CISPR 11 Class B EMI Filter Design for Inverters

  • Guide
  • 2 min read
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EMIcommon-mode interferenceCISPR 11filter designPCB layoutsafety compliance

Introduction to High-Power Inverter EMC#

High-power inverters in industrial and renewable energy systems use fast switching devices that create steep voltage and current transitions. These transitions generate broadband electromagnetic interference (EMI) that can exceed CISPR 11 Class B limits. The EMI filter must attenuate both differential-mode and common-mode noise while respecting safety insulation and leakage current limits.

This guide explains the design logic for an EMI filter that meets Class B. It covers noise path analysis, common-mode choke selection, X and Y capacitor calculations, and PCB layout techniques that reduce high-frequency coupling. The goal is a repeatable workflow from specification to validation.

Understanding Common-Mode and Differential-Mode Noise#

Differential-mode (DM) noise flows in the power lines and returns through the opposite conductor. It is driven by switching cell commutation and bus capacitor impedance. Common-mode (CM) noise flows through parasitic capacitances to earth, such as semiconductor-to-heatsink capacitance, motor winding capacitance, and PCB stray capacitance. CM current returns through the protective earth, creating radiated and conducted emissions.

At high power, CM noise dominates above a few hundred kHz because high dv/dt on the switching node injects displacement current through parasitic capacitors. The CM path impedance is often dominated by the Y capacitors and the common-mode choke. The DM path is dominated by the X capacitors, bus inductance, and line impedance stabilization network (LISN) impedance.

A useful first step is to measure the noise spectrum with a LISN and separate CM and DM components using a noise separator or mathematical transformation. This data determines whether the filter needs more CM attenuation, DM attenuation, or both.

Common-Mode Choke Selection and Calculation#

The common-mode choke presents high impedance to CM current and low impedance to DM current. Its impedance is Z_cm = 2 * pi * f * L_cm, where L_cm is the CM inductance per line and f is frequency. Required attenuation A_dB =

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