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APPLICATION NOTE 7 Using the Bead Models Beads are used in many situations today to control EMI. One of the more useful configurations is to provide power supply decoupling on a per board basis. A typical expression is shown above. This is a standard circuit that may be used on a digital or analog PCB power entry. The bead used is a 0.3 OD x 0.1 ID x 0.3 L #73 core; the capacitors are modeled for series resistance and inductance as described in reference [2]. The resulting filter response is plotted versus the number of turns on the bead. An interesting thing happens with this circuit. As the number of turns is increased, the filter attenuation increases at high frequencies as expected; but at 10 kHz, the Q of the first pole also increases. This results in more peaking in the filter response which may or may not cause a problem for the circuit. It is, however, nice to know the full ramifications of increasing turns on the bead. It can also be seen that increasing the turns on the bead above two does not significantly increase the filter rejection. This is due to the effects of the DC bias increasing proportionally to N. The core can only support so many amp-turns before saturation—hence, the diminishing returns of attenuation as the turns are increased above a certain point. The Fair-Rite #43 and #73 bead models presented in this article contains the equivalent schematics and symbol files for use in Schematics-based Design. References [1] Fair-Rite Soft Ferrites, 11th edition catalog, POB J, One Commercial Row, Wallkill, NY 12589 [2] Hageman, S. "Improve Simulation Accuracy When Using Passive Components," The Design Center Source, April 1994, MicroSim Corporation, Irvine, CA. [3] Acknowledgments: Special thanks go to William Kimmel of Kimmel Gerke Associates for laying the groundwork for modeling EMI ferrites in his article, "Wide Frequency Impedance Modeling of EMI Ferrites," published in the IEEE 1994 Symposium on EMC. Mr. Kimmel is a well known EMI/EMC consultant and lecturer. © Copyright 2015 Cadence Design Systems, Inc. All rights reserved. Cadence, the Cadence logo, and Spectre are registered trademarks of Cadence Design Systems, Inc. All others are properties of their respective holders. 1766 12/13 CY/DM/PDF