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 (SMPS) which commonly use standard pulsewidth modulation (PWM). In this paper, Sigma-Delta (Ca) modulation is proposed as an alternative switching technique to reduce conducted EMI from SMPS. The result of using Ca 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 Ca modulators are presented to prove the viability of the scheme for EMI mitigation. These modulators are then applied to a DC-DC converter in an off-the-shelf computer power supply and experimental results show a reduction of roughly 5-10dB mu V in EMI emissions over standard PWM modulators.