Electro Static Discharge (ESD):
I believe most of us have experienced the static electric shock some point of time and are aware about how static electricity is formed. Hence I'm not going to bore you by going into those theories again. Also we know that the generation of static electric voltage is more effective in a dry climatic condition. For your information, the following table shows different means of static electricity generation and the respective voltage levels (Source: ESD Association).
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Means Of Generation--------------V (10-25% RH)-----V (65-90% RH)
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Walking across carpet------------------35,000-----------------1,500
Walking across vinyl tile---------------12,000-------------------250
Poly bag picked up from bentch------20,000----------------1,200
Getting up from chair with
Urethane foam-----------------------------18,000----------------1,500
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In the manufacturing environment, ICs are only specified to survive 2KV HBM (Human Body Model: standard intended to simulate a person becoming charged and discharging from a bare finger to ground through the circuit under test), although some have been specified as high as 8KV while others - particularly newer parts in very small geometry processes - can be 500V or less.
The standard IEC 61000-4-2 specifies the test methodologies & measurement techniques to test the electronic products (& systems), which simulates the real industrial environment having possible ESD hazards. Electronic products are required to be tested for ESD immunity (IEC 61000-4-2) to insure their continued reliable operation if subjected to ESD after commissioned in the field. The European Union’s EMC Directive 2004/108/EC (replaced 89/336/EEC) mandates ESD immunity testing for virtually all electrical and/or electronic products as a requirement for obtaining the CE Mark before shipping to a member state of the European Union. As per IEC 61000-6-2: 2005, Electromagnetic Compatibility (EMC): Generic Standard, a device shall be tested as per IEC 61000-4-2 to verify that it passes the following test levels (at minimum):
1. Air Discharge: 8KV
2. Direct (Contact) Discharge: 4KV
The standard for ESD, IEC 61000-4-2 specifies the standard strike waveform. It is worth to study the nature of the waveform before designing the necessary protection in the circuit.
The ESD waveform specified in IEC 61000-4-2 has a rise time of 0.7 ns to 1.0 ns, resulting in a noise bandwidth (1/πtr) of approximately 450 MHz. This is an important factor to consider while choosing appropriate circuit components.
Protection against ESD:
There could be several way we could protect our devices from ESD. Where ever applicable, one could use series blocking resistor, ceramic capacitor, opto-isolation, bare guard trace, dual-rail clamping diodes, zener diodes, MOVs, TVS diodes or a combination of some of these techniques. The articles published in the following links could be useful as references:
It is not only about selecting a suitable protection device but also important to pay careful attention to the layout design. It is necessary to follow cautious approach in implementing a proper ground system such that the returning current through the protection circuit doesn't interfere with the rest of the circuit in operation. In case of a TVS, the common practice to refer the protection circuit to chassis ground or, in case of systems with non-conductive enclosures, power ground. Mount the TVS closest (as possible) to the entry point, use short and wide traces and used dedicated ground plane if possible, avoid loops and other best practices of high speed design.
Next, we are going to continue our discussion on "Electrical Transient Burst (EFT)' in part III.
