Electrostatic Discharge Modelling

Electrostatic Discharge (ESD) is the abrupt release of charge from one object (often a person) to another. Such a discharge can permanently damage or otherwise disturb the function of sensitive electronic circuits. Electronic products are tested for ESD immunity to insure their continued reliable operation if subjected to realistic levels of ESD after being placed in service. The European Union's EMC Directive mandates ESD immunity testing for virtually all electrical and electronic products as a condition for obtaining the CE Mark before shipping to a member state of the European Union. Generic immunity standards, product standards and product family standards require that ESD tests be performed in accordance with specific basic EMC Standards IEC 801-2, IEC 61000-4-2, or EN 61000-4-2.

ESD generators are widely used for testing the robustness of electronic equipment against human-metal ESD, which can disturb systems by its current and the associated fields. To predict the susceptibility of a system, a sufficiently accurate model of the ESD generator (gun) is needed. A simplified ESD generator model [1] is shown in Figure 1. The model contains metal and dielectric parts which reproduce the typical form of an ESD generator, and lumped circuit elements to simulate; a reference discharge current. The lumped element values were chosen to simulate the body and arm effects of a person causing the ESD. An ideal 25 Ohm current source excites the model using a user defined pulse with a rise-time of 1 ns, which simulates the slow charging, switching and rapid discharging behaviour of the ESD generator.

The model of the ESD generator was optimised to match measurements of current injection by the gun into a large metallic wall of a shielding enclosure, inside which an oscilloscope was placed to measure the currents in a target. Measured and simulated results from the optimised ESD model are shown in Figure 2. The fast (< 1 ns) rise time is reproduced well, and the simulated signal shows similar characteristics to the measured one in the first 15 ns before again matching the decaying signal very well to 200 ns. The differences are mainly caused by the imprecise modelling of the geometry of the grounding strap.

A classical example of this sort of ESD susceptibility study is the penetration of an ESD generated field through a slot in a shielded enclosure [2]. In the set-up shown in Figure 3, the electromagnetic field due to the ESD current between the points 1 and 3 excites the enclosure, and the square loop inside the cavity senses the internal electromagnetic field.

A comparison of numerical results obtained using CST MWS and measured results is shown for the first 10 ns in Figure 4. The agreement is close in magnitude and time, though the measured results show more ripple than the simulated ones.

The close agreement between measured and simulated results shows that CST MICROWAVE STUDIO® is ideally suited to EMC simulations where a user defined time-domain signal - in this case the pulse output of an ESD generator - is required.

This model was created and measured by Spartaco Caniggia and Fransescaromana Maradei in work supported by Italtel S.p.A. Thanks to the authors for allowing us to publish these results.

References:

[1] S. Caniggia, F. Maradei, "Circuit and Numerical Modeling of Electrostatic Discharge Generators," IEEE Transactions on Industry Applications, Vol. 42, pp. 1350-1357, Nov. 2006.

[2] G. Cerri, R. de Leo, R. de Rentiis, V. M. Primiani, "ESD Field Penetration Through Slots into Shielded Enclosures: A Time Domain Approach," IEEE Trans. on EMC, Vol. 39, No. 4, pp. 377-386, Nov. 1997.