Electronics Guide

Sources of Common-Mode Noise

Switch-mode power supplies generate significant common-mode noise through high-frequency voltage transitions coupling capacitively from switching elements to the chassis or ground plane. The parasitic capacitance between switching transistors and their heatsinks, typically mounted to chassis ground, creates a direct path for common-mode current injection. This capacitive coupling becomes more efficient as switching frequencies increase, contributing to the challenges of EMC compliance in modern power electronics.

Ground potential differences between equipment connected by cables generate common-mode currents on the cable conductors. Even small voltage differences at power frequencies, combined with the low impedance of cable shields, can produce significant common-mode currents. At higher frequencies, the inductance of ground connections allows ground potential differences to develop from normal circuit currents, driving common-mode noise onto signal cables.

Radiated electromagnetic fields induce common-mode voltages on cable runs exposed to the field. The cable acts as an antenna, with the induced voltage appearing equally on all conductors relative to ground. Long cable runs and cables routed parallel to the dominant field polarization exhibit the highest coupling efficiency. This mechanism allows external RF sources to introduce common-mode noise into otherwise well-designed systems.

Common-Mode Current Paths

Common-mode currents require a return path to complete their circuit, and this path often includes unintended routes through chassis structures, cable shields, and parasitic elements. In a typical installation, common-mode current flows from noise source through the equipment ground, onto the power cable, through the building ground system, and back to the source through parasitic capacitance or ground connections.

The high-frequency characteristics of common-mode current paths significantly affect filtering requirements. At frequencies where cable length approaches a significant fraction of a wavelength, standing wave effects create current and voltage maxima at specific points along the cable. These resonances can significantly increase common-mode current at particular frequencies, requiring broad-spectrum filtering approaches.

Operating Principle

A common-mode choke consists of two or more windings on a shared magnetic core, with the windings oriented so that differential currents produce opposing magnetic fluxes that cancel in the core. For differential signals, the choke presents only the relatively small leakage inductance. Common-mode currents produce additive magnetic flux, causing the full inductance to oppose the common-mode current flow.

The differential cancellation occurs because the go and return signal currents flow in opposite directions through windings on the same core. These opposing currents create opposing magnetic fields that substantially cancel, leaving only the field from any imbalance in winding inductance or placement. The result is a component that can provide tens of dB of common-mode attenuation while introducing minimal insertion loss for differential signals.

Core Materials

Core material selection determines the frequency range and impedance characteristics of common-mode chokes. High-permeability materials including nanocrystalline alloys and manganese-zinc ferrites provide high inductance in compact packages, offering excellent performance at lower frequencies (typically 10 kHz to 1 MHz). As frequency increases, core permeability typically decreases, reducing inductance but also reducing core losses.

Nickel-zinc ferrites with lower permeability extend useful performance to higher frequencies (1 MHz to several hundred MHz) where manganese-zinc materials become lossy. The increasing resistive component of impedance at high frequencies actually aids filtering by dissipating noise energy rather than reflecting it back to the source. Selection of core material should consider the frequency spectrum of the noise to be filtered.

Powdered iron and iron powder cores offer lower permeability with excellent high-frequency characteristics and saturation resistance. These materials are commonly used in power applications where DC current or low-frequency AC might saturate higher-permeability materials. The distributed air gap inherent in powder cores provides stable inductance over varying current levels.

Winding Configurations

The simplest common-mode choke uses bifilar winding, where both conductors are wound together around the core. This technique ensures nearly identical inductance in both windings and maximizes the coupling coefficient for differential cancellation. Bifilar winding is standard for signal line common-mode chokes and smaller power line chokes.

For high-voltage power applications, separate windings with adequate spacing provide the required isolation between conductors. The windings must be balanced in turns and positioning to maintain differential cancellation. Sector winding, where each winding occupies a defined angular portion of the core, provides controlled coupling while meeting safety spacing requirements.

Multi-line common-mode chokes accommodate more than two conductors, providing filtering for multi-phase power or multi-conductor signal cables. Three-phase power common-mode chokes wind all three phases on a single core, providing common-mode filtering while remaining transparent to the normal three-phase currents that sum to zero in a balanced system.

Y Capacitor Function

Y capacitors connect from line conductors to ground, providing a controlled capacitive path for common-mode currents. At high frequencies where the capacitor impedance is low, common-mode noise currents flow through the capacitor to ground rather than propagating along the cable. The capacitor voltage rating must exceed the peak line voltage plus any expected transients.

