Performance improvement of current-controlled PWM inverter by means of dithering

This paper describes a novel current control strategy of a voltage-source PWM (pulse width modulation) inverter by means of dithering. There have been two principal methods to achieve the current control, i.e. a sinusoidal PWM (sub-harmonic) method by using PI regulators and a triangular wave carrier, and a relay (bang-bang) control method by using hysteresis elements as current regulators. It is inherently impossible to achieve PWM-pattern optimization, quick response and current-error minimization with those conventional methods at the same time. The method proposed in this paper is based on dithering of the hysteresis bands in the current regulators, where thresholds of the hysteresis elements are modulated by a triangular wave with minute amplitude and high frequency. The control characteristics of this system mostly depend on the amplitude of the dither signal and all the drawbacks of the conventional methods can be simultaneously overcome by setting the amplitude as small as the hysteresis bands. This paper presents a basic concept of the proposed method and the operating characteristics compared with the commonly known current control methods. Consequently, remarkable performance improvement by the proposed method has been confirmed through experimental tests as well as computer simulations.