Conformity - November 2008 - (Page 27) A variety of full wave models have been published for ESD prediction. As a result of cooperation with the authors of this article [4], a model has been published that is suitable to operate on standard electromagnetic software such as CST Microwave Studio, and that also has been ported to FlowEMC (Microstripes). This model emulates the structure of the ESD generator coarsely and excites the structure with a step function having 1 ns rise time. Here it contrasts from real ESD generators in an important point. The voltage collapse in the high voltage relays, usually used, occurs in 50-100 ps. As the standard asks for 700 – 1000 ps rise time of the current at the discharge tip, a low pass filter is present in every ESD generator. Thus, the injected current mainly contains frequency components below 300 MHz (it may also contain some higher frequencies if the current rise is not smooth). However, the fast voltage collapse in the relay excites the structure of the ESD generators. This causes radiation of strong ESD transient fields with frequency components greater than 1 GHz, often leading to soft errors. It needs to be noted that this is strongly affected by the detailed construction of each ESD generator model. Due to the fast voltage collapse, ESD generator models should model the relay and the associated low pass filters, including its radiating structures, with sufficient detail to predict the radiated fields accurately. Without correctly modeling the voltage collapse in the relay, the results may not match the high frequency behavior. An example is shown in Figure 1. The simulated wave form is smooth relative to the measured wave form; thus, the high frequency components that are present during testing have not been modeled with sufficient accuracy. A highly detailed approach has been published by K.Wang [2]. However, this was based on special FDTD codes from Zeland Software and a code developed by the MST EMC laboratory that allowed time varying material constants to emulate the breakdown within the relay. To expand the usability of a detailed modeling approach, we created a detailed model, using more generally used software Microstripes. This model includes the details of the relay and the low pass filter and has been verified by: • Injected current • Voltages induced in a loop close to the generator • Currents on a cable attached to a small, hand held device • Voltages induced within a small mobile device. This article explains the model, its results, and discusses the limits. Numerical Model Our objective is to create a full wave model that allows simulating the coupling into systems for frequencies up to at least 2 GHz, having calculation times of hours (not days). An overview of the model is shown in Figure 2. An off-the-shelf ESD generator was used for the full wave model. The model consists of the following major parts: • Voltage source: As source, a step function has been used. Its rise-time is about 200 ps. It has been designed based on an integrated Gaussian pulse to ensure a smooth rising edge. Alternative approaches would base the rise on the functional behavior of the spark resistance inside the relay (e.g., as expressed by Rompe & Weizel’s law). The shape and rise time of this pulse allows us to adopt the RF spectrum of the ESD generator to approximately accommodate other ESD generator models. • Low pass filter: The relay connects to the other parts of the ESD generator via an R-C-R low pass filter. The resistors are discrete carbon composition resistors, while the november 2008 Conformity 27 http://www.fair-rite.com http://www.fair-rite.com http://www.fair-rite.com
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