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Conducted Emissions

Conducted Emissions

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In most cases, conducted emissions should be easier to control and avoid than radiated emissions. Being lower frequency, it is less influenced by parasitics than higher frequency problems. However, they are still an issue and must be considered. Thus, causes of conducted emissions, and the solutions for them, are usually easier to understand than those for radiated emissions. Most conducted emissions are due to switch-mode power supplies (SMPS), and the best power supply designs are usually adequately filtered at the power input. However, many OEM power supplies are poorly designed, have horrible emissions, yet have FCC and CE markings. When these power supplies are loaded into a reactive load, instead of the nice resistive load it was designed for the power supply may start to get unstable or noisy, and additional measures are usually needed to keep it in compliance. Also, most commercial line filter modules or filter circuitry is designed to cover frequencies up to only 30 MHz. Therefore, there is the possibility that harmonics generated by the switching devices or rectifier switching transients can still make it through the filter. In addition, with all the high-speed digital circuitry inside electronic products today, it is possible for higher frequency harmonics to contaminate the system power supply and leak out through the filter and back out the power line. Therefore, while our experience demonstrates that most well-designed filters are sufficient, always be on guard for situations where the filter is compromised - either by design or by system design issues, such as poor internal cable routing, filter or power supply placement, or poor connection to chassis or signal returns. Typically, failure modes will be minimal to the product itself, but high emissions can upset sensitive measuring equipment or communications receivers nearby or connected to the same power line circuit.

Chapter Contents:

  • 5.1 Introduction to Conducted Emissions
  • 5.2 Conducted Emissions Checklist
  • 5.3 Typical Failure Modes
  • 5.4 Troubleshooting at the Test Lab
  • 5.5 Troubleshooting at Your Facility
  • 5.5.1 Circuits and Filters
  • 5.6 Special Cases and Issues
  • 5.7 DIY Tricks and Low-Cost Tools
  • 5.8 Typical Fixes

Inspec keywords: switched mode power supplies; network routing; electromagnetic interference; digital circuits; failure analysis

Other keywords: high-speed digital circuitry; failure modes; filter circuitry; switch-mode power supplies; line filter modules; system design; conducted emissions; internal cable routing; power line circuit

Subjects: Electromagnetic compatibility and interference; Power electronics, supply and supervisory circuits; Digital circuit design, modelling and testing; Reliability; Power convertors and power supplies to apparatus

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