The location of Y capacitors relative to common-mode chokes affects filter performance. Capacitors on the source side of the choke provide a low-impedance termination that improves the choke's effectiveness at shunting common-mode current. Capacitors on the load side reduce the noise level seen by connected equipment. Many filter designs include Y capacitors on both sides of the common-mode choke.

Safety Considerations

Y capacitors in AC mains applications carry special safety requirements because failure of these components could connect line voltage directly to accessible grounded parts. Safety-agency-rated Y capacitors are designed to fail open circuit and must maintain their ratings after defined stress tests including voltage surges and endurance testing.

Y1 capacitors are rated for line-to-line and line-to-ground applications in circuits without basic insulation to ground. Y2 capacitors are suitable for line-to-ground applications where basic insulation to ground exists. The safety class affects both the maximum capacitance value and the creepage and clearance distances in the component construction.

Leakage current requirements limit the total Y capacitance that can be used. Ground fault protection devices may trip if leakage current exceeds their threshold. Medical equipment and information technology equipment standards specify maximum earth leakage currents that constrain Y capacitor selection. Touch current in equipment with the ground removed must also be considered.

Determining Attenuation Requirements

The required common-mode filter attenuation is the difference between the measured or estimated noise level and the applicable limit, plus appropriate margin. EMC limits are typically specified in dB above one microvolt or in absolute units, while noise measurements provide the baseline for filter specification. A margin of 6 dB or more accounts for measurement uncertainty and production variation.

Frequency-dependent requirements often call for filters providing maximum attenuation in a specific frequency range. Conducted emission limits have different values in different frequency bands, and noise spectra vary with source characteristics. Analysis of both requirements and noise spectrum identifies the critical frequency ranges for filter optimization.

Impedance Considerations

Filter effectiveness depends on the source and load impedances at both the input and output. A filter designed for 50-ohm terminations may perform differently when connected to the actual source and load impedances, which vary with frequency and may differ significantly from 50 ohms. The line impedance stabilization network (LISN) used for conducted emissions testing presents a defined impedance, but this may not represent actual installation conditions.

Common-mode source impedance is typically high at lower frequencies (dominated by parasitic capacitance) and decreases at higher frequencies as capacitive reactance falls. This frequency-dependent impedance affects optimal filter topology. At frequencies where source impedance is high, capacitive input stages provide better attenuation. At frequencies where source impedance is low, inductive input stages may be more effective.

Practical Design Approach

A systematic approach to common-mode filter design begins with noise characterization to identify the magnitude and frequency content of the common-mode noise. This characterization can be based on measurements of existing equipment or predictions from circuit simulation and experience with similar designs.

Selection of filter topology follows from the frequency range and attenuation requirements. A simple LC filter using a common-mode choke and Y capacitors provides 40 dB per decade roll-off above the cutoff frequency. Additional filter stages can provide steeper roll-off when required, though practical considerations often favor distributed filtering with components at multiple locations.

Component selection must account for frequency-dependent behavior. Common-mode choke inductance decreases at high frequencies due to falling core permeability. Capacitor impedance is affected by equivalent series inductance (ESL) and equivalent series resistance (ESR). Selection of components rated for the application frequency range ensures filter performs as designed.

Component Placement

Common-mode filters should be placed at the boundary between noise-generating and noise-sensitive regions, typically at cable entry points or at the interface between power stages and sensitive circuits. Placement close to the noise source reduces the opportunity for noise to radiate before filtering. Placement at enclosure boundaries maintains the shielding integrity of compartmentalized designs.

Y capacitors require short, low-inductance connections to the ground reference. Any inductance in the ground path adds to the capacitor's impedance and reduces its effectiveness at high frequencies. On PCBs, capacitor ground pads should connect directly to the ground plane with multiple vias. In chassis-mounted assemblies, capacitors should mount directly to the chassis or use very short leads to a ground lug.

Avoiding Bypass Coupling

Physical separation between filter input and output prevents capacitive coupling that allows noise to bypass the filter. Input and output traces should not run parallel, and input and output terminals should be separated. For high-attenuation filters, shielding between input and output may be necessary to achieve the designed attenuation.

Common-mode choke lead dress affects high-frequency performance. Long leads add inductance that may be desirable for filtering but also add parasitic capacitance that allows high-frequency bypass. For best high-frequency performance, leads should be short and routed away from each other to minimize inter-winding capacitance.