EMI reduction by optimizing the output voltage rise time and fail time in high-frequency soft-switching converters

In order to ensure the compatibility between devices, several electromagnetic compatibility (EMC) standards specify the maximum permitted levels of radiated noise. To comply with these standards, hard switching power converters need to use expensive solutions, namely shielding, Filtering techniques and snubbers, because the voltages have low values of rise and fall times. This fact leads into significant harmonic content at high frequencies. In this case, the most cost effective way to deal with radiated noise is to reduce it at the source. A way to achieve this goal is to impose moderate values of output voltage rise and fall times. This paper theoretically and experimentally explains, that the harmonic content at high frequencies can be significantly reduced with a selection of a suitable value of voltage rise time and fall time. Based on the study of a generic trapezoidal waveform, simple but precise equations, that permit to easily evaluate the envelope of the frequency spectrum, are derived. These equations also permit to analyze the importance and the influence of the rise and fall times in the spectrum envelope. Finally, with this simple analysis, the influence of the values of rise time and fall time in more complex waveforms are shown, as the output voltage waveform of a high frequency cycloconverter. The validity of the theoretical analysis is confirmed by experimental results.