Bidirectional CLLC Research Articles

Bidirectional Resonant CLLC Charger for Wide Battery Voltage Range: Asymmetric Parameters Methodology

The CLLC bidirectional resonant converter has significant potential in battery chargers and dc microgrids, due to its bidirectional power transfer capability. To ensure uniform characteristics for bidirectional operation, secondary LC resonant tank components are usually designed to equal the primary LC components after reflection. This conventional design method is regarded as the symmetric design. It is used in applications where wide voltage regulation is not required, such as CLLC dc transformers. However, for bidirectional battery charger applications, the battery has a wide voltage range variation, and gain requirements during charging and discharging are different. These asymmetric characteristics could lead to undesirable large switching frequency range if a conventional symmetric CLLC design is employed. To address this issue, a detailed asymmetric parameters methodology (APM) is proposed in this article. It can design gain curves for charging and discharging modes separately. This enables overlapping of the switching frequency range for both modes, thereby reducing the bidirectional frequency range variation. It brings lower switching loss caused by excessive high frequency, and relieve the extra conduction loss and current stress of power switches as well. Finally, a detailed analysis of the proposed APM is provided and validated with simulations and experiments.

DC/DC Converter for Bipolar LVdc System With Integrated Voltage Balance Capability

Bipolar LVdc distribution is a promising solution for better integration of renewables and enhancement of system efficiency and reliability, but it also faces the challenge of pole voltage imbalance. By simple modifications of the bidirectional CLLC dc/dc converter, this article presents a dc/dc converter with integrated pole voltage balance capability. The converter presents less number of semiconductors, smaller overall footprint, and lower power loss compared to the employment of additional pole voltage balancers. Operating principle of this dc/dc converter and its inherent pole voltage balancing capability are explained, while further control scheme is also developed to compensate the small pole voltage difference caused by the dead time effect. Simulation results with 150 kW/±375 V bipolar output demonstrate the effectiveness of the converter, and the experimental results from a 500 W/±50 V downscaled prototype further prove the proposed concept.

Modelling of phase‐shift modulated bidirectional CLLC resonant converter

The study proposes the application of two modelling techniques for analysis of bidirectional CLLC resonant converters. The state-variable and cyclic-averaging techniques are applied for converters operating under two types of phase-shift modulation: single-phase-shift and pulse-phase modulation. The converter is analysed considering forward and reverse power flow directions and a state-variable equation description is obtained for both modes. The models are first validated through simulation, comparing the state-variable and cyclic-averaging results to a simulation program with integrated circuit emphasis (SPICE)-based simulation. Additionally, a low power prototype is built, experimental results are presented and the influence of parasitic elements and system delays is discussed. Simulation and experimental results show the models can accurately represent the behaviour of CLLC converters for both types of phase-shift modulation. In addition, using the cyclic-averaging technique results in a considerably faster execution compared to state-variable and SPICE-based models.

Sliding Mode Control Scheme for a CLLC Resonant Converter

In this paper, a comprehensive sliding mode control (SMC) loop design for a CLLC resonant converter is proposed. The major objectives of the proposed SMC are to improve the converter dynamics and to achieve a tight output voltage regulation with respect to any parameter variations and external disturbances. The sliding surface coefficients are selected to ensure both large- and small-signal stability for the robustness of the converter under different operating conditions. Furthermore, system dynamic performances considering the error dynamics and overshoots are investigated. To validate the proposed algorithm, a hardware prototype of a bi-directional CLLC resonant converter for plug-in electric vehicle applications is developed and tested up to 1 kW, and the effectiveness of the proposed control solution is verified by the load transients and start-up tests. At a 100% step-change in load power, the SMC achieves 1 ms settling time, which is approximately 0.9 ms faster than the conventional proportional integral control strategy.

