Bidirectional CLLC Research Articles

A bidirectional resonant CLLC converter combining three‐level characteristic and bipolar DC structure

SummaryIn order to meet the efficient and convenient usage requirements for bidirectional DC‐DC converters in some fields such as DC microgrids and vehicle power conversion systems, a bidirectional resonant CLLC converter combining three‐level characteristic and bipolar DC structure is used and analyzed in this paper. A three‐level topology is adopted in the resonant CLLC converter, which can achieve stable and wider range voltage regulation through the frequency adjustment combined with duty cycle and phase shift modulation, and can also increase the flexibility of control strategy. The application of bipolar DC structure enables this resonant CLLC converter to have multi‐port on one side, and the method of using two interleaved inductors combined with synchronous rectification enables the output voltage of the converter to maintain a certain balance. In this paper, the voltage gains and running states of the three‐level regulation method are analyzed, and then these operation modes and the bipolar DC voltage balance effect of the converter under synchronous rectification control are verified by the experiment results.

A Low-Cost and High-Efficiency Active Cell-Balancing Circuit for the Reuse of EV Batteries

In this paper, a high-efficiency and low-cost active cell-to-cell balancing circuit for the reuse of electric vehicle (EV) batteries is proposed. In the proposed method, a battery string is divided into two legs to transfer the charge from each cell in one leg to that in the other and a bidirectional CLLC resonant converter is used to transfer energy between the selected cells. Thanks to the proposed structure, the number of bidirectional switches and gate drivers can be reduced by half compared to the conventional direct cell-to-cell topologies, thereby achieving lower cost for the system. The CLLC converter is used to transfer the charge, and it is designed to work at resonant frequencies to achieve zero-voltage zero-current switching (ZVZCS) for all the switches and diodes. Consequently, the system’s efficiency can be enhanced, and hence, the fuel economy of the system can also be improved significantly. To verify the performance of the proposed active cell-balancing system, a prototype is implemented for balancing the three EV battery modules that contain twelve lithium-ion batteries from xEV. The maximum efficiency achieved for the charge transfer is 89.4%, and the balancing efficiency is 96.3%.

On‐board charger applications: A new hybrid control strategy for bidirectional CLLC resonant converter

AbstractThis paper proposes a new hybrid control strategy to solve the problem of bidirectional wide voltage range regulation in CLLC converters. On the (LLC) side (G2V direction), the SSSA control method is employed for voltage boost, combined with bus voltage and burst control to achieve wide voltage regulation. On the (C) side (V2G direction), the SSSA and full/half‐bridge mode switching control are used to achieve V2G direction wide voltage regulation. This hybrid control strategy also enables high conversion efficiency in both directions without the need for additional components to reduce the converter power density. This is particularly suitable for OBC applications with strict requirements for converter volume and weight. A detailed analysis with mathematical derivation, hybrid control strategy, and converter operation principles is introduced in this paper. The following subchapters supplement the converter voltage gain, soft switching implementation conditions, and design considerations. Finally, the feasibility of the proposed control strategy was verified through an experimental prototype with an input voltage ranging from 340 to 430 V to an output voltage ranging from 225 to 450 V and bidirectional rated power of 6.6/3.3 kW. This prototype achieved a maximum conversion efficiency of 97.7% in the G2V direction and 97.1% in the V2G direction.

A Hybrid Bidirectional Full-Bridge CLLC Resonant Converter with Dual-Resonant Frequency for Enhancing Efficiency at Light-to-Medium Loads

A Hybrid Bidirectional Full-Bridge CLLC Resonant Converter with Dual-Resonant Frequency for Enhancing Efficiency at Light-to-Medium Loads

A Dynamic Smooth Transition Control Integrated With Hybrid Modulation for Wide Output Voltage Range Bidirectional CLLC Resonant Converters

A Dynamic Smooth Transition Control Integrated With Hybrid Modulation for Wide Output Voltage Range Bidirectional CLLC Resonant Converters

Fundamental impedance-based digital synchronous rectification scheme for bidirectional CLLC resonant converters

Fundamental impedance-based digital synchronous rectification scheme for bidirectional CLLC resonant converters

Bidirectional Control With Fitting Model-Based Synchronous Rectification and Input Ripple Current Feedforward for SiC Bidirectional CLLC EV Charger

