Feed-forward Compensator Design with countermeasures against the non-realizable modulation delay inversion for Current Control of PWM converters

Abstract

The disturbance rejection capability of a PWM linear current controller can be enhanced if the feedback controller is augmented with a feed-forward compensator that processes the measured external disturbances. Indeed, the inverse model of the plant is needed to construct the transfer function of the feed-forward compensator. In case of PWM based-linear control, the modulator forms inherently part of the plant and it is typically approximated in the continuous domain as a dead-time element. The consequence is that the feed-forward compensator design for the PWM linear controller implies to deal with the non-realizable delay inversion problem. For this reason, conventional approaches for the design of the feedforward compensator disregard the non-realizable part and only implement the realizable part that most of the times ends in the implementation of a purely static gain. In fact, feed-forward compensators based on a lead-lag compensator structure and a static gain that interacts with the feedback controller can be used to tackle the feed-forward design problem when the non-realizable delay inversion arises. The goal is to improve the disturbance rejection capability of the entire current control system by minimizing the integrated absolute error (IAE) against disturbance changes. This paper covers the design of interactive feed-forward compensators with the lead-lag structure for the current control of PWM converters. Moreover, comparative evaluations between the interactive structure and conventional feedforward compensators are carried out. As main conclusion from the comparative evaluation, it was found that the interactive lead-lag feed-forward structure reduces the IAE index in case of step disturbances in a 40% to 60% in comparison with conventional feed-forward structures. Additionally, the interactive lead-lag structure avoids output current overshoots and provides faster settling times against disturbance changes.