What is the difference between common-mode and differential-mode EMI?

Differential-mode noise flows in opposite directions on the line and neutral conductors and is caused by the pulsating current drawn by switching converters. Common-mode noise flows in the same direction on both conductors and returns through earth ground, caused by parasitic capacitances to chassis. DM noise typically dominates below 1-2 MHz, while CM noise dominates above 2 MHz.

How much attenuation does a single-stage LC EMI filter provide?

A single-stage LC low-pass filter provides 40 dB per decade of attenuation above its cutoff frequency (equivalent to 12 dB per octave or second-order rolloff). Adding a second LC stage doubles this to 80 dB per decade. The cutoff frequency is determined by the inductance and capacitance values: fc = 1/(2*pi*sqrt(LC)).

What are X and Y capacitors in EMI filters?

X capacitors connect between line and neutral to suppress differential-mode noise, with typical values from 100 nF to 2.2 uF. Y capacitors connect from line or neutral to earth ground to suppress common-mode noise, with values limited to approximately 4.7-47 nF due to safety leakage current requirements. Both types are safety-rated to withstand specific voltage and surge conditions.

What CISPR 32 conducted emissions limits apply to consumer products?

Consumer products must meet CISPR 32 Class B limits, which are the stricter of the two classes. At 150 kHz, the quasi-peak limit is 66 dBuV and the average limit is 56 dBuV. Limits decrease with frequency, dropping roughly 13 dB between 150-500 kHz, then holding relatively flat from 500 kHz to 5 MHz. Class B is 13 dB tighter than Class A (industrial) limits.

EMC / ComplianceApril 11, 202610 min read

EMI Filter Design: LC Filter Calculations for CISPR Compliance

Design EMI filters for conducted emissions compliance. Covers LC filter topology selection, cutoff frequency calculation, common-mode vs differential-mode filtering, and CISPR 32 limits.

Contents

The Conducted Emissions Problem
Understanding CISPR Limits
Differential-Mode vs. Common-Mode Noise
LC Filter Fundamentals
Worked Example
Component Selection for EMI Filters
Inductors
Capacitors
Multi-Stage Filter Topologies
Practical Layout Tips That Make or Break Your Filter
Summary

The Conducted Emissions Problem

You've built a switch-mode power supply that works beautifully on the bench. It regulates perfectly, efficiency is great, thermal performance is solid. Then you take it to the EMC lab and it fails conducted emissions by 15 dB at 300 kHz. Welcome to the club.

Conducted emissions are the noise currents that your product pushes back onto the AC mains or DC supply lines. Every switching converter, motor driver, LED driver, and digital circuit generates high-frequency noise that travels back through the power cord and potentially interferes with other equipment on the same grid. This is why every country on earth regulates conducted emissions, and why EMI filter design is a critical skill for any power electronics engineer.

The good news: LC filters are remarkably effective at suppressing conducted emissions once you understand the physics. The bad news: getting the component values and topology right requires more thought than most engineers give it. Use the EMI Filter LC calculator to quickly iterate on filter designs as we work through the concepts.

Understanding CISPR Limits

CISPR 32 (which replaced CISPR 22) defines conducted emission limits from 150 kHz to 30 MHz. There are two limit classes:

Class	Environment	Quasi-peak Limit (150 kHz)	Average Limit (150 kHz)
A	Industrial	79 dB $\mu$ V	66 dB $\mu$ V
B	Residential	66 dB $\mu$ V	56 dB $\mu$ V

Limits decrease as frequency increases, dropping by roughly 13 dB between 150 kHz and 500 kHz, then holding relatively flat from 500 kHz to 5 MHz, and decreasing another 10 dB from 5 to 30 MHz.

Class B is the one that hurts. Consumer products, IT equipment, anything used in a residential setting must meet Class B. That's 13 dB tighter than Class A across the board. Many engineers design to Class B even for industrial products, because getting the Class B mark opens up more markets.

FCC Part 15 Subpart B has similar limits, but uses CISPR 22 methodology. If you pass CISPR 32 Class B, you'll almost certainly pass FCC.

Differential-Mode vs. Common-Mode Noise

This is the single most important concept in EMI filter design, and getting it wrong is the number one reason filters don't work as expected.

Differential-mode (DM) noise flows in opposite directions on the line and neutral conductors. It's caused by the pulsating current drawn by the switching converter itself. A buck converter chopping current at 500 kHz produces strong DM harmonics at 500 kHz, 1 MHz, 1.5 MHz, and so on. Common-mode (CM) noise flows in the same direction on both line and neutral, returning through the earth ground. It's caused by parasitic capacitances from switching nodes to chassis ground — the heatsink-to-MOSFET capacitance, transformer interwinding capacitance, and PCB parasitic coupling.

The key insight: DM noise dominates below 1-2 MHz, while CM noise dominates above 2 MHz. This generalization holds for most switch-mode converters and tells you where to focus your filtering effort at each frequency.

Measuring the split: use a LISN (Line Impedance Stabilization Network) on each line, then calculate: $V_{DM} = (V_L - V_N)/2$ and $V_{CM} = (V_L + V_N)/2$ . Some EMC receivers have a CM/DM discrimination network that does this automatically.