High-Frequency and High-Efficiency Isolated Two-Stage Bidirectional DC–DC Converter for Residential Energy Storage Systems

The main aim of this article is to develop an isolated bidirectional CLLC converter that achieves a wide output-voltage range for residential energy storage systems (ESS), while maintaining high efficiency and achieving high power density. The circuit is a two-stage architecture, where the first stage is an isolated CLLC converter with open-loop fixed-frequency control and the second stage is a buck converter used for output voltage regulation. The influence of the bus voltage on the efficiency of the system in both forward and reverse operation modes is analyzed. Then, an asymmetric resonant tank design is developed such that different voltage gains are achieved in each mode with high switching frequency operation at the resonance point to optimize efficiency. For high power density, an improved integrated transformer structure with controllable leakage is designed for the high step-down ratio specification of the ESS application. The core design is optimized based on core loss, copper loss, and core volume. A 1-kW experimental prototype using GaN switches was implemented with an input voltage of 380 V, output voltage of 35–50 V, highest efficiency of 94.55%, and power density of 5.56 W/cm3. The front-end stage CLLC converter operates at 700 kHz with a maximum efficiency of 97.02%.

Analysis and Design of High-Efficiency Bidirectional GaN-Based CLLC Resonant Converter

A bidirectional CLLC resonant converter (CLLC-BRC) based on GaN transistors is analyzed and designed in this paper. Similar resonant topologies are listed and commented on, with the CLLC topology showing competitiveness in bidirectional energy transmission. The analysis of the aforementioned converter has been provided, including the reveal of resonant frequencies of the CLLC topology and an improved zero-voltage switching (ZVS) condition with operation principles of the reverse mode and relevant parasitic parameters taken into account. The design methodology of the aforementioned converter based on pulse frequency modulation (PFM) is further discussed in detail. A prototype with a rated power of 400 W and a maximal operating frequency that is larger than 0.5 MHz was built to verify the proposed design methodology. The highest conversion efficiency of the prototype was 97.02% in the forward mode, and it was 95.96% in the reverse mode.

Research on characteristics of bidirectional CLLC DC–DC transformer used in DC microgrid

In the DC microgrid system, the energy storage system must have both high power density and high energy density, and it is difficult for a single type of energy storage device to meet this requirement. The battery and the super capacitor are highly complementary in performance. Here, the battery and the super capacitor are, respectively, connected to the DC bus through a bidirectional DC–DC converter to form a hybrid energy storage system. For a 380V DC microgrid system, a new type of symmetrical circuit topology structure with two inductors and two capacitors (CLLC) resonant network is proposed. Since the symmetrical CLLC resonant network has the zero voltage switching capability of the main power switch and the soft commutation capability of the output rectifier, the converter can operate at high power conversion efficiency. Finally, the effectiveness of the system control strategy is verified through simulation and experiments.

State‐variable and cyclic‐averaging analysis of bidirectional CLLC resonant converters

The study proposes the application of state variable and cyclic-averaging modelling techniques for analysis of bidirectional, dual active bridge, CLLC resonant converters. The techniques are applied for converters operating under single phase-shift modulation in forward and reverse modes and the equation description is obtained for both models. The design of the converter is presented and the simulation results obtained are compared to a Spice-based simulation to verify the accuracy of the proposed models. Results show the models are suitable to represent the behaviour of CLLC converters under single phase-shift modulation. In addition, when using the cyclic-averaging technique integration is not required to solve the model, which results in a considerably rapid analysis when compared to the execution times of state variable and Spice-based models.

Design of a 1-MHz High-Efficiency High-Power-Density Bidirectional GaN-Based CLLC Converter for Electric Vehicles

In this paper, a bidirectional CLLC converter operating at MHz-level switching frequency is proposed to achieve high power density and high efficiency. An optimized stacked printed circuit board winding based planar transformer with adjustable leakage inductance is modeled and designed for the proposed converter. It is enabled using gallium nitride devices to achieve fast switching with low switching losses. A detailed power loss model is established for the efficient thermal design of the converter. A proof-of-concept hardware of 3.3 kW, 400–450 VDC input, 250–420 VDC output CLLC converter with 1 MHz operating frequency is built and tested over a wide range of loading conditions. The power density of the system is 9.22 W/cm3 (151.1 W/in3) and the peak efficiency achieves over 97%.

