New-Tech Europe Magazine | August 2017

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 systemmust 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 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

Figure 1: Conducted and radiated emissions limits

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

30 MHz and greater. Test procedures and tools are slightly different for conducted versus radiated 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

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