LLC Converter Research Articles

Study of the decoupling magnetic integrated high‐frequency transformer for DC/DC converter

AbstractThe isolated DC/DC converter plays an irreplaceable role in the development of the DC microgrid. However, the multiple discrete high‐frequency transformers occupy considerable volume in power converters due to the connection of multiple isolated converters. In order to reduce the number of discrete transformers and increase the power density as well as the reliability of the power converter, a three‐port isolated DC/DC converter with a decoupling magnetic integrated high‐frequency transformer (DMIHFT) is proposed for the DC microgrid in this paper. The semiconductor count of the proposed converter is reduced by combining one DAB converter and two LLC converters. Three discrete transformers are integrated into a single common core, and magnetic decoupling is achieved among each transformer by rational design at the same time. In addition, the magnetic circuit model derivation process of the proposed DMIHFT and the operation principles of the converter are described in detail. The design guidance of the proposed DMIHFT is present. And then, a 15 kW/20 kHz DMIHFT is designed based on the design guideline, and the simulation is carried out. Finally, a 15 kW/20 kHz DMIHFT prototype is fabricated, and the effectiveness of the proposed DMIHFT is verified by experimental results.

A novel two‐stage solution for low‐power 48‐V voltage regulator module (VRM) using high step‐down hybrid LLC with integrated planar transformer

SummaryIn this paper, a novel two‐stage solution based on a 1‐MHz high step‐down hybrid LLC converter is proposed for low‐power 48 V voltage regulator modules (VRM). Since the hybrid LLC DCX converter with high step‐down ability is employed as the first‐stage, lower bus voltage can be obtained with the same transformer turns, which contributes to enhancing the duty cycle and efficiency of the second‐stage multi‐phase buck converter. Thus, an overall efficiency improvement can be achieved in the proposed two‐stage system, especially in low‐power applications. To further improve efficiency and power density, a novel integrated planar transformer is adopted for the double resonant tanks of the hybrid LLC converter. To validate the analysis, a 150‐W hardware prototype of the hybrid LLC converter is constructed in the laboratory. To ensure a fair comparison with existing LLC DCX designs, the output voltage is set at 6 V. The measured efficiency results of the first‐stage demonstrate an efficiency advantage at low power output levels compared to the similar full bridge LLC designs, especially at an output of around 100 W. Under the nominal 48‐V input, the first‐stage achieves a peak efficiency of 98.5% and a full load efficiency of 94.62%. Furthermore, a 4‐phase buck converter with nominal 6‐V input and 1.6‐V output is constructed to evaluate the efficiency performance of the overall system. When the second stage operates at 500 kHz, the proposed two‐stage system achieves a peak efficiency of 93.2% at 40A load, while maintaining an efficiency of 91.2% at 65A output. The calculated efficiency of the proposed two‐stage solution exhibits higher performance compared to the majority of single‐stage schemes.

PCB-Based Magnetic Integration and Design Optimization for Three-Phase LLC

The LLC converter is the most efficient topology in server and telecom applications. Furthermore, three interleaved LLC converters have been shown to yield efficiency advantages at power levels of several kilowatts. The magnetic components of a multiphase LLC, on the other hand, are complex, laborious, and difficult to build cost-effectively. To solve these issues, a high-frequency GaN-based three-phase LLC converter is used in this research. With GaN working at 500 kilohertz, all magnetic components, including six inductors and six transformers, can be housed in a single structure, while all magnetic windings are accommodated in a 4-layer PCB with 3 oz. of copper. Furthermore, EMI can be improved by virtue of three-phase by 20 dB and additional two layers is added to gain additional 20-30 dB reduction from a low frequency up to 30 MHz. The proposed construction is simple and economically mass-producible. A 6kW 400V/48V 3-phase prototype is implemented, with a peak efficiency over 99% and a power density of 1000 W/in3 (61 kW/L).

