Modelling, design, control, and implementation of advanced isolated DC/DC converters for renewable energy applications

Demand for high-efficient isolated DC/DC converters to achieve energy transfer among renewable energy sources, energy storage elements, and loads is increasing because of renewable energies’ increasing market penetration. Traditional converters pose significant challenges due to the wide voltage range operation nature of these components. Interest in improving the power converters’ performances for renewable energy applications is growing rapidly. Researchers are exploring accurate analysis methods, optimal design, and novel control strategies to improve the power converters’ performance. These strategies can improve the converters’ performance without any modifications to the traditional hardware. In addition, design automations are drawing more and more attention. By introducing new control strategy, the converter performance over a wide voltage range can be significantly improved. Furthermore, novel topology is another effective approach to cover wide voltage gain range operation.

From the power converter application perspective, stability, reliability, and fault tolerant operation are of great importance. The existing literature still lacks simple yet effective small signal models for those popular isolated DC/DC converters. Stability and reliability analyses are required to ensure safe operation of the system. This Special Issue published original research related to modelling, design, control, and implementation of advanced isolated DC/DC converters for renewable energy applications with improved performance.

In this Special Issue, we have received seventeen papers, all of which underwent peer review. The overall submissions were of high quality and all of them have been accepted.

Zhou et al. present a two-phase interleaved LLC converter with reduced switch count and precise current balancing by using switching-control-capacitor (SCC) technology. At the same time, the design of the SCC circuit parameters is given to reduce the SCC circuit voltage stress. To verify the proposed method, a 2.16 kW GaN-based two-phase interleaved SCC-LLC converter is established. The experimental results show that this method achieves excellent current sharing under all the tolerance conditions with cheap ceramic capacitor.

Zhou et al. explore solutions to address the voltage ringing issue across the synchronous rectifier switches (SRs) in low voltage dc-dc converters (LDCs) used in electric vehicles (EVs). The paper proposes an accurate ringing model for LLC resonant converters and a simple, lossless, and low-cost filter circuit to eliminate the negative effects of voltage ringing. The theoretical analysis is validated by simulated and experimental results, and the SR works efficiently and reliably with the proposed method at high load current.

Luo et al. introduce a method using multiple coil branches instead of a whole large coil to solve thickness problem and save space in the inductive power transfer (IPT). The paper analyzes key factors causing current unbalance and studies the equivalent circuit model of IPT with secondary multiple parallel branches. At the same time, this paper gives two kinds of current balance methods and the detailed parameter design criteria. The results show that this method can obtain nearly ideal current balance effect.

Zhou et al. present a multi-objective optimization design method for LLC converter to overcome the shortcomings of traditional design that cannot obtain the real optimal solution. Artificial intelligence methods are used to alleviate computation complexity, such as NSGA-II algorithm and APA. A simplified surrogate model is substituted into NSGA-II algorithm to obtain optimal parameter scheme. Comprehensive comparison analysis and experimental results show that the proposed method features highest efficiency of 95%.

Guo et al. present an accurate time-domain model of LLC series resonant converter with frequency and single phase shifted control, which has higher accuracy than traditional FHA method. Based on the proposed model, two simple fixed-frequency SPS control strategies are proposed. The first one makes the switching frequency fixed at 0.5f_n to realize infinite soft-switching margin with no restrictions on the transformer magnetizing inductance. The second one makes the switching frequency fixed at 0.8f_n to improve the efficiency. The experimental results under a 1 kW prototype verify the proposed model accuracy and control methods.

Tian et al. propose a segmented-region optimum modulation (SROM) scheme based on an extended-phase-shift (EPS) control for dual-active-bridge (DAB) operations under light load conditions or high voltage gain conditions to solve the restricted soft-switching range problem. This paper uses unified parameter algorithm to accurately establish the boundaries of different EPS operation regions. Based on accurate boundaries, the most suitable optimization considering both soft-switching range and efficiency can be selected by SROM. A SROM design example is proposed and the results show the superiority of the proposed method.

Liu et al. present an integrated charging equalizing converter (ICEC) based on Cuk converter to achieve load voltage converting and battery/supercapacitor charging and equalizing simultaneously in Solar Home System (SHS). The ICEC uses a voltage multiplier (VM) to replace the capacitor in Cuk convertor as a charging equalizer and the VM can be driven by its own voltage ripple, which saves extra switches and magnetic elements. A prototype for four SCs is built to verify the feasibility of the proposed model and the efficiency is improved to 87%. Furthermore, a quantitative comparison demonstrates the proposed method can reduce the system size and cost effectively.

