Evaluation of switching frequency modulation in EMI emissions reduction applied to power converters

Although most of the power converters are currently designed to operate by using a constant switching frequency and a variable duty-cycle, some attempts were made in order to verify the effect of modulating the switching frequency [F. Lin et al., 1992] and how this modulation affected the power converters EMI emissions. As known, spread spectrum clock generation (SSCG) modulates the originally constant switching frequency by following a certain modulation profile in order to spread the single harmonic energy into an amount of side-band harmonics having the same energy but much smaller amplitudes. This reduction technique has been used and implemented for high frequencies (as those related to clock frequencies in communications and microprocessors systems). This paper is dedicated to SSCG applied to the reduction of EMI emissions in power converters, focusing on the effectiveness of frequency modulation in EMI reduction as a function of the different switching frequency ranges and modulation profiles. Theoretical results were obtained just modulating a sine pure wave following several modulation profiles [A. Santolaria et al., 2002], this one representing each one of the harmonics composing the real square PWM-signal controlling the power converter. A practical arrangement was implemented to generate and measure EMI emissions.