Appliance Design - February 2009 - (Page 34)

SHIELDING & EMI the contribution of the test fixture can be removed and the inductance values of various DUTs extracted [1]. As seen in Fig. 3, inductance comparisons of same size 0603 MLCCs and X2Y capacitors show that X2Y capacitors have approximately 1/10th the inductance, and as low as 1/25th the inductance of larger component sizes (1206, 1812, etc.), which are often used to filter motors and power converters. The lower inductance characteristic of X2Y is due to the added side terminations that form four small opposing current loops when X2Y is attached incircuit, and a cancellation effect that reduces the net inductance to the circuit. In addition, the device is inherently balanced by design, so when properly mounted in-circuit between the +/- power lines and grounded, an X2Y capacitor provides tightly matched line-to-ground shunt capacitance, which greatly reduces EMI [2]. The combination of low inductance and tight matching results in dramatically improved EMI filter performance and a significant cost savings achieved by reducing passive components and their associated assembly costs. Fig. 2. A three-node X2Y capacitor compared to a two-node conventional capacitor. (Illustration/image courtesy Johanson Dielectrics.). Radiated emissions examples Many appliance manufacturers use DC brush motors for motion control in their products. Brush commutated motors are spark generators that combine with transmission lines such as power cables, wires, traces, to radiate broadband electromagnetic interference into a larger electrical system. (See Fig. 4.) The EMI can cause malfunction when coupled to other nearby circuit components that are susceptible to interference. In one example, application engineers at X2Y Attenuators were contacted by a blender manufacturer that was plagued by integrated circuit (IC) latch-up. The manufacturer’s engineers had isolated the source of the problem as the blender’s DC motor, which gave them a choice of placing the noise filter at the IC that was susceptible to EMI, or at the EMI source, the DC motor. Attempts to filter the DC motor with conventional capacitors had failed to stop the IC malfunction. However, a single X2Y filter capacitor placed between the DC motor’s power leads as they exit the motor housing (I/O) completely eliminated the malfunction issue. An added benefit of suppressing EMI at the source is that it may solve susceptibility issues with other compo- Fig. 3. Extracted inductance, X2Y capacitors compared to MLCCs. Fig. 4. Applying X2Y capacitors in DC motors. 34 applianceDESIGN February 2009 www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - February 2009 Contents Editorial Shipments/Forecasts News Watch Force-Sensing Technology Offers Designers an Alternative Approach to Creating Touch-Control Interfaces A Flammable Vapor Sensing Technology that was Originally Developed for use in Water Heaters has Evolved for use in a Broader Array of Applications Latches, Hinges, and Slides Must Reliably Secure Parts Together, Sometimes for Hundreds of Thousands of Cycles. Beyond their Utilitarian Functions, They Can Also Change Consumer Perceptions Elastomeric Electrical Connectors Can Solve Space Constraint Problems in Electronic Products and Provide Some Cost Advantages as Well A Multi-Layer Ceramic Capacitor with a Novel Electrode Design Can Suppress Electromagnetic Interference, Improve Circuit Performance, and Reduce the Number of Needed Components Design Marts Association Report: AHRI Advertisers’ Index

Appliance Design - February 2009

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