A comparative study of electric power distribution systems for spacecraft

The electric power distribution systems for spacecraft are compared concentrating on two interrelated issues: the choice between dc and high frequency ac, and the converter/inverter topology to be used at the power source. The relative merits of dc and ac distribution are discussed. Specific converter and inverter topologies are identified and analyzed in detail for the purpose of detailed comparison. Finally, specific topologies are recommended for use in dc and ac systems

Analysis, Design and Implementation of a Resonant Solid State Transformer

This thesis discusses the design of a full-bridge resonant LLC Solid State Transformer. The proposed topology uses a high-frequency transformer which helps minimizing its cost and size, and enables operating at varying load conditions. By using a resonant circuit, soft switching is achieved. Commutation techniques are discussed, namely ZVS and ZCS. Both concepts are applied on different legs of the H-bridge. Pulse frequency modulation (PFM) and Phase Shifting Modulation (PSM) are utilized to control this resonant converter. One of the requirements of this work is to achieve a tightly regulated DC bus voltage. This was shown to be achieved using the proposed controller. An experimental setup was assembled and the controller was tested, the results match the simulation and calculation results. The SST setup was tested for two different power levels. The outputs confirm the validity of the controller in feeding the load and regulating the voltage within the desired switching frequency interval, while maintaining soft switching. A thermal analysis was conducted to calculate losses, and a conversion efficiency of 97.18% was achieved. Using a high frequency transformer, a reduction in size and cost is achieved as compared to conventional low frequency transformers that usually are large in size and require more material to be assembled (copper and iron). Design requirements and limitations, the proposed control scheme, modeling and implementation, and test results are provided in this thesis

Pushing the Boundary of the 48 V Data Center Power Conversion in the AI and IoT Era

openThe increasing interest in cloud-based services, the Internet-of-Things and the take-over of artificial intelligence computing require constant improvement of the power distribution network. Electricity consumption of data centers, which drains a consistent slice of modern world energy production, is projected to increase tremendously during the next decade. Data centers are the backbone of modern economy; as a consequence, energy-aware resource allocation heuristics are constantly researched, leading the major IT services providers to develop new power conversion architectures to increase the overall webfarm distribution efficiency, together reducing the resulting carbon footprint and maximizing their investments. As higher voltage distribution yields lower conduction losses, vendors are moving from the 12 V rack bus to 48 V solutions together with research centers and especially data center developers. As mentioned, efficiency is crucial to address in this scenario and the whole conversion chain, i.e. from the 48 V bus to the CPU/GPU/ASIC voltage, must be optimized to decrease wasted energy inside the server rack. Power density for this converters family is also paramount to consider, as the overall system must occupy as less area and volume as possible. LLC resonant converters are commonly used as IBCs (intermediate bus converters), together with their GaN implementations because of their multiple advantages in efficiency and size, while multiphase-buck-derived topologies are the most common solution to step-down-to and regulate the final processor voltage as they're well-know, easy to scale and design. This dissertation proposes a family of non-isolated, innovative converters capable of increasing the power density and the efficiency of the state-of-the-art 48 V to 1.8/0.9 V conversion. In this work three solutions are proposed, which can be combined or used as stand-alone converters: an ASIC on-chip switched-capacitor resonant voltage divider, two unregulated Google-STC-derived topologies for the IBC stage (48 V to 12 V and 48 V to 4.8 V + 10.6 V dual-output) and a complete 48 V to 1.8 V ultra-dense PoL converter. Each block has been thoroughly tested and researched, therefore mathematical and experimental results are provided for each solution, together with state-of-the-art comparisons and contextualization.The increasing interest in cloud-based services, the Internet-of-Things and the take-over of artificial intelligence computing require constant improvement of the power distribution network. Electricity consumption of data centers, which drains a consistent slice of modern world energy production, is projected to increase tremendously during the next decade. Data centers are the backbone of modern economy; as a consequence, energy-aware resource allocation heuristics are constantly researched, leading the major IT services providers to develop new power conversion architectures to increase the overall webfarm distribution efficiency, together reducing the resulting carbon footprint and maximizing their investments. As higher voltage distribution yields lower conduction losses, vendors are moving from the 12 V rack bus to 48 V solutions together with research centers and especially data center developers. As mentioned, efficiency is crucial to address in this scenario and the whole conversion chain, i.e. from the 48 V bus to the CPU/GPU/ASIC voltage, must be optimized to decrease wasted energy inside the server rack. Power density for this converters family is also paramount to consider, as the overall system must occupy as less area and volume as possible. LLC resonant converters are commonly used as IBCs (intermediate bus converters), together with their GaN implementations because of their multiple advantages in efficiency and size, while multiphase-buck-derived topologies are the most common solution to step-down-to and regulate the final processor voltage as they're well-know, easy to scale and design. This dissertation proposes a family of non-isolated, innovative converters capable of increasing the power density and the efficiency of the state-of-the-art 48 V to 1.8/0.9 V conversion. In this work three solutions are proposed, which can be combined or used as stand-alone converters: an ASIC on-chip switched-capacitor resonant voltage divider, two unregulated Google-STC-derived topologies for the IBC stage (48 V to 12 V and 48 V to 4.8 V + 10.6 V dual-output) and a complete 48 V to 1.8 V ultra-dense PoL converter. Each block has been thoroughly tested and researched, therefore mathematical and experimental results are provided for each solution, together with state-of-the-art comparisons and contextualization.Dottorato di ricerca in Ingegneria industriale e dell'informazioneopenUrsino, Mari

