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TRANSIENT
VOLTAGE SUPPRESSION
Lightning
protection falls into two broad categories—building
protection and circuit protection. When lightning
strikes, it creates an electromagnetic flux that radiates
from the point of impact.
Like
the windings of a transformer, this flux impulse induces
a voltage on nearby conductors and electronic circuits.
Depending on the proximity of the stroke, this transient
voltage can be hundreds or even thousands of volts.
A
number of techniques are available to protect electronic
circuitry from the effects of voltage transients.
These include the use of passive components (resistors
and inductors) or devices with specific conduction
characteristics to limit peak voltages.
The
latter category includes gas discharge tubes (GDTs),
reverse voltage breakdown diodes (TVSs), and zinc
oxide varistors (MOVs). We’ll just give you an
overview for now, but next month, Joe DiBartolomeo
starts a four-part MicroSeries with an in-depth look
at surge suppression.
A
GDT is a sealed tube containing an inert gas and two
electrodes. When a high voltage appears across the
terminals, the gas ionizes and a spark bridges the
gap between the terminals, allowing current to flow.
The
gas tube is like a crowbar device that short circuits
the applied voltage down to less than 20 V. GDTs have
very high surge capacity—on the order of 20 kA.
When
a GDT is used across the AC power line, however, it
must be combined with a circuit breaker. Once triggered
by a transient, the GDT’s 20-V clamping action
effectively shorts out the 120 VAC as well.
The
only way to reset the GDT is by blowing the breaker.
GDTs are robust and efficient devices, but resetting
the breaker from the crowbar action is a nuisance.
Typically,
GDTs are used as main-power lightning arresters—often
referred to as primary transient protection. Because
their function involves a physical spark gap, they
generally trigger at higher voltages than other protection
devices. They can handle high current surges repeatedly
without degradation.
Unfortunately,
since their operation usually results in tripping
the circuit breaker, GDTs are typically reserved for
applications where a crowbar across the power line
is a benefit rather than a nuisance.
Avalanche
diodes are called by many different names (e.g., SAD,
Transorb, TVS, etc). Basically, they’re all just
specialized zeners.
Their
large PN junction blocks current flow until the voltage
reaches a specific level when there is an avalanche
of current flow. While considerably faster than GDTs,
avalanche diodes are relatively low-current devices
by comparison.
Their
clamping characteristics are repeatable and do not
degrade with continuous use (unless you exceed their
surge-current rating). Avalanche diodes are ideal
for low-voltage logic protection.
While
low-voltage avalanche diodes have some application
as secondary surge protection, they are primarily
used to protect semiconductor circuits from fast transients
and ESD (electrostatic discharge). Avalanche diodes
are frequently integrated within semiconductor components
(e.g., communications line drivers) as well as attached
across I/O lines and connecting wires.
Since
they’re designed to instantly clamp a transient
and sacrifice themselves, avalanche diodes should
be thought of as final protection.
MOVs
are made of grains of zinc oxide bonded together in
a disc form. They exhibit basic PN-junction zener
characteristics. The typical 130-VRMS MOV (170 Vpeak)
has an initial conduction point of 205 V at 1 mA.
Within 25 ns of reaching breakdown voltage, the internal
resistance reduces from 5 MW to a level where as much
as 10 kA can flow.
At
this maximum current threshold, however, the MOV’s
clamping voltage can go as high as 600 V. This clamping
threshold is determined by the MOV’s grain density,
and there is a wide range of operating voltages. MOVs
are ideal for use both on the AC power line as well
as low-voltage logic.
Because
gas-tube crowbars and avalanche diodes need replacement
when they sacrifice themselves, MOVs are used in 99%
of power-protection products. Quite often, they’re
the only protective component in the device.
The
only real downside to MOVs is that repeated high-current
surges degrade their performance over time. In an
application where a wide level of transient and surges
are expected, GDTs are often used along with the MOVs.
