New-Tech Europe | Sep 2017 | Digital Edition

Figure 1: Conducted and radiated emissions limits

during system integration. Because the requirements vary with each application, these recommendations are left to the discretion of the designer; this way each design incorporates only the components required for the specific application. Similarly, most wall plug and desktop versions of external power supplies are also designed and tested to meet EMC regulations as stand-alone units. If the power supply customer is a manufacturer combining the power supply with a load then they will be required to perform testing to ensure the complete system meets EMC regulations. As the circuitry is housed in an enclosed case, adding external components to address EMC issues will be more challenging for wall plug and desktop versions as compared to internally mounted power supplies. EMC regulatory testing of power supplies is performed with static resistive loads, but almost all power supplies are based upon switching regulator topologies. A switching regulator inherently produces conducted and radiated emissions which need to be mitigated in the design of the supply. The load

applied to the power supply may create additional emissions. The uncertainty of the conducted and radiated emissions from the combined power supply and load is addressed by allowing a margin in the stand-alone power supply test results to take into account variations when a load is applied to the power supply. The Case for Early Testing Often EMC testing is put off until the end of a project due to time, cost and workload constraints. Unfamiliarity with compliance testing also contributes to the perception of the difficulty of such testing. While the required equipment and facilities for EMC compliance testing can be unique, many testing labs are available with experienced staff to assist in the testing. The costs associatedwith compliance testing often become a ‘pay me now or pay me more later’ event. As testing is usually done at the end for full certification this cost can be high, but for preliminary screening the cost is minimal. Availability of lab time can be an issue as many

emissions and the filter components used to mitigate the EMC issues are similar but differ in electrical values. The conducted emissions frequency band is lower than the radiated emissions frequency band and thus the filter components used to address conducted emissions will be electrically and physically larger than those required to address radiated emissions. EMC for Power Supplies Most internally mounted power supplies are designed and tested to meet EMC regulations and the testing is performed with the supply configured as a stand- alone product. After the power supply has been installed into a system the completed system must also be tested to ensure it meets EMC regulations. Incorporating compliant power supplies into systems minimizes the potential for EMC related issues during system testing, but does not guarantee that the completed system will pass emissions testing. Many vendors of internally mounted power supplies will provide recommended circuits to address EMC issues encountered

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