Pinpoint And Plan For EMI Sources In Automobiles

AUTOMOBILES now come with more electrical and electronic systems, such as control-area networks (CANs) and infotainment systems. The growing complexity of such electronics systems has made automotive wiring systems more expensive. They also are a major point of failure. As a result, wireless technology is replacing wire harnesses in many cases. Because automotive electronic systems are located in a confined space, electromagnetic interference (EMI) can be a problem. In "Automotive EMI Shielding Controlling Automotive Electronic Emissions and Susceptibility with Proper EMI Suppression Methods," Laird Technologies discusses the various electronic systems found in automobiles while addressing the need to contain and suppress their EMI potential with each other.

The 14-page white paper is authored by John Noto, Gary Fenical, and Colin Tong. The authors begin by defining EMI and pinpointing its many sources and potential receptors within a vehicle. Automotive electronics also may be affected by harsh external EMI environments, such as highpower transmitters and cellular phones. Designers must provide immunity to such threats via optimal electromagnetic-compliance (EMC) design and the correct selection of EMI shielding materials.

The paper provides an equation for emissions from a basic circuit. It then analyzes that equation to show that frequency is the major issue, as emissions increase as the square of frequency rises. For current, emissions increase linearly. The same is true for loop area. The authors also show that emission levels are a function of frequency squared and inversely related to shielding effectiveness. Susceptibility to EMI is directly related to loop area, bandwidth, and the transmitted frequency and field strength. It is inversely related to shielding effectiveness.

The white paper asserts that EMI in automobiles can be reduced in a number of ways. For example, components should be moved onto the printed-circuit board (PCB). Groundplanes also may be added and/or changed. The length of noisy PCB traces and wires should be reduced as well. To cancel magnetic signals and reduce loop area, it is advisable to match driver and return circuit traces or cable lines. It also is helpful to add special components, such as inductors, capacitors, and resistors, to filter problem signals. In addition, the paper advises the addition of ferrite products, which absorb EMI energy by dissipating it as small amounts of heat. Most of all, the selection of shielding materials is critical to controlling or mitigating EMI in automobile electronic systems.

Laird Technologies, 16401 Swingley Ridge Rd., Suite 700, Chesterfield, MO 63017; (636) 898-6000, FAX: (636) 898-6100, Internet:

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