LC Filter Fundamentals

A basic LC low-pass filter provides 40 dB/decade attenuation above its cutoff frequency. That's 12 dB per octave, or roughly 40 dB of attenuation for every decade you go above the cutoff. The cutoff frequency is:

f_c = \frac{1}{2\pi\sqrt{LC}}

For a single-stage LC filter, the insertion loss at a frequency $f$ well above cutoff is approximately:

\text{IL}(f) \approx 40 \log_{10}\left(\frac{f}{f_c}\right) \text{ dB}

Design procedure:

Measure (or estimate) your unfiltered conducted emissions
Compare against the applicable limit
Determine the required attenuation at the worst-case frequency
Add 6-10 dB of margin (components degrade, parasitics eat performance)
Choose $f_c$ to provide the needed attenuation
Select L and C values to hit that $f_c$

Worked Example

Your 100 kHz buck converter shows 85 dB $\mu$ V at 300 kHz on the LISN. The CISPR 32 Class B quasi-peak limit at 300 kHz is approximately 60 dB $\mu$ V. You need:

\text{Required attenuation} = 85 - 60 + 6 = 31 \text{ dB (with 6 dB margin)}

The noise is at 300 kHz and your switching frequency is 100 kHz, so you want the filter cutoff well below 300 kHz. For 31 dB at 300 kHz:

31 = 40 \log_{10}(300/f_c)

f_c = 300 / 10^{31/40} \approx 300 / 10^{0.775} \approx 300 / 5.96 \approx 50 \text{ kHz}

So you need an LC filter with a cutoff around 50 kHz. Let's choose $C = 1\,\mu$ F (a standard X2 safety capacitor value):

L = \frac{1}{(2\pi f_c)^2 C} = \frac{1}{(2\pi \times 50 \times 10^3)^2 \times 10^{-6}} \approx 10 \text{ mH}

A 10 mH common-mode choke or differential-mode inductor, paired with a 1 $\mu$ F capacitor, gives you the filtering you need. Verify this with the EMI Filter LC calculator.

Component Selection for EMI Filters

Inductors

Common-mode chokes are wound with both line and neutral on the same core, with opposing windings. Normal load current (differential-mode) cancels in the core, so the inductor doesn't saturate under load. Only common-mode currents see the full inductance. Typical values: 1-47 mH for mains filters. Core materials: nanocrystalline (best broadband performance), MnZn ferrite (good to 1 MHz), NiZn ferrite (good above 1 MHz). Differential-mode inductors must carry the full load current without saturating. This limits the inductance value for a given core size. Typical values: 10-1000

\mu

H. Powdered iron cores are common because they have a soft saturation characteristic.

The practical gotcha: inductor impedance has a self-resonant frequency (SRF) above which the component becomes capacitive and stops filtering. Always check the SRF is above your highest frequency of concern.

Capacitors

X capacitors go between line and neutral (across the mains). They suppress differential-mode noise. Safety rated X1, X2, or X3 depending on voltage and surge requirements. Typical values: 100 nF to 2.2

\mu

F. X2 is the most common rating for consumer products. Y capacitors go from line or neutral to earth ground. They suppress common-mode noise. Safety ratings Y1, Y2, Y3, Y4 — the numbers indicate impulse voltage withstand. Y2 is typical for consumer electronics. Leakage current limits restrict Y capacitor values to roughly 4.7 nF for medical equipment and 10-47 nF for commercial products. Exceeding these values risks failing the touch current (leakage) safety test.

For a deeper analysis of how shielding complements filtering in a complete EMC strategy, see the Cable Shield Effectiveness calculator.

Multi-Stage Filter Topologies

A single-stage LC filter gives 40 dB/decade. Need more? Add another stage for 80 dB/decade:

f_c = \frac{1}{2\pi\sqrt[4]{L_1 C_1 L_2 C_2}}

Common topologies for AC mains EMI filters, from simplest to most effective:

C only — just X and Y capacitors. Quick fix, limited attenuation. Good for 10-15 dB.
LC (pi section) — one inductor + capacitors. The workhorse topology. Good for 30-50 dB.
CLC (pi-LC) — capacitor-inductor-capacitor. Adds another 20 dB without much size increase.
LCLC (two-stage) — two inductor-capacitor stages. 80 dB/decade rolloff. Used when you need serious attenuation.

Each additional stage adds component cost and board space but dramatically increases high-frequency attenuation. For most products, a well-designed single-stage filter with a common-mode choke, X capacitors, and Y capacitors is sufficient.

Practical Layout Tips That Make or Break Your Filter

Keep input and output separated. The single most common filter layout mistake is running the unfiltered input traces close to the filtered output traces. Capacitive and inductive coupling between them can bypass your filter entirely, destroying 20+ dB of performance. Ground Y capacitors to a low-impedance earth point. Long traces or wires from Y capacitors to chassis ground add inductance that reduces common-mode filtering at high frequencies. Use short, wide traces directly to a chassis ground screw or spring clip. Place the filter at the power entry point. The filter should be the first thing the mains connection sees, before any PCB traces that could radiate. Many products mount the filter on the power entry module itself. Use a ground plane under the filter. If the EMI filter is on the main PCB, a solid ground plane underneath reduces parasitic coupling and provides a return path for common-mode currents.

The Conducted Emissions Filter designer helps you model multi-stage filters and verify that your component values provide the required attenuation across the full CISPR frequency range.

Summary

Designing an effective EMI filter for conducted emissions compliance follows a logical process:

Separate DM and CM noise — they require different filter components and topologies
Calculate required attenuation from your measured emissions minus the limit, plus 6-10 dB margin
Set the cutoff frequency using $f_c = 1/(2\pi\sqrt{LC})$ $f_{c} = 1/ (2 π L C)$ to provide the needed attenuation at your worst-case frequency
1. Select safety-rated components — X capacitors for DM, Y capacitors for CM, common-mode chokes for CM inductance
2. Watch out for parasitics — capacitor ESR, inductor SRF, and PCB layout coupling all degrade real-world filter performance
The difference between a filter that works on paper and one that works in the lab is almost always layout and component parasitics. Start with the math, verify with the EMI Filter LC calculator, then validate on hardware with proper high-frequency measurement techniques.