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1. Design and Implementation of a LLC Resonant Solid State Transformer

   https://doi.org/10.1109/TIA.2020.2982847  

   Rashidi, Mohammad; Altin, Necmi; Ozdemir, Saban; Bani-Ahmed, Abedalsalam; Sabbah, Mohammad; Balali, Farhad; Nasiri, Adel (March 2020, IEEE Transactions on Industry Applications)

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   In this study, analysis and control of a highly efficient, high-power full-bridge unidirectional resonant LLC solid-state transformer (SST) are discussed. A combination of pulse frequency modulation and phase-shift modulation is utilized to control this resonant converter for a wide load range. The converter is designed to maintain soft switching by using a resonant circuit to minimize the switching loss of the high-frequency converter. Zero-voltage-switching (ZVS) is achieved for the H-bridge converter. The ZVS boundary for the proposed combined control method is also analyzed in detail. The experimental setup for the suggested configuration was implemented, and the performance of the proposed control scheme and resonant LLC SST have been verified with test results. The proposed combined control scheme improves control performance. The obtained results show that, the proposed system can regulate output voltage and maintain soft switching in a wide range of load. Thus, the efficiency of the system is improved and an efficiency of 97.18% is achieved. 

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2. A Novel Solar PV Inverter Topology Based on an LLC Resonant Converter

   https://doi.org/10.1109/ECCE.2019.8912924  

   Altin, Necmi; Ozdemir, Saban; Nasiri, Adel (September 2019, IEEE Energy Conversion Conference and Expo)

   In this paper, a new topology for grid-connected solar PV inverter is proposed. The proposed topology employs an LLC resonant converter with high frequency isolation transformer in the DC-DC stage. The DC-DC converter stage is controlled to generate a rectified sine wave voltage and current at the line frequency. An unfolder inverter interfaces between this DC stage and the grid. Both phase-shift and frequency control methods are used to control the LLC resonant converter. The switching frequency is determined depending on the phase-shift angle to extend the zero-voltage switching (ZVS) region. The transformer leakage and magnetization inductances are also properly designed to provide ZVS for wide operation area. The LLC converter operates in the ZVS region except the narrow band around the zero-crossings of the inverter output current. Since the LLC resonant converter has a high frequency transformer, the line frequency transformer requirement is eliminated, and thus more compact and efficient design is obtained. The proposed topology is validated by the simulation and experimental results. 

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3. A Modulation Scheme for Differential-Mode ZVS Resonant-Switched-Capacitor Rectifier

   https://doi.org/10.1109/TPEL.2024.3477388  

   Rad, Sadegh Esmaeili; Gupta, Shantanu; Mazumder, Sudip K (January 2025, IEEE Transactions on Power Electronics)

   This article proposes a predictive modulation scheme for a differential mode resonant switched capacitor rectifier (DMRSCR) to achieve high efficiency and power factor correction (PFC) for wide voltage gain. The modulation scheme ensures extensive zero-voltage switching (ZVS) turn-ON on all the switches under varying sinusoidal input voltage without requiring additional circuits or sensors. Four key control parameters, namely, phase shift ratio, duty cycle ratios, and switching frequency, are controlled for the converter to maintain ZVS turn-ON, PFC, output voltage regulation, and reduced resonant inductor current ripple. The article outlines a detailed DMRSCR model to deduce the dependency of the four control and converter design parameters on the converter operation. Based on the model, a complete converter design process is provided. A DMRSCR prototype rated at 1.1 kW was built using the underscored design methodology to validate the proposed modulation scheme, reaching a peak efficiency of 98.27% 

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4. A Matrix Autotransformer Switched-Capacitor Converter for Data Center Application

   https://doi.org/10.1109/TPEL.2023.3308888  

   Qiu, Maohang; Wei, Mengxuan; Liu, Xiaoyan; Meng, Haoran; Cao, Dong (December 2023, IEEE Transactions on Power Electronics)

   This article proposes a matrix auto-transformer switched-capacitor dc–dc converter to achieve a high voltage conversion ratio, high efficiency, and high power density for 48-V data-center applications. On the high-voltage side, the proposed converter can fully leverage the benefits of high-performance low voltage stress devices similar to the multilevel modular switched-capacitor converter. Compared with the traditional isolated LLC converter with a matrix transformer, the proposed solution utilized a matrix autotransformer concept with merged primary and secondary side windings, thus leading to reduced transformer winding loss. The resonant inductor could be integrated into the transformer similar to the LLC converter. Because of the matrix autotransformer design, it can achieve a current doubler rectifier on the low voltage side. For less than 8-V low output voltage application, the current doubler rectifier design can fully utilize the best figure-of-merit 25-V device, which is more efficient than the full-bridge rectifier solution using two 25-V devices during the operation. All the devices can achieve zero voltage switching or zero current switching and can be naturally clamped without additional clamping circuits. A 500-W 48-V to 6-V dc–dc converter hardware prototype has been developed with optimized device selection and integrated matrix autotransformer design. Both simulation and experiment results have been provided to validate the features and benefits of the proposed converter. The maximum efficiency of the proposed converter can reach 98.33%. 

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5. A High Efficiency, Decoupled On-board Battery Charger with Magnetic Control

   https://doi.org/10.1109/ICRERA.2018.8566835  

   Wei, Yuqi; Altin, Necmi; Luo, Quanming; Nasiri, Adel (October 2018, International Conference on Renewable Energy and APplications)

   In this paper, a high efficiency, decoupled on-board battery charger is proposed. The proposed topology is composed of two LLC resonant converters sharing the same full-bridge inverter with constant switching frequency. The outputs of two LLC resonant converters are connected in series. One of the LLC resonant converters is operated at the resonant frequency, which is the highest efficiency operation point. The magnetic control is adopted for the second LLC resonant converter to fulfill the closed-loop control of charge voltage and current for constant voltage (CV) and constant current (CC) charge modes. The proposed topology can achieve zero voltage switching (ZVS) for all primary switches and zero current switching (ZCS) for all secondary diodes during both CC and CV modes. Furthermore, thanks to constant frequency operation, the electromagnetic interference (EMI) filter design is simplified. Simulation and experimental studies are presented to verify the feasibility and validity of the proposed topology. 

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