EMI reduction in switched power converters by means of spread spectrum modulation techniques

Spread spectrum clock generation techniques (SSCG) were originally developed to reduce EMI in communications and microprocessor systems working in the range of hundreds of MHz. The working principle consists of modulating the original constant clock frequency, in order to spread each single harmonic energy into a certain frequency band, thus reducing amplitudes at each individual frequency. Nowadays, the switching frequency of power converters has been increasing up to values that make feasible the application of such techniques to reduce EMI emissions in power circuits. This paper deals with SSCG applied to reduction of EMI emissions in switched power converters, following certain modulation profiles. It is focussed on studying the effectiveness of EMI reduction as a function of frequency ranges, modulation profiles and other modulation parameters. A buck converter, whose switching frequency can be swept from 100 kHz to 1 MHz, has been used for experimental validation. Conducted disturbances produced by the converter, using different modulation parameters, were measured and compared with those produced by the same converter when driven with the classical constant frequency, variable duty, switching pattern. Significant reductions in conducted EMI may be observed. Optimal modulation profiles and parameters are identified.