High Gain DC-DC and Active Power Decoupling Techniques for Photovoltaic Inverters

abstract: The dissertation encompasses the transformer-less single phase PV inverters for both the string and microinverter applications. Two of the major challenge with such inverters include the presence of high-frequency common mode leakage current and double line frequency power decoupling with reliable capacitors without compromising converter power density. Two solutions are presented in this dissertation: half-bridge voltage swing (HBVS) and dynamic dc link (DDCL) inverters both of which completely eliminates the ground current through topological improvement. In addition, through active power decoupling technique, the capacitance requirement is reduced for both, thus achieving an all film-capacitor based solution with higher reliability. Also both the approaches are capable of supporting a wide range of power factor. Moreover, wide band-gap devices (both SiC and GaN) are used for implementing their hardware prototypes. It enables the switching frequency to be high without compromising on the converter efficiency. Also it allows a reduced magnetic component size, further enabling a high power density solution, with power density far beyond the state-of-the art solutions. Additionally, for the transformer-less microinverter application, another challenge is to achieve a very high gain DC-DC stage with a simultaneous high conversion efficiency. An extended duty ratio (EDR) boost converter which is a hybrid of switched capacitors and interleaved inductor technique, has been implemented for this purpose. It offers higher converter efficiency as most of the switches encounter lower voltage stress directly impacting switching loss; the input current being shared among all the interleaved converters (inherent sharing only in a limited duty ratio), the inductor conduction loss is reduced by a factor of the number of phases. Further, the EDR boost converter has been studied for both discontinuous conduction mode (DCM) operations and operations with wide input/output voltage range in continuous conduction mode (CCM). A current sharing between its interleaved input phases is studied in detail to show that inherent sharing is possible for only in a limited duty ratio span, and modification of the duty ratio scheme is proposed to ensure equal current sharing over all the operating range for 3 phase EDR boost. All the analysis are validated with experimental results.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Design of Power Switched-Capacitor Converters and Their Performance Analysis in a Soft-Charging Operation

Switched-capacitor (SC) converters have gained more interest due to their high power density and appropriateness for small circuit integration. Building a SC DC-to-DC converter with only capacitors and switches is the main reason to seek a higher power density achievement. However, the SC converters suffer dominant losses related to their capacitors and switches. These losses can be determined and optimized by calculating the converter\u27s output impedance in its two asymptotic limits. We proposed a high voltage gain and a very low output impedance power switched-capacitor converter (PSC) with a lower number of components compared to other step-up switched-capacitor topologies. The high output efficiency and the higher power density are two fundamental aspects of the PSC converter. We can eliminate the current transient by applying the soft-charging technique that results a higher power density and a higher efficiency in PSC. The soft-charging operation is more preferable to the soft-switching technique (resonant operation) since it does not require any auxiliary components. Furthermore, soft-charging helps to resize capacitors and reduce the switching frequency of the PSC converter. Furthermore, a split-phase control design is proposed to achieve the complete soft-charging operation in a PSC. The control diagram was designed for a 1-to-4 PSC (two levels of the PSC) which controls eight switches to exhibit eight modes of operation. The complete soft-charging accomplishes a 96% efficiency due to the lower output impedance and the dead time switching. LT-spice software has been used to verify the proposed control, and the results were compared with hard-charging and incomplete soft-charging operations. In this research, we also proposed a two-level power switched-capacitor boost converter (PSC-boost) for a high voltage gain application by integrating a PSC converter and a conventional boost converter. The PSC switched-capacitors and the conventional boost converter are respectively cascaded as a primary and a secondary side of the proposed converter. Without alerting of the secondary side (conventional boost), the conversion ratio can be increased by adding more switched-capacitors cells. The proposed converter similarly acts as an MBC; however, it can maintain the rated voltage gain at a higher duty cycle. Unlike the MBC converter, the simulated voltage gain is closer to the calculated voltage gain for PSC-boost converter. In addition to the switched-capacitors insertion, a switched inductor model is used instead of the single inductor in the traditional boost converter. Five switches, five capacitors, seven diodes, and three inductors are used to build a PSC-boost switched-inductor converter. The PSC-boost converter accomplishes 94% efficiency which a higher rated power

MODELING AND CONTROL OF DIRECT-CONVERSION HYBRID SWITCHED-CAPACITOR DC-DC CONVERTERS

Efficient power delivery is increasingly important in modern computing, communications, consumer and other electronic systems, due to the high power demand and thermal concerns accompanied by performance advancements and tight packaging. In pursuit of high efficiency, small physical volume, and flexible regulation, hybrid switched-capacitor topologies have emerged as promising candidates for such applications. By incorporating both capacitors and inductors as energy storage elements, hybrid topologies achieve high power density while still maintaining soft charging and efficient regulation characteristics. However, challenges exist in the hybrid approach. In terms of reliability, each flying capacitor should be maintained at a nominal `balanced\u27 voltage for robust operation (especially during transients and startup), complicating the control system design. In terms of implementation, switching devices in hybrid converters often need complex gate driving circuits which add cost, area, and power consumption. This dissertation explores techniques that help to mitigate the aforementioned challenges. A discrete-time state space model is derived by treating the hybrid converter as two subsystems, the switched-capacitor stage and the output filter stage. This model is then used to design an estimator that extracts all flying capacitor voltages from the measurement of a single node. The controllability and observability of the switched-capacitor stage reveal the fundamental cause of imbalance at certain conversion ratios. A new switching sequence, the modified phase-shifted pulse width modulation, is developed to enable natural balance in originally imbalanced scenarios. Based on the model, a novel control algorithm, constant switch stress control, is proposed to achieve both output voltage regulation and active balance with fast dynamics. Finally, the design technique and test result of an integrated hybrid switched-capacitor converter are reported. A proposed gate driving strategy eliminates the need for external driving supplies and reduces the bootstrap capacitor area. On-chip mixed signal control ensures fast balancing dynamics and makes hard startup tolerable. This prototype achieves 96.9\% peak efficiency at 5V:1.2V conversion and a startup time of 12$\mu s$, which is over 100 times faster than the closest prior art. With the modeling, control, and design techniques introduced in this dissertation, the application of hybrid switched-capacitor converters may be extended to scenarios that were previously challenging for them, allowing enhanced performance compared to using traditional topologies. For problems that may require future attention, this dissertation also points to possible directions for further improvements