Dutta et al. introduce a comparative study on the design and optimization of two feasible Power Pulsating Buffer (PPB) including buck and boost PPB for single stage isolated grid-connected PV microinverter system. The design process considers multi-objective optimization to achieve great performance. Besides, outer voltage-inner current feedforward control is used to enhance the system overall dynamic stability. According to the experimental results, the buck PPB outperforms the boost PPB at low switching frequency. However, at high frequency region, the boost PPB has superior performance, achieving an efficiency of 99.15% compared to the buck PPB 98.79%. The comparison shows the selection between buck and boost PPB can be optimized on the specific operating conditions.

Ibanez et al. propose an analysis method which uses Fourier decomposition for the main power signals to separate the real and imaginary parts. Based the fundamental harmonic analysis, a general control method for regulating the power at different ports is presented to minimize the recirculating current. Simulation and experimental results verify the feasibility of the control method.

Bachman et al. explore the comparison and selection of feasible topologies for deployment in high power and high efficiency DC microgrids. All topologies have been tested on the same components and controller concept. The comparison results show: (1) The DAB 3-phase has lower efficiency than the DAB 1-phase. (2) The Star-Delta structure reduces EMC interference. (3) Star-Star configuration has superior dynamic and efficiency performance, resulting in nearly a 5% reduction in THD. (4) The reverse gear ratio results in significant difference in efficiency.

Moradi et al. present an offline data-driven predictive control approach called the iterative feedback predictive controller (IFPC) to avoid extracting an accurate model of complex system. The proposed method is applied to a less-than-ideal buck converter. And robust stability analysis is performed to investigate the stability of the proposed controller. Simulation studies are conducted to evaluate the proposed controller under different scenarios compared with well-known model-based and data-enabled predictive controller approaches. It is discovered that the proposed controller has better robustness behavior even when the elements are perturbed by 10%.

Ren et al. propose an improved auxiliary circuit for IPT systems to achieve inherent CC-to-CV transition and load fault-tolerant operation, which is easy to achieve load short-circuit and open-circuit protection. This method can address the cross-coupling issue and increase the design freedom of the loosely coupled transformer (LCT). Besides, this paper discussed the operating principle and parameters design of the improved auxiliary circuit. The experimental results show the maximum system efficiency is up to 95.31%, which demonstrates the generality and feasibility of the proposed method.

Wei et al. propose an adaptive SR strategy based on the constant voltage gain characteristic at resonant frequency point to compensate the circuit components’ variations for SR LLC DCX. The experimental results validate the proposed strategy is applicable for all different scenarios with easy implantation and low cost.

Oh et al. present a bidirectional push–pull/H-bridge DC/DC converter for a low-voltage energy storage system, which is composed of the push–pull converter, the phase-shifted H-bridge converter, and the transformer. Compared with the traditional DAB, the proposed converter only has two power switches with a common ground and simple gate-driving circuits in the low-voltage side. The experimental results have demonstrated the validity of the proposed converter, which can achieve high efficiency in the step-down and step-up power flow operations, with 95.69% and 95.66%, respectively.

Santoro et al. explore a TAB prototype for NanoGrid (NG) applications, analyzing the possibility of a direct interface of PV modules, storage units, and DC loads, without the use of intermediate conversion stages. An experimental TAB converter with GaN devices is established and the overall efficiency is analyzed. Besides, a proper control strategy has been applied to provide a stable DC bus. The efficiency and transient characteristics under different operating conditions are tested and the results show TAB has a promising application.

Davoodi et al. propose a new DC–DC bipolar resonance converter that combines a dual-active-bridge and a multi-port resonance Buck-Boost converter, which has many advantages including bidirectional power exchange, high efficiency, integration of transformer parasitic elements, and the modular capability. According to the experimental results, the converter had a high efficiency of 94.7% in the nominal mode for a sample of 200 W and 2 A in each output port, which verifies the superiority of the proposed converter.

Zhang et al. present a method to enhance the small signal stability of a DCMG cluster by optimizing the main control parameters of the system. This paper utilizes the participation factor method based on the system-level linearized state-space model to identify the significant control parameters, specifically the voltage loop PI parameters and the droop coefficient. And an improved genetic algorithm and fuzzy membership function method are used to search for the optimal solution of important control parameters. The proposed method has been verified on a three-sub DCMG test system with droop control and the results demonstrate that the optimized DCMG cluster system has a better stability margin and damping.

