LLC Converter Research Articles

Multi-Constraint Optimization and Co-Design of a 2-MHz All-GaN Based 700 W 95.6% Efficient LLC Converter

This paper analyzes and develops a multi-variable & multi-constraint design optimization approach with the goal of minimizing power losses in a 2 MHz LLC resonant converter for next-generation data center applications. For a thorough co- design of a multi-MHz resonant converter, intricately curated performance constraints and associated design-based trade-offs are presented. In addition, accurate characterization, and parametric minimization of the AC resistance by optimal selection of transformer winding configuration, while achieving a controllable leakage flux for the resonant inductor integration into the high frequency planar transformer (HFPT) thereby reducing the effective winding losses by 6%. An all-GaN based 700 W, high power density (6.2 W/cm3) experimental proof-of-concept was built for a conversion from a variable input bus voltage (380–420 V) to 12 V stiff output at a resonant frequency of 2 MHz. The results portrayed a steady state peak efficiency of 95.65%, with an improvement of 2.2% over the state-of-the-art (SOA) operable at MHz frequency

A novel hybrid half‐bridge LLC resonant converter with double resonant tanks for high step‐down applications

AbstractConventional half‐bridge (HB) LLC converters have been a good candidate for the first stage in the two‐stage conversion structure because of its high conversion efficiency and high frequency operation ability. However, the voltage gain is only 0.5 exclusive of the transformer, resulting in the reduced efficiency for high step‐down applications. In order to solve this issue, in this paper, a novel hybrid HB LLC converter with double resonant tanks is firstly presented for high step‐down ratio applications. The distinctive features of the proposed converter are as follows: (1) the voltage gain of 0.25 is achieved by incorporating the switched capacitor circuit. Thus, the primary turns number for the transformer is halved compared with conventional HB LLC converters. Correspondingly, the smaller primary side winding losses and transformer volume can be implemented. (2) The power splitting and low input current ripple can be obtained due to the employed double resonant tanks. Thus, the uniform thermal distribution and low input filter capacitance are the second advantage for the proposed converter. (3) Soft switching over the entire operation range still can be maintained with larger magnetics inductance. The detailed operation principles, circuit analysis, and resonant tank design for the proposed converter are introduced and presented. A hardware prototype with 36–60 V input, 3 V/50 A‐output is built to demonstrate the declared features of the proposed hybrid converter.

Photovoltaic Energy Storage System Based on Bidirectional LLC Resonant Converter Control Technology

Bidirectional DC/DC converters are widely adopted in new energy power generation systems. Because of the low conversion efficiency and non-isolation for conventional, bidirectional DC/DC converters in the photovoltaic energy storage complementary system, this paper proposes a bidirectional isolation LLC converter topology, with compensating inductance for the energy storage system; it has excellent characteristics, such as wide input voltage range and soft switching in full-load range. First, an AC equivalent model based on the fundamental wave equivalent method is established to derive the voltage gain. Meanwhile, a small signal model is also built to design a reasonably closed-looped controller. Finally, the improved bidirectional LLC resonant converter is applied to the photovoltaic energy storage complementary system. The correctness and feasibility for the bidirectional LLC converter topology under the proposed charging and discharging control strategy of the DC bus are verified by simulation and experimental results.

Pareto-Optimal Design of Litz-Wire Gapped-Core High-Frequency Transformer for LLC Converters

The optimization of Litz-wire gapped-core high-frequency transformer (LGHT) for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> converters is a challenging task owing to application requirements and complex relationship between multiple design-objectives and constraints. This necessitates the use of multiobjective optimization-techniques to obtain tradeoff design-solutions; however, these are cumbersome methods that require many iterations and resources. Therefore, this article proposes an improved and efficient Pareto-optimization method for LGHT-design. It combines multiobjective whale-optimization algorithm and analytical core-geometry factor model based core and winding selection method while keeping in consideration the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> converter requirements. The proposed method utilizes benefits of both method-types; it obtains the best LGHT-design in shortest-time and minimum-iterations with optimized volume and losses, integrated magnetizing-inductance, and satisfied thermal-limit. The method is made resource-efficient by carefully selecting independent and necessary LGHT design-parameters as optimization variables keeping in view interdependency among the parameters and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> converter performance; these include peak-flux-density, LGHT design-frequency, core-window utilization-factor, core-material, and core-shape. For best design-selection, supplementary decision functions (total-loss and loss-and -volume-product) are used together with the objective functions (core-loss, winding-loss, and total-volume) to enhance the optimal Pareto-solutions’ diversity and to assist designers in decision-making. The method is proven via comparison with existing benchmark-methods and validated through performance demonstration of LGHT-prototype in 400 Vdc-to-12 Vdc <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> converter.