A High-Efficiency High-Density Wide-Bandgap Device-Based Bidirectional On-Board Charger

This paper proposes a novel two-stage topology for a 6.6-kW on-board charger. The first stage, employing an interleaved bridgeless totem-pole ac/dc in critical conduction mode to realize zero-voltage switching, is operated at over 300 kHz. A bidirectional CLLC resonant converter operating at 500 kHz is chosen for the second stage. A variable dc-link voltage is adopted to track the wide battery voltage range, so that the CLLC resonant converter can always operate at its most efficient point. The 1.2-kV SiC devices are adopted for the ac/dc stage and the primary side of dc/dc stage, while 650-V GaN devices are used for the secondary side of dc/dc stage. In addition, PCB winding coupled inductors and integrated transformer are implemented in ac/dc stage and dc/dc stage, respectively, for the purpose of high density and manufacture automation. The proposed structure is demonstrated to have 37-W/in3 power density and above 96% efficiency over the entire battery voltage range, which far exceeds the current practice.

Bi-Directional CLLC Converter With Synchronous Rectification for Plug-In Electric Vehicles

This paper presents an innovative technique for synchronous rectification in a bidirectional CLLC converter with an integrated transformer for plug-in electric vehicle applications. To improve efficiency and power density, the integrated transformer is introduced and simulated using finite element analysis. In addition, synchronous rectification is implemented by controlling the turn- on and turn- off timings based on the phase difference between primary gate pulse and secondary resonant current. Furthermore, a simplified closed-loop design is discussed and implemented on a digital signal processor (TMS320F28335). The effectiveness of the control loop is verified through a 3.3 kW proof-of-concept prototype with peak efficiency of 97.5%.

Design Methodology of Bidirectional CLLC Resonant Converter for High-Frequency Isolation of DC Distribution Systems

A bidirectional full-bridge CLLC resonant converter using a new symmetric LLC-type resonant network is proposed for a low-voltage direct current power distribution system. This converter can operate under high power conversion efficiency because the symmetric LLC resonant network has zero-voltage switching capability for primary power switches and soft commutation capability for output rectifiers. In addition, the proposed topology does not require any snubber circuits to reduce the voltage stress of the switching devices because the switch voltage of the primary and secondary power stage is confined by the input and output voltage, respectively. In addition, the power conversion efficiency of any directions is exactly same as each other. Using digital control schemes, a 5-kW prototype converter designed for a high-frequency galvanic isolation of 380-V dc buses was developed with a commercial digital signal processor. Intelligent digital control algorithms are also proposed to regulate output voltage and to control bidirectional power conversions. Using the prototype converter, experimental results were obtained to verify the performance of the proposed topology and control algorithms. The converter could softly change the power flow directions and its maximum power conversion efficiency was 97.8% during the bidirectional operation.

High-Efficiency Isolated Bidirectional AC–DC Converter for a DC Distribution System

A high-efficiency isolated bidirectional ac–dc converter is proposed for a 380-V dc power distribution system to control bidirectional power flows and to improve its power conversion efficiency. To reduce the switches’ losses of the proposed nonisolated full-bridge ac–dc rectifier using an unipolar switching method, switching devices employ insulated-gate bipolar transistors, MOSFETs, and silicon carbide diodes. Using the analysis of the rectifier’s operating modes, each switching device can be selected by considering switch stresses. A simple and intuitive frequency detection method for a single-phase synchronous reference frame-phase-locked loop (SRF-PLL) is also proposed using a filter compensator, a fast period detector, and a finite impulse response filter to improve the robustness and accuracy of PLL performance under fundamental frequency variations. In addition, design and control methodology of the bidirectional full-bridge CLLC resonant converter is suggested for the galvanic isolation of the dc distribution system. A dead-band control algorithm for the bidirectional dc–dc converter is developed to smoothly change power conversion directions only using output voltage information. Experimental results will verify the performance of the proposed methods using a 5-kW prototype converter.