Testing and reducing electromagnetic interferences in switched mode power suplies

One of the most preferable types of the switched mode power supplies (SMPS) is the flyback converter due to its wide application areas. The properties such as low cost, providing multiple output terminals, higher efficiencies make it important, hence its usage in cell phone chargers, PCs, set-top boxes, TVs, etc. increases. Due to its working principles and electromagnetic interference (EMI) performance, this type of converter requires extra care in the design process. This thesis reviews the most common methods of mitigating EMI in SMPS and discusses the spread spectrum frequency modulation (FM) technique, which is increasingly used in recent years to achieve this goal. The modulation technique that is applied to the pulse width modulation (PWM) switching signal of the flyback converter is analyzed and the effect on EMI performance is discussed. The spectrum of a frequency modulated PWM signal has theoretically been derived by using Fourier analysis. Based on the FM theory and the operation principle of the flyback converter, two application circuits have been built in this work. A 24W AC-DC flyback converter with the L6566B integrated circuit which has a dedicated FM pin, has been built as a prototype. A 5W DC-DC flyback converter that is driven by an FM modulated PWM signal has also been designed. The EMI spectrums of these two converters have been measured and test results are given. According to these results, FM technique provides better EMI performance.