Conventional CLLC Synchronous Rectification (SR) typically adopts detection circuits to sense the high frequency signals. It can hardly be used in the high voltage SiC applications directly as it is sensitive to high dv/dt and may cause false operation. A bidirectional control is proposed for the SiC CLLC charger to achieve the SR function and mitigate the input ripple current. Considering the switching frequency and load, the SR on-time is calculated by the proposed three-order fitting model with low calculation source and high immunity to high dv/dt. Moreover, an input ripple current feedforward control is proposed in the discharging mode, which adds the extracted double line-frequency ripple current to the bus voltage reference. It not only achieves low SR conduction loss, but also reduces the input ripple current largely in the discharge. A 6.6-kW SiC bidirectional CLLC charger was built. Compared to the conventional SR, the CLLC efficiency of proposed SR improves 0.28% at 6.6 kW in the charge. In the discharge, the reduction of CLLC input ripple current at the double line-frequency is up to 91%. The overall charging and discharging efficiencies are 95.4% and 96.1% at full load, respectively.

Variable Frequency Isolated Bidirectional CLLC Resonant Converter With Voltage Controlled Variable Capacitors

This paper proposes a novel variable frequency isolated bidirectional CLLC resonant converter with voltage-controlled variable capacitors (VCVCs) and auxiliary clamp circuits (ACCs). By employing the class II ceramic capacitors as VCVCs, the converter's modes in one switching cycle and the transient process to set up the DC voltage bias on VCVCs are analyzed detailly to explain the fundamental principles and lossless characteristics of ACCs in the steady state. Suitable ranges of capacitor voltage stress and current stress and gain characteristics of the converter are also given to design the resonant parameters. The converter's soft-switching characteristics, fast transient process, and high efficiency are well demonstrated by the experimental results obtained on a 500 W prototype. The high-efficiency range of the proposed converter with the VCVCs is 20%∼100%, which is at least 10% wider than the others without VCVCs.

Fabrication Tradeoff Based Optimal Synthesis of Winding Configurations for Planar Transformer in CLLC DC–DC Converter

Adhering to the objective of modelling and selecting the most optimal winding configuration for a high frequency planar transformer (HFPT) for auxiliary charging systems for more electric aircrafts (MEA) this paper elucidates numerous fabrication and design-based constraints and correlations to enable parametric modelling of various magnetic components. Instead of providing generalized solutions, this paper thoroughly characterizes all possible winding configurations for HFPT employed in a bidirectional CLLC DC/DC converter. A detailed analytical study is presented for each component and verified using several instances of 3D Finite Element Analysis (FEA) based model to synthesize the effective field and current density distribution in the windings. Several design-based trade-offs are graphically explained with various criteria pertaining to optimal winding selection to study the interdependence of the resultant parameters on hardware specifications, such as the PCB thickness and its fabrication layout, air gaps and conductor thickness. The paper presents a comprehensive comparison between the analytical, simulation and experimental analysis that portrays a strong agreement between these models with an average mismatch of up to 6.2%. An experimental prototype of CLLC converter rated for 1kW nominal power, operating at 500kHz resonant frequency is developed with the optimal HFPT characteristics that yields a peak efficiency of 98.49% with magnetics stage efficiency of 99.31%.

A Multimode Control Scheme for Output Regulation and Voltage Balancing in a Stacked-Switch Resonant Converter With Extended High Efficiency for Wide Gain Range Applications

In this article, a new hybrid comprehensive multimode control system is proposed for a stacked-switch bidirectional CLLC resonant converter for wide voltage gain applications. By configuring the presented leg with different switching patterns, the presented leg in the converter can operate in different modes: full-wave (FW), half-wave (HW), or voltage-doubler (VD) through the converter’s built-in circuit redundancy. The proposed hybrid control scheme consists of different control modes: variable frequency modulation, pulse-width modulation (PWM), and asymmetric PWM (APWM) control. Correspondingly, four voltage gain curves are derived from the proposed converter. With the proposed approach, the converter is able to achieve constant voltage regulation while constraining the range of switching frequency, allowing very high efficiencies to be realized for a wide range of input voltages. Soft-switching is achieved for all the semiconductor devices for the entire operating range. The steady-state and dynamic performances of the proposed hybrid comprehensive control system are verified through a proof-of-concept 400-V <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim ~1$ </tex-math></inline-formula> -kV/ 700-V, 1-kW laboratory prototype. Results confirmed that the efficiency is maintained between 95.03% and 96.97% throughout the specified input voltage range.