Yuqi Wei (Member, IEEE) was born in Henan, China, in 1995. He received his B.S. degree in Electrical Engineering from Yanshan University, Hebei, China, in 2016, and his M.S. degree in Electrical Engineering from the University of Wisconsin-Milwaukee (UWM), Wisconsin, U.S.A., in 2018. He received another M.S. degree in Electrical Engineering from Chongqing University, Chongqing, China, in 2019. He was a Visiting Researcher with Kiel University, Germany from September to December 2021. He received his Ph.D. degree in Electrical Engineering from the University of Arkansas, Fayetteville, U.S.A, in 2022. He was a Post-doc researcher at University of Arkansas in 2022. Since 2023, he has been a Full Professor with Xi'an Jiaotong University. His current research interests include wide band gap devices, design automation and cryogenic power electronics. Dr. Wei received the 2020 IEEE Power Electronics Society Transactions Second Place Prize Paper Award, 2021 IEEE IFEEC best conference paper award, 2022 IEEE TEC Prize Ph.D. Thesis Talk Award and 2022 IEEE ECCE prize paper award.

Quanming Luo (Member, IEEE) was born in Chongqing, China, in 1976. He received the B.S., M.S., and Ph.D. degrees in electrical engineering from Chongqing University, Chongqing, China, in 1999, 2002, and 2008, respectively. Since 2005, he has been with the College of Electrical Engineering, Chongqing University, where he is currently a Professor. He is the author or coauthor of more than 40 papers in journal or conference proceedings. His current research interests include LED driving systems, communication power systems, power harmonic suppression, and power conversion systems in electrical vehicles.

Di Mou (Member, IEEE) was born in Lichuan, Hubei Province, China, in 1994. He received his B.S. degree in electrical engineering from the Three Gorge University, Yichang, China, in 2017, and the Ph.D. degree in electrical engineering from the Chongqing University, Chongqing, China, in 2021. He is currently a Postdoctoral Fellow with Tsinghua University, Beijing, China. He has authored or co-authored more than 30 papers in journal or conference proceedings. His research interests include multiport power electronic transformers, bidirectional dc–dc converter, and electrical vehicles.

Shuang Zhao (Member, IEEE) received the B.S. and M.S. degrees in electrical engineering from Wuhan University, Wuhan, China, in 2012 and 2015, respectively, and the Ph.D. degree in electrical engineering from the University of Arkansas, Fayetteville, AR, USA, in 2019. In 2018, he was an intern with ABB US Corporate Research Center, Raleigh, NC, USA. In 2019, he joined Infineon Technologies, El Segundo, CA, USA, where he was a Product Application Engineer for ATV. Since 2022, he has been based at Hefei University of Technology, Hefei, China, where he is currently an Associate Professor with the Department of Electrical Engineering.

Marco Liserre (Fellow, IEEE) received his M.Sc. and Ph.D. degrees in electrical engineering from Bari Polytechnic, Bari, Italy, in 1998 and 2002, respectively. He has been an Associate Professor at Bari Polytechnic. Since 2012, he has been a Professor in reliable power electronics at Aalborg University, Aalborg, Denmark. Since 2013, he has been a Full Professor at Kiel University, Kiel, Germany, where he holds the Chair of Power Electronics. He has authored 500 technical articles (1/3 of them in international peer-reviewed journals) and a book. These works have received more than 35 000 citations. He has been listed in ISI Thomson report “The world's most influential scientific minds” since 2014. He has been awarded with an ERC Consolidator Grant for the project “The Highly Efficient And Reliable smart Transformer (HEART), a new Heart for the Electric Distribution System.”

H. Alan Mantooth (Fellow, IEEE) received his B.S. and M.S. degrees in electrical engineering from the University of Arkansas (UA), Fayetteville, AR, USA, in 1985 and 1986, respectively, and his Ph.D. degree from the Georgia Institute of Technology, Atlanta, GA, USA, in 1990. He then joined Analogy, a startup company in Oregon, where he focused on semiconductor device modeling and the research and development of modeling tools and techniques. In 1998, he joined the faculty of the Department of Electrical Engineering, UA, where he currently holds the rank of Distinguished Professor. He helped to establish the National Center for Reliable Electric Power Transmission (NCREPT), UA, in 2005. He serves as the Executive Director of NCREPT and two of its centers of excellence: the NSF Industry/University Cooperative Research Center on GRid-connected Advanced Power Electronic Systems (GRAPES) and the Cybersecurity Center on Secure, Evolvable Energy Delivery Systems (SEEDS), funded by the U.S. Department of Energy. In 2015, he also helped to establish the UA's first NSF Engineering Research Center entitled Power Optimization for Electro-Thermal Systems (POETS) that focuses on high-power density systems for transportation applications. His current research interests include analog and mixed-signal IC design and CAD, semiconductor device modeling, power electronics, and power electronic packaging. Dr. Mantooth holds the 21st Century Research Leadership Chair in Engineering. He serves as the Immediate Past-President of the IEEE Power Electronics Society from 2019 to 2020 and the Editor-in-Chief of IEEE OPEN JOURNAL OF POWER ELECTRONICS. He is a member of Tau Beta Pi and Eta Kappa Nu and a registered professional engineer in Arkansas.