LLC Converter for Multi-Output LED for Output Current Balancing in case of LED Module Failure

LLC Converter for Multi-Output LED for Output Current Balancing in case of LED Module Failure

A new detailed loss model and design approach for LLC resonant converter with phase shift control to overall optimization of converter loss at whole battery charging process

AbstractThis paper proposes a new loss modeling and design method for phase shift controlled resonant LLC converter with optimization applied to minimize the overall loss during the whole battery charging process. In this work, the loss model has been formulated more accurately for phase shift controlled LLC converter. One of the contributions in accurate loss modeling is all switching modes of power switches have been investigated in detail in the aspect of zero voltage switching (ZVS) and ZCS occurrence and their related dynamic loss are updated in loss calculation during the whole charging process. In addition, to optimize the copper and core losses of high‐frequency transformer and resonant inductor, a design trend is included in optimization process to calculate the best physical dimensions and winding parameters of magnetic components. A genetic algorithm has been employed for the optimization during the charging process. In this work, the first harmonic approximation (FHA) model was developed and solved by nonlinear Newton–Raphson method in any operating point along one complete charging process. The target of optimization was designing a wall mounted type level 2 battery charger (7.4 kw/32A) for charging a battery pack with specs according to the Nissan Leaf 2011 electric vehicle. For verification of the proposed method, simulation results are provided and also experimental results for a low‐scale prototype are proposed. In this study, the average efficiency along the overall charging process obtained up to 95% in simulation and 84% in experiments.

Real Time Hardware-in-Loop Implementation of LLC Resonant Converter at Worst Operating Point Based on Time Domain Analysis

The inductor inductor capacitor (LLC) resonant topology has become more popular for deployment in high power density and high-efficiency power converter applications due to its ability to maintain zero voltage switching (ZVS) over a wider input voltage range. Due to their ease of operation and acceptable accuracy, frequency domain-related analytical methods using fundamental harmonic approximation (FHA) have been frequently utilized for resonant converters. However, when the switching frequency is far from the resonant frequency, the circuit currents contain a large number of harmonics, which cannot be ignored. Therefore, the FHA is incapable of guiding the design when the LLC converter is used to operate in a wide input voltage range applications due to its inaccuracy. As a result, a precise LLC converter model is needed. Time domain analysis is a precise analytical approach for obtaining converter attributes, which supports in the optimal sizing of LLC converters. This work strives to give a precise and an approximation-free time domain analysis for the exact modeling of high-frequency resonant converters. A complete mathematical analysis for an LLC resonant converter operating in discontinuous conduction mode (DCM)—i.e., the boost mode of operation below resonance—is presented in this paper. The proposed technique can confirm that the converter operates in PO mode throughout its working range; in addition, for primary MOSFET switches, it guarantees the ZVS and zero current switching (ZCS) for the secondary rectifier. As a function of frequency, load, and other circuit parameters, closed-form solutions are developed for the converter’s tank root mean square (RMS) current, peak stress, tank capacitor voltage, voltage gain, and zero voltage switching angle. Finally, an 8 KW LLC resonant converter is built in the hardware-in-loop (HIL) testing method on RT-LAB OP-5700 to endorse the theoretical study.

An LLC Converter with Capacitive Insulation

Offline power converter products must apply for and pass the national electrical safety code before they can be marketed. The transformers of offline power converters must be made from certificated materials with a high voltage rating and high electrical insulation, which increases the volume of the transformers and the printed circuit boards. In most studies, the miniaturization of a power converter is usually achieved by increasing the conversion efficiency and reducing the heat sink, or by increasing the switching frequency to reduce the size of the transformers. In this paper, the insulation material is reduced to miniaturize the transformer of the LLC converter. The resonant capacitor of the LLC converter is used to meet the requirements of insulation voltage, leakage current, creepage, and clearance. A prototype with the specifications of 12 V and 10 A rated output was built to verify the proposed method.