Transient response improvement of half‐bridge LLC resonant converter with full‐bridge rectifier for DC microgrid

AbstractLLC resonant converter has the features of low noise, high efficiency and power density, which is suitable to be integrated into DC microgrid. Due to severe stresses in the resonant tank, achieving fast and safe transient performance has been challenging for resonant converters. With less switch number and voltage stress, half‐bridge LLC resonant converter with full‐bridge rectifier (H‐F LLC resonant converter) are more suitable for high‐voltage application. To improve the transient response of the converter, this paper proposes a method based fixed center state trajectory for the converter to achieve soft start‐up. To reduce dynamic response time during start‐up, each step of fixed center state‐trajectory is designed optimally for the shortest path with symmetrical current limitation. In addition, the proposed method is also applicable to full‐bridge LLC converters. Moreover, according to the state‐plane analysis, load stepping‐up and stepping‐down transient state trajectories with the shortest path are carried out as well. To minimize the adverse effect of digital delay, fast load transient control based on 3D look‐up table is given based on FPGA‐EP3C25E144I7, which would also significantly reduce the computation requirement of the hardware controllers. The experimental results are verified on a 380V/12V LLC converter prototype at 130 kHz.

A Discrete Coupled Multiphase Interleaved LLC Converter With Symmetrical Components Analysis

This paper proposes a coupling structure and a generalized analysis approach for multi-phase interleaved LLC converters that applies to all odd phases that can simultaneously improve the current balancing in all interleaved phases. Firstly, a discrete coupled inductor array (DCIArray) structure is proposed, and its magnetic circuit model is analyzed in detail; then, the generalized symmetrical component theory is introduced to decouple the inductance matrix to the sequence impedance, with expansion to all odd-phase LLC converters, and then the sequence impedance, coupling coefficient, components tolerance impacts, and extended voltage gain with control architecture are discussed under the corresponding models; Finally, the recommended design for three-phase and five-phase DCIArray was discussed. Experiments based on both three-phase and five-phase prototypes have proved that the proposed scheme has excellent current sharing in almost all frequency ranges.

Adaptive droop control strategy of parallel LLC converters

AbstractWhen the resonant converters are connected in parallel, the output current is unbalanced without additional current‐sharing measures due to differences in resonant component. In the input‐parallel output‐parallel resonant converter system, the output voltage accuracy of the converter under the traditional droop control is analyzed when the load changes. In this paper, an adaptive droop control is proposed, in which the droop coefficient is adaptively adjusted as the load changes. The control method proposed in this paper realizes the current sharing between the converters and solves the shortcomings of the output voltage changing under different loads in the conventional droop control. Taking two parallel LLC converters as an example, the control block diagram of the parallel control method for adaptive adjustment of the droop coefficient proposed in this paper is given, and the stability of the control loop is analyzed. Finally, two parallel prototypes of LLC converters with input 200 V/output 24 V input are built by digital control, and the experiment shows that the proposed control method improves the shortcomings of excessive output voltage droop caused by traditional droop control during dynamic load switching, which verifies the effectiveness of the method.

A High-Capacity Interleaved Parallel Full-Bridge LLC Converter for External Charging Applications

This paper explores innovative converter designs tailored to meet the evolving demands of modern electric vehicle (EV) charging and power conversion systems. The first investigation centres on an AC to DC converter featuring an input series inductor and an output capacitor, enhancing power factor correction, reducing harmonics, and optimizing voltage regulation through advanced simulation tools. Comparative analysis highlights its superior power factor maintenance and efficiency. The second proposal introduces an interleaved parallel LLC (Inductor-Inductor-Capacitor) DC/DC converter, incorporating hybrid pulse frequency modulation (PFM) and pulse width modulation (PWM) control. This design extends the adjustable output voltage range, ensuring precise voltage control. A meticulous analysis and parametric design demonstrate an output voltage range of 50–80 V and a maximum power of 20 kW through simulation results. These innovations hold promise for various applications, including renewable energy, electronic device power supplies, and EV charging, offering practical solutions to enhance energy efficiency and power quality across modern electrical systems and EV infrastructure.

A Nonlinear Load Current Feedforward Strategy for the Charge-Controlled LLC Converter and Its Digital Implementation to Improve the Dynamic Response

Compared with the traditional single voltage loop control strategy in LLC converters, the charge-controlled LLC converter can significantly improve its dynamic response due to its model having the first-order feature. Generally, a load current feedforward method could improve the converter's dynamic response. However, for the LLC converter, it is almost impractical due to the unclear relationship between the load current and the control variable. To build this relationship, a mathematical expression between the load current change and the control variable is derived that can precisely offset the load disturbance and achieve faster dynamic response. The derived relationship is nonlinear, related to both input and output voltages and switching frequency, and is suitable for digital implementation. To further analyze the dynamic response, the output impedances with and without the load current feedforward control are established and analyzed, which visually demonstrates the dynamic improvement in the time domain through the calculation of the output impedance and load change spectrum in the frequency domain. Finally, a 120W LLC prototype with nominal 48V input and 12V output is built to verify the nonlinear load current feedforward control and its analysis.

