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Design007-Mar2019

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60 DESIGN007 MAGAZINE I MARCH 2019 The Proximity Effect Skin effect and the proximity effect are man- ifestations of the same principle—magnetic lines of flux cannot penetrate a good conduc- tor. The difference between them is that skin effect is a reaction to the magnetic fields gen- erated by current flowing within a conductor (Figure 1), while proximity effect is generat- ed by current flowing in other nearby traces or planes. The frequency at which both effects begin to occur is the same. In this month's column, I will focus on the proximity effect. Please see "Beyond Design: Effects of Surface Roughness on High-speed PCBs" for further in- formation on skin effect. The Proximity Effect In multilayer PCBs, these effects start to take hold at rather low frequencies on the order of ~30 MHz. Below that frequency, due to chang- ing currents in the traces, the magnetic forces are too small to influence the pattern of current flow. In a low-frequency or DC circuit, the re- turn current takes the path of least resistance filling the entire cross-sectional area of the trace. As it returns to the source via the pow- er/ground planes, this current tends to spread throughout the wide, flat sheet of copper. How- ever, as the frequency increases, the magnetic forces surrounding a trace become significant and the return current takes the path of least inductance. This high-frequency distribution follows a tight path directly above and/or be- low the trace in the reference plane(s). As represented in Figure 1, magnetic fields distribute current to a shallow depth around the perimeter of the trace (red), increasing the apparent resistance of the trace; this is the skin effect. The magnetic fields also distribute cur- rent around the perimeter of the trace in a non- uniform manner when referenced to a plane; this is the proximity effect. This draws current toward the side of the trace facing the refer- ence plane and forms the return current into a narrow band directly above and/or below the trace. Figure 2 shows microstrip return current density. In an asymmetric stripline configura- tion (Figure 3), the proximity effect draws cur- rent in an uneven distribution towards the near and far reference planes. Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Skin vs. proximity effect. Figure 2: Microstrip return current density.

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