Conformity Magazine - June 2008 - (Page 57)

And, at 100 MHz Z = 179.8 ohms Φ = 0 XL = 0 ohms Rs = 179.8 ohms The same approach may be used for different materials, dimensions, and frequencies. The previous discussion assumed that the core of choice was cylindrical. If the ferrite core being used is for flat ribbon, or bundled cable, or a multi-hole plate, the calculation for the Lo becomes more difficult, and fairly accurate figures for the cores path length and effective area must be obtained in order to calculate the air core inductance. This can be done by mathematically sectioning the core and summing the calculated path length and magnetic area for each section. In all cases though, an increase or decrease in impedance will be directly proportional to an increase or decrease in the height/ length of the ferrite core.5 Relationship Between Impedance and Attenuation As stated, most manufacturers are specifying cores for EMI applications in terms of impedance, but often the end user needs to know the attenuation. The relationship that exists between these two parameters is: Attenuation = 20 log10 ((Zs + Zsc + ZL) / (Zs + ZL)) dB Where: Zs = Source impedance Zsc = Suppressor core impedance ZL = Load impedance The relationship is dependent on the impedance of the source generating the noise and the impedance of the load receiving it. These values are usually complex numbers that can be infinite in scope and not easily obtained by the designer. Selecting a value of one ohm for both the load and the source impedance (as may be the case when the source is a switch mode power supply and the load many low impedance circuits) simplifies the equation and allows comparison of ferrite cores in terms of attenuation. Under these conditions, the equation reduces to: Attenuation = 20 log10 (Zsc/2) dB The graph in Figure 12 is a family of curves that show the relationship between the shield bead impedance and the attenuation for a number of commonly used values of the load plus the generator impedance. Figure 13 is the equivalent circuit of an interference source with an internal impedance of Zs, generating an interference signal through the series impedance of the suppressor core Zsc and the load impedance ZL. The Environment Temperature As stated, ferrite’s magnetic parameters can be affected by temperature and field strengths. Figures 14 and 15 are graphs of impedance vs. temperature for the same three ferrite materials. The most stable of these materials is the 61 material, with a decrease in impedance of 8% at 100° C and 100 MHz. This is compared to a 25% drop in impedance for the 43 material at the same frequency and temperature. These curves, when supplied, may be used to adjust the specified room temperature impedance if desired attenuation is to be at elevated temperatures. Field Strength As in the case of temperature, dc and 50 or 60 Hz power current will also affect the same intrinsic ferrite characteristics which, in turn, will result in lowering of the impedance of the core. Figures 16, 17 and 18 are typical of curves that illustrate the effect of biases on impedance a ferrite material. The curve depicts the degradation of impedance as a function of field SOLAR ELECTRONICS COMPANY 10866 Chandler Blvd., North Hollywood, CA 91601 (800) 952-5302 • (818) 755-1700 • Fax (818) 755-0078 See our catalog online at www.solar-emc.com Contact us at sales@solar-emc.com Innovative EMI Solutions Since 1960 The Solar Model 2654-1 Lightning Generator synthesizes electrical impulses needed for testing susceptibility to transients induced in aircraft equipment by lightning strikes. The peak voltage and current requirements of RTCA/DO-160E, Section 22 are met at Levels 1 through 3. Using available accessory probes, pulses can be applied by pin injection, cable bundle injection, and ground injection. JUnE 2008 Conformity 57 http://www.solar-emc.com http://www.solar-emc.com

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Conformity - June 2008 Contents Editor's Note 700 MHz Auction Raises a Record $20 Billion FCC Modifies PCS and AWS Power Limit Rules for Broadband Wireless Commission Designates Spectrum Test Bed FCC Releases Report on Wireless Competition Commission Proposes $5 Million Fine for Slamming Using EDX for Non-Destructive Detection of Lead in Consumer Products ESD Open Forum Challenges in Testing - Improving Election Security and Accuracy Part 1 Challenges in Testing - Human Body Model: The Hidden Challenges High-Speed Signal Integrity Considerations for ESD Components Focus On...EMC Components Buyer's Guide FCC Levies $2.6 Million Fine for Junk Faxes FDA Updates Guidance on its Product Review Process EU Issues Updated Energy Star Regulations EU Repeals Directive on Veterinary Electro-Medical Equipment New Product Announcements Updated Standards List for the EU's PPE Directive CPSC Announces New Effort to Keep Out Hazardous Products CPSC Actions in the News IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity - June 2008

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