EMI (Electromagnetic Interference)

Notes

Conducted emissions in the 150 kHz to 30 MHz band are where most switching converters fail their first compliance test.

Overview

EMI (electromagnetic interference) is unwanted electromagnetic energy from one circuit that disturbs another, either conducted along wires and supply lines or radiated through space. In electronics it comes mainly from fast switching edges in power converters, clocks and high-speed interfaces. Electromagnetic compatibility (EMC) is the wider goal that a product neither emits too much interference nor is upset by interference from others.

How it works

Every EMI problem has a source, a coupling path and a victim. Fast changes in voltage (dv/dt) and current (di/dt) are the sources; the switching stage of a DC-DC converter is a classic example. Energy couples by direct conduction, through stray capacitance and mutual inductance between nearby conductors, or by radiation when cables and current loops act as antennas. Commercial limits typically cover conducted emissions from 150 kHz to 30 MHz and radiated emissions above 30 MHz.

Conducted noise is split into two modes. Differential-mode current flows out on one line and back on the other, like the supply current itself, and is reduced with series inductors and capacitors across the lines. Common-mode current flows the same way on both lines and returns through stray capacitance to ground, often driven by high dv/dt; it is reduced with common-mode chokes and Y-capacitors to ground. Spread-spectrum operation, which dithers the switching frequency, lowers peak emissions by spreading energy over a wider band.

Products are tested against limits in standards, using a line impedance stabilisation network (LISN) to present a defined source impedance and receivers with peak, quasi-peak and average detectors. The IEC 61967 series defines how to measure emissions from an integrated circuit itself, and automotive components are tested to CISPR 25, which protects radio receivers on the same vehicle.

Examples

• A buck converter whose input filter combines a differential-mode LC stage with a common-mode choke to pass conducted emission limits.
• An automotive power or interface chip measured on a test board with the CISPR 25 component methods before release to vehicle makers.
• Spread-spectrum switching in a regulator, lowering the peak emission at any one frequency.
• IC-level emission measurements to the IEC 61967 series, giving system designers comparable data for different chips.

Why it matters

EMI failures tend to be found late and are expensive to fix: a module that fails its emissions test may need a board respin, extra filtering or shielding. EMC engineers, power IC designers and application engineers therefore design for low emissions from the start, and chip suppliers offer features such as spread spectrum and slew-rate control to help.

Related terms

• EMC
• Common-mode choke
• dv/dt
• CISPR 25
• Y-capacitor

Where it appears on the Hub

Used on 28 pages of the Hub. In each group, the pages that mention it most come first.