AN9768 Datasheet, PDF (3/8 Pages) Littelfuse – Transient Suppression Devices and Principles

Application Note 9768
corresponding change is only 330V to 400V. In other words,
a variation of only 21% in the protective level achieved with a
nonlinear suppressor occurs for a 10 to 1 error in the
assumption made on the transient parameters, in contrast to
a 447% variation in the protective level with a linear
suppressor for the same error in assumption. Nonlinear
voltage-clamping devices give the lowest clamping voltage,
resulting in the best protection against transients.
Crowbar Devices
This category of suppressors, primarily gas tubes or carbon-
block protectors, is widely used in the communication field
where power-follow current is less of a problem than in
power circuits. Another form of these suppressors is the
hybrid circuit which uses solid-state or MOV devices.
In effect, a crowbar device short-circuits a high voltage to
ground. This short will continue until the current is brought
to a low level. Because the voltage (arc or forward-drop)
during the discharge is held very low, substantial currents
can be carried by the suppressor without dissipating a
considerable amount of energy within it. This capability is a
major advantage.
Volt-Time Response - When the voltage rises across a spark
gap, no significant conduction can take place until transition
to the arc mode has occurred by avalanche breakdown of
the gas between the electrodes.
Power-Follow - The second characteristic is that a power
current from the steady-state voltage source will follow the
surge discharge (called “follow-current” or “power-follow”).
Voltage-Clamping Devices
To perform the voltage limiting function, voltage-clamping
devices at the beginning of the section depend on their
nonlinear impedance in conjunction with the transient source
impedance. Three types of devices have been used: reverse
selenium rectifiers, avalanche (Zener) diodes and varistors
made of different materials, i.e., silicon carbide, zinc oxide,
etc. [1].
Selenium Cells - Selenium transient suppressors apply
the technology of selenium rectifiers in conjunction with a
special process allowing reverse breakdown current at high-
energy levels without damage to the polycrystalline
structure. These cells are built by developing the rectifier
elements on the surface of a metal plate substrate which
gives them good thermal mass and energy dissipation
performance. Some of these have self-healing
characteristics which allows the device to survive energy
discharges in excess of the rated values for a limited number
of operations characteristics that are useful, if not “legal” in
the unsure world of voltage transients.
The selenium cells, however, do not have the clamping
ability of the more modern metal-oxide varistors or
avalanche diodes. Consequently, their field of application
has been considerably diminished.
Zener Diodes - Silicon rectifier technology, designed for
transient suppression, has improved the performance of
regulator-type Zener diodes. The major advantage of these
diodes is their very effective clamping, which comes closest
to an ideal constant voltage clamp.
Since the diode maintains the avalanche voltage across a
thin junction area during surge discharge, substantial heat is
generated in a small volume. The major limitation of this type
of device is its energy dissipation capability.
Silicon Carbide Varistors - Until the introduction of metal-
oxide varistors, the most common type of “varistor” was
made from specially processed silicon carbide. This material
was very successfully applied in high-power, high-voltage
surge arresters. However, the relatively low a values of this
material produce one of two results. Either the protective
level is too high for a device capable of withstanding line
voltage or, for a device producing an acceptable protective
level, excessive standby current would be drawn at normal
voltage if directly connected across the line. Therefore, a
series gap is required to block the normal voltage.
In lower voltage electronic circuits, silicon carbide varistors
have not been widely used because of the need for using a
series gap, which increases the total cost and reproduces
some of the characteristics of gaps described earlier.
However, this varistor has been used as a current-limiting
resistor to assist some gaps in clearing power-follow current.
Metal-Oxide Varistors - A varistor functions as a nonlinear
variable impedance. The relationship between the current in
the device, I, and the voltage across the terminals, V is
typically described by a power law: I = kV ^α. While more
accurate and more complete equations can be derived to
reflect the physics of the device, [2, 3] this definition will
suffice here. A more detailed discussion will be found in
Application Note AN9767, “Littelfuse Varistors - Basic
Properties, Terminology and Theory”.
The term α (alpha) in the equation represents the degree of
nonlinearity of the conduction. A linear resistance has an
α = 1. The higher the value of a, the better the clamp, which
explains why α is sometimes used as a figure of merit. Quite
naturally, varistor manufacturers are constantly striving for
higher alphas.
This family of transient voltage suppressors are made of
sintered metal oxides, primarily zinc oxide with suitable
additives. These varistors have α values considerably
greater than those of silicon carbide varistors, typically in the
range of an effective value of 15 to 30 measured over
several decades of surge current.
The high exponent values (α) of the metal-oxide varistors
have opened completely new fields of applications by
providing a sufficiently low protective level and a low standby
current. The opportunities for applications extend from low-
power electronics to the largest utility-type surge arresters.
10-104