Use of sigma-delta modulation to control EMI from switch-mode power supplies

Conducted electromagnetic interference (EMI) is a major cause of concern in switch-mode power supplies (SMPSs) which commonly use standard pulsewidth modulation (PWM). In this paper, Sigma-Delta (Sigma Delta) modulation is proposed as an alternative switching technique to reduce conducted EMI from an SMPS. The result of using Sigma Delta modulation is a spread in the spectrum of the conducted emissions so that large concentrations of power at discrete frequencies are avoided. Experimental time-domain waveforms and spectra of the switching function of first-order and second-order Sigma Delta modulators are presented to prove the viability of the scheme for EMI mitigation. These modulators are then applied to a dc-ac converter in an off-the-shelf computer power supply and experimental results show a reduction of roughly 5-10 dB.muV in EMI emissions over standard PWM modulators.