Varistor Metal Oxide Varistor Products Page 8 Varistor Metal-Oxide Varistor Products

2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 09/14/17 Metal-Oxide Varistors (MOVs) 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 -cm. Designing a varistor for a given nominal varistor voltage, ( V N ), is basically a matter of selecting the device thickness 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: The varistor voltage, ( V N ), is defined as the voltage across a 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. 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 N O grains. 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 N O grains, thus forming a space 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 3 . In 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 INTERGRANULAR BOUNDARY CURRENT ELECTRODES SCHEMATIC DEPICTION OF THE MICROSTRUCTURE OF A METAL-OXIDE VARISTOR, GRAINS OF CONDUCTING ZnO (AVERAGE SIZE d) ARE SEPARATED BY INTERGRANULAR BOUNDARIES. d FIGURE 3. between electrodes and, varistor thickness, D = (n + 1)d where, d = average grain size V N d 3 ----------------- R X V I --- - = TABLE 1. VARISTOR VOLTAGE AVERAGE GRAIN SIZE n GRADIENT DEVICE THICKNESS VOLTS MICRONS V/mm AT 1mA mm 150V RMS 20 75 150 1.5 25V RMS 80 (Note) 12 39 1.0 NOTE: Low voltage formulation. Varistor Characteristics, Terms and Consideration Factors (continued)

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