Advanced resampling techniques for PWM amplifiers in real-time applications

Abstract

Regularly sampled pulse width modulation (PWM) has been a mainstay of the power electronics community since the advent of digital controllers. In this form of PWM the modulating signal is sampled only at either the peaks and/or the troughs of the triangular carrier waveform, then held constant until the next sampling instant, which allows ample time for the calculation of switching instants. Unfortunately, it produces phase and amplitude distortion of the modulating signal that can be significant at low pulse numbers. In applications where the desired modulating signal is not known a priori, this phase delay can become a critical limitation. The analogue alternative to regular sampling is naturally sampled PWM, where the modulating signal is permitted to vary as a continuous waveform. This form of PWM does not apply any phase or amplitude distortion to the modulating signal. True natural sampling is, however, impossible to implement on the digital control platforms that are used in modern power electronics applications. In this paper, a type of hold circuit known as a First-Order Hold (FOH) circuit is used in conjunction with the technique known as resampled regular PWM to improve upon the limitations of regular sampling. It is found that this augmentation to the PWM strategy improves both the linearity and phase delay of the modulator as well as (for high pulse numbers) harmonic performance. The FOH circuit has an overshoot in its transfer function which should be avoidable if the modulating signal is sampled at a sufficiently high rate.