AN34 Ver la hoja de datos (PDF) - Cirrus Logic

Número de pieza

componentes Descripción

Lista de partido

AN34

Secondary Line Protection for T1 and E1 Line Cards

Cirrus Logic 

T1/E1 Line Protection

nals with fast rise times the tube will conduct

within nanoseconds after the DC firing voltage is

reached, however the fast rise time can allow the

signal to reach several times the DC firing volt-

age in a few nanoseconds. [11] To specify a gas

tube that will provide effective transient protec-

tion it is necessary to know the magnitude and

rise time of the transients it must suppress.

Once a gas tube begins conduction in the arc re-

gime, the voltage drop across the device is

typically between 10 V to 30 V. Conduction will

continue until the voltage falls below the mini-

mum extinguishing voltage (typically 60% to

70% of the DC firing voltage). [13] If sufficient

current (as little as 10 µA for some devices) is

available the gas tube can continue to operate in

the glow regime with between 50 V to 100 V

across its terminals. [11] It is important that the

signal fall below this voltage after a transient to

extinguish this follow current. In T1/E1 applica-

tions the 3 V AMI encoded signal will not

sustain follow current, although follow current is

a problem in AC power line protection applica-

tions.

Gas tube arrestors have high surge current han-

dling capability (e.g., 15 kA for an 8/20 µS

waveform), but the large current increase that

occurs when the device goes from the insulating

to the conducting state can produce a great deal

of radiated energy potentially upsetting other

system components. [11]

Gas tube arrestors have the lowest capacitance of

the common shunt type protection devices which

makes them ideal for use as primary protectors

in T1 and higher rate digital communications ap-

plications. These devices are often used as

primary protectors because of their high break-

down voltages (at least 60 to 100 V) and large

surge handling capability, but some models are

suitable for use as board level secondary protec-

tors.

Like MOVs, gas tubes require current limiting

protection in communications applications which

are exposed to AC power cross faults. When

subjected to a power cross fault, the gas arrestor

can get very hot and become permanently dam-

aged. The hot expanding gas inside can break

the package seal letting the gas escape and dra-

matically increasing the firing voltage. In some

cases, the hot gas can even shatter the case.

Fuses

Fuses have traditionally been used for inexpen-

sive, non-resettable series overcurrent protection

in telecom applications. A fuse consists of a con-

ductive element of precise length, diameter and

composition designed to melt when subjected to

a specified current flow for a specified period of

time. [3] For North American T1 applications,

UL listed/CSA certified fuses (commonly in a

1/4" x 1/4" cylindrical glass tube package) are

used. For European applications, VDE and/or

SEMKO certified fuses (commonly in a 5mm x

20mm package) are required. However, both

types of fuses are available in other packages.

For example, the TR5 and TE5 fuses from

Wickmann USA use a small, sealed plastic pack-

age making them suitable for automated wave

soldering and cleaning procedures. Because the

parts are pin-compatible, one PCB layout can

support the UL/CSA approved TR5 and the

VDE/SEMKO certified TE5.

Fuses are specified by: an operating current rat-

ing, breaking capacity rating, maximum voltage

rating, and a fusing time characteristic. [3]

The operating current rating defines the maxi-

mum current at which the fuse element is stable

over time. Operating current must be derated for

operation at high ambient temperatures. The fuse

element has a small finite resistance (less than

0.1 Ω) below its rated current and exhibits a

positive temperature coefficient. A current larger

than the rated current causes the element tem-

10

AN34REV1

Share Link: