Low Frequency Analysis of EMI Filters

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Dr. Kovácsházy Tamás
Department of Measurement and Information Systems

Nowadays during the designing phase of an electrical device we have to place a heavy emphasis on the suppression of EMI (Electronic Magnetic Interference), which is solved by using EMI filters. This is especially true for the automotive industry, where besides the safety-critical designing, the EMC (Electronic Magnetic Compatibility) limit requirements of the industrial standards have to be observed. So far, the EMI filters have been built with tantalum or electrolyte capacitor, but with the appearance of the MLCC (Multi-layer Ceramic Capacitor), the development of this field has been started. In the high frequency domain the MLCC-s have more favorable properties, however we do not have to forget about the low frequency domain behavior of these capacitors.

In this thesis, I determine the noise sources and mathematical model of different filter structures and I calculate the voltage transient levels before and after the filter.

After this I design and build a measurement system, what is capable of different noise sources generations with changeable parameters (PWM frequency, Duty Cycle), then I will verify my calculation result with measurements.

Finally I will connect a DC/DC converter buck after the examined filters with low voltage supply and I try to determine if the MLCC EMI filter could cause self-oscillation or an accidental brown out reset.


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