A Control Design Technology of Isolated Bidirectional LLC Resonant Converter for Energy Storage System in DC Microgrid Applications

This paper presents a new control method for a bidirectional DC–DC LLC resonant topology converter. The proposed converter can be applied to power the conversion between an energy storage system and a DC bus in a DC microgrid or bidirectional power flow conversion between vehicle-to-grid (V2G) behavior and grid-to-vehicle (G2V) behavior. Furthermore, such a converter can be applied to energy storage systems for decentralized renewable energy generation systems, such as solar and wind power. In addition, this converter can be combined with a bidirectional inverter to allow energy storage in the system to improve the safety, stability, and power quality of the microgrid. In the proposed circuit structure, we use a bidirectional DC–DC LLC, which has the advantages of a higher voltage conversion ratio, lower part count, simpler control than similar converters such as DAB, CLLC, and L–LLC converters, and bidirectional power flow and electrical isolation. Specifically, to extend the battery life, it can be employed as a control strategy for discharging the energy stored in the battery (SOC) and reducing the temperature rise generated by the internal solid electrolyte interphase (SEI) when discharging the battery under the variation in distributed energy resource (DER) generation and load demand. To realize the bidirectional power conversion without using any auxiliary inductor and only changing the control strategy, the forward step-down power conversion was based on pulse frequency modulation (PFM) control, and the reverse step-up power conversion was based on pulse width modulation (PWM) control. In this paper, we introduce the bidirectional converter topology and its control strategy for the DC microgrid battery energy storage system. Finally, a 500 W prototype is built to verify the effectiveness of the proposed converter.

Design of Planar Transformers for LLC Converters in High Power Density On-Board Chargers for Electric Vehicles

This article presents a planar transformer design and optimization method for high power density on-board chargers (OBCs) utilized in electric vehicles (EVs). Owing to considerations of electrical safety, OBCs require an isolated converter, leading to a substantial increase in volume due to the inclusion of a transformer. To address this issue and achieve high power density, a planar transformer is used, and an optimized design method is proposed for pattern arrangement, width, and core shape. The feasibility of the design is verified through the development of a 3.3 kW OBC prototype. Consequently, when compared to conventional transformers, the design method in this article results in a 27% reduction in the transformer’s height and a 20% reduction in its overall volume. This reduction is advantageous for meeting the requirements of high power density OBCs.

Sliding Mode Control of a 2-MHz All-GaN-Based 700-W 95.6% Efficient LLC Converter

Sliding Mode Control of a 2-MHz All-GaN-Based 700-W 95.6% Efficient LLC Converter

A Switching-State Equivalent Gain Method for the Design of Dual-Transformer Wide Gain Range LLC Resonant Converter

Dual transformer resonant tank (DTRT) structure has been increasingly applied in the LLC resonant DC-DC converters to meet the demand for ultra-wide gain range in the utilization of renewable energy. Due to the structural particularity, the analysis of its gain characteristics and the design of topology are still rather complicated. In this paper, the parameter equivalent model of DTRT structure is derived from the traditional LLC converter to optimize the design of resonant elements. On this basis, the port theory based on mathematical model is proposed to quantify the relationship between the gain characteristics and the switching-states of the front-stage inverter. Inductively, the switching-state equivalent gain (SSEG) method was abstracted to broaden the total gain range within limited frequency range by mode splicing, and to guide the design of topology and operation strategy. To verify the theoretical analysis and the performance of proposed method, a 40-320V DC input, 96V DC output, 300W prototype was built up and applied in various inverter configurations. Analysis and experimental results indicate that the SSEG method is applicable to all kinds of DTRT based converters, representing the operating characteristics consistent with of the traditional converter but with an ultra-wide gain range.

