Galvanic isolation between the grid and energy storage unit is typically required for bidirectional power distribution systems. Due to the recent advancement in wide-bandgap semiconductor devices, it has become feasible to achieve the galvanic isolation using bidirectional isolated DC/DC converters instead of line-frequency transformers. A survey of the latest generation SiC MOSFET is performed. The devices were compared against each other based on their key parameters. It was determined that under the given specifications, the most suitable devices are X3M0016120K 1.2 kV 16 mohm and C3M0010090K 900 V 10 mohm SiC MOSFETs from Wolfspeed. Two of the most commonly utilized bidirectional isolated DC/DC converter topologies, dual active bridge and CLLC resonant converter are introduced. The operating principle of these converter topologies are explained. A comparative analysis between the two converter topologies, focusing on total device loss, has been performed. It was found that the CLLC converter has lower total device loss compared to the dual active bridge converter under the given specifications. Loss analysis for the isolation transformer in the CLLC resonant converter was also performed at different switching frequencies. It was determined that the total converter loss was lowest at a switching frequency of 250 kHz A prototype for the CLLC resonant converter switching at 250 kHz was then designed and built. Bidirectional power delivery for the converter was verified for power levels up to 25 kW. The converter waveforms and efficiency data were captured at different power levels. Under forward mode operation, a peak efficiency of 98.3% at 15 kW was recorded, along with a full load efficiency value of 98.1% at 25 kW. Under reverse mode operation, a peak efficiency of 98.8% was measured at 17.8 kW. The full load efficiency at 25 kW under reverse mode operation is 98.5%. / Master of Science / Electrical isolation between the grid and energy storage unit is typically required for bidirectional power distribution systems. Traditionally, this isolation is achieved via line-frequency transformers, which tend to be bulky and heavy. This imposes a limit on the overall system power density, which is a crucial performance metric for bidirectional power distribution systems. Alternatively, the required electrical isolation can be implemented through bidirectional power converters. As a result, the overall system power density can be drastically improved. However, the losses incurred by the semiconductor devices in such converters could significantly reduce the overall system efficiency, which is another important performance metric. Due to the recent advancement in semiconductor devices, it has become feasible to design the required bidirectional power converters with high efficiency and high power density. A survey of the latest generation semiconductor devices is performed. A 25 kW converter prototype was designed and built using the selected semiconductor devices. Experimental testing was conducted for the converter prototype and efficiency values exceeding 98% were captured across the entire load range. The converter prototype has a power density of 78 W/in³.

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Modern data center power architecture developing trend is analyzed, efficiency improvement method is also discussed. Literature survey of high frequency isolated power conversion system which is also called solid state transformer is given including application, topology, device and magnetic transformer. Then developing trend of this research area is clearly shown following by research target. State of art wide band gap device including silicon carbide (SiC) and gallium nitride (GaN) devices are characterized and compared, final selection is made based on comparison result. Mostly used high frequency high power DC/DC converter topology dual active bridge (DAB) is introduced and compared with novel CLLC resonant converter in terms of switching loss and conduction loss point of view. CLLC holds ZVS capability over all load range and smaller turn off current value. This is beneficial for high frequency operation and taken as our candidate. Device loss breakdown of CLLC converter is also given in the end. Medium voltage high frequency transformer is the key element in terms of insulation safety, power density and efficiency. Firstly, two mostly used transformer structures are compared. Then transformer insulation requirement is referred for 4160 V application according to IEEE standard. Solid insulation material are also compared and selected. Material thickness and insulation distance are also determined. Insulation capability is preliminary verified in FEA electric field simulation. Thirdly two transformer magnetic loss model are introduced including core loss model and litz wire winding loss model. Transformer