Design of High-Gain DC-DC Converters for High-Power PV Applications

Renewable energy sources are penetrating the market in an ever increasing rate, especially in terms of Wind and Solar energies, with the latter being more suitable for the GCC region. Typically, Photovoltaic (PV) strings’ output voltage is limited to ~ 1500 V due to safety constraints, and thus requires boosting to higher DC levels (non-isolated step-up DC-DC transformer) suitable for High-Voltage DC (HVDC) and AC grid applications in order to provide the required DC-Link voltage level. Nevertheless, conventional non-isolated DC-DC converters provide a limited practical gain due to their parasitic elements. Other options include isolated DC-DC converters that utilize costly high-frequency transformers with limited power capability. Moreover, the isolation requirements of transformers in HVDC significantly increase the footprint of the converters. High-frequency transformers for high-power applications are hard to design and are usually associated with higher losses. Alternatively, connecting conventional DC-DC converters in different combinations can provide higher gains to the required levels, while maintaining the high efficiency requirements. This thesis proposes the cascade and/or series connection of DC-DC modules as a solution to the high-conversion ratio requirement, based on Cuk and Single-Ended Primary Inductor Converter (SEPIC) topologies, whose continuous input current is suitable for PV applications, and reduces the bulky capacitor filters at the input side. Detailed theoretical models of the proposed topologies are first derived, then their trends are practically verified by low power prototypes. Sensitivity analysis is also performed to assess the effect of small variations to the parasitic inductors’ resistances on the overall system gain, where the input inductor is found to have a considerable effect, especially at higher duty ratios (i.e. higher gains). High-power applications’ scenarios with their considerations are simulated to compare the different topologies and the results show a comparable efficiency of the proposed converters for a 1 –MW application with efficiencies higher than 90%

Isolated Single-stage Power Electronic Building Blocks Using Medium Voltage Series-stacked Wide-bandgap Switches

The demand for efficient power conversion systems that can process the energy at high power and voltage levels is increasing every day. These systems are to be used in microgrid applications. Wide-bandgap semiconductor devices (i.e. Silicon Carbide (SiC) and Gallium Nitride (GaN) devices) are very promising candidates due to their lower conduction and switching losses compared to the state-of-the-art Silicon (Si) devices. The main challenge for these devices is that their breakdown voltages are relatively lower compared to their Si counterpart. In addition, the high frequency operation of the wide-bandgap devices are impeded in many cases by the magnetic core losses of the magnetic coupling components (i.e. coupled inductors and/or high frequency transformers) utilized in the power converter circuit. Six new dc-dc converter topologies are propose. The converters have reduced voltage stresses on the switches. Three of them are unidirectional step-up converters with universal input voltage which make them excellent candidates for photovoltaic and fuel cell applications. The other three converters are bidirectional dc-dc converters with wide voltage conversion ratios. These converters are very good candidates for the applications that require bidirectional power flow capability. In addition, the wide voltage conversion ratios of these converters can be utilized for applications such as energy storage systems with wide voltage swings

Advanced topologies of high step-up DC-DC converters for renewable energy applications

This research is focused on developing several advanced topologies of high step-up DC-DC converters to connect low-voltage renewable energy (RE) sources, such as photovoltaic (PV) panels and fuel cells (FCs), into a high-voltage DC bus in renewable energy applications. The proposed converters are based on the combinations of various voltage-boosting (VB) techniques, including interleaved and quadratic structures, switched-capacitor (SC)-based voltage multiplier (VM) cells, and magnetically coupled inductor (CI) and built-in-transformer (BIT). The proposed converters offer outstanding features, including high voltage gain with low or medium duty cycle, a small number of components, low current and voltage stresses on the components, continuous input current with low ripple, and high efficiency. This research includes five new advanced high step-up DC-DC converters with detailed analyses. First, an interleaved converter is presented, which is based on the integration of two three-winding CIs with SC-based VM cells. Second, a dual-switch converter is proposed, which is based on the integration of a single three-winding CI with SC-based VM cells. Third, the SC-based VM cells are utilized to present three new Z-source (ZS)-based converters. Fourth, two double-winding CIs and a three-winding BIT are combined with SC-based VM cells to develop another interleaved high step-up converter. Finally, two double-winding CIs and SC-based VM cells are adopted to devise an interleaved quadratic converter with high voltage gain. The operating and steady-state analyses, design considerations, and a comparison with similar converters in the literature are provided for each converter. In addition, hardware prototypes were fabricated to verify the performance of the proposed converters --Abstract, page iv