Depth comparison of common inductor and common capacitor passive current sharing methods for multiphase LLC converter

AbstractCommon inductor (CIM) and common capacitor (CCM) passive current‐sharing methods have the advantages of simple implementation and low cost. However, there is still a lack of detailed comparative research on the two methods, which makes researchers confused when choosing the two methods. To provide clear scheme guidance, this paper provides a detailed comparison of the performance of the two methods. Firstly, this paper reveals the difference between the current waveforms of the two methods, that is, the current waveform of the CIM method is distorted, while the CCM method is non‐distorted. Then, the mechanism of the above phenomenon is analysed in depth. Secondly, for comparing the current sharing performance of the two methods, the first harmonic approximation (FHA) equivalent circuit model considering parasitic parameters is established to obtain the system's current sharing error model, and the parameter sensitivity analysis is provided. Finally, the relationship between the current waveform, current sharing error, and load ripple of the two methods is discussed. Then, based on the two methods, a more attractive common resonate tanks (CRT) current‐sharing method is proposed. The correctness of the view in this paper has been verified by extensive simulations and experiment results.

A Fault-Tolerant LLC Converter With High Reliability and Low Cost for Two-Stage Converters

The popularity of electric vehicles is getting higher, and a large number of DC-DC converters are deployed. LLC converters have witnessed a rapid deployment due to their remarkable merits, often employed as the isolation stage in two-stage converters. However, the contradiction between the increasing demand for reliability and the limited space is becoming prominent. Therefore, ensuring fault tolerance capacity with fewer devices is highly desired. A fault-tolerant LLC converter, which can keep working after multiple unexpected faults, is designed to take over half-bridge LLC converters. The pre-fault converter behaves like a full-bridge LLC converter and would be reconstructed to retain uninterrupted power output after faults. The reconstructed converter after the first fault behaves like two parallel half-bridge LLC converters and would be further reconstructed into a single half-bridge converter after the second fault. The faults can both be handled, including short-circuit faults and open-circuit faults. The transition processes of reconstruction are extremely short. The spike current during the transition processes is well limited, and the output voltage has slight fluctuation. The proposed converter could drastically reduce the need for redundancy and has obvious cost advantages. Finally, a prototype is developed to verify the superiority of the proposed converter.

A Structure-Reconfigurable LLC Resonant Converter With Wide Gain Range

In this paper, a structure-reconfigurable LLC (SR-LLC) resonant converter is proposed for wide output voltage applications. The converter can operate in half-bridge mode or full-bridge mode by modulating the driving signal of the switches. Four operation modes with scaled voltage gains are obtained by the independent or combined operation of resonant tanks. Compared with the conventional full-bridge LLC converter, the voltage gain range is greatly extended without extra switches. The consistent resonance parameters of two tanks allow four operating modes to have the same resonant frequency, which facilitates the design of magnetic components. Moreover, zero-voltage switching (ZVS) of the primary-side MOSFETs can be achieved in all modes. A 3.3-kW prototype with 75 V-450 V output voltage is built to verify the feasibility and effectiveness of the proposed converter. The peak efficiency of the prototype is 97.3%, and the efficiency curve is smooth in the ultra-wide output voltage range.

Design of a Flying-Capacitor-Based Voltage Divider LLC Resonant Converter for USB-C PD Charger in Aircraft Applications

Portable electronic devices have grown dramatically at an incredible speed and include many new and sophisticated functions. Thanks to the advent of universal serial bus (USB) power delivery (PD) specifications, power delivery is increased as is the number of applications that can be powered using a USB cable in aircraft. However, the combination of the wide-voltage-gain operation required by the USB PD specification, high step-down ratio, and the lower-pressure environment associated with aircraft puts a lot of stress on converter optimization. This paper presents the DC-DC stage of an integrated, highly compact USB type-C PD charger for aircraft. First, a flying-capacitor-based voltage divider (FCVD) switching bridge is proposed to replace the conventional switching bridge in an LLC converter to improve its efficiency. Second, a printed circuit board (PCB) winding planar transformer is adopted to increase the power density. Gallium nitride devices are used for both the FCVD and the synchronous rectifiers, further shrinking the size and reducing the loss. Critical areas of the converter prone to a high electrical field (E-field) are identified and preemptively addressed with the use of the E-field control methodology that follows the Paschen-curve-based insulation coordination. The prototype is built and tested, and it is verified that the unit successfully operates in rated conditions to achieve a power density of 73.2 W/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and an efficiency of over 90% at all nominal outputs. A high-altitude partial discharge test is also conducted, showing that the proposed prototype meets the partial discharge inception voltage (PDIV) at 10,000 ft.