turn number is determined based on core loss and winding loss trade-off. Different core loss density and working frequency impact is carefully analyzed. Different materials show their best performance among different frequency range. Transformer prototype is developed following designed parameter. We test the developed 15 kW 500 kHz transformer under 4160 V dry type transformer IEEE Std. C57.12.01 standard, including basic lightning test, applied voltage test, partial discharge test. 500 kHz 15 kW CLLC converter gate drive is our design challenge in terms of symmetry propagation delay, cross talk phenomenon elimination and shoot through protection. Gate drive IC is carefully selected to achieve symmetrical propagation delay and high common mode dv/dt immunity. Zero turn off resistor is achieved with minimized gate loop inductance to prevent cross talk phenomenon. Desaturation protection is also employed to provide shoot through protection. Finally 15 kW 500 kHz CLLC resonant converter is developed based on 4160V 500 kHz transformer and tested up to full power level with 98% peak efficiency. / Master of Science / Modern data center power architecture developing trend is analyzed, efficiency improvement method is also discussed. At the same time high frequency operation is preferred to reduce reactive component size like transformer and capacitor. To achieve better trade-off between high efficiency and high frequency in our research. Literature survey of high frequency isolated DC/DC power converter is given including application, circuit topology, power electronics device and magnetic transformer. Then developing trend of this research area is clearly shown following by research target. State of art advance material based power electronics devices are characterized and compared, final selection is made based on comparison result. Mostly used high frequency high power DC/DC converter topology dual active bridge (DAB) is introduced and compared with novel CLLC resonant converter in terms of converter loss. CLLC holds smaller converter loss. This is beneficial for high frequency operation and taken as our candidate. Medium voltage high frequency transformer is the key element in terms of insulation safety, power density and efficiency. Firstly, two mostly used transformer structures are compared. Then transformer insulation requirement is referred for 4160 V application according to IEEE standard. Solid insulation material are also compared and selected. Material thickness and insulation distance are also determined. Thirdly transformer loss model are introduced including core loss model and winding loss model. Transformer turn number is determined based on transformer loss and volume trade-off. Transformer prototype is developed following designed parameter. We test the developed transformer under IEEE standard requirement and pass all the test. Converter gate drive is one of our design challenge. We need to achieve symmetrical propagation delay between command signal and final drive circuit output, suppress interference from other high frequency switching devices, and protect device under short circuit condition. Gate drive IC is carefully selected to achieve symmetrical propagation delay and suppress other’s interference. Device conduction voltage is employed to compare with threshold value to determine whether it is under short circuit condition. Finally 15 kW 500 kHz CLLC resonant converter is developed based on 4160V 500 kHz transformer and tested up to full power level with 98% peak efficiency.

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Rising fuel costs and concerns about air pollution have significantly increased interest in electric vehicles (EVs). EVs are equipped with rechargeable batteries that can be fully recharged by connecting to an external electrical source. However, the wider adoption of EVs is hindered by the need for an on-board charger system that is both lightweight and efficient. EVs utilize two main charging methods: on-board chargers (OBC) for regular charging and off-board (fast) chargers for quick refills of battery pack. Most EVs currently use 400V battery packs paired with 6.6kW or 11kW OBCs, while larger vehicles with over 100 kWh battery packs employ 16.5kW or 19.2kW OBCs, constrained by household voltage and current limits. Some manufacturers are transitioning to 800V battery packs to lower costs and enhance fast charging capabilities, necessitating the development of 800V OBCs with high efficiency and power density. For household use, EVs can charge via OBC in a grid-to-vehicle transfer and can supply energy back to the home or grid (vehicle-to-grid) for emergency use or to support smart grid functionalities, requiring bidirectional OBCs. Advanced power semiconductor devices have been instrumental in advancing power conversion technology. The introduction of