because of it’s high energy capability (up to 20KA), is ideal for these applications. Typically, 5KA would be the lowest rating for a Primary Protector used in an outside plant application.

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GDT Telecom Applications Protection Examples

Telecom 3 Point Protection Solution (typically employed in outside plant protectors)

3 Terminal GDT

2 x 2 Terminal GDT

Three terminal Gas Plasma devices are constructed using a common gas chamber, so in differential mode (a transient appearing on both Tip and Ring) the resulting differential transient is smaller.

Two terminal tubes are often easier, from a mechanical standpoint, to incorporate in module designs.

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GDT Telecom Applications Protection Examples

MDF Protection Modules

MDF modules perform some level of Primary Protection, but often incorporate secondary level features as well. Such devices are are of Hybrid construction incorporating more than one technology.

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GDT Telecom Applications Protection Examples

Telecom 5 Point Protection Solution (Hybrid design commonly deployed in MDF modules)

Depending on geography, the level of primary protection provided in the MDF varies. In certain regions of the world the effects of lightning can be more severe. In these cases, the MDF module will need a surge rating of several thousand Amperes (KA); the solution here is to use a Gas Plasma device. A typical rating in this application is 5KA.

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GDT Telecom Applications Protection Examples

3 Terminal Device (to provide protection against isolation breakdown on transformer)

In order to transfer high speed data over distance and without loss or corruption, the protector has a exhibit low ‘insertion loss’ values (normally measured in dB). To some extent, the capacitance value has a direct relationship with insertion loss: higher capacitance = higher insertion loss.

Littelfuse “broadband optimized” protectors offer very low insertion loss, even up to 1.8Ghz, making them virtually transparent to the protected system.

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GDT Telecom Applications Protection Examples

Global Lab Capabilities

  • Qualification of all LF products

  • UL-Approved Customer Testing in ISO 17025 Lab (Des Plaines)

    • High power (AC/DC up to 1KV/50KA) UL approvals available in DP

    • Telcordia approvals in DP planned (2008)

  • Verification of Telcordia, ITU, IEC, FCC, and other industry, regulatory, and safety standards

    • Verification to various OC and OV standards

      • Insure application meets standards before submitting for approval

  • Customer Application testing

    • Assistance with design-in and performance verification

      • Help with selection of appropriate technology and rating

    • Application troubleshooting

      • Assistance insuring proper OV/OC and primary/secondary protection coordination

    • Competitive evaluations

      • Competitive or technology performance comparisons

    • Reliability & Tin Whisker data/testing

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Telecom GDT Training

Section 4 GDT Telecom Applications Product Selection

GDT Product Selection

  • Select power rating: 5A/5KA, 10A/10KA or 20A/20KA

  • Select package outline: 2 Pole, 3 Pole, Axial, Radial or SM

  • Choose a device with a minimum DC Breakdown higher than the maximum system voltage

  • Select the corresponding device voltage type

Gdt telecom applications product selection
GDT Telecom Applications Product Selection






Response Time



Slower response time




As low as 1pF



Current surge rating


As high as 500A for 200 impulses



Electrical Characteristic


Degrade with time

Fatigue after multiple pulses



Principle Overvoltage in telecom, datacom circuit

Telecom application

Useful in AC applications

Secondary Protectors

Overvoltage Protection Comparison

Gdt telecom applications product selection1
GDT Telecom Applications Product Selection

Characteristics of Transient Voltage Suppressor Technology

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Telecom GDT Training

Section 5 Littelfuse GDT Product Road Map

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Telecom GDT Training

Section 6 Telecom GDT Technology Challenges

  • Higher Surge Rating and Smaller Packaging

  • Multiple Elements in One Package

  • GDT Technology Combined with Other Technologies in Same Package

  • Improved De-rating Characteristics