Varistor Metal Oxide Varistor Products Page 7 Varistor Metal-Oxide Varistor Products

2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 09/14/17 Metal-Oxide Varistors (MOVs) Physical Properties MOVs are designed to protect sensitive circuits against external transients (lightning) and internal transients (inductive load switching, relay switching and capaci- tor discharges). And other high level transients found in industrial, AC line application or lower level transients found in automotive DC line applications with peak current rating ranging from 20A to 500A and peak energy rating from 0.05J - 2.5J. An attractive property of the MOV is that the electrical char- acteristics are related to the bulk of the device. Each ZnO grain of the ceramic acts as if it has a semiconductor junc- tion at the grain boundary. A cross-section of the material is shown in Figure 2, which illustrates the ceramic micro- structure. Varistors are fabricated by forming and sintering Zinc Oxide-based powders into ceramic parts. These parts are then electroded with either thick film Silver or arc/flame sprayed metal. The ZnO grain boundaries can be clearly observed. Since the nonlinear electrical behavior occurs at the boundary of each semiconducting ZnO grain, the varistor can be considered a "multi-junction" device composed of many series and parallel connections of grain boundaries. Device behavior may be analyzed with respect to the details of the ceramic microstructure. Mean grain size and grain size distribution play a major role in electrical behavior. The varistor body structure consists of a matrix of con- ductive Z N O grains separated by grain boundaries provid- ing P-N junction semiconductor characteristics. These boundaries are responsible for blocking conduction at low voltages and are the source of the nonlinear electrical conduction at higher voltages. The symmetrical, sharp breakdown characteristics shown in Figure 1, enable the varistor to provide excellent tran- sient suppression performance. When exposed to high voltage transients the varistor impedance changes many orders of magnitude from a near open circuit to a highly conductive level, thus clamping the transient voltage to a safe level. The potentially destructive energy of the incom- ing transient pulse is absorbed by the varistor, thereby protecting vulnerable circuit components. Since electrical conduction occurs, in effect, between Z N O grains distributed throughout the bulk of the device, the Littelfuse Varistor is inherently more rugged than its single P-N junction counterparts, such as Zener diodes. In the varistor, energy is absorbed uniformly throughout the body of the device with the resultant heating spread evenly through its volume. Electrical properties are con- trolled mainly by the physical dimensions of the varistor body which is sintered in various form factors such as discs, chips and tubes. The energy rating is determined by volume, voltage rating by thickness or current flow path length, and current capability by area measured normal to the direction of current flow. Varistor Characteristics, Terms and Consideration Factors V I PER VERT DIV 1mA PER HORIZ PER STEP DIV 50V gm PER DIV FIGURE 1. TYPICAL VARISTOR V-I CHARACTERISTIC 100 FIGURE 2. OPTICAL PHOTOMICROGRAPH OF A POLISHED AND ETCHED SECTION OF A VARISTOR

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