power semiconductor devices based on wide bandgap (WBG) materials marks a revolutionary shift, offering potential improvements over silicon-based devices. These WBG devices are capable of achieving higher efficiency, and higher power density in power conversion at higher operation frequency. Elevating the switching frequency diminishes the voltage-second across the transformer, facilitating the utilization of printed-circuit-board (PCB) technology for the windings as opposed to Litz wire implementations. Compared to traditional Litz wire-based transformers, the manufacturing process is significantly streamlined, and the management of parasitic is considerably more straightforward. Furthermore, the integration of resonant inductors with PCB-based transformer results in a reduction in the overall number of magnetic components and improved power density. This dissertation focuses on the DC/DC conversion stage of a bi-directional battery charger. It aims to achieve high power density and high efficiency using a PCB-based integrated transformer, enhancing manufacturing processes. The dissertation details the specific accomplishments in this area: Firstly, a two-stage on-board charger structure for 800 V battery EVs is proposed. The first stage is a four-phase bridgeless totem pole AC/DC converter working at critical conduction mode (CRM) so that soft switching can be achieved for all the fast switches. The second stage is single phase CLLC (1PCLLC) converter which is attractive due to its less component counts of devices and driver circuits. A novel matrix integrated transformer with controllable built-in leakage inductance for bi-directional 1PCLLC converter was proposed. Integrating three UI-core-based (1UI-based) elemental transformers with non-perfectly interleaved winding structures into one 3UI-based integrated transformer can reduce the core loss significantly with a smaller footprint compared with three EI-core-based integrated transformers. The proposed integrated magnetics can be scalable for higher voltage and higher power converters by assembling more 1UI-based elemental transformers. A SiC-based 1PCLLC converter prototype operating at 250-kHz switching frequency for 11-kW OBC applications was built with the proposed integrated transformer, and it can achieve a power density of 250 W/in3 with maximum efficiency of 98.4%. Secondly, the challenge of increased common mode (CM) noise after adopting PCB-based windings in the design was discussed. The inter-winding capacitors between the primary and secondary windings act as a conduction path for high dv/dt CM noise, which can lead to electromagnetic interference (EMI) issues. To address this, a winding cancellation method for an integrated matrix transformer in a 1PCLLC converter was proposed and validated. This approach was tested in an 11-kW 1PCLLC converter. The EMI measurement results align with the analysis, confirming the effectiveness of the proposed method, which achieved a reduction in CM noise by 17dB. Furthermore, the 1PCLLC converter, incorporating the proposed planar matrix integrated transformer and winding cancellation technique, attained a power density of 420 W/in³ and a peak efficiency of 98.5%. Thirdly, to enhance efficiency further, the 1PCLLC converter is substituted with the proposed three-phase CLLC (3PCLLC) resonant converter equipped with three-phase rectifiers. The 3PCLLC converter becomes more promising for high power applications as its lower RMS current stress and automatic current sharing capabilities. It can achieve soft switching under all conditions. In addition, due to the symmetrical resonant tank, it is more suitable for bi-directional operation. Variable DC-link voltage is adopted so that the DC/DC stage can always work at its optimized point, providing best efficiency for the entire battery voltage. An improved core structure for the three-phase integrated transformer was proposed to reduce the core loss and simplify the magnetic components by integrating three primary resonant inductors, three secondary resonant inductors and three transformers into one magnetic component. A systematic method of converter design which includes the design of integrated transformer, converter loss optimization was adopted to design an 11kW 3PCLLC resonant converter. A SiC-based 3PCLLC converter prototype operating at 250-kHz switching frequency for 11-kW OBC applications was built with the proposed integrated transformer, and it can achieve a power density of 330 W/in3 with peak efficiency of 98.7%. Fourthly, the power level of OBC continues to increase to make up the large capacitance battery pack inside the EVs to relief the concern of mileage range. To address this challenge of higher power, a scalable matrix integrated transformer for multi-phase CLLC converter was proposed. A universal method of integrating magnetizing inductance