AN34 Просмотр технического описания (PDF) - Cirrus Logic

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AN34

Secondary Line Protection for T1 and E1 Line Cards

Cirrus Logic 

T1/E1 Line Protection

efficient. The negative temperature coefficient

can cause thermal runaway because as current

flow heats the device its resistance decreases

thereby increasing the current and further heating

the device. Thermal runaway ultimately results

in a short circuit failure of the device. For this

reason, MOVs used to provide the low clamping

voltages for board level secondary protection

must be protected from the continuous current

possible during an AC power cross fault. A fuse

or PTC resistor can provide this overcurrent pro-

tection.

Although MOVs are commonly used in analog

subscriber line applications because of their low

cost and high surge handling capability, they are

not suitable for use on digital T1 line cards be-

cause of their high parasitic capacitance.

Spark Gaps

The spark gap is another shunt (or parallel) pro-

tector which provides over-voltage limiting.

The spark gap consists of two movable elec-

trodes separated by an air gap. A spark gap is

specified by its breakdown voltage which is the

potential difference between the plates just large

enough to breakdown the air gap and trigger a

spark across the gap. [11] The gap size is ad-

justed to control the breakdown voltage of the

device which is typically several hundred Volts.

When the voltage across the gap is below the

breakdown voltage the device presents a high

impedance between its terminals. Once the ter-

minal voltage exceeds the breakdown voltage the

arc which results across the terminals creates a

low impedance path for current which persists

until the current falls below the minimum extin-

guishing current level. During operation in the

arc regime, the voltage across the spark gap is

limited at just above the breakdown voltage.

The carbon block arrestor is inexpensive and has

fast response time, but also has significant disad-

vantages. It must be replaced after handling a

AN34REV1

high energy transient because of electrode ero-

sion which widens the gap and significantly

increases the firing voltage leaving downstream

circuitry unprotected. [11] Also, the accumula-

tion of carbon dust between the electrodes after

conduction eventually leads to erratic operation.

Because spark gaps provide clamping at high

voltages and handle high currents, they are used

as primary protection devices rather than board

level secondary protectors.

Gas Filled Surge Arrestors

Gas filled arrestors, commonly called gas tubes,

are devices which incorporate electrodes inside a

sealed glass or ceramic package filled with a no-

ble gas. Gas filled arrestors are parallel

over-voltage protectors based on the same prin-

ciple as the spark gap without the mechanical

instability and contamination problems common

in air gap devices. Since these arrestors can fail

as an open circuit, some gas tube devices also

include a parallel air gap for backup protection.

Gas filled protectors are specified by: DC firing

voltage, surge firing voltage, arc extinguishing

voltage, max. terminal voltage (during an arc),

and max. surge current. [13]

The DC firing voltage is the voltage required to

trigger an arc in response to a slowly increasing

gap voltage (typically 75 V to 300 V specified at

dv/dt = 500 V/s). Since the gas in the gap re-

quires time to ionize, the breakdown voltage

increases for signals with faster rise times. [13]

Often, the surge firing voltage (typically speci-

fied at dv/dt = 1000 V/µS) is several times

higher than the DC firing voltage. However the

response time also decreases for signals with

faster rise times. Both of these factors determine

how much transient energy is allowed to pass

through a gas tube. For signals with slow rise

times the tube will conduct within microseconds

after the DC firing voltage is reached. For sig-

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