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MMT10B310T3
Thyristor Surge Protectors
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Prepared by: Alfredo Ochoa, Alex Lara, and Gabriel Gonzalez
Thyristor Applications Engineers
INTRODUCTIONThe current Telecom infrastructure in special the two
wire systems are exposed to the outside environment and
are susceptible to any electromagnetic disturbance caused
by lightning strikes during a thunderstorm or by Power
Induced due to High Voltage power lines. These situation
generates in the Telecom wire system very fast Transients
of V oltage of Several hundreds or Volts that can harm
people if they are using an unprotected telephone handset
during these situations, and destroying the telecom
equipment connected to the telephone line.
Lightning causes a large shift in ground potential near a
lightning strike. These shifts can reach as high as 50 KV.
Large magnetic fields from lightning strikes may also cause
problems.
In addition to the traditional gas–discharge tubes (GDT),
metal oxide varistors (MOVs), and fuses, very effective
semiconductor devices are appearing that protect on–board
electronics against real–world insults. Protecting your
products against lightning, electrostatic discharge (ESD),
electrical fast transient (EFT), inductive load switching, and
AC line fluctuations has the obvious advantage of making
them more rugged, thereby lowering your warranty costs.
A gas discharge tube (GDT) is simply two electrodes that
are held at a close distance within a gas filled tube. When
high voltages (150 V
to the electrodes, the gas inside ionizes and current begins
to flow. In the off–state, the resistance across the tube is
very high; when conducting, the resistance is low. GDTs
are relatively slow, but can handle very large levels of
surge. Unfortunately, some gas tubes’ electrodes burn out
after a few hundred hits.
A MOV is an inexpensive, rugged device capable of
absorbing very large currents without damage. Every time
they conduct, however, their capacity degrades and they are
known to fail
Semiconductor–based suppressors, on the other hand, do
not have a wear–out mechanism, if properly specified. The
MOV, a compressed–powder products is
thought to have slower reaction time than semiconductor
protection devices, but this is not necessarily true.
Transient voltage suppressor (TVS) are
specifically designed to provide overvoltage protection.
TVS diodes are characterized by their high surge capability,
low operating and clamping voltages and virtually
instantaneous response time. These characteristics make
them ideal for use as board level protection.
Ideally, the capacitance of a shunt protection device
should be zero. Typical numbers of gas–discharge tubes are
from 1 to 1.25 pf. Semiconductor devices range from 100 to
5000 pf.
Now, one of the most reliable semiconductor device used
in the actual telecom infrastructure for protection purposes
against overvoltage conditions is the TSPD. TSPD is the
abbreviation of “Thyristor Surge Protective Device” and it
is a two terminal solid state device capable to drain a surge
current pulse to ground when a transient voltage appears in
between its two terminals when a specific maximum
voltage delimited by the maximum breakover voltage of
the device is reached.