with built-in leakage inductance based on multiple perfectly coupled transformers (PCTs). The integration of built-in leakage inductance can be achieved by connecting several PCTs using a standardized core type for cost considerations or can be further integrated into a customized core with interleaved magnetomotive force polarities across transformer legs to achieve better flux distribution and smaller core loss. The proposed concept can be applied to single-input single-output, and multiple-inputs multiple-outputs integrated transformer applications. A 3x3 PCTs-based integrated transformer built with PCB windings was designed for a 3PCLLC resonant converter, which integrates three primary resonant inductors, three secondary resonant inductors, and three transformers into one magnetic core to simplify the complexity of the converter. The effectiveness of the proposed concept was demonstrated through a high-efficiency, high-power density 3PCLLC DC/DC converter for an 800V 16.5kW OBC. The designed converter can achieve a power density of 500 W/in3 and a peak efficiency of 98.8%. / Doctor of Philosophy / Rising fuel costs and concerns about air pollution have significantly increased interest in electric vehicles (EVs). EVs are equipped with rechargeable batteries that can be fully recharged by connecting to an external electrical source. However, the wider adoption of EVs is hindered by the need for an on-board charger system that is both lightweight and efficient. The dissertation presents advances in OBC technology to address these challenges, focusing on the development of efficient, high-power density OBCs suitable for various EV applications. EVs utilize two main charging methods: on-board chargers (OBC) for regular charging and off-board (fast) chargers for quick refills of battery pack. Most EVs currently use 400V battery packs paired with 6.6kW or 11kW OBCs, while larger vehicles with over 100 kWh battery packs employ 16.5kW or 19.2kW OBCs, constrained by household voltage and current limits. Some manufacturers are transitioning to 800V battery packs to lower costs and enhance fast charging capabilities, necessitating the development of 800V OBCs with high efficiency and power density. For household use, EVs can charge via OBC in a grid-to-vehicle transfer and can supply energy back to the home or grid (vehicle-to-grid) for emergency use or to support smart grid functionalities, requiring bidirectional OBCs. Advanced power semiconductor devices have been instrumental in advancing power conversion technology. The introduction of power semiconductor devices based on wide bandgap (WBG) materials marks a revolutionary shift, offering potential improvements over silicon-based devices. These WBG devices are capable of achieving higher efficiency, and higher power density in power conversion at higher operation frequency. Elevating the switching frequency diminishes the voltage-second across the transformer, facilitating the utilization of printed circuit board (PCB) technology for the windings as opposed to Litz wire implementations. Compared to traditional Litz wire-based transformers, the manufacturing process is significantly streamlined, and the management of parasitic is considerably more straightforward. Furthermore, the integration of resonant inductors with PCB-based transformer results in a reduction in the overall number of magnetic components and improved power density. Addressing cost concerns, a novel, cost-effective single-phase converter design was proposed, achieving high efficiency with integrated magnetics. Additionally, the research tackled the challenge of electromagnetic interference (EMI) through a winding cancellation technique, significantly reducing common-mode noise and further improving the converter's performance. The research introduces an improved core structure for a three-phase integrated transformer, significantly reducing core loss and simplifying the design by combining multiple components into a single unit. This approach facilitated the creation of a high-efficiency, SiC-based converter prototype, demonstrating remarkable power density and peak efficiency compared with state-of-the-art solutions. To accommodate the increasing power requirements of OBCs, a scalable, matrix integrated transformer design was developed for multi-phase converters, optimizing cost and performance. This design simplifies the converter architecture, enhancing efficiency and power density, and is adaptable to both single and multiple output applications. These advancements offer promising solutions to the challenges hindering the wider adoption of EVs. The dissertation underscores the potential of advanced power conversion technologies, including the application of WBG devices, integrated magnetics to streamline converter design and enhance both the efficiency and power density of battery chargers.

