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Design007-Oct2020

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18 DESIGN007 MAGAZINE I OCTOBER 2020 Electromagnetic energy is all around us from TV to radio and communication waves and the Earth's magnetic field and plasma shield that protect us from the sun's solar flares (Figure 1). This activity is driven by the sun's mag- netic field. Like all physical phenomena, they tend to follow the same rules. A parallel plate capacitor (or a planar pair) has two conductors separated by a dielectric layer. Most of the energy in the structure is concentrated directly between the plates. How- ever, some of the energy spills over into the area outside the plates. The electric field lines associated with this effect are called fringing fields. In this month's column, I will look at how electromagnetic radiation can be emitted from the edges of planes in multilayer PCBs by the fringing fields, possibly causing electro- magnetic compatibility (EMC) issues. Fringing is the bending of the electric flux lines near the edge of the parallel plate capaci- tors. Fringing is also known as the edge effect. Normally, the flux lines inside the capacitor are uniform and parallel. But at the edges, the flux lines are not straight and bend slightly outward due to the geometry. Also, in a plane pair, signals passing through the cavity may intensify fringing fields (Figure 2). When the return current flows through the impedance of a cavity between two planes, it generates voltage. Although quite small (typi- cally in the order of 5 mV), the accumulated Fringing Fields Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: The sun's corona discharge (solar flare) and Earth's magnetic field.

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