Optimization of Active-Bridge Based Modular Power Converters

Majmunovic, Branko

Graduate Thesis Or Dissertation

Optimization of Active-Bridge Based Modular Power Converters Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/dj52w624m

Abstract

Modularization is one of the essential concepts used in today’s power electronics integration. Modular converters are often realized based on active bridges, arranged around high-frequency passive components. Due to soft-switching properties, these converters feature high efficiency and high power density, especially at the nominal conversion ratio. This thesis covers optimization of the aforementioned converters from different angles, including topology configuration, selection of semiconductor devices, modulation strategies, as well as optimization and integration of magnetic components. The thesis is organized into two parts based on different target applications.

The first part of the thesis considers the design and optimization of stacked active bridge (SAB) converters for high-conversion ratio dc-to-dc applications. Firstly, a transformerless high step-down SAB dc-dc converter is introduced. The SAB converter consists of series-stacked, capacitively coupled inverter modules and parallel-connected rectifier modules. The nominal step-down conversion ratio is determined by the number of inverter modules, while the output current capability scales with the number of paralleled rectifier modules. SAB operating principles, including phase-shift control and soft switching, achieved through series inductors, are similar to those of transformer-isolated dual active bridge (DAB) converters. It is found that the best size-versus-loss trade-off is achieved by the integration of the series inductors on a custom core. Furthermore, it is shown how soft switching contributes to natural voltage sharing among the series-stacked inverter modules. The approach is verified by experimental results on a 400-to-48 V, 3 kW SAB prototype using GaN devices and featuring 400W/in³ power density. A flat efficiency curve is obtained, with 99% peak efficiency, 97.5% full-load efficiency, and 98% efficiency at 20% load. Second, a galvanic isolated version of the SAB converter is discussed. To achieve galvanic isolation, transformers are inserted between the inverters and rectifier bridges. The nominal step-down conversion ratio is determined by the number of inverter modules and the turns ratio of the transformer. In order to reduce the footprint of the magnetic components, the transformers are coupled on a single core, and the series inductances are realized as controllable leakage inductances within the same magnetic structure using a novel custom core and planar winding arrangement, a solution unique to the iSAB configuration. The approach is verified by experimental results on a 400-to-48 V, 3 kW, 400 kHz iSAB prototype using GaN devices and having 96.7% peak efficiency.

The second part of the thesis is centered around the topic of modular SiC-based string inverters for medium-voltage transformer-less photovoltaic (PV) systems. The interface of lowvoltage (LV) DC to medium-voltage (MV) three-phase AC grid is often based on series-stackable modular converter architectures. To minimize energy storage requirements, it is advantageous to employ a quadruple active bridge (QAB) stage operating as a “dc transformer” (DCX) in each stackable module. The QAB stage offers three isolated dc link voltages, which then allow flexible stacking of three single-phase dc-to-ac inverter stages. Each of the module phases processes a pulsating power having a component at twice the line frequency. This presents a challenge in maintaining zero-voltage switching (ZVS) on the secondary sides of the QAB during low-power portions of the line cycle. The design of the QAB stage is covered in this thesis. A detailed analysis of ZVS switching waveforms is presented, including the effects of nonlinear device capacitances. It is shown how ZVS can be achieved at all times using a relatively small circulating current provided by the magnetizing inductance of the high-frequency transformer. Analytical expressions are given for the optimal values of the magnetizing inductance and the dead times of the QAB primary and secondary bridges. The approach is verified by experimental results on a 1 kV, 10 kW SiC-based prototype, demonstrating a relatively flat efficiency curve with a peak efficiency of 97.1% at 75% load.

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