An increase of a down-hole nuclear magnetic resonance tool’s reliability and accuracy by the cancellation of a multi-module DC/AC converter's output’s higher harmonics

Abstract: Described in this paper is a method for improving higher harmonic cancellation in Nuclear Magnetic Resonance (NMR) transmitters, which are used in oil and gas well logging tools operating at 175°C. Multi-module multi-level topology which combines the outputs of several identical power modules operating at 50% duty cycle at the fundamental frequency provide the versatility needed for both low harmonic sine voltage synthesis and amplitude control. Cancellation of the output voltage higher harmonics is achieved by creating fixed relative phase shifts between the individual modules of the multi-module converter. The amplitude control employs the Chireix-Doherty outphasing modulation principle with added feed forward correction circuitry. The possibilities of a 20% increase of the tool signal to noise ratio (SNR), as compared to that of a two-module transmitter has also demonstrated significant increase in the tool life expectancy

Grid-Forming Converter Control Method to Improve DC-Link Stability in Inverter-Based AC Grids

As renewable energy sources with power-electronic interfaces become functionally and economically viable alternatives to bulk synchronous generators, it becomes vital to understand the behavior of these inverter-interfaced sources in ac grids devoid of any synchronous generation, i.e. inverter-based grids. In these types of grids, the inverters need to operate in parallel in grid-forming mode to regulate and synchronize their output voltage while also delivering the power required by the loads. It is common practice, therefore, to mimic the parallel operation control of the very synchronous generators that these inverter-based sources are meant to replace. This practice, however, is based on impractical assumptions and completely disregards the key differences between synchronous machines and power electronic inverters, as well as the dynamics of the dc source connected to the inverter. This dissertation aims to highlight the shortcomings of conventional controllers and derive an improved grid-forming inverter controller that is effective in parallel ac operation without sacrificing dc-link stability. This dissertation begins with a basis for understanding the control concepts used by grid-forming inverters in ac grids and exploring where existing ideas and methods are lacking in terms of efficient and stable inverter control. The knowledge gained from the literature survey is used to derive the requirements for a grid-forming control method that is appropriate for inverter-based ac grids. This is followed by a review and comparative analysis of the performance of five commonly used control techniques for grid-forming inverters, which reveal that nested loop controllers can have a destabilizing effect under changing grid conditions. This observation is further explored through an impedance-based stability analysis of single-loop and nested-loop controllers in grid-forming inverters, followed by a review of impedance-based analysis methods that can be used to assess the control design for grid-forming inverters. An improved grid-forming inverter controller is proposed with a demonstrated ability to achieve both dc-link and ac output stability with proportional power-sharing. This dissertation ends with a summary of the efforts and contributions as well as ideas for future applications of the proposed controller

Dynamic analysis and QFT-based robust control design of switched-mode power converters

The use of switched-mode power converters is continuously growing both in power electronics products and systems, e.g. in Telecom applications, commercial grid systems etc. The switching converters are required to provide robust behavior and to operate without instability under a variety of operation conditions. Hence the converter system may be subject to disturbances due to load, input voltage, and system parameter variations. In the thesis a robust control design procedure based on the QFT method (Quantitative Feedback Theory) is applied successfully for switching-mode DC-DC converters in order to achieve robust output in spite of different uncertainties. Simulation results are presented to demonstrate and validate the control design, showing good dynamic performance of the QFT controller. When designing large-scale systems it is often impractical to analyze and design the system as a whole. Instead, it is desirable to divide the system into manageable subsystems which can then be designed independently. The subsystems may then be connected together to form a complete integrated system. One of the major difficulties in integrated subsystems is the stability performance degradation due to the interaction between the subsystems. A formalism to analyze the interaction between subsystems using the unterminated two-port small-signal representation is derived. Two-port models are first defined as unterminated models, where the effect of load is excluded but may be easily included using the developed reflection rules. The use of the impedance ratio as a minor loop gain, which can be used to check system stability, is outlined. Recently, there has been increasing interest in the parallel operation of DC-DC converters for reasons of increasing system reliability, facilitating system maintenance, allowing for future expansion, and reducing system design cost. However, paralleled DC-DC converters require a systematic modeling methodology and a categorical current-sharing mechanism to improve a performance of the overall system. In order to achieve desirable characteristics when operating converter modules in parallel, a unified systematic approached for modeling of parallel DC-DC converter with current-sharing control, is proposed, developed, and analyzed