Conducted Emissions
Conducted emissions are electromagnetic disturbances that propagate through physical conductors such as power lines, signal cables, and ground connections rather than radiating through free space. These emissions represent a critical aspect of electromagnetic compatibility, as unwanted electrical noise traveling along conductors can interfere with other equipment sharing the same power distribution network or connected through common signal interfaces.
Unlike radiated emissions that diminish with distance according to electromagnetic wave propagation, conducted emissions can travel considerable distances along power and signal cables with relatively little attenuation at lower frequencies. This makes conducted emissions particularly significant in environments where multiple electronic devices share common power infrastructure, such as industrial facilities, office buildings, data centers, and residential installations connected to the public utility grid.
Fundamentals of Conducted Emissions
Conducted emissions manifest in two primary forms: differential-mode noise and common-mode noise. Differential-mode emissions flow in opposite directions on the line and neutral conductors, representing noise that appears across the power input terminals. Common-mode emissions flow in the same direction on both line and neutral conductors, returning through the ground connection. Most practical electronic devices generate both types of noise, though switching power supplies and digital circuits often produce dominant common-mode components that prove more challenging to filter.
The frequency range of regulatory concern for conducted emissions typically spans from 150 kHz to 30 MHz, though some standards extend the lower limit to 9 kHz for certain product categories. This frequency range captures the fundamental switching frequencies and lower harmonics of most switching power converters while stopping before radiated emission measurements become more relevant. Understanding the spectral content of conducted emissions helps engineers identify noise sources and select appropriate filtering strategies.
Sources of Conducted Emissions
Switching power supplies represent the most common source of conducted emissions in electronic equipment. The rapid switching of transistors in buck, boost, flyback, and forward converter topologies generates current pulses rich in harmonic content. The fundamental switching frequency and its harmonics appear directly on the input power lines unless adequate filtering attenuates them. Synchronous rectification, while improving efficiency, can introduce additional high-frequency noise components.
Variable frequency motor drives and inverters generate substantial conducted emissions due to their high-power switching operations and long cable runs to motors. Digital circuits with fast edge rates contribute conducted noise through their power supply decoupling networks, while electronic lighting ballasts, battery chargers, and any equipment with rectifier inputs produce characteristic harmonic patterns. Even linear power supplies generate some conducted emissions from rectifier switching transients, though at much lower levels than switching converters.
Measurement Techniques
Conducted emission measurements employ a line impedance stabilization network (LISN) to provide a defined, repeatable impedance at the equipment under test power input while isolating the measurement from ambient noise on the power mains. The LISN presents a standardized 50-ohm impedance for conducted emission measurements while allowing the passage of normal AC power to the equipment. Measurements are taken separately on each power conductor and compared against applicable limits.
EMI receivers or spectrum analyzers with appropriate detector functions measure the amplitude of conducted emissions across the specified frequency range. Quasi-peak and average detectors weight the measurements according to the repetition rate and duration of noise pulses, reflecting the actual interference potential to broadcast receivers that originally drove EMC regulation development. Pre-compliance measurements during development can identify problems early, allowing correction before costly formal compliance testing.
Filtering and Mitigation
EMI input filters represent the primary defense against conducted emissions, employing combinations of inductors and capacitors to attenuate noise before it reaches the power lines. Differential-mode filtering uses series inductance and parallel capacitance between line and neutral, while common-mode filtering employs common-mode chokes wound on single cores and Y-capacitors connected to the safety ground. Effective filter design requires understanding both the noise source characteristics and the source and load impedances at relevant frequencies.
Beyond dedicated input filtering, design practices that reduce noise generation at the source improve overall system performance. Proper power supply design with optimized switching transitions, appropriate snubber circuits, and careful layout minimizes the noise requiring filtration. Shielding of noisy circuits and proper grounding techniques prevent noise from coupling to power input traces. A comprehensive approach addressing both noise generation and filtration achieves compliance with appropriate margins while minimizing filter size and cost.
Regulatory Requirements
Conducted emission limits are specified by regulatory standards that define maximum allowable noise levels at each frequency within the measurement range. CISPR publications establish international limits adopted in European and many Asian markets, while FCC Part 15 governs products sold in the United States. Different limits apply to Class A equipment intended for commercial and industrial environments versus Class B equipment suitable for residential use, with Class B limits being more stringent.
Product-specific standards may impose additional or different conducted emission requirements. Medical devices follow IEC 60601 series standards with particular attention to patient safety environments. Automotive equipment must meet manufacturer specifications and regional standards for conducted emissions on both power and signal lines. Understanding which standards apply to a product and designing to meet those requirements ensures market access while protecting the electromagnetic environment from excessive pollution.