Varistor Metal Oxide Varistor Products Page 4 Varistor Metal-Oxide Varistor Products

2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 09/14/17 Metal-Oxide Varistors (MOVs) Transient Threats - What Are Transients? Voltage transients are defined as short duration surges of electrical energy and are the result of the sudden release of energy that was previously stored, or induced by other means, such as heavy inductive loads or lightning strikes. In electrical or electronic circuits, this energy can be released in a predictable manner via controlled switching actions, or randomly induced into a circuit from external sources. Repeatable transients are frequently caused by the operation of motors, generators, or the switching of reactive circuit components. Random transients, on the other hand, are often caused by Lightning (Figure 1) and Electrostatic Discharge (ESD) (Figure 2). Lightning and ESD generally occur unpredictably, and may require elaborate monitoring to be accurately measured, especially if induced at the circuit board level. Numerous electronics standards groups have analyzed transient voltage occurrences using accepted monitoring or testing methods. The key characteristics of several transients are shown below in Table 1. t1 t2 Vp Vp/2 t Figure 1. Lightning Transient Waveform VOLTAGE CURRENT RISE-TIME DURATION Lighting 25kV 20kA 10 s 1ms Switching 600V 500A 50 s 500ms EMP 1kV 10A 20ns 1ms ESD 15kV 30A <1ns 100ns Table 1. Examples of transient sources and magnitude Characteristics of Transient Voltage Spikes Transient voltage spikes generally exhibit a "double exponential" wave form, shown in Figure 1 for lightning and figure 2 for ESD. The exponential rise time of lightning is in the range 1.2 s to 10 s (essentially 10% to 90%) and the duration is in the range of 50 s to 1000 s (50% of peak values). ESD on the other hand, is a much shorter duration event. The rise time has been characterized at less than 1 ns. The overall duration is approximately 100ns. 100 90 % I 30n 60n t r = 0.7 to 1.0ns Current (I) % 30 60 Figure 2. ESD Test Waveform Why are Transients of Increasing Concern? Component miniaturization has resulted in increased sensitivity to electrical stresses. Microprocessors for example, have structures and conductive paths which are unable to handle high currents from ESD transients. Such components operate at very low voltages, so voltage disturbances must be controlled to prevent device interruption and latent or catastrophic failures. Sensitive devices such as microprocessors are being adopted at an exponential rate. Microprocessors are beginning to perform transparent operations never before imagined. Everything from home appliances, such as dishwashers, to industrial controls and even toys, have increased the use of microprocessors to improve functionality and efficiency. Vehicles now employ many electronics systems to control the engine, climate, braking and, in some cases, steering systems. Some of the innovations are designed to improve efficiency, but many are safety related, such as ABS and traction control systems. Many of the features in appliances and automobiles use modules which present transient threats (such as electric motors). Not only is the general environment hostile, but the equipment or appliance can also be sources of threats. For this reason, careful circuit design and the correct use of overvoltage protection technology will greatly improve the reliability and safety of the end application. Table 2 shows the vulnerability of various component technologies. Device Type Vulnerability (volts) VMOS 30-1800 MOSFET 100-200 GaAsFET 100-300 EPROM 100 JFET 140-7000 CMOS 250-3000 Schottky Diodes 300-2500 Bipolar Transistors 380-7000 SCR 680-1000 Table 2: Range of device vulnerability. Introduction to Overvoltage Suppression (continued)

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