TMOV25SP385M Littelfuse Inc, TMOV25SP385M Datasheet - Page 15

• Manufacturer Part Number: TMOV25SP385M
• Description: TMOV VARISTOR PB FREE 25S
• Manufacturer: Littelfuse Inc
• Series: iTMOV®r

Specifications of TMOV25SP385M

• Varistor Voltage: 682V
• Current-surge: 20kA
• Number Of Circuits: 1
• Maximum Ac Volts: 385VAC
• Energy: 430J
• Package / Case: Disc 25mm 3-Lead
• Suppressor Type: Varistor
• Peak Surge Current @ 8/20µs: 20000A
• Varistor Case: 25mm DISC
• Clamping Voltage Vc Max: 1010V
• Peak Energy (10/1000us): 430J
• Voltage Rating Vdc: 682V
• Voltage Rating Vac: 385V
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Maximum Dc Volts: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Specifications are subject to change without notice.
Please refer to www.littelfuse.com for current information.
©2009 Littelfuse, Inc.
Varistor Microstructure
The bulk of the varistor between contacts is comprised of
ZnO grains of an average size "d" as shown in the schemat-
ic model of Figure 3. Resistivity of the ZnO is <0.3 Ω–cm.
Designing a varistor for a given nominal varistor voltage,
(V
such that the appropriate number of grains, (n), are in se-
ries between electrodes. In practice, the varistor material is
characterized by a voltage gradient measured across its
thickness by a specific volts/mm value. By controlling
composition and manufacturing conditions the gradient
remains fixed. Because there are practical limits to the
range of thicknesses achievable, more than one voltage
gradient value is desired. By altering the composition of
the metal oxide additives it is possible to change the grain
size "d" and achieve the desired result.
A fundamental property of the ZnO varistor is that the
voltage drop across a single interface "junction" between
grains is nearly constant. Observations over a range of
compositional variations and processing conditions show a
fixed voltage drop of about 2V-3V per grain boundary
junction. Also, the voltage drop does not vary for grains of
different sizes. It follows, then, that the varistor voltage
will be determined by the thickness of the material and the
size of the ZnO grains. The relationship can be stated very
simply as follows:
and, varistor thickness, D = (n + 1)d
where,
FIGURE 3.
N
), is basically a matter of selecting the device thickness
SCHEMATIC DEPICTION OF THE
MICROSTRUCTURE OF A METAL-OXIDE
VARISTOR, GRAINS OF CONDUCTING
ZnO (AVERAGE SIZE d) ARE SEPARATED
BY INTERGRANULAR BOUNDARIES.
R
CURRENT
X
=
V
--- -
I
d = average grain size
≈
between electrodes
ELECTRODES
INTERGRANULAR
BOUNDARY
V
---------------- -
N
3
×
d
d
Varistor Products
Revision: November 5, 2009
11
The varistor voltage, (V
varistor at the point on its V-I characteristic where the
transition (v) is complete from the low-level linear region
to the highly nonlinear region. For standard measurement
purposes, it is arbitrarily defined as the voltage at a current
of 1mA. Some typical values of dimensions for Littelfuse
Varistors are given in Table 1.
NOTE: Low voltage formulation.
Theory of Operation
Because of the polycrystalline nature of metal-oxide semi-
conductor varistors, the physical operation of the device is
more complex than that of conventional semiconductors.
Intensive measurement has determined many of the de-
vice’s electrical characteristics, and much effort continues
to better define the varistor’s operation. However from the
user’s viewpoint, this is not nearly as important as under-
standing the basic electrical properties as they relate to
device construction.
The key to explaining metal-oxide varistor operation lies in
understanding the electronic phenomena occurring near
the grain boundaries, or junctions between the Z
While some of the early theory supposed that electronic
tunneling occurred through an insulating second phase
layer at the grain boundaries, varistor operation is prob-
ably better described by a series-parallel arrangement of
semiconducting diodes. In this model, the grain boundaries
contain defect states which trap free electrons from the
n-type semiconducting Z
charge depletion layer in the ZnO grains in the region adja-
cent to the grain boundaries. (See reference notes on the
last page of this section).
Evidence for depletion layers in the varistor is shown in Fig-
ure 4, where the inverse of the capacitance per boundary
squared is plotted against the applied voltage per boundary.
This is the same type of behavior observed carrier concen-
tration, N, was determined to be about 2 x 1017 per cm
addition, the width of the depletion layer was calculated to
be about 1000 Angstrom units. Single junction studies also
support the diode model.
It is these depletion layers that block the free flow of
carriers and are responsible for the low voltage insulating
behavior in the leakage region as depicted in Figure 5. The
leakage current is due to the free flow of carriers across
VARISTOR
VOLTAGE
150V
25V
VOLTS
RMS
RMS
GRAIN SIZE
AVERAGE
MICRONS
80 (Note)
20
N
), is defined as the voltage across a
TABLE 1.
N
O grains, thus forming a space
75
12
n
GRADIENT
V/mm AT
1mA
150
39
THICKNESS
DEVICE
N
mm
1.5
O grains.
1.0
3
. In