Innovative Digital dc-dc Architectures for High-Frequency High-Efficiency Applications

The new generation of automotive controllers requires a space-constrained and high-efficiency step-down architecture. Hence, recently a potential alternative for the conventional step-down topologies is highly demanded. The new architecture should meet the high power density, high efficiency, wide operating ranges, high EMI capabilities, and low-cost requirements. This thesis, developed at the University of Padova and sponsored by Infineon Technologies, aims at investigating potential candidate topologies for automotive step-down conversion capable of eliminating or offsetting some of the common shortcomings of conventional solutions currently in use. Many research effort is paid for the soft switching quasi-resonant topologies in order to miniaturize the passive components through the switching frequency increase. However, the variable switching frequency, increased components count, and narrow operating ranges prevent the wide adoption of the quasi-resonant topologies in the target application. The first objective of this project is to investigate the quasi-resonant buck converter topology in order to stand on the limitations and operating conditions boundaries of such topology. The digital efficiency optimization technique, which is developed in this work, extends the operating ranges in addition to reduce operating frequency variations. On the other hand, the multilevel hybrid topologies are potentially able to meet the aforementioned requirements. By multiplying ripple frequency and fractioning voltage across the switching node the multilevel topologies have the direct advantage of reduced passive components. Moreover, multilevel topologies have many other attractive features include reduced MOSFET voltage rating, fast transient response, a Buck-like wide range voltage conversion ratio, and improved efficiency. These features candidate the multilevel topologies, in particular, the three-level flying-capacitor converter, as an innovative alternative for the conventional topologies for the target application. Accordingly, the three-level flying-capacitor converter (3LFC) is investigated as a second objective for this project. Flying-capacitor (FC) voltage balancing in such topology is quite challenging. The 3LFC under valley current mode control shows an interesting performance, where the FC voltage is self-balanced. In this work, the stability of the converter under valley and peak current mode control is studied and a simplified stability criterion is proposed. The proposed criterion address both current loop static stability and FC voltage stability. The valley current mode modulator results to be inherently stable as soon as the current static instability is compensated with an external ramp. On contrary, the FC voltage in peak current mode control (P-CMC) will never be balanced unless the converter operated with relatively high static peak-to-peak inductor current ripple. Since P-CMC has an inherent over-current protection feature, P-CMC based architectures are widely used in the industrial applications. However, in practice the peak current controlled three-level converter is inherently unstable. Consequently, the instability of the P-CMC 3LFC is addressed. A sensorless stabilizing approach, with two implementation methodologies, is developed in this work. The proposed technique eliminates the instability associated with the FC voltage runaway, in addition to FC voltage self-balancing. Moreover, the proposed methodology offers reduced size, less complexity, and input voltage independent operation. Besides, the proposed approach can be extended to system with a higher number of voltage levels with minimal hardware complexity. The proposed techniques and methodologies in this work are validated using simulation models and experimentally. Finally, in the conclusions the results of the Ph.D. activity are summarized and recommendations for the further development are outlined

Analysing integrated renewable energy and smart-grid systems to improve voltage quality and harmonic distortion losses at electric-vehicle charging stations

Due to environmental impacts of fossil fuels, a move towards using Electric-Vehicles (EV) to reduce carbon emissions and fossil fuels is regarded as a good solution to the climate change problem. In recent years, a dramatic increase of EV and charging stations has raised voltage quality and harmonic distortion issues that are affecting the electrical grid network. To address these issues there is a need to redesign the integrated renewable energy and smart grid network by applying new methodologies. The aim of this work is to propose an isolated smart micro grid, which connects renewable energy generation units to the electric vehicles charging station without degrading voltage quality or causing harmonic distortion losses. A topology has been identified for the smart grid that is simulated with the intention of implementing it with the integration of modern communication technologies that enables the components to produce and reflect data in an efficient way to assist better regulation in the power flow. The power flow is investigated by simulating unpredictable renewable energy and by using car batteries at the electric vehicle charging station. It is investigated how micro grid parameters are affected in the presence of super capacitors, car batteries and the use of larger power electronic converters. In the simulations, an electrical power control system is implemented at power conversion units which generates the correct duty cycle of the converter switches and controls the power flow operation at the smart grid. Then the proposed electrical power control system is compared with other systems such as maximum power point tracking (MPPT) algorithm and space vector pulse width modulation (SVPWM). A smart sensor system and smart protection are connected to protect the grid and to maintain system stability over a long time. The research focuses on developing a smart grid that performs the communication among the converters, performs power sharing, and does preventive management. It also monitors the energy efficiently and balances the energy in the grid irrespective of load or power generation variations. A mathematical model is developed to predict grid behaviour and is validated via MATLAB simulation of the grid. It is noticed that an improvement is made in the efficiency of renewable energy transmission to the electric-vehicle charging station