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In today's industrial power converters, converter reliability is essential, and converter topologies are well-established. Without a doubt, the power electronic industry continues to seek efficient power delivery and high power density. Resonant converters, especially LLC converters, have been intensively studied and applied in DC-DC converters. One of the most demanding applications for LLC converters is data centers. To date, LLC Resonant converters, are deployed in many applications for improved efficiency, density, and reliability. With the introduction of WBG devices coupled with the soft switching feature, the switching frequency can be extended beyond Mega-Hertz. With the significant increase in operating frequency, complicated magnetic components can be broken down into a cellular structure, each with a few number of turns. They can be easily implemented using 4-6 layers of PCB windings. Moreover, integrating the cellular cores using flux cancellation can further improve the power density. The proposed integrated magnetics can be automated in the manufacturing process. The magnetic size is reduced at this frequency, and planar magnetics using PCB winding become more relevant. PCB magnetics feature multiple advantages over Litz wire. The benefits are summarized as follows: The labor-intensive manufacturing process can be automated, thus reduction of cost. There is much reduced CM noise by using the shield layer. They have parasitics with much-improved reproducibility in large quantities. PCB windings feature less leakage between transformer windings because of the flexibility of the winding interleaving and the reduced number of turns. There is better thermal management due to the increased surface-to-body ratio. The design has a low profile and high-power density. However, it is not without its own limitations. There are challenges for high frequency PCB-magnetic magnetic design for the LLC converter. Firstly, With the recently developed high frequency core material, a phenomenon referred to as the dimensional resonant is observed. The effects of dimensional resonance were discussed in the literature when using an unusually large core structure; however, it can be observed more frequently under high excitation frequency, particularly with integrated magnetics. This dissertation discusses the dimensional effects of core loss on a PCB-based magnetics structure. A case study is presented on a 3-kW 400-to-48-V LLC prototype running at 1 MHz. The converter utilizes a low-profile matrix of two integrated transformers with a rectangular and thin cross-section area for reduced core loss. Specific solutions are presented. % Secondly, The matrix transformer is suitable for an LLC converter with high output current. However, the matrix transformer also increases the core size and core losses. The core loss degrades the LLC converter's light load and peak efficiency. In this dissertation, We discuss the design process and implementation of the DC-DC stage of the power supply unit for narrow range 48 V data center bus architecture. The optimization takes into account the number of elemental transformers, number of transformer turns, switching frequency, and transformer dimensions, namely winding width and core cross-section area. The optimization process results in a nearly 99% efficient 400-to-48-V LLC with a very high-power density and low profile fully integrated on PCB. A matrix of four transformers is used to reduce the termination loss of the secondary synchronous rectifier and achieve better thermal management. The number of secondary turns is optimized to achieve the best trade-off between winding loss, core loss, and power density. Another challenge arises for magnetic integration when multiple magnetic components with different characteristics come together. For instance, in the case of a transformer and an inductor on the same PCB. The PCB transformer is designed with perfectly interleaved primary and secondary layers to utilize the full PCB layer thickness. As a rule of thumb, the transformer winding layer is designed within 1 to 2 times the skin depth. On the other hand, the inductor's winding lacks interleaving and suffers from high MMF stress on layers. This makes the inductor prone to high eddy currents and eddy loss. Furthermore, this dissertation addresses the challenges associated with the high winding and core loss in the Integrated Transformer-Inductor (ITL). To overcome these challenges, we propose an improved winding design of the ITL by utilizing idle shielding layers for inductor integration within the matrix transformer. This method offers full printed circuit board (PCB) utilization, where all layers are consumed as winding, resulting in a significant reduction in the winding loss of the ITL. Moreover, we propose an improved core structure of the ITL that offers better flux distribution of the leakage flux within the magnetic core. This method reduces the core loss by more than 50% compared to the conventional core structure. We demonstrate the effectiveness of our proposed concepts by presenting the design