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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)

   null (Ed.)

   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. Design and implementation of a series resonant solid state transformer

   https://doi.org/10.1109/ECCE.2017.8095937  

   Rashidi, Mohammad; Sabbah, Mohamad; Bani-Ahmed, Abedalsalam; Nasiri, Adel; Balali, Mohammad Hasan (October 2017, 2017 IEEE Energy Conversion Congress and Exposition (ECCE))

   The focus of this study is on the design of a full-bridge unidirectional resonant LLC Solid State Transformer. The proposed topology uses a high-frequency transformer to optimize the size and weight of the converter. This converter has the capability of operating at fluctuating load conditions while it keeps the voltage regulated in different operation point. The converter is designed to maintain soft switching by using a resonant circuit in this design to minimize the switching loss of the high frequency converter. ZVS in the leading leg for turn on mode and ZCS commutation in the lagging leg for all of the modes are achieved in the H-bridge through the suggested circuitry which is analyzed mathematically in detail in this study. A combination of Pulse frequency modulation (PFM) and Phase Shifting Modulation (PSM) are utilized to control this resonant converter. The experimental setup for the suggested configuration was implemented and the results of the simulation and calculations have been verified with test results. The hardware set up was tested with two different power levels and the output results confirm that the control method works properly to feed the load and keeps the converter working in the expected frequency range and maintaining the soft switching to decrease switching loss. The results shows conversion efficiency of 97.18% is achieved. 

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3. Optimal Operation of Multilevel Modular Resonant Switched-Capacitor Converter

   https://doi.org/10.1109/WiPDA.2018.8569135  

   Li, Yanchao; Ni, Ze; Cao, Dong (November 2018, 2018 IEEE 6th Workshop on Wide Bandgap Power Devices and Applications (WiPDA))

   Multilevel modular resonant switched-capacitor converter can achieve either zero-current switching (ZCS) or zero-voltage switching (ZVS) by utilizing different converter control strategies. This paper presents a comprehensive way to compare the root mean square (RMS) value of current flowing through switching devices in both ZCS operation and ZVS operation. The study shows that with appropriate converter parameter design, the ZVS operation allows the RMS value of switch current at most 10% lower than that in ZCS operation. Therefore, the converter operating at ZVS mode has the potential to achieve higher efficiency comparing to the converter that operates at ZCS mode due to less semiconductor conduction loss. Furthermore, the ZVS operation can reduce the power loss due to MOSFET output capacitance. A 6x converter with 54V input voltage, 9V output voltage and 600W power rating is used as an example to show the detailed design procedure. Simulation results are provided to verify the theoretical analysis. Also, a 600W lab prototype that has 6 to 1 voltage conversion ratio has been built to verify the theoretical analysis. 

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4. 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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5. Analysis and Design of High-Efficiency Modular Multilevel Resonant DC-DC Converter

   https://doi.org/10.1109/OJPEL.2022.3215581  

   Li, Yanchao; Wei, Mengxuan; Lyu, Xiaofeng; Ni, Ze; Cao, Dong (January 2022, IEEE Open Journal of Power Electronics)

   This paper demonstrates a high-efficiency modular multilevel resonant DC-DC converter (MMRC) with zero-voltage switching (ZVS) capability. In order to minimize the conduction loss in the converter, optimizing the root-mean-square (RMS) current flowing through switching devices is considered an effective approach. The analysis of circuit configuration and operating principle show that the RMS value of the current flowing through switching devices is closely related to the factors such as the resonant tank parameters, switching frequency, converter output voltage and current, etc. A quantitative analysis that considers all these factors has been performed to evaluate the RMS current of all the components in the circuit. When the circuit parameters are carefully designed, the switch current waveform can be close to the square waveform, which has a low RMS value and results in low conduction loss. And a design example based on the theoretical analysis is presented to show the design procedures of the presented converter. A 600 W 48 V-to-12 V prototype is built with the parameters obtained from the design example section. Simulation and experiments have been performed to verify the high-efficiency feature of the designed converter. The measured converter peak efficiency reaches 99.55% when it operates at 200 kHz. And its power density can be as high as 795 W/in 3 . 

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