EMC & CIRCUIT PROTECTION FEATURE MODERN GAS DISCHARGE TUBES


The latest protection devices from Littelfuse provide high levels of protection against fast-rising transients caused by lightning disturbances. Here’s more on the design capabilities of these devices


selection and assembly in the automatic production of printed circuit boards.


D


ue to comprehensive digitisation, more and more data is being stored on servers,


in the cloud and at data centers. Companies must still deliver rapid, secure and reliable access to this information. Protection from overvoltage transients, which are mainly caused by lightning strikes, is a vital part of this. However, the transient protection capacitance of such bandwidth applications can impair signal strength, which reduces speed. But not protecting G.fast modems and circuits from lightning overvoltage transients is also no solution. Three protection options are basically


available: Gas Discharge Tubes (GTD), SPAs (Silicon Protection Arrays) and SIDACtor Protection Thyristors. GTDs offer the advantages of a current resistance of up to 20kA and a low off-state capacitance of up to 1pF at 0V. Due to their resistance to very high impact currents, they are often used for primary protection. Their low interference with high-frequency applications also lends itself to high-speed data connections. There are disadvantages for G.fast applications, however, due to their size and high input threshold. The SH Series GDT from Littelfuse is


designed to provide high levels of protection against fast-rising transients caused by lightning disturbances. It was specifically designed for protection of electrical, multimedia and communication equipment with surface mounting applications against overvoltage transients. It has a5kA (8/20μs) surge capability. Offered in square packaging


with surface mounting (5.0 × 5.0 × 4.2mm). The extremely low off-state capacitance


value of ≤0.7pF is compatible with high bandwidth applications for high data transmissions and remains constant even with signal voltage fluctuations. Other features include an excellent response to fast-rising transients and a frequency in the GHz range. The SH Series GDTs are non-radioactive, lead- free and compatible with RoHS (Restriction of Hazardous Substances), an EU directive to facilitate scrapping by avoiding hazardous substances. In addition, the SH Series is recognised by UL (Underwriters Laboratories Inc.. The series is suited for all bandwidth


applications. These include G.fast, xDSL, 10GbE, 10/100/1000 Base-T Ethernet port protection and general telecommunication devices for the telecom/datacom market; coaxial cable protection in satellites, CATV equipment and set top boxes for consumer products; industrial automation interfaces such as Ethernet, RS- 485, RS-232 and others for the industrial market; as well as all other bandwidth devices such as power inverters/Variable Frequency Drives (VFDs). This allows companies to benefit from numerous advantages. The SH Series is ideally suited for protection


against high overvoltage transients in high- density applications such as the power supply of radio base stations and cable modems. It minimises insertion loss and demodulation, making it compatible with a variety of high- speed bandwidth applications, such as G.fast and xDSL. It also simplifies the process for


/ ELECTRONICS Figure 1:


The SH Series GDT from Littelfuse is designed to provide high levels of protection against fast- rising transients caused by lightning disturbances


COAXIAL CABLE PROTECTION One possible application area for GDTs is the protection of long coaxial cable guides. These are susceptible to overvoltage transients caused by lightning disturbances in the vicinity. Such current surges can reach magnitudes of 400A at the moment of impact. Lightning strikes usually have a heavy initial blow followed by several impacts tapering in strength. These successive impacts usually lead to a higher surge in the coaxial cable since the mutual inductance and current rise time with the number of impacts provides for a rising current strength. Subsequent impacts can therefore lead to surges of up to 750A. A rise in earth potential and the resulting increase in ground current, however, may increase a surge on the cable by an additional 600A. This depends on the proximity of the lightning strike, the characteristics of the earth resistance and other factors. Various phenomena require a protective application that can withstand three to five lightning disturbances within a short period of time. As its rating of 5kA 8/20 certifies, the SH Series


is suitable for such a demanding environment. If the coaxial cable transmits an RF signal in the GHz range, a low off-state capacitance is necessary for any protective application to remain viable. The SH Series satisfies this requirement with a maximum capacity of 0.7pF. G.fast is a DSL protocol with standard line lengths of up to 250 meters and bandwidths of 106MHz – the next generation of G.fast will have 212MHz. TDD (Time Division Duplex) coding results in data transfer rates of 1Gb/s per 50 meters. Lightning disturbances can lead to overvoltage transients in the range of 10kV/5kA. Due to its high rating of 5kA for overvoltage transients and its low off-state capacity of a maximum of 0.7pF, the SH Series is ideally suited for these high-speed data connections. Low voltage versions are ideally switched


from behind line capacitors. Otherwise, when placed on the line side, SH Series GDTs with higher thresholds would be required to remain compatible with direct current voltage on tip and ring. Reliable high-speed data transmission protocols such as RS 485 are often used for industry-specific receivers.


Littelfuse www.littelfuse.com T: +1 773-628-1000


ELECTRONICS | APRIL 2017 25


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