An Improved Grouping Transformer's Secondary Windings Current-Sharing Method for Multiphase LLC Converter

Current balancing performance of the existing grouping transformer's secondary windings (GTSW) method is determined by leakage inductance mismatches. Nevertheless, the transformer's leakage inductance parasitic parameters in practice are difficult to accurately design, especially in high-frequency occasions, causing a large current-sharing error between phases. To solve this problem, this paper proposes an improved GTSW (IGTSW) current-sharing method. By ingeniously changing the connection mode of transformer's secondary windings, the influence of secondary leakage inductance mismatch on current sharing performance is eliminated, and the current sharing performance of secondary side is significantly improved. The first harmonic approximation (FHA) analysis method is used to establish the current sharing error model of the primary side and secondary side of the system. Furthermore, the current sharing performance of the existing GTSW method and the proposed method is quantitatively compared. Then, the effectiveness of the proposed method is verified by simulation and the two-phase 480W experiment results. Finally, some great performances of the proposed method are discussed, such as advantages in high-frequency applications, no effect of parasitic capacitance on the current sharing performance, the potential scalability in the case of other DC-DC topology and high current scenarios, and modular expansion capabilities, etc.

Synchronous rectification of LLC resonant converters based on resonant inductor voltage

Synchronous rectification (SR) technology has been a critical technology for LLC converters to achieve high efficiency and power density. However, conventional SR driving methods face challenges in terms of light-load condition, module size, switching accuracy, and circuit complexity. This paper proposes an SR driving strategy based on resonant inductor voltage (RLV) to address those issues. This RLV-SR driving strategy does not require current sensors and is insensitive to rectifier parasitic parameters. In addition, the RLV-SR driving strategy can be applied in a relatively wide operating frequency range and load conditions. Experimental results based on a 100-W/24-V LLC converter are presented to verify the effectiveness of the proposed RLV-SR driving strategy. Furthermore, the error of turn-on time caused by stray inductance is significantly reduced compared with the conventional VDS-ON sensing method, which improves the power converter’s efficiency.

Design of Magnetic Integrated LLC Transformer

The direct current/direct current (DC/DC) converter of the LLC (inductor-inductor-capacitor) converter is an important part of affecting the work efficiency, volume, and weight of the device. It not only has the functions of traditional transformers but is also able to solve the problems of traditional power transformers’ high price, huge volume, prodigious no-load loss, and inflexible control. This paper studies the DC/DC converter mainly, according to the given indexes, the magnetic integrated LLC resonant transformer is designed in detail. The magnetic integrated transformer greatly reduces the converter volume, and the selection of devices is completed based on parameters design. In addition, according to design parameters, losses and the efficiency of the LLC resonant transformer are calculated. The results meet the efficiency requirements. A test platform of a full-bridge LLC resonant converter is built according to theoretical research. The correctness and effectiveness of theoretical research and design methods of the DC/DC converter are verified by analyzing the experimental waveforms.

Design and Comparative Analysis of an Ultra-Highly Efficient, Compact Half-Bridge LLC Resonant GaN Converter for Low-Power Applications

For low-power applications, this paper presents the development and design of a compact and ultra-highly efficient half-bridge LLC resonant converter. By using Galium Nitride (GaN) devices and high-efficient magnetics, the efficiency and power density of resonant converters can be improved. Compared to Silicon MOSFETs, GaN high-electron-mobility transistors (GaN HEMT) have a lower output capacitance and gate charge, resulting in lower driving loss and shorter dead times. Consequently, the proposed LLC converter based on GaN devices has excellent performance characteristics such as ultra-high efficiency, low switching losses, compact size, high voltage endurance, high operating temperature and high operating frequency. Furthermore, the proposed resonant converter features soft switching properties that ensure that the switches and diodes on the primary side are always switched at zero voltage and current. By doing so, LLC resonant converter switching losses are significantly reduced by up to 3.1%, and an overall efficiency of 98.5% is achieved. The LLC resonant converter design with GaN HEMT has great advantages over Si MOSFET solution regarding efficiency, overall losses, switching loose and power factor correction. A 240 W, 240 V to 60 V half-bridge GaN HEMT LLC resonant converter is simulated with a switching frequency of 75 KHz, along with the comparative analysis of the Si metal oxide semiconductor field effect transistor (MOSFET) solution. Moreover, the design and analysis of highly efficient magnetics with a power factor of 0.99 at full load is presented. A 240-Watt single stage LED driver with power factor correction is also designed to verify and compare the performance of proposed LLC resonant converter.