With the development of electric vehicle (EV) charging systems, energy storage systems (ESS), data center power supplies, and solid-state transformer (SST) systems, the fixed-ratio isolated DC-DC converter, namely the DC transformer (DCX), has gained significant popularity. Similar to the passive AC transformer, DCX can bidirectionally convey DC power with very high efficiency. Due to zero-voltage switching (ZVS) and a small root mean square (RMS) current, the open-loop CLLC resonant converter operating at the resonant frequency is a promising candidate for DCX with a constant voltage transfer ratio. In Chapter 2, to solve unsmooth bidirectional power flow and current distortion in the traditional CLLC-DCX with synchronization rectification (SR) modulation, a dual-active-synchronization (DAS) modulation is adopted with identical driving signals on both sides. First, the switching transition of this modulation is thoroughly analyzed considering the large switch's output capacitances. After comparing different transitions, a so-called sync-ZVS transition is more desirable with ZVS, has no deadtime conduction loss, and almost has load-independent voltage gain. An axis and center symmetric (ACS) method is proposed to achieve this switching transition. Based on this method, an overall design procedure of CLLC-DCX with DAS modulation is also proposed. However, designing a high-power and high-frequency transformer for CLLC-DCX presents significant challenges due to the trade-off between thermal management, leakage inductance minimization, and insulation requirements. To overcome this trade-off between power rating and operation frequency, a scalable electronic-embedded transformer (EET) with a low-voltage bridge integrated into the transformer windings is proposed in Chapter 3. The EET addresses the challenge through simple open-loop control and natural current sharing, enabling easy parallel connection and scaling to different power ratings. Based on this concept, a bidirectional, EET-based DC transformer (EET-DCX) is proposed to solve the transformer-level paralleling and resonant point shift issues in traditional LLC-DCX designs. By employing the embedded full bridge, the EET-DCX effectively cancels out the impedance of the leakage inductance, ensuring optimal operation at any frequency. Additionally, the EET-DCX retains the inherent advantages of the LLC-DCX, such as load-independent voltage gain, simple open-loop control, full-load range ZVS, and low circulating current. Leveraging these advantages, the proposed EET-DCX solution has the potential to push the boundaries of transformer performance to the MHz operation frequency range with hundreds of kilowatts of power capability. Moreover, to address the significant RMS current problem of the CLLC-DCX, a trapezoidal current modulation is also proposed in Chapter 3. Compared to the CLLC-DCX with a sinusoidal current, an EET-DCX with a trapezoidal current can reduce the total conduction loss by up to 23%. This total conduction loss includes semiconductor loss on both high-voltage and low-voltage bridges and transformer winding loss. In light of this EET concept, another resonant commutation (RC) EET-DCX is proposed to streamline the circuit. First, it replaces the embedded full bridge with a low-voltage bidirectional AC switch. Second, it introduces a resonant current commutation to realize a quasi-trapezoidal transformer current with a smaller RMS value. Compared to the triangular current produced by the original EET-DCX, the RMS current can be decreased by 15%. By incorporating only one embedded bidirectional AC switch, the high-frequency transformer leakage inductance impedance is fully neutralized. As a result, the rated power of the proposed RC EET-DCX can be readily scaled up through transformer-level parallelism. Furthermore, the RC EET-DCX maintains the benefits of a typical LLC/CLLC-DCX, including load-independent voltage gain, full load range ZVS, and low circulating current. However, either in EET-DCX or RC EET-DCX, the trapezoidal current modulation will increase the voltage stress on the low-voltage full bridge or bidirectional AC switch, especially when the leakage inductance is large and variable, such as in the high-power wireless charging application. To address this trade-off between RMS current and voltage stress, this paper proposes the concept of a hybrid resonant-type EET-DCX with a series resonant capacitor. Following this concept, two specific topologies, hybrid EET-DCX and hybrid RC EET-DCX, are proposed. The main difference between these topologies is that the former adopts a full bridge. In a hybrid RC EET-DCX, a resonant current commutation scheme is developed. Among these topologies, since the passive capacitor can mainly cancel the leakage inductance impedance, the full bridge or AC switch only needs to handle the remaining impedance. Thus, the voltage stress on active components can