Parameters analysis and control strategy combined with synchronous rectification technology for bidirectional CLLC resonant converter

Parameters analysis and control strategy combined with synchronous rectification technology for bidirectional CLLC resonant converter

Parasitic Component Inclusive Optimum Phase-Frequency Contour Enabled Synchronous Rectification of Asymmetric CLLC Resonant Converter

With an objective to reduce the switching losses in a bidirectional resonant CLLC DC/DC converter for electric vehicle (EV) charging applications, this paper presents an elaborate frequency dependent general harmonic approximation (GHA) based secondary side turnoff current minimization technique by investigating the optimum operating point to achieve synchronous rectification (SR). Formulation of an accurate all-inclusive gain model is presented, specifically focusing on effect of parasitic components on the resultant gain-frequency trend, backed with thorough experimental validation. Further, meticulous modeling of the GHA based state equations is presented to obtain a contour of feasible operational frequencies and corresponding phase shifts to ascertain accurate SR operation with a multi-dimensional optimization technique to ensure reduced switching losses. In addition to that, to precisely characterize the resonant tank equivalent circuit, stressing on its effect on SR phase calculation, a detailed 3D finite element analysis (FEA) based R-L-C modelling of the employed high frequency planar transformer (HFPT) is explained. To validate and benchmark the performance of the proposed gain model while ensuring accurate SR operation, a 1kW all-GaN based CLLC experimental prototype is developed for a resonant frequency of 500kHz, with a power density of 106 W/inch <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . Experimental waveforms at corner conditions are presented for a wide-gain bidirectional operation, portraying a peak converter efficiency of 98.49%.

Design Methodology of Bidirectional Resonant CLLC Charger for Wide Voltage Range Based on Parameter Equivalent and Time Domain Model

The bidirectional CLLC resonant converter has distinguished potential in battery chargers and energy storage systems for its advantages in soft switching and bidirectional power flow capability. However, traditional CLLC converters generally adopt symmetrical design to maintain bidirectional symmetrical characteristics, which means the secondary <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> network is designed to be equal to the primary <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> network after reflection. The symmetrical design is only suitable for voltage grade matching scenarios where a wide voltage range is not required, such as CLLC-type dc transformers. Whereas in the field of bidirectional chargers, due to the wide voltage range of battery, there are significant differences in the characteristics required between charging and discharging mode, which makes symmetrical design no longer applicable. To cope with this issue, this paper proposes a novel design methodology of CLLC based on parameter equivalent and time domain model. With the parameter equivalent principle, the CLLC resonant tank with arbitrary parameters is investigated to satisfy the requirements of wide voltage range for bidirectional charger application. Compared with the symmetrical design, the proposed method can meet the requirements of bidirectional gain within preset frequency range and guarantee the achievement of zero-voltage switching under required load conditions with the minimum reactive power. In addition, the area product capacity of the magnetic part of the CLLC resonant tank is minimized based on the parameter equivalent principle. Finally, experiments have been performed on a 1 kW prototype to confirm the validity and feasibility of the proposed design methodology.

Research on Automotive Bidirectional CLLC Resonant Converters Based on High-Order Sliding Mode Control

A CLLC resonant converter’s gain is easily influenced by the operating frequency, and when the operating frequency is adjusted over a wide range, the efficiency of the converter is greatly reduced. Traditional closed-loop control strategies also have disadvantages such as slow dynamic response and vulnerability to load. In this paper, a high-order sliding mode control (SMC) design method is proposed based on the current problems and the characteristics of automotive CLLC resonant converters. A sliding mode surface based on the output voltage characteristics of the CLLC converter includes higher-order differential terms for voltage and current and an error integral term for the output voltage, which reduces the operating frequency range of the converter and improves its dynamic responsiveness, thus increasing its efficiency. In order to verify the accuracy of the algorithm, a simulation model is built in MATLAB to verify the stability of the controller by varying the input voltage and the magnitude of the load and to verify the dynamics by abruptly varying parameters such as load and voltage. Comparing high-order SMC with PID control also shows that high-order SMC is more suitable for automotive converters.

Integrated current balancing cells based IPOP bidirectional CLLC resonant converter modules for high‐power applications

Integrated current balancing cells based IPOP bidirectional CLLC resonant converter modules for high‐power applications

Fault-Tolerant Operation of a Multimode Stacked Switch Rectifier Leg Through Built-In Circuit Redundancy

In this paper, the open-circuit fault-tolerant operation of a multi-mode stacked switch rectifier leg is presented and investigated. A fault-tolerant control technique is proposed through the built-in circuit redundancy operation of the presented multi-mode stacked switch rectifier leg. A bidirectional CLLC resonant DC-DC converter that employs the presented leg is used to demonstrate the features of the presented control scheme. The proposed control is able to allow post fault operation of the converter to continue to deliver the demanded load power with soft-switching operation in case any switch experiences an open-circuit fault. In addition, the built-in circuit redundancy of the leg allows the resonant converter to operate in different operating modes, resulting in an enhanced control in regulating the output voltage while avoiding the switching frequency to diverge far from the resonant frequency, leading to better performance and improved efficiency with the complete soft-switching operation before and after the fault. To highlight the merits of the proposed fault-tolerant control in the multi-mode stacked switch rectifier leg, experimental results are provided on a 300W resonant converter prototype with an input of 250V and an output voltage of 400V.

