Theoretical and experimental results of power converter frequency modulation

Most of the power converters are controlled by a PWM signal based on a constant frequency and variable duty-cycle. A very large harmonic is expected at this frequency, spreading itself to the rest of the system. This paper is dedicated to spread spectrum clock generation (SSCG) theory applied to the reduction of EMI when a carrier or controlling constant frequency signal is present. SSCG is used to modulate square clock lines, PWM signals in a converter or data frames in a communication system to reduce EMI emissions. Instead of maintaining a constant frequency, SSCG systems modulate the switching (or clock) frequency following certain modulation profiles. Two goals are intended in the paper: (a) review the theoretical fundamentals of SSCG; and (b) compare the results of theoretical simulation with experimental tests to evaluate the possibilities of the method. For the experimental part, a "pure" sinusoid was modulated following several modulation profiles by using an arbitrary signal generator. The results were analyzed with an EMI compliant spectrum analyzer.