EMC Filter Calculator
Emissions vs CISPR Limit
Filter Component Values
Y capacitors at safety limit (4.7 nF). Verify leakage current per IEC 60950-1 / IEC 62368-1.
Power-line EMI filters keep a product's conducted emissions below regulatory limits and protect it from incoming noise. This calculator compares your measured emissions against the CISPR 11/32 Class A or Class B quasi-peak limits across 150 kHz to 30 MHz, finds the worst-case required attenuation with a design margin, and sizes the common-mode choke, differential-mode inductor, and X and Y capacitors.
Formula
Areq = Emeasured − Lcispr + 6 dB; Lcm = (Zsource × 10^(Areq/20)) / (2π·f)
- Emeasured
- Measured conducted emission level (dBµV)
- Lcispr
- CISPR 11/32 quasi-peak limit at the worst-case frequency (dBµV)
- Areq
- Required filter attenuation including a 6 dB design margin (dB)
- Zsource
- Source impedance (commonly the 50 Ω LISN value)
- Lcm
- Required common-mode choke inductance (H)
How it works
- Enter the frequency range to evaluate, your measured conducted emissions in dBµV, the CISPR class (A for industrial, B for residential), the source and load impedances, and whether the filter is single or two-stage.
- Across the band the engine subtracts the CISPR limit from your emissions and adds a 6 dB design margin to find the worst-case required attenuation, then locates the frequency where it is largest.
- At that worst-case frequency it sizes a common-mode choke, a differential-mode inductor, an X capacitor (line-to-line), and a safety-limited Y capacitor (line-to-ground), then sweeps the resulting filter to check the worst-case compliance margin.
Worked example
Measured emissions of 90 dBµV evaluated over 150 kHz–30 MHz against CISPR Class B, with 50 Ω source and load impedances and a single-stage filter.
- Worst-case Class B limit in the upper band is 56 dBµV (0.5–5 MHz). Required attenuation: 90 − 56 + 6 = 40 dB.
- Common-mode choke at the worst-case frequency: Lcm ≈ 1.49 mH.
- Differential-mode inductor ≈ 148.7 µH; X capacitor ≈ 0.059 µF.
- Y capacitor is held at its 4.7 nF safety limit, and the swept filter leaves a worst-case margin of about 10.3 dB.
Required attenuation 40 dB → CM choke ≈ 1.49 mH, DM inductor ≈ 148.7 µH, X-cap ≈ 0.059 µF, Y-cap 4.7 nF; design is compliant with ~10.3 dB worst-case margin.
Frequently asked questions
- What is the difference between common-mode and differential-mode noise?
- Differential-mode noise flows in opposite directions on line and neutral (like the normal load current) and is suppressed by X capacitors and differential inductors. Common-mode noise flows in the same direction on both lines returning through ground, and is suppressed by a common-mode choke and Y capacitors.
- Why are Y capacitors limited in value?
- Y capacitors connect line to protective earth, so any leakage current they pass flows through the ground conductor and can become a shock hazard. Safety standards such as IEC 62368-1 cap the total Y capacitance, which is why this tool limits each Y capacitor to 4.7 nF.
- What do CISPR Class A and Class B mean?
- Class A applies to equipment intended for industrial or commercial environments and allows higher emission limits. Class B is stricter and applies to equipment used in residential settings, where nearby radios and TVs need more protection, so a Class B product needs more filter attenuation.
- Why add a 6 dB design margin?
- Component tolerances, parasitics, layout, and test-to-test variation all erode real-world attenuation. Designing for 6 dB more than the bare requirement provides headroom so the product still passes formal certification testing rather than just barely meeting the limit on paper.