Considering EMC Emissions Early in the Design Process
If developers of power supplies do not want to experience an unpleasant surprise during the prescribed EMC conformity test, it is a good idea to consider the EMC as early as possible.
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One of the design activities often left to the end of a project is verifying that the product meets electro-magnetic compatibility (EMC) emissions requirements. EMC regulations help to ensure unintentional electro-magnetic conducted and radiated emissions do not interfere with other electronic devices. While delaying EMC compliance testing until the end of the project is a common practice, unexpected costs and project delays can be avoided by considering EMC compliance earlier in the design process.
Electro-magnetic conducted and radiated emissions are the radio frequency (RF) energy emitted by a product. The level of RF emissions are regulated to ensure they do not cause unreasonable harm to other electronic products. At low frequencies (less than about 30 MHz) the conductors and cables of most electronic devices are ineffective as antennas and thus radiated emissions are not an issue.
At these low frequencies the conductors and cables can conduct RF energy through shared power sources or loads and cause issues with other electronic products, while at high frequencies (above about 30 MHz) the impedances of the conductors and cables attenuate the conducted energy sufficiently to prevent it from being an issue. However, at these higher frequencies the conductors and cables can serve as antennas and radiate the RF energy with the ability to cause interference with nearby electronic products.
Most industrial and consumer electronic products sold in the United States are required to meet conducted and radiated emissions standards as described in FCC regulations Title 47 Part 15, often referred to as FCC Part 15. Similar standards for products sold in Europe are governed by European regulations CISPR 22/EN 55022. Both sets of these regulations describe limits for conducted and radiated emissions and are applied to the final system, including the internal or external power supply. While these two sets of regulations are created and administered by separate organizations they have been constructed to be similar or “harmonized”.
One benefit of harmonizing these regulations is that designing a product to meet one set of regulations typically ensures it will also satisfy the requirements set forth in the other set of regulations. Conducted radiation specifications cover emissions in the frequency range of 150 kHz through 30 MHz. A separate set of radiated emissions specifications covers the spectrum of 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 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 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.