Non-Isolated High Step-Down DC-DC Converters for LVDC Application

In the early stage, AC prevailed over DC mainly because of the following reason: the presence of the low frequency transformer, which enables to step-up AC voltages for transmission purposes, and which enabled to generate the electricity centrally, far from the main economical and populational centers (AC offered lower losses for longer distances at that time). From that point on, AC was massively used to implement our today's generation, transmission and distribution systems. Nowadays, however, due to the advent of renewable energy sources in our modern grid, DC voltage is proving to be an alternative for our next generation grid design. The main DC energy technologies available and being introduced in the grid are the following: photovoltaics, batteries, fuel cells, super capacitors, among others. To integrate these technologies in a DC system, power electronic converters are essential to adjust and regulate the voltage levels accordingly; to control and adjust the power flow; to maximize the energy generation, e.g. maximum power point tracking in photovoltaics; to guarantee grid stability, e.g. voltage balancer converters; and for safe operation and protection, e.g. in DC circuit breakers. All the applications mentioned above are based on power electronic converters and, therefore, these devices are the breakthrough technology for massive DC grid penetration in our generation, transmission and distribution systems. In the DC world, different applications require different converter specifications and structures. The DC-link voltages of these systems are being implemented in a broad range e.g. between 48-800 V. Additionally, low voltage/low power equipment's are massively present in the grid. Therefore, high step-down DC-DC conversion is required to comply with these voltage profiles. Different strategies to step-down voltages can be used to achieve systems requirement, e.g. application of high frequency isolated transformers, coupled-inductor techniques, voltage/current multiplier/divider cells, to name a few. Isolated topologies and topologies that implement coupled-inductor techniques are able to achieve high ratios by selecting the proper transformer/coupled-inductor winding ratio. However, these techniques present high magnetic complexity. To achieve high ratios without the implementation of transformer and coupled-inductor techniques, the voltage/current multiplier/divider cells mentioned above are used. These techniques are investigated in this thesis. The introduction of these strategies increase the power stage complexity and component part count of the circuits. Therefore, a careful analysis is important to maximize the performance of these converters. The main objective of this thesis is to propose new solutions for DC-DC conversion with high step-down voltage capability for applications in, e.g. industrial and residential systems, datacenters, electric vehicles, aircrafts, among others. To do so, converters with high step-down capability applying voltage and current multiplier/divider cells are the main topic tackled in this thesis. As previously mentioned, converters that apply these techniques often involve a high power stage complexity and component part count. To fully address the advantages of these converters, it is important to evaluate the switch technologies available on the market, as for instance Silicon and Gallium Nitride semiconductors, and their performance in highly complex circuits. Gallium-Nitride semiconductors are proven to have better theoretical characteristics compared to Silicon. These devices are, therefore, theoretically and practically compared to each other in some of the high step-down converters proposed in this thesis and conclusions related to their overall performance are discussed. This thesis proposes 12 new converters and the high step-down capability of each of them is evaluated. These converters are based on multiplier/divider cells and the performance of Silicon and Gallium Nitride technologies applied in these circuits is addressed. There is a strong emphasis on the prototype development and practical validation of the proposed converters. Discussions on the circuit's performance are shown together with a comparison between the proposed solutions and state-of-the-art converters, showing their benefits and drawbacks. In the end, it has been proved that, with a careful theoretical analysis, the correct implementation of voltage divider cells and converter validation, a high step-down DC-DC conversion operating with satisfactory performance is obtained, enabling its applications in the next generation of power electronics-based DC grids.

Publication year:2020

Accessibility:Open