Analysis of Random Pulse-Width Modulation Techniques for Power Electronic Converters

The research documented in this thesis addresses random pulse-width modulation (PWM) and its applications in hard-switched power electronic converters. This emerging PWM technology has gained considerable interest in academia during the last decade, although industrial applications are still few in number. The key property that differentiates random PWM from classic PWM, which generates time-periodic switching functions, is that random PWM produces switching functions that have a non-deterministic (random) component. For a well-designed random PWM technique it is, nonetheless, still possible to get an accurate synthesis of the commanded reference waveform, i.e. random PWM behaves exactly as its deterministic counterpart with respect to generating a switching function that allows the reference signal to be extracted by, typically, low-pass filtering. As a consequence of the non-repetitive switching functions, the frequency-domain spectra for randomized modulators are very different from the spectra caused by a deterministic modulation strategy. Essentially, the spectrum for classic PWM consists of discrete frequency components clustered around multiples of the PWM carrier frequency, whereas random PWM — as least partially — transfers the power carried by the harmonics into the continuous density spectrum. This spectral spreading property is the key to understand the current interest in random PWM. In particular, it has been demonstrated by experiments in the literature that the otherwise annoying tonal acoustic noise emitted from ac motors and other magnetic components in converter-based systems operating with a carrier frequency in the audible range may be alleviated substantially from a psycho-acoustic point of view in an inexpensive manner by using random PWM instead of deterministic PWM. Other investigations have shown that random PWM may also be used to obtain compliance with standards for electro-magnetic compatibility with less filtering/shielding efforts, because the spectral peaks are reduced compared to deterministic PWM operation. Deterministic modulators are well understood, but many details relating to random PWM have not yet been properly analyzed from a theoretical point of view. This research attempts to narrow this gap by providing thorough analyses of different random PWM schemes that are considered possible candidates for industrial applications. The main emphasis is put on the standard three-phase voltage-source converter which is widely used for ac drives, active mains rectifiers, uninterruptible power supplies, etc., although much of the work is of general validity in power electronics. Prior to giving a detailed analysis of specific random PWM strategies, the basic principles for random PWM are discussed. It is shown how randomized modulators may be categorized and how new modulators may be constructed by concatenation of random PWM schemes belonging to different categories. From a practical point of view, however, only a few of the resultant random PWM techniques are of any immediate relevance. Therefore, the main focus is put on simple strategies that rely on dithering of either the PWM carrier frequency, the pulse positions, or possibly the pulse widths.