Engineer Column

Harmonizing Circuit Design and EMC DesignPart 13 EMC Overview (1) What is Electromagnetic Compatibility?


Hello! I’m Inagaki, at ROHM

Beginning with this 13th installment, we commence our long-awaited (!?) intermediate-level discussion of electromagnetic compatibility (EMC). From this point on, I would like to focus on electromagnetic compatibility (EMC) and dive rather more deeply into the details. Please keep me company!

This time, partly for review purposes, I would like to talk about the nature of electromagnetic compatibility. In the first column of our introduction-level discussion, it was explained as follows.

“Electromagnetic compatibility (EMC) mainly involves two types of phenomena: electromagnetic interference (EMI or emission), which is the effect that equipment itself exerts on other equipment and on humans due to electromagnetic noise, and electromagnetic susceptibility (EMS or immunity), which refers to the malfunctioning of equipment due to electromagnetic effects from outside. Here ‘compatibility’ refers to the need to address both, so that neither of these two phenomena cause harm.”

The text definition is more or less as above, but it’s a bit hard to understand. I’ll explain it so that it is intuitively easy to grasp. This explanation will be centered on the semiconductor integrated circuits (LSIs, ICs) that you are all acquainted with.

We’ll begin with electromagnetic interference (EMI or emission). Various types of LSIs and ICs are being developed and sold. For purposes of this explanation, they can be broadly classified as follows.

  • ① Direct-current power supply-related devices that we have had from the beginning, such as three-pin power supplies (7805, 7905, etc.) and low-saturation power supplies (LDOs). The signals handled by these devices are direct current (DC) signals.
  • ② Differential operational amplifiers (op-amps), voltage comparators, devices for audio signal processing, and so on. Signals handled include analog signals based on sine waves, and linear signals.
  • ③ Microcomputer, memory devices, logic chips, etc. These devices handle digital signals.
  • ④ Switching power supplies, charge pump power supplies and other power supply-related devices that have come into widespread use, as well as LED drivers, LCD drivers and other display-related devices, and PWM motor drives and other driver devices. Such LSIs and ICs are products that handle switching operations.

Among these, electromagnetic interference (EMI) is not generated by ① and ②, but is generated by ③ and ④. As a simple way of understanding things, I think we can intuitively recognize that analog LSIs and linear LSIs do not generate electromagnetic noise, but digital LSIs and switching LSIs do produce electronic noise.

DC voltages do not have a fundamental wave or harmonic components, and in sine waves there are few harmonic components (components that are N times the frequency of the fundamental), so that electromagnetic noise is not easily generated. On the other hand, digital LSIs and switching LSIs handle square waves (pulse waves), and therefore harmonics with frequencies up to, for example, 1 GHz or so (mainly odd harmonics) are generated. This is the essence of electromagnetic interference (EMI). Put another way, in digital and switching LSIs, circuits that produce electromagnetic interference (EMI) are being operated. Of course, digital operations have as merits fast and large-scale computations, low-power operation for extended battery-driven lifetimes, and other features. These devices are in widespread use because such advantages outweigh the disadvantages.

Next, electromagnetic susceptibility (EMS or immunity) refers to the ability or inability of semiconductor integrated circuits (LSIs, ICs) to withstand electromagnetic noise; the robustness needed to prevent malfunctions and the like is required. Electromagnetic susceptibility (EMS) can be considered from two different perspectives.

The first of these considers the voltage axis. The lowering of power supply voltages through ever-finer manufacturing processes tends to result in malfunctions. In times past, 5-volt logic was the mainstream; at present, however, devices that use power at 0.9 V are not uncommon. In logic ICs, internal threshold voltages (the voltage used within an IC to discriminate between H level and L level) have dropped from for example 2 V to 0.4 V. Devices using 5 V logic did not malfunction upon the occurrence of 1 V external electromagnetic noise, but products using 0.9 V logic will easily malfunction under the same conditions. But manufacturers nonetheless use 0.9 V logic because of the advantages accruing from designs using lower power levels.

The second perspective considers the frequency axis. Semiconductor integrated circuits (LSIs, ICs) are never used by themselves. They are mounted on printed circuit boards (PCBs) as part of a circuit that is configured and operated. A printed circuit board (PCB) has numerous parasitic components due to wiring, including the interiors of LSIs. Simpler parasitic components are parasitic resistances R (wiring resistances), parasitic capacitances C (stray capacitances), and parasitic inductances L (DC inductances). One often hears about an equivalent series resistance (ESR) and an equivalent series inductance (ESL) as representative quantities. And among various parasitic components, it is capacitance and inductance that are particularly troublesome. This is because the parasitic capacitances and parasitic inductances that are present within LSIs and all over printed circuit boards (PCBs) give rise to resonance phenomena. LC series resonances and parallel resonances occur at various frequencies, from the low to the high frequency range. At these resonance frequencies, because the impedance goes to zero or to infinity, malfunctions tend to occur. These are one cause of electromagnetic susceptibility. I write “one” because there are many other causes as well, such as circuit configurations and board artwork that tend to induce malfunctions. It is generally said that measures to address electromagnetic susceptibility (EMS) are more difficult than electromagnetic interference (EMI) countermeasures; this is because there are multiple causes of electromagnetic susceptibility (EMS), and time and techniques are needed to discover which among these are dominant.

I would next like to discuss transmission paths of electromagnetic noise; but this is a fairly voluminous subject, so I will save it for next time.

Thank you very much for your attention.

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Downloadable materials, including lecture materials from ROHM-sponsored seminars and a selection guide for DC-DC converters, are now available.