What is it about?
A fully digital, nonlinear, wide-bandwidth current controller for dc-ac and dc-dc voltage source converters is presented in this paper. Exploiting oversampling, the controller mimics an analog hysteresis current controller, but it does not employ analog comparators, digital-to-analog converters, or any other analog signal pre- or postprocessing circuitry. Indeed, it fully virtualizes the hysteresis controller's operation and, based only on a nonlinear, efficient current error processing algorithm, drives the power converter at almost constant switching frequency. Overall, it offers the same excellent dynamic performance of the analog hysteresis controller and, at the same time, solves most of the related problems. Because the current error sample processing algorithm is inherently parallel in structure, the controller is suited for VHDL synthesis and field-programmable gate-array implementation, which guarantees flexibility and low cost, together with minimum computation and signal conversion delays. Its intended application areas include active filters, uninterruptible power supplies, microgrid distributed energy resource controllers, laboratory battery testers, and welding machines.
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Why is it important?
In this paper, a new digital current controller is presented that operates as a virtual analog hysteresis current controller, taking advantage of a high-performance analog-to-digital converter (ADC) and a field-programmable gate-array (FPGA) circuit implementation. It is designed as a nonlinear current error filter and requires a considerable oversampling factor, as compared with the natural synchronous sampling of pulsewidth-modulation (PWM)-based current controllers. It overcomes the typical limitations of conventional hysteresis controllers, guaranteeing regulated switching frequency, with a few percent accuracy in the steady state, together with minimum sensitivity to dead times, sampling, and computation delays. The paper organization comprises, in the first place, the detailed explana- tion of the algorithm operation and of its design criteria. Suc- cessively, the effectiveness of the proposed solution is verified, first through simulations and then on a 3-kVA single-phase full- bridge inverter. Experimental results are presented that allows assessing the static and dynamic performances of the controller.
Perspectives
The controller is designed to approximate the steady state and dynamic performance offered by an analog hysteresis current controller. It is implemented on an FPGA chip and exploits the oversampling of the current error signal allowed by a fast ADC. In addition, it effectively compensates the variable switching frequency effect that typically characterizes hysteresis current controllers yet minimizing analog signal processing. This is limited to what is strictly required for the current error signal acquisition. Accordingly, no DACs or analog comparators are required. Its operation results into an almost constant switch- ing frequency in the steady state and, at the same time, into the wide small-signal bandwidth and minimum large-signal response delay typical of nonlinear controllers.
Dr Tommaso Caldognetto
University of Padova
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This page is a summary of: A Nonlinear Wide-Bandwidth Digital Current Controller for DC–DC and DC–AC Converters, IEEE Transactions on Industrial Electronics, December 2015, Institute of Electrical & Electronics Engineers (IEEE),
DOI: 10.1109/tie.2015.2465351.
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