Theoretical spectra for narrow-band random PWM waveforms

Pulse-width-modulated (PWM) power electronics systems often produce objectionable acoustic noise at the PWM switching frequency and multiples of it. Subjectively this noise may be reduced by randomising the switching frequency, thereby spreading the acoustic energy over a wider bandwidth. The paper presents a theoretical basis for predicting the output spectra of PWM power converters under these random switching conditions. The technique is developed for a DC chopper system where theoretical spectral bands can be compared with exact spectra obtained by direct Fourier transform methods, but is then extended to three-phase applications. The paper also discusses strategies for selecting random switching sequences to improve the noise dispersion while reducing the power converter´s complexity