of the ITL used in a high-efficiency, high-power-density 3-kW 400-to-48-V LLC module. The proposed converter achieves a peak efficiency of 98.7% and a power density of 1500 W/in3. This dissertation presents the concept of matrix inductors to solve such problems. A matrix of four resonant inductors is also designed to reduce the proximity effect between inductor windings and reduce inductor PCB winding loss. The matrix inductor provides a solution for high thermal stress in PCB-based inductors and reduces the inter-winding capacitance between inductor layers. This dissertation solves the challenges in magnetic design in high-frequency DC-DC converters in offline power supplies and data centers. This includes the transformer and inductor of the LLC converter. With the academic contribution in this dissertation, Wide-bandgap devices WBG can be successfully utilized in high-frequency DC-DC converters with Mega-Hertz switching frequency to achieve high efficiency, high power density, and automated manufacturing. The cost will be reduced, and the performance will be improved significantly. / Doctor of Philosophy / Industrial power converters need to be reliable and efficient to meet the power industry's demand for efficient power delivery and high power density. Research should focus on improving existing converter designs to improve fabrication, efficiency, and reliability. Resonant converters have been found to be effective in power conversion, especially in data centers where energy consumption is high. Three-element Resonant converters (LLC) are already used to improve efficiency, density, and reliability. By using Wide Bandgap devices and soft switching, the switching frequency can be extended beyond MHz, simplifying magnetic components and improving power density. The proposed integrated magnetics can be automated during the manufacturing process, further improving power density. At higher frequencies, planar magnetic components made with PCB winding are more effective than Litz wire. They are cheaper to make because of automation, have less common-mode noise, and are more reproducible in large quantities. PCB winding also has a low profile, high-power density, and better thermal management. However, it is not without its own limitations. There are challenges for high frequency PCB-magnetic magnetic design for the LLC converter. Firstly, With the recently developed high frequency core material, a phenomenon referred to as the dimensional resonant is observed. The effects of dimensional resonance were discussed in the literature when using an unusually large core structure; however, it can be observed more frequently under high excitation frequency, particularly with integrated magnetics. This dissertation discusses the effects of core loss on a PCB-based magnetics structure and presents solutions, including a case study on a 3-kW 400-to-48 V LLC prototype running at 1 MHz. Another challenge arises for magnetic integration when multiple magnetic components with different characteristics come together. For instance, in the case of a transformer and an inductor on the same PCB. The PCB transformer is designed with perfectly interleaved winding and low Ohmic loss. On the other hand, the inductor's winding lacks interleaving and suffers from a high proximity field. This makes the inductor prone to high eddy currents and eddy loss. This dissertation presents the concept of matrix inductors to solve such problems. A matrix of four resonant inductors is also designed to reduce the proximity effect between inductor windings and reduce inductor PCB winding loss. The matrix inductor provides a solution for high thermal stress in PCB-based inductors and reduces the inter-winding capacitance between inductor layers. Furthermore, this dissertation addresses the challenges associated with the high winding and core loss in the Integrated Transformer-Inductor (ITL). To overcome these challenges, we propose an improved winding design of the ITL by utilizing idle shielding layers for inductor integration within the matrix transformer. This method offers full printed circuit board (PCB) utilization, where all layers are consumed as winding, resulting in a significant reduction in the winding loss of the ITL. Moreover, we propose an improved core structure of the ITL that reduces the core loss by more than 50% compared to the conventional core structure. We demonstrate the effectiveness of our proposed concepts on a high-efficiency, high-power-density 3-kW 400-to-48-V LLC module. The proposed converter achieves a peak efficiency of 98.7% and a power density of 1500 W/in3. This dissertation solves the challenges in magnetic design in high-frequency DC-DC converters in offline power supplies and data centers. This includes the transformer and inductor of the LLC converter. With the academic contribution in this dissertation, Wide-bandgap devices WBG can be successfully utilized in high-frequency DC-DC converters with Mega-Hertz switching frequency to achieve high efficiency, high power density, and automated manufacturing. The cost will be reduced, and the performance will be improved significantly.

