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Surge Protection Devices basics | Surge Protection Device types

This page covers Surge Protection Devices Basics including selection criteria. It also mentions Surge Protection Device types viz. GDT (Gas Discharge Tube), TSPD (Thyristor Surge Protection Devices), TVS(Transient Voltage Suppressor), MOV(Metal Oxide Varistor), PESD (Polymer device) etc. and comparison between them.

What are surge protection devices?

• The devices used to prevent damage to electronic or electrial equipments from voltage spikes or transients are known as surge protector or surge suppressor.
• It is inserted in AC utility line and/or telephone line.
• It is also called transient suppressor.
• Surge protectors provide protection by dissipating any excess power and preventing it to reach connected equipments/devices. Electronic equipments such as desktop, laptop or hard drive are designed to operate with certain power range. If too much voltage is passed to them, they will get permanent damage.
• It assists protection of equipments against transients originating from lightning and from other switching sources.

Surge Protection Device

• The figure-1 depicts surge protection device in the network. As shown the device is connected in parallel on the power supply circuit of the loads which has to be protected. It diverts current waves to the earth and hence limit the amplitude of the overvoltage to a value which is not hazardous for electrical installation.

Following are the three types of Surge Protection Devices (SPDs).
• Type-1 SPD: It is characterized by a 10/350 µs current wave.
• Type-2 SPD: It is characterized by an 8/20 µs current wave.
• Type-3 SPD: It is characterized by a combination of voltage waves (1.2/50 µs) and current waves (8/20 µs).

Surge protection device types

Following are over voltage protection devices. Let us understand construction and functions of these surge protection device types.
Thyristors: They are crowbar devices used as surge protection devices. They are based on pair of intertwined bipolar transistors created by four layer stack of n and p doped silicon regions. Refer TVS diode >>, TVS Thyristor selection guide >> for more information.

Transient voltage suppressors:They are based on avalanche diodes and zener diodes. Diodes are made of semiconductors usually silicon, at a junction between n and p doped regions. TVS devices provide protection with combination of forward bias and reverse bias breakdown conduction. Refer ESD protection diode >>

MOV (Metal Oxide varistor) : Varistors are usually made of ceramic of zinc oxide grains in a matrix of other oxides. The grains form diodes with surrounding matrix, creating a complex array of parallel and anti-parallel diodes. At higher voltages, the individual diodes begin to conduct and the resistance of the varistor drops dramatically.

Polymer ESD devices: Polymer devices are bidirectional crowbar devices. They have very high turn on voltages, often over 200 Volts, but they turn on quickly which limits exposure to high voltage. Polymer ESD devices consist of a polymer embedded with conducting particles.

Gas Discharge Tubes: They are usually formed with ceramic body filled with gas mixture containing neon and argon and two or more electrodes. When voltage across the electrodes exceeds a specified value an arc occurs within the tube which provides low current path.

Fuses: They are used as over current protection devices.

Following table mentions comparison between various surge protection devices.

High Power Surge Events - 8x20µs, 10x1000µs, ect.

Type Speed Accuracy Current capability Size on board Lowest crowbar trigger voltage Wearout
Gas Discharge Tube (GDT) Slow Fair High Large 75 V Will fail after numerous stresses depending on severity.
Thyristor Surge Protection Devices (TSPD) Fair Good Medium Small 80V NO
Transient Voltage Suppressor (TVS) Fast Good Low Small NA NO
Metal Oxide Varistor (MOV) Fair Poor Medium Small NA Yes

Very Fast Surge Events - ESD (IEC61000-4-2)

Type Speed Accuracy Current capability Size on board Lowest crowbar trigger voltage Wearout
PESD (Polymer Device) Fast Poor Low Small ~100V Yes
MOV (Metal Oxide Varistor) Fair Poor Medium Small NA Yes
Silicon ESD suppressor (TVS) Fast Good Medium Small NA NO

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