Unified Modeling, Analysis, and Design of Isolated Bidirectional CLLC Resonant DC–DC Converters

Due to the bidirectional power transmission, high power density, and high efficiency, bidirectional CLLC resonant converters are attracting more and more research attention in electric vehicles and energy storage systems. However, the existing models are difficult to unify the bidirectional circuits to analyze and design the circuit factors, and thus the parameter design approach and bidirectional synchronous rectification (SR) become rather complex and cumbersome. Especially in asymmetric parameter design, a lot of calculations or iterations are needed, which seriously increase the difficulty of design. To address this concern, a bidirectional unified modeling, analysis, and design method is proposed in this article. By selecting appropriate and quantitative bidirectional circuit factors, this model unifies the forward and reverse circuits and establishes the relationship between them, which greatly simplifies the calculation or iteration of the parameter design and the complexity of bidirectional SR. In addition, this parameter design method makes the bidirectional gain reach a higher range and reduces the range of bidirectional switching frequency. Finally, this unified modeling, analysis, and design method is verified in bidirectional CLLC resonant converters including symmetrical full-bridge, asymmetrical full-bridge, symmetrical half-bridge, and asymmetrical half-bridge CLLC, respectively.

Current-Sharing Worst-Case Analysis of Three-Phase CLLC Resonant Converters

Three-phase CLLC resonant converters provide higher power conversion capability compared to half-bridge and full-bridge structures. In addition to the unique features of CLLC converters for bi-directional applications, the three-phase structure provides significantly reduced output current ripple (smaller output capacitor), parallel power processing (reduced components size and current peak stress), and better thermal distribution (smaller heatsinks). However, with practical, i.e. non-zero, resonant component tolerances, these benefits are normally less, and sometimes significantly less than expected in the ideal case. In this paper, the unbalanced behavior of the converter with fifteen unknown resonant components is identified and analyzed. A new analysis methodology is proposed to investigate the worst-cases of current-sharing among above 32000 possible scenarios in three-phase CLLC resonant converters. In addition, the paper shows that phase-shifting techniques can be effective to mitigate the unbalanced behavior of the converter. The proposed analysis in this paper helps to determine the highest admissible tolerance in the components to keep the converter working within a certain range of unbalanced behavior without requiring any balancing techniques. The proposed analytical framework is verified with experimental and simulation results of a 3-kW bidirectional three-phase CLLC experimental prototype.

Bidirectional CLLC Resonant Converter Employing PLC Capability and EM Noise Reduction Technique for Small-Sized ESS Application

An energy storage system (ESS) requires high reliability with low electromagnetic (EM) interference, communication capability between ESS applications, and bidirectional power conversion for charging and discharging operations. However, a conventional ESS separately has power converters, communication devices, and large-sized EM interference filters for its stable operation. In this article, a current-fed capacitor-inductor-inductor-capacitor (CLLC) resonant converter employing asymmetric pulsewidth modulation and spread spectrum modulation is proposed to obtaining three functions, bidirectional power conversion with tight output voltage regulation, EM noise mitigation, and power line communication capability among ESSs, simultaneously. It can improve the cost-effectiveness, system's form factor, and communication flexibility. The proposed integration of three functionalities has design constraints on the implementation of each function. Design methodologies of power stage and control algorithm are also proposed for achieving the simultaneous operation of each function. The performance of the proposed converter and control algorithm are verified through a 20-W prototype converter.

Bidirectional Current-Fed CLLC Resonant Converter Employing Asymmetric PWM

A power systems of wearable application requires high power conversion efficiency and bidirectional power flows to obtain the compatibility with various power sources and loads. The conventional flyback converter has a simple structure and bidirectional power conversion capability. However, it shows poor power conversion efficiency caused by hard-switching operation, high DC offset current on the transformer, snubber losses, and large peak current. In this paper, a current-fed CLLC resonant converter is proposed to obtain the high power conversion efficiency and the bidirectional power conversion capability. The soft-switching operation, zero DC-offset current, and small peak current can induce high power conversion efficiency. The asymmetric pulse width modulation (APWM) regulates the bidirectional power flows and the output voltage. The design methodology of the proposed converter is analyzed by introducing its operational principles. The performance of the proposed converter is experimentally verified with a 20 W prototype.