App note: Power supply layout and EMI

in app notes by DP | 1 comment


Reduce EMI by determining large alternating currents carrying areas and making them compact, this minimizes radiation and spilling of unwanted signal to other part of your PCB. More of these methods are shown from Linear Technology’s app note.

PC-board layout determines the success or failure of every power supply project. It sets functional, electromagnetic interference (EMI), and thermal behavior. Switching power supply layout is not black magic, but is often overlooked
until it is too late in the design process. Fortunately physics
is on your side. Functional and EMI requirements must
be met, and in a world of trade-offs in power supply unit
layout, what is good for functional stability is good for
EMI. Good layout from first prototyping on does not add
to cost, but actually saves significant resources in EMI
filters, mechanical shielding, EMI test time and PC board
runs. This application note focuses primarily on nonisolated
topologies, but will examine some isolated topologies as
well. You will learn to make the optimum choices regarding
PC-board layout for solid power supply designs.

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  1. Drone says:

    Nice app-note reference. Also keep in-mind “modern” switching converters have options for “dithering” the switching frequency (pseudo-random typically) to reduce EMI spurious spectral density. This is especially important when dealing with higher switching frequencies which are more popular these days due to reduced charge storage requirements (simply-put smaller physical inductors). With cutting edge designs (e.g., smart phones and tablets), your processor(s), GPU(s), and/or memory management system will also dither the high speed busses. You can run into “mixing” or “adding” spectral density problems with high-speed bus noise if your power supply (and even power management) system frequencies are carefully chosen. One of the overlooked reasons why on-die busses exist (e.g. AMBA bus) in modern processors is to avoid exposing high-speed edges at the board level (e.g., GPIO/USB etc.), thereby causing EMI issues. The Devil truly lives in the details on this subject.

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