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In this paper, dithered sigma-delta modulation (DSDM) is proposed as an effective modulation scheme for the switching converter, to suppress harmonic spikes. The proposed scheme has a simple configuration, which comprises a 1/sup st/-order sigma-delta modulator and a random dither generator. The 1/sup st/-order sigma-delta modulator produces a switching pulse waveform that has the same low-frequency component as the reference input, while the random dither spreads the distribution of sampling numbers per switching cycle, and thus diminishes harmonic spikes in the output. The relationship between the harmonic spectra and sampling numbers per switching cycle is studied through computer simulations, and performances are confirmed in an experimental setup.