Faster charging and availability of charging infrastructure are the two main challenges facing an accelerated transition to sustainable electri ed transportation. Both challenges can be solved by developing modular charging systems that are future proof all while having low running and installation costs. As such, this thesis focuses on developing modular and e cient DC/DC charging solutions with a wide charging voltage range capability to meet the needs of existing and next generation plug-in electric vehicles. The thesis starts with describing its motivation and gives an overview on the impact of charging technologies on the electri fication movement. Then, specifi c objectives and research contributions are laid out to narrow the focus of the reader. A review on existing charging systems, standards, architecture and features is presented. Existing isolated and on-isolated power converter topologies for DC-chargers are analyzed and research gaps in power converters with a wide charging voltage range are highlighted. A new single stage DC/DC converter topology and operation scheme is proposed to extend the charging voltage range. Modeling and analysis of the proposed solution was used to select the transition point between different operating modes. Impedance tolerance and pulse distortion was modeled to analyze the passive current sharing error at light and full load operation. The combination of the proposed topology and unique operating scheme reduced the voltage and current stress per device allowing the use of lower kVA rated devices leading to higher cost savings compared to other solutions. An experimental setup has been developed which showed the excellent performance of the proposed topology. The design and optimization strategy for the proposed dual-secondary dual-active bridge (DAB) converter topology is presented. A converter loss model is developed to take in to account: magnetic, switching, and conduction loss. Then, the design process and quantization scheme to quantize charging pro les into discrete energy points is explained, which entails parametric optimization using a genetic algorithm (GA) to minimize energy loss across widely varying charging pro les based on actual charging data. Comprehensive experimental testing was carried out to validate the proposed design strategy and excellent performance was achieved over an extended operating range. After the review of power magnetics used in isolated chargers, high parasitic capacitance in planar transformers was identi ed as an obstacle in the way of development of chargers, especially in charging applications that demand high switching frequencies or extended low power operation. Therefore, a novel planar transformer structure was proposed with ultra-low winding capacitance. The proposed co-planar transformer was compared to three other planar types to highlight the differences and bene fits. Four different prototypes of planar transformers were built with the same target speci fications, to compare the proposed structure against previous solutions. Impedance testing of the planar prototypes was carried to measure the winding stray capacitance and frequency response. Experimental power testing using a DAB converter setup showed excellent results in reducing voltage overshoot, high frequency oscillations, and power losses. Finally, a 30-kW dual-secondary DAB charging module was designed, implemented, and tested. The purpose of this work is to bridge the engineering gap between a proof of concept and a higher Technology Readiness Level (TRL) mature charging module, focusing more on regulatory standards and control system development. Experimental validation of the liquid cooled module showed excellent performance characteristics. / Thesis / Doctor of Philosophy (PhD)

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Distributed energy is an attractive alternative to typical centralized energy sources specifically for remote locations not accessible to the electricity grid. With the continued advancement into new renewable technologies like solar, wind, fuel cell etc., off-grid standalone systems are becoming more attractive and even compeating on a cost basis for rural locations. Along with the environmental and sustainable movement, these technologies are only going to get more and more popular as time goes on. Power electronic converters are also advancing which will help the shift in electricity options. Creating innovative power electronic systems will be important when moving toward smaller, more e cient and higher power density solutions. As such, this thesis will aim to design and create an integrated power electronic system for an o -grid standalone solar application designed for remote rural locations with no access to electricity, or in locations which could bene t from such a system. It is designed for a DC input source from 24V-40V, such as a solar panel, and can operate four di erent loads; one 12V-24V 100 W DC load, charge a 48V battery, run three 5V cell phone charger outputs and run one 230V, 50Hz, 1 kW AC load. A boost converter, buck converter, phase shifted full bridge isolated DC-DC converter and a single phase inverter are implimented in the integrated system to achieve these outputs. A comparison of similar products on the market are presented and compared with the proposed design by showing the product speci cations, advantages and disadvantages of each. A discussion of each converter in the system is presented and will include operation, design and component selection. An in-depth design process for the inductor within the boost converter is presented and will cover core, winding design and an optimization algorithm using the Genetic Algorithm (GA) is used to compare di erent ferrite based C-C shaped inductors. More speci cally, the core material selected is Ferroxcube 3C97 and the inductor comparions are between di erent Litz bundled windings from New England Wire Tecnologies and a customized rectangular winding. The GA optimizes around the lowest volume by comparing the di erent inductor designs using the di erent Litz winding constructions and the custom rectangular winding constrictuion. The rectangular winding achieves the lowest volume and will be compared with a three phase interleaved boost design implimenting a CoilCraft inductor. The buck converter is the simplest converter and is designed using the traditional methods in literature. An in-depth design process for the phase shifted full bridge converter is also done wherein the zero voltage switching (ZVS) is achieved. The DC-AC inverter is the last converter designed within the integrated system and covers input capacitor sizing, and output lter design. There are speci c distributed energy standards that must be followed when connecting loads to the system and so the purpose of the lter is to lter out the voltage harmonics. The control techniques for each converter is also discussed and shown to operate in both simulation and in experimentally. The losses within the system are discussed and the required equations are de ned / Thesis / Master of Applied Science (MASc)

This work explores the design, construction, and analysis of a novel DC-DC converter which incorporates combinations of switching capacitors and inductors to achieve an integer voltage divider function, without the need for a feedback loop controller to achieve the desired output voltage. The proposed Hybrid Voltage Divider additionally provides zero voltage switching (ZVS) at turn on transitions which yields improved overall efficiency of the converter. Besides a proof-of-concept via computer simulations, another primary goal of this thesis is to demonstrate the functionalities of the proposed Zero Voltage Switching Hybrid Voltage Divider (ZVS-HVD) through hardware prototyping. The proposed ZVS-HVD was designed and constructed to provide a 2:1 division with 24V input voltage at 120W maximum output power utilizing 500kHz switching frequency. Findings from simulations and hardware tests verify that the converter effectively provides the desired 12V output at varying loads with less than 5% voltage ripple. The efficiency of the converter reaches 95.02% at full load and peak efficiency of 96.33% at 55% load. Moreover, the converter consistently maintains the ZVS operations across all switches under varying loads. Overall, results verify the feasibility of the proposed ZVS HVD converter as an alternative solution in providing high efficiency DC voltage division without the need for complex feedback circuitry.

