Mlv Design Guide Page 2 Introduction-Common Transient Events

Here are some examples of the voltages that can be generated, depending on the relative humidity (RH): Walking across a carpet: 35kV @ RH = 20%; 1.5kV @ RH = 65% Walking across a vinyl floor: 12kV @ RH = 20%; 250V @ RH = 65% Worker at a bench: 6kV @ RH = 20%; 100V @ RH = 65% Opening a vinyl envelope: 7kV @ RH = 20%; 600V @ RH = 65% Picking up a polymer bag from a desk 20kV @ RH = 20%; 1.2kV @ RH = 65% The level of ESD generated by everyday activities like these can be far higher than the vulnerability threshold of standard semiconductor protection technologies. Following figure illustrates the ESD waveform defined in the IEC 61000-4-2 test specification. 100 90 % Current (I) % tr = 0.7 to 1.0ns 30ns 60ns I 60 I 30 Figure 1. ESD Test Waveform Inductive Load Switching The switching of inductive loads generates high energy transients that increase in magnitude with increasingly heavy loads. When the inductive load is switched off, the collapsing magnetic field is converted into electrical energy that takes the form of a double exponential transient Depending on the source, these transients can be as large as hundreds of volts and hundreds of amps, with durations of 400ms. Typical sources of inductive transients include: Generators Motors Relays Transformers Sources like these are extremely common in electrical Surface Mount Multilayer Varistors (MLVs) Design Guide 2016 Littelfuse Multilayer Varistors (MLVs) Design Guide www.littelfuse.com Introduction - Common Electrical Transient Events Introduction Over many decades, the demands imposed on electronic systems designers have remained consistent. The challenge to design circuits and systems that include more functionality and capabilities in smaller sizes continues. This ongoing miniaturization trend has given rise to other challenges. As the size of the components used continues to shrink, their sensitivity to electrical stresses increases. Integrated circuits, especially microprocessors, have become instrumental components in modern electronic systems. However, their vulnerability to common electrical threats, such as electrostatic discharge (ESD) events, has forced circuit designers to take special measures to protect these devices. Modern vehicles employ many electronic systems to control their various parts, including the engine, climate control, braking and, in some cases, steering systems. Transient electrical threats, such as in-rush currents for starting electric motors or ESD events, can also prove detrimental to sensitive automotive electronic systems. A Multilayer Varistor (MLV) is a compact, surface-mountable chip that is voltage dependent, nonlinear, and bidirectional. It has an electrical V-I characteristic similar to back-to-back zener diodes, offering symmetrical protection in both forward and reverse directions. The sharp, symmetrical breakdown characteristics of an MLV device provide excellent protection from damaging voltage transients. When exposed to high voltage transients, the impedance of the MLV changes by many orders of magnitude, from a near open circuit to a highly conductive state. MLV technology has high surge handling capability and exhibits extremely low leakage current, so it is a preferred solution commonly used for protection against ESD events, inductive switching transients, and general surge protection. Common Electrical Transient Events Voltage transients are defined as short-duration surges of electrical energy and are the result of the sudden release of energy previously stored or induced by other means, such as heavy inductive loads or lightning. In electrical or electronic circuits, this energy can be released in a predictable manner through controlled switching actions or randomly induced into a circuit from external sources. ESD (Electrostatic Discharge) Electrostatic discharge is characterized by very fast rise times and very high peak voltages and currents. This energy is the result of an imbalance of positive and negative charges between objects. 2

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