Power-Conversion Techniques for Complying with Automotive Emissions Requirements
This paper introduces the sources and types of emissions typically seen in power supplies. Various methods to mitigate emissions are presented including filtering, component selection, printed circuit board (PCB) layout and circuit techniques. The goal is to enable the reader to design power converters that pass EMI test requirements. A case study is included to experimentally demonstrate the relative effectiveness of the mitigation techniques discussed in an example application. The example converter is designed for a nominal 13.5 V input and 3.3 V, 5 A output and achieves CISPR 25 Class 5 limit compliance for conducted emissions.
Automotive environments present challenges for making low EMI power solutions. Vehicles are known for considerable vibration and mechanical stress. Certain construction methods used in other applications to reduce parasitic inductance are not able to withstand the stresses of the automotive environment. Components with leaded packages are preferred at times for low-cost manufacturing and inspection but tend to increase parasitic inductance. Sometimes two series-connected ceramic capacitors are placed on a PCB at a 90 degree angle to provide redundancy in case one of them fails short. This technique also increases parasitic inductance. The operating temperature range is also quite wide, often from -40°C to 125°C, and cooling fans may not be an option. Despite these challenges, design of low EMI power converters is achievable.
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