Analysis and control of wide output voltage multi-level buck converters

Power supplies are a critical element of modern electrical systems. For emerging applications, conventional fixed-output power supplies may be unable to meet increasingly diverse load regulation requirements, particularly when it comes to ensuring the power supply’s output does not damage the load from unexpected voltage spikes or uncontrolled output current. This thesis investigates the development of a Wide Output Voltage converter capable of delivering a broad voltage and current range with precise control of the output voltage and current, that could be used in a modular, multiple-output ac-dc power supply, enabling a solution that combines the capabilities of multiple power converters into one. A comprehensive review of state-of-the-art wide output voltage architectures identified the Point of Load stage as the most effective location to increase the output voltage range without compromising independent operation. The Multi-Level buck converter emerged as a promising topology, offering improved efficiency, reduced component stress, and lower filter requirements compared to conventional “Two-Level” converter designs. A detailed loss analysis of multiple wide-output voltage converters using analytical models confirmed that the Three-Level buck converter outperforms equivalent single- and two-phase topologies across the whole output range, with significant gains in thermal headroom and potential power density. The power loss calculations were verified using three prototype converters, designed for a 1 V to 30 V, 0 A to 16 A, 300 W output range, which covers nearly 90 % of the duty cycle range of the converters. The literature review found that for wide output voltage operation, the control schemes available for the three-level converter were limited, particularly those that could provide cycle-by-cycle current control over the full output range. To determine if this affected the output regulation of the three-level converter, its dynamic performance was compared with that of other converters, using both Voltage Mode Control and Current Mode Control schemes to quantify their output regulation performance fully. This was achieved analytically using simulated models to characterise the frequency-domain and time-domain performance of the converters in both load and voltage steps. The dynamic performance evaluation revealed that existing control schemes for Three-Level converters suffer from poor transient response, particularly during large voltage steps, which was verified with the same prototype converters. To address this, a novel Hysteretic Current Mode Control scheme was developed, enabling proper cycle-by-cycle current regulation over the entire output range while maintaining high efficiency and robust stability. This control method was validated through simulation and experimental prototypes, demonstrating compatibility with an existing modular ac–dc system with minimal impact on its own regulation during load transients or input voltage variations. The results establish that a Three-Level buck converter with HCMC can deliver a wide range of output voltages while providing higher efficiency and robust load handling in a modular system. The work described in this thesis presents a validated topology and control scheme that can be applied to wider voltage and current output converters, including potential extensions to higher-level converter architectures to leverage the benefits of multi-level converters further.

Funder

Research Ireland
Advanced Energy

Publisher

University of Galway

Rights

CC BY-NC-ND

cbn

Collections

University of Galway Theses (PhD Theses)