Analysis and spectral characteristics of a spread-spectrum technique for conducted EMI suppression

Frequency modulation (FM) and random switching methods have been used for r educing conducted electromagnetic interference (EMI) in power converters. L imited theoretical studies and comparisons of these schemes, however, are a vailable. In this paper, a detailed analysis and the spectral characteristi cs of a random carrier-frequency (RCF) technique for suppressing conducted EMI in an offline switched-mode power supply are presented. The analysis pr ovides a theoretical platform for studying the characteristics of this rand om switching scheme. The level of randomness is defined for the RCF scheme and varied in the converter example so that its effects on the power spectr a can be demonstrated. Theoretical predictions of the spectral characterist ics of this scheme are confirmed with measurements. The RCF scheme has been compared with the standard constant-frequency pulsewidth modulation (PWM) scheme and the FM scheme. Comparisons of their spectral performance show th at the RCF scheme has better conducted EMI suppression than the FM and stan dard PWM schemes.