Aumov LV Ultramov Varistor Design Guide Page 19 Varistor Design Guide For DC & Automotive Applications

DC Application Varistor Design Guide 2015 Littelfuse DC Application Varistor Design Guide Series and Parallel Operation of Varistors In most cases, a designer can select a varistor that meets the desired voltage ratings from the standard models listed in the catalog. Occasionally, however, the standard catalog models do not fit the requirements of the application, either due to voltage ratings or energy/current ratings. When this happens, two options are available: varistors can be arranged in series or parallel to make up the desired ratings or a "special" can be requested from the manufacturer to meet the unique requirements of the application. Series Operation of Varistors Varistors are applied in series for one of two reasons: to provide voltage ratings higher than those available or to provide a voltage rating between the standard model voltages. As a side benefit, higher energy ratings can be achieved with series connected varistors over an equivalent single device. For instance, assume the application calls for a radial leaded varistor with a VDC rating of 75VDC and an I TM peak current capability of 4000A. The designer would like to have the varistor size fixed at 14mm. When we examine the LV UltraMOV Varistor series voltage ratings for 14mm size discs, part number V14E35P has a maximum voltage of 45VDC. In order to support a 75VDC requirement, we will need to place two MOVs in series. In this basic example, we would have the additive effects of both varistors to get a total stand-off voltage of 45V + 45V = 90VDC. Therefore, we get greater than 20% tolerance headroom over 75VDC, so this solution should be okay. The clamping voltage (V C ) is now the sum of the individual varistor clamping voltages or 220V at 10A. The peak current capability is still 4000A because the surge current will be conducted through both varistors in series mode. Parallel Operation of Varistors Application requirements may necessitate higher peak currents and energy dissipation than the high energy series of varistors can supply individually. When this occurs, the logical alternative is to examine the possibility of configuring varistors in parallel. Fortunately, all Littelfuse varistors have a property at high current levels that makes this feasible. This property is the varistor's series resistance, which is prominent during the "upturn region" of the V-I characteristic. This upturn is due to the inherent linear resistance component of the varistor characteristic. It acts as a series balancing (or ballasting) impedance to force a degree of sharing that is not possible at lower current For example, at a clamp voltage of 600V, the difference in current between a maximum specified sample unit and a hypothetical 20% lower bound sample would be more than 20 to 1. Therefore, there is almost no current sharing and only a single varistor carries the current. Of course, at low current levels in the range of 10A-100A, this may well be acceptable. Series and Parallel Operation of Varistors 18

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