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Designing a low-EMI power supply

As electronic systems become increasingly dense and interconnected, reducing the effects of electromagnetic interference (EMI) is becoming an increasingly critical system design consideration. EMI can no longer be an afterthought, as it can cause significant setbacks late in the design phase that cost both time and money. TI offers multiple features and technologies to mitigate EMI in all of the frequency bands of interest. Our devices and technologies can help designers not only improve filter size and cost but also reduce design time and complexity. To help you design a more efficient power supply that meets EMI requirements, start here with our comprehensive training series where we first provide the fundamentals of EMI and then dive deeper into the specific IC and package innovations, design tips regarding external component selection and layout and conclude with application-specific EMI topics.

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      Hello and welcome to TI's Low EMI Video Series. In this brief overview, I'd like to introduce the basics of electromagnetic interference, the relevant standards that need to be adhered to, and the advanced techniques utilized with TI's new generation of devices to help mitigate EMI and provide an easy-to-use solution that does not compromise on efficiency or power density.

      As electronic systems get more sophisticated, diverse circuits are packed in close proximity to one another, improving the functionality and reducing the form factor of the eventual solution. Take the example of this compact camera module for automotive driver monitoring systems. It combines a 2-megapixel imager with a 4-gigabits-per-second serializer and 4-channel power management integrated circuit, all packed into a small form factor.

      The byproduct of this improvement in complexity and density is that sensitive circuitry like the imager and signal processing elements sit very close to the power management IC, which carries large currents and voltages. This inevitably would lead to a set of circuits electromagnetically interfering with the functionality of the sensitive elements if careful attention is not paid during design of these systems.

      Electromagnetic interference can manifest itself in two forms. As illustrated by this diagram, the sensitive system, which in this case is the radio, is affected through conductor means by the interfering motor since they both share the same outlet for power. The motor also affects the working of the radio through electromagnetic radiation that's caught below the air and is picked up by the radio's antenna.

      When end equipment manufacturers integrate components from various sources, the only way to guarantee that the interfering and sensitive circuits can co-exist is through the establishment of a common set of rules where the interference is limited to a certain level, and the sensitive circuits are capable of handling that level of interference.

      These rules are established in industry standard specifications such as CISPR 25 for automotive equipment and CISPR 32 for multimedia equipment. CISPR standards are critical for EMI design, as they will dictate the targeted performance of any EMI mitigation technique. The standards are broken into conducted and radiated limits, depending on the mode of interference. The bars and the plots here represent the maximum conducted emission limits that can be tolerated while measured through standard EMI measuring equipment for the device under test.

      Similar standards also exist for radiated interference limits, as shown for automotive equipment in this chart. More details about the exact standard and the test methodology can be found in the link shown.

      Now that we have seen what typically EMI standards need to be adhered to, let's take a closer look at what are the primary causes of EMI. One of the most common circuits in modern electronic systems is the switch mode power supply-- SMPS-- which provides drastic improvements in efficiency over linear regulators in most applications. But this efficiency comes at a price, as the switching of power FETs in the SMPS causes it to be a major source of EMI.

      The nature of switching in the SMPS leads to discontinuous input currents, fast edge rates on switching nodes, and additional ringing along switching edges, caused by parasitic inductances in the power loop. While the discontinuous currents impact EMI in the sub 30 megahertz bands, the fast edges of the switching node and the ringing impact EMI in the 3,200 megahertz and the greater than 100 megahertz bands, respectively.

      In conventional designs, the EMI generated by the switching converters are mitigated using two main methods, both of which have an associated penalty. To deal with the low frequency emissions and meet appropriate standards, large passive are placed at the input of the switching converters, leading to a more expensive, less power-dense solution.

      The high frequency emissions, on the other hand, are typically mitigated by slowing down the switching edges by effective gear drive design. While this helps in reducing the EMI in the greater than 100 megahertz band, the reduced edge rates lead to increased switching losses and hence, a lower efficiency solution. Thus, effectively, there's an inherent density and efficiency trade-off that needs to be made for low EMI solutions.

      To break this trade-off and obtain the combined benefits of high power density, high efficiency, and low EMI, a host of techniques, as listed here on this slide, are employed by the switching converters and controllers built by Texas Instruments. These techniques are tailored to specific frequency bands of interest and are described in depth in the following videos that are available in this Low EMI Video Series.

      Topics
      Expand all
      The fundamentals of EMI (2)
      Reducing EMI through IC and package innovations (7)
      Reducing EMI through input filter design, PCB and BOM optimization (5)
      Application-specific EMI considerations (5)
      View series

      Designing a low-EMI power supply

      Expand all
      The fundamentals of EMI (2)
      Reducing EMI through IC and package innovations (7)
      Reducing EMI through input filter design, PCB and BOM optimization (5)
      Application-specific EMI considerations (5)