EMI REDUCTION

• IntroductionIntroduction
• ApproachesApproaches
• OptimizationOptimization

The term electromagnetic interference (EMI) is typically used to refer to any unintentional power transfer between a source and a victim circuit. Spread spectrum is a technique introduced for mitigating EMI problems in many class of circuits. The main idea grounding this technique is the introduction of a controlled jitter in the timing signal (usually by means of Frequency Modulation) controlling the source circuit to avoid the perfect periodic behavior of all waveforms, thus spreading the power of their spectral components over a bandwidth which depends on the jitter characteristics. A classic example is shown in the following figure, showing both a downspread approach (case a) and center-spread approach (case b)

Fabio Pareschi, Riccardo Rovatti, and Gianluca Setti, "EMI Reduction via Spread Spectrum in DC/DC Converters: State of the Art, Optimization, and Tradeoffs," IEEE Access, Vol. 3, pp. 2857-2874. 2015. ISSN: 2169-3536. doi: 10.1109/ACCESS.2015.2512383

The most common solutions to achieve spreading is by using either a cubic (also known as Hershey-Kiss) modulation profile, or a triangular modulation profile. The first solution has been proposed and patented by Hardin, Fessler and Bush from Lexmark International, Inc. (a spin-off from IBM) and presents some advantages with respect to the standard and more common triangular profile (adopted in many standard, such as the Serial-ATA, PCI-express, etc.). Both solutions ensure, under the assumption of a slow modulation (i.e., a slowly varying cubic or triangular signal) a well-optimized EMI emission, with an almost flat EMI spectrum.

In recent years we focused on a different approach, i.e., we have proposed a pulse-amplitude modulated (PAM) spreading signal. This ensure a few advantages, including a continuous EMI spectrum (while classical solutions generate a discrete spectrum), and increased possibilities for a better spectrum shaping. See more details in references below.

Fabio Pareschi, Gianluca Setti, Riccardo Rovatti, and Giovanni Frattini, "A Spread Spectrum Clock Generator Based on a Short-Term Optimized Chaotic Map," Proceedings of 37th European Solid-State Circuits Conference (ESSCIRC2011), pp. 507-510. ISBN: 978-1-4577-0704-9. Helsinki (Finland), September 12-16, 2011. doi: 10.1109/ESSCIRC.2011.6044933

Sergio Callegari, Riccardo Rovatti, and Gianluca Setti, "Spectral properties of chaos-based FM signals: Theory and simulation results," IEEE Transactions on Circuits and Systems I: Fundamental Theory and Applications, Vol. 50, No. 1, pp. 3-15, January 2003. ISSN: 1549-8328. doi: 10.1109/TCSI.2002.807510 - Winner of the 2004 IEEE CAS Darlington award  

Compulsory international regulations impose/guarantee the electromagnetic compatibility (EMC) of any electronic device. According to these, EMC is linked with: i) the ability to fit the power spectrum of any interfering signal radiated/conducted by a circuit/system under a prescribed mask; ii) the ability of a circuit/system to withstand incoming electromagnetic interference at a given level. International regulations also require measurements to be taken with an EMI receiver, that is basically an analog spectrum analyzer with some additional input filters. Furthermore, they impose a prescribed setting measurements method.

In order to investigate optimization of spread spectrum systems, we developed a simple but effective mathematical model of the EMI receiver. A Matlab code simulating an EMI receiver based on this model is also available for download here . With this, we were able to find which modulation parameters set ensures, under the assumption that measurements are taken accordingly to international regulations, to maximized the EMI reduction. We are able to investigate both the standard triangular case, where we can determine the optimal triangular waveform frequency maximizing performance, and the random PAM modulation, where we can find which random signal statistical features ensure optimal performance.

Fabio Pareschi, Gianluca Setti, Riccardo Rovatti, and Giovanni Frattini, "Short-term Optimized Spread Spectrum Clock Generator for EMI Reduction in Switching DC/DC Converters," IEEE Transactions on Circuits and Systems I - Regular Papers, Vol. 61, No. 10, pp. 3044-3053. October 2014. ISSN: 1549-8328. doi: 10.1109/TCSI.2014.2327273

Fabio Pareschi, Gianluca Setti, Riccardo Rovatti, and Giovanni Frattini, "Practical Optimization of EMI Reduction in Spread Spectrum Clock Generators with Application to Switching DC/DC Converters," IEEE Transactions on Power Electronics, Vol. 29, No. 9, pp 4646-4657. September 2014. ISSN: 0885-8993. doi: 10.1109/TPEL.2013.2286258