Design and Demonstration of a 540 V/28 V SiC-Based Resonant DC–DC Converter for Auxiliary Power Supply in More Electric Aircraft

Efficient and robust power electronic converters are vital to the success of the electrification of aircraft. Especially, low voltage auxiliary converters, which usually supply high current and low voltage loads, are not readily available industrially and need special attention. In terms of energy density and efficiency, LLC converters are among the most commonly used and efficient topologies for automotive and aerospace applications. In the case of aerospace applications, a fault-tolerant topology is highly desirable to reduce the need for redundant components and weight by removing backup systems. To solve this issue, this study introduces a new 2.0 kW LLC-based converter with a reconfigurable fault-tolerant architecture. With the help of a specially designed secondary side, the proposed converter can reconfigure itself so that even if one of the semiconductor switches fails permanently, the converter can still maintain power at nominal voltage levels, ensuring that the aircraft’s vital functionality is preserved. This paper also describes the basic operation principle, component-design aspects, conduction loss reduction techniques, and control system algorithm. Finally, a 2.0 kW experimental prototype is built to verify and demonstrate the operation of the proposed reconfigurable LLC converter.

Optimized Design of Integrated Planar Matrix Transformer for LLC Converter in Consumer Electronics

LLC resonant converter gets more and more applications in consumer electronics as it can convert the high voltage-low current state induced by wireless power transfer (WPT) into a low voltage-high current state effectively and efficiently. In this work, a planar matrix transformer structure and a new winding method are proposed in an LLC resonant converter to achieve a voltage conversion of 40–5 V after WPT in consumer electronics. This article focuses on modeling such a transformer and finding an optimal size to minimize the copper loss and the core loss with a total thickness of 2 mm. The theoretical analysis shows that the proposed winding method can minimize magnetomotive force (MMF) in both positive and negative periods and reduce the fringing magnetic fields’ influence around air gaps. The winding’s ac resistance can be reduced dramatically by such a printed circuit board (PCB) winding method. Meanwhile, such a structure can decrease copper loss by integrating the secondary winding in the UI core. An 80-W prototype is built to validate the proposed method’s feasibility with 1.5-MHz operating frequency, 40-V input voltage, and 5-V output voltage. The system owns the efficiency of 93.1% at full load condition.

Optimal design of GaN HEMT based high efficiency LLC converter

LLC converter has the advantages of large range voltage regulation ability, zero voltage switch (ZVS), magnetic isolation and is prevalent in data center power supply. As the increasing of power consumption in data center, the power density and efficiency of LLC converter is being pushed higher and higher. With the adoption of wide bandgap gallium nitride high electron mobility transistor (GaN HEMT), power device switching loss and conduction loss have been largely decreased and the switching frequency can be increased significantly. Thus the efficiency and power density promotion of LLC converter mainly dependent on the optimal design of deadtime and resonant parameters. In this paper, the ZVS working frequency boundary and the analytical calculation equations of both resonant and rectifier currents with the consideration of deadtime are deduced first, based on which a GaN device based LLC converter optimal design method is proposed with the realization of high efficiency and ZVS under wide input voltage range. An accurate GaN HEMT device model based LLC virtual prototype has been further constructed to verify the correctness of the proposed optimal design method. At last, the correctness of the proposed design method has been verified by experiment results.

A novel analog controller design scheme for LLC resonant converter used in battery charging application

A new generation of batteries must be charged according to the charging curves recommended by battery manufacturers. For high reliability, low cost, high speed, and pulse by pulse protection ability, the controller of these chargers are commonly designed by analog circuits. Analog controllers can be implemented by internal circuits of analog controller ICs with additional passive elements. This paper proposes a controller design approach for battery charger applications with an LLC converter. The proposed approach stands on a novel method for applying the well-known least-squares (LS) identification method for designing the classic controller with changing the identification problem to the design problem. Compared with other methods, the proposed approach benefits from design a controller with desired structure and desired performance, simultaneously. Here, modelling of LLC converter has been carried out by the first harmonic approximation (FHA) method. For controller design, the linear model has been extracted by linearizing the non-linear FHA model. In addition, for providing the reduced switching losses and increased efficiency, the passive elements of the resonance tank and isolation transformer ratio have been designed to operate the converter in the zero-voltage switching (ZVS) region. To verify the controller and designed converter, some simulation and experimental results are provided.