be dramatically decreased. Additionally, these two proposed topologies can retain all the advantages of previous EET-DCX designs, including natural current sharing, load-independent voltage gain, simple open-loop control, and full-load range ZVS. The comparison between these two topologies is thoroughly studied. Finally, a 12-kW DCX testbench is built to verify all the analysis and performance in Chapter 3. If output voltage regulation is required, DCX can cooperate with other voltage regulators to realize high conversion efficiency and power density. In Chapter 4, two DCX applications are implemented: an 18-kW 98.8% peak efficiency EV battery charger with partial power processing and a 50-kW symmetric 3-level buck-boost converter with common-mode (CM) noise reduction. In the first battery charger, a large portion of the power is handled by an 18 kW CLLC-DCX, and the remaining partial power goes through a 3-phase interleaved buck converter. The proposed switching transition optimization in Chapter 2 is adopted in the 18-kW CLLC-DCX to realize 98.8% peak efficiency. To handle the step-up and step-down cases at the same time, a symmetric 3-level buck-boost converter with coupled inductors is also studied as a post regulator. With symmetric topology and quadrangle current control, the converter can achieve a CM noise reduction and full load range ZVS with a small RMS current. To further optimize the performance and simplify the control, a mid-point bridging with a better CM noise reduction and a split capacitor voltage auto-balance is implemented. A 50-kW prototype is built to verify the above analysis. To summarize, Chapter 2 first proposes a switching transition optimization for CLLC-DCX. Later, to address the intrinsic trade-off between transformer rating power and frequency, an EET concept and its corresponding soft-switching DCX family are found in Chapter 3. Finally, to handle voltage regulation, two examples for practical applications are studied in Chapter 4 —one is an 18-kW partial power converter, and the other is a 50-kW 3-L buck-boost converter. Finally, Chapter 5 will draw conclusions and illustrate future work. / Doctor of Philosophy / With the development of electric vehicle (EV) charging systems, energy storage systems (ESS), data center power supply, and solid-state transformer (SST) systems, the fixed-ratio isolated dc-dc converter, namely dc transformer (DCX), has gained significant popularity. However, designing a high-performance DCX still has many challenges, such as large dead time loss, poor current sharing, and sensitivity to parameter tolerance. Firstly, the state-of-the-art resonant CLLC-DCX is optimized in Chapter 2. With an optimal switching frequency and dead time, both the primary and secondary sides of zero voltage switching (ZVS) can begin and finish simultaneously, which means dead time loss caused by current through the body diode can be eliminated. Therefore, the efficiency of CLLC-DCX can be improved. However, designing a high-power and high-frequency CLLC-DCX transformer still presents significant challenges due to the trade-off between thermal management, leakage inductance minimization, and insulation requirements. To overcome this trade-off, in Chapter 3, a scalable electronic-embedded transformer (EET) concept with a low-voltage bridge integrated into the transformer windings is proposed. The EET addresses the challenge through its simple open-loop control and natural current sharing, enabling easy parallel connection and scaling to different power ratings. In light of this EET concept, a new family of soft-switching DCXs is proposed for different applications, such as high-power wireless charging systems. All these EET-based DCXs retain the merits of typical CLLC-DCX, such as small circulating current ringing, small turn-off current, full load range ZVS, and load-independent gain. After realizing a desirable design for DCX, Chapter 4 presents two DCX applications with voltage regulation. Firstly, an 18 kW 98.8% peak efficiency battery charger is designed with partial power processing. Most of the power will go through an optimized DCX, and the remaining small portion of power will go through a 3-phase interleaved buck converter. On the other hand, DCX can also be adopted as a front-end or rear-end converter in a typical two-state DC-DC converter. As for another stage, a non-isolated DC-DC converter with a large output range can be used to handle voltage regulation. Following this structure, a 50-kW symmetric 3-L buck-boost converter with coupled inductors and reduced common emission is proposed. To summarize, the state-of-the-art CLLC-DCX is optimized in Chapter 2. Afterward, a new concept of EET-DCX and its corresponding DCX family is proposed in Chapter 3. After obtaining an optimized DCX, two practical applications with DCX are implemented in Chapter 4. Finally, Chapter 5 will draw conclusions and illustrate future work.