EMC & Circuit Protection

Making the right choice

Rob Ashton reviews and compares some of the protection products available to system designers

C

ustomer satisfaction demands that electrical systems from mobile phones and

entertainment systems to high-end computers not affected by electrostatic discharge (ESD). To ensure system robustness against ESD, products are tested, against standards such as IEC 61000-4-2 and system designers use a variety of techniques to ensure compatibility with prevailing ESD standards including: case design, board design, component selection and even software fixes. One important tool is the use of protection components on critical circuit nodes such as input and output connectors. Protection components for ESD are often called transient voltage suppressors (TVS).

TVS basics for ESD

Many ICs have potentially sensitive inputs making them susceptible to damage if they are subject to an input voltage significantly above the normal

as occurs during an ESD stress. Between the normal operating voltage range and the onset of device damage is a region of safe overvoltage. There is some overlap in the safe overvoltage and device damage regions because larger over voltages can be tolerated if their duration is only very short. The job of a TVS is to maintain Vinput within the safe overvoltage range during an ESD event, without compromising system performance during normal operation. The TVS device is placed near to where an ESD event is likely to enter a system and is intended to limit the voltage on sensitive nodes and to direct current to less sensitive nodes such as ground. To perform this function the TVS must have high resistance for the normal

operating voltage range. Outside of the normal operating voltage range the TVS must have low resistance, such that current is directed away from the sensitive node and voltage transients are limited. The basic requirements for a TVS are

application specific but in general are:



Ability to survive the expected ESD stress

   

High resistance (low leakage) in the normal voltage range

Low resistance outside of the normal voltage range

Turn on voltages that are correct for the application

Fast transition from high resistance to low resistance during stress



Capacitance that is not too high for the intended application

There are two classification

categories which need to be understood before comparing specific types of TVS devices:

Unidirectional versus Bidirectional

Both unidirectional and bidirectional TVS devices can protect for both positive and negative stress. The differences between the terms is best understood in terms of voltage range over which the TVS maintains a high resistance, low leakage state. This voltage range determines the types of circuit nodes the TVS device can be used to protect. A bidirectional TVS has symmetrical properties about zero volts, as can be seen in Figure1. Bidirectional devices are best for

Figure 1: Unidirectional and bidirectional TVS devices compared

12 April 2010

Components in Electronics

protecting circuit nodes whose voltage is symmetric, or bidirectional, around zero volts. Unidirectional TVS products

meanwhile have asymmetric behaviour around zero volts as shown in Figure 1. Unidirectional TVS devices are perfect for protecting circuit nodes whose voltage always has the same polarity, for example a 0V to 5 V signal.

Clamping versus Crowbar

Voltage clamping devices work by changing from a high resistance at low voltage to a low resistance above a turn on voltage without a region of negative resistance as shown in Figure 2. The device works by clamping the voltage above the turn-on voltage by providing a low resistance path to ground. Crowbar devices also have high

resistance at low voltages. At higher voltage, however, a new conductance mechanism is triggered and an increase in current is accompanied by a drop in voltage. A crowbar device therefore has a region of negative resistance. For some crowbar devices the trigger voltage can be quite high. If the crowbar TVS triggers fast enough it can often provide protection, even though the voltage appears to have reached a level that could cause damage. Crowbar devices are sometimes called ‘snapback’ devices because the voltage snaps back. There are three primary technologies

used for ESD TVS devices: varistors, polymers and silicon diodes.

Metal Oxide Varistors (MOV)

At low currents and voltages varistors have a high resistance but at higher voltages and currents their resistance drops dramatically; they are therefore voltage clamping devices. Varistors are bidirectional and are

manufactured with a very wide range of current and voltage capacities for

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