Smart Efficiency Tracking for Novel Switch—LLC Converter for Battery Charging Applications

In this study, a novel smart efficiency tracking (SET) algorithm for GaN-based LLC resonant converter for quick battery charging of light electric vehicles is proposed. Conventional frequency modulation (FM) method, single pulse width modulation method (S-PWM) and discontinuous operation mode method (DOM) are used to track the converter efficiency which varies for different loading conditions during the battery charging process. First, the charging modes are explained to define the efficiency variation clearly. Then, the theoretical background of GaN-based LLC resonant converter is presented and the details of design study are given. Third, the principles of three different switching control strategies, the modeling study and the simulation results are put forward. Finally, the details of the implementation of three different switching strategies are presented for the validation of the study. Furthermore, the implementation of the design and its impact on battery charging are discussed. The proposed efficiency tracking method is validated with a 20% efficiency increase for the low loading conditions of GaN-based LLC converter.

Ripple Behavior of LLC EV Charger in Boost Mode With Two-Capacitor Series Filter

This paper aims to explore the ripple behavior of the recommended connection method between the converter output and the filter, and especially the connection to a two-capacitor series filter (TCSF) used to boost LLC output voltage as an EV charger. The study confirmed that the form of ripple current in the capacitor significantly determines the integral ripple voltage and the equivalent series resistance (ESR) ripple voltage. The most effective way to reduce ripple current is to phase-shift the converter output prior to connecting to the filter. This will effectively achieve a smaller ripple current to improve and maintain the reliability and quality of the capacitor. In this study, the ripple behavior of the three connection modes between two interleaved LLC converters (ILLC) and TCSF is studied, analyzed and compared, including general connection, in-phase parallel (IPP) connection, and out-of-phase cross (OPC) connection. To facilitate the analysis, the first harmonic approximation (FHA) method and graphical description are used to model the LLC output as a quasi-sinusoidal current source and analyze the ripple behavior. In addition, we attempt to connect a third capacitor in parallel between TCSF and the load. Experiments proved that the load energy is adjustable, the ripple current in TCSF is halved, the reliability and quality of the capacitor are further augmented and maintained, and EMI immunity is improved. Finally, a 4kW/400V EV charger composed of two ILLCs connected to TCSF was implemented to verify the estimated ripple performance in various modes. Compared with the experimental results, the recommended OPC-connection mode 3 has the most significant ability to achieve the lowest ripple current and ripple voltage.

A Wide Output Voltage Range LLC Resonant Converter Based on Topology Reconfiguration Method

This article proposes a wide output voltage range LLC resonant converter based on topology reconfiguration method (TRM). First, with different driving signals two operational modes of inverter circuit and rectifier circuit of the proposed topology are analyzed, respectively. On this basis, the converter in whole voltage range has three variable operating modes considering voltage gain by different combination between two kinds of inverting modes and two kinds of rectifying modes. Thus, compared with conventional LLC converter, the voltage gain range can be extended by four times within the same switching frequency range using TRM plus pulse-frequency modulation (PFM). Comprehensive analysis of circuit states and soft-switching conditions in these three modes are presented. Moreover, the design of voltage area division in three modes is discussed to arrive a better tradeoff between efficiency and power density. A design flowchart is given to help to implement an efficiency optimization algorithm. In addition, a smooth mode transition technique is proposed to avoid the rapid frequency change during reconfiguring modulation strategy. Finally, a 2 kW 100–900 V output prototype is built, and main experimental results are provided to verify the effectiveness of the proposed topology and TRM.

Grid and PV Fed Uninterruptible Induction Motor Drive Implementation and Measurements

Motors powered directly from solar panels are becoming more and more popular in pump applications. However, solar panels can be the source of operational issues due to varying irradiance, ambient temperature, weather. This paper shows how it is worth expanding a solar induction motor drive to provide an uninterrupted flow of electricity to the motor. In addition, the main components of the uninterruptible induction motor drive are presented, including the LLC (inductor-inductor-capacitor) converter, the three