With the fast advance in VLSI technology, smaller, more powerful digital system is available. It requires power supply with higher power density, lower profile and higher efficiency. PWM topologies have been widely used for this application. Unfortunately, hold up time requirement put huge penalties on the performance of these topologies. Also, high switching loss limited the power density achievable for these topologies. Two techniques to deal with hold up time issue are discussed in this dissertation: range winding solution and asymmetric winding solution, the efficiency at normal operation point could be improved with these methods. To reduce secondary rectifier conduction loss, QSW synchronous rectifier is developed, which also helps to achieve ZVS for symmetrical half bridge converter. Although with these methods, the efficiency of front end DC/DC converter could be improved, the excessive switching loss prohibited higher switching frequency. To achieve the targets, topologies with high switching frequency and high efficiency must be developed. Three resonant topologies: SRC, PRC and SPRC, are been investigated for this application because of their fame of low switching loss. Unfortunately, to design with hold up requirement, none of them could provide significant improvements over PWM converter. Although the negative outcome, the desired characteristic for front end application could be derived. Base on the desired characteristic, a thorough search is performed for three elements resonant tanks. LLC resonant topology is found to posses the desired characteristic. From comparison, LLC resonant converter could reduce the total loss by 40% at same switching frequency. With doubled switching frequency, efficiency of LLC resonant converter is still far better than PWM converters. To design the power stage of LLC resonant converter, DC analysis is performed with two methods: simulation and fundamental component simplification. Magnetic design is also discussed. The proposed integrated magnetic structure could achieve smaller volume, higher efficiency and easy manufacture. To make practical use of the topology, over load protection is a critical issue. Three methods to limit the stress under over load situation are discussed. With these methods, the converter could not only survive the over load condition, but also operate for long time under over load condition. Next small signal characteristic of the converter is investigated in order to design the feedback control. For resonant converter, state space average method is no longer valid. Two methods are used to investigate the small signal characteristic of LLC resonant converter: simulation and extended describing function method. Compare with test results, both methods could provide satisfactory results. To achieve both breadth and depth, two methods are both used to reveal the myth. With this information, compensator for feedback control could be designed. Test circuit of LLC resonant converter was developed for front end DC/DC application. With LLC topology, power density of 48W/in3 could be achieved compare with 13W/in3 for PWM converter. / Ph. D.

Fuel cells are a clean, high-efficiency source for power generation. This innovative technology is going to penetrate all aspects in our life, from utility distributed power, transportation applications, down to power sources for portable devices such as laptop computer and cell phones. To enable the usage of fuel cell, developing power converters dedicated for fuel cells becomes imminent. Currently, the full-bridge converter is the dominating topology in high power dc/dc applications. Although multiphase converters have been proposed, most of them are dealing with high input-voltage systems, and their device characteristic is not suitable for a low voltage source such as a fuel cell. For a high power fuel cell system, high voltage conversion ratios and high input currents are the major obstacles to achieving high-efficiency power conversions. This dissertation proposes a novel 3-phase 6-leg dc/dc power converter with transformer isolation to overcome these obstacles. Major features of the proposed converter include: (1) Increase converter power rating by paralleling phases, not by paralleling multiple devices; (2) Double the output voltage by transformer delta-wye connection, thus lowering the turns-ratio; (3) Reduce the size of output filter and input dc bus capacitor with interleaved control; (4) Achieve Zero-Voltage Zero-Current Switching (ZVZCS) over a wide load range without auxiliary circuitry. High conversion efficiency above 96% is verified with different measurement approaches in experiments. This dissertation also presents the power stage and control design for the proposed converter. Control design guideline is provided and the design result is confirmed with both simulation and hardware experiments. When using the fuel cell for stationary utility power applications, a low-frequency ripple interaction was identified among fuel cell, dc/dc converter and dc/ac inverter. This low frequency ripple tends to not only damage the fuel cell, but also reduce the source capability. This dissertation also investigates the mechanism of ripple current propagation and exploits the solutions. A linearized ac model is derived and used to explain the ripple propagation. An active ripple reduction technique by the use of the current loop control is proposed. This active current loop control does not add extra converters or expensive energy storage components. Rather, it allows a reduction in capacitance because the ripple current flowing into the capacitor is substantially reduced, and less capacitance can be used while maintaining a clean dc bus voltage. The design process and guideline for the proposed control is suggested, and the effectiveness of this active control is validated by both simulation and experimental results. / Ph. D.

With the increasing demand for clean and renewable energy, new distribution network concepts, such as DC microgrids and distributed power generation networks, are being developed. One key component of such networks is the grid-interfacing DC-DC power converter that can transfer power bi-directionally while having a wide range of voltage step-up and step-down capabilities. Also, with the proliferated demand for electric vehicle chargers, battery energy storage systems, and solid-state transformers (SST), the bi-directional high-power DC-DC converter plays a more significant role in the renewable energy industry. To satisfy the requirements of the high-power bi-directional wide-range DC-DC converter, different topologies have been compared in this thesis, and the four-switch buck-boost (FSBB) converter topology has been selected as the candidate. This work investigates the operation principle of the FSBB converter, and a digital real-time low-loss quadrangle current mode(QCM) control implementation, which satisfies the zero-voltage-switching (ZVS) requirements, is proposed. With the QCM control method, the FSBB converter efficiency can be further increased by reducing the inductor RMS current and device switching loss compared to traditional continuous current mode(CCM) control and discontinuous current mode(DCM) control. Although the small signal model has been derived for FSBB under CCM control, the small ripple approximation that was previously used in the CCM model no longer applies in the QCM model and causing the model to be different. To aid the control system compensator design, QCM small signal model is desired. In this thesis, a small signal model for FSBB under QCM control is proposed. A 50 kW silicon carbide (SiC) based grid-interfacing converter prototype was constructed to verify the QCM control implementation and small signal model of the FSBB converter. For driving the 1.2kV SiC modules, an enhanced gate driver with fiber optic (FO) based digital communication capability was designed. Digital on-state and off-state drain-source voltage sensors and Rogowski coil-based current sensors are embedded in the gate driver to minimize the requirement for external sensors, thus increasing the power density of the converter unit. Also, Rogowski-coil-based current protection and drain-source voltage-based current protection is embedded in the gate driver to prevent SiC switching device from damage. / Master of Science / The renewable energy sector is driving the development of new distribution networks, such as DC microgrids and distributed power generation networks. One crucial component of these networks is the grid-interfacing DC-DC power converter, which can transfer power in both directions while maintaining a wide voltage range. This study evaluates various topologies and selects the four-switch buck-boost (FSBB) converter topology to meet the demands of high-power, bi-directional, and wide-range DC-DC converters. This work analyzed the operation of the FSBB converter and proposed a novel simplified quadrangle current mode (QCM) control implementation. With the QCM control method, the FSBB converter efficiency can be further improved by reducing losses compared to conventional control methods. This study also provides a small signal model, which can be used to aid the control loop compensator design where application of FSBB converter is required. A 50 kW silicon carbide (SiC) based grid-interfacing converter prototype, which was constructed to validate the proposed QCM control implementation and small signal model of the FSBB converter. As part of the converter unit,the enhanced gate driver design and implementation is presented in this thesis. This gate driver is designed with fiber optic-based digital communication, drives the wide bandgap SiC modules. The gate driver also features embedded digital on-state and off-state drain-source voltage sensors and non-intrusive current sensors to minimize external sensor requirements, thereby increasing the power density of the converter unit. The gate driver also incorporates high bandwidth current protection and drain-source voltage-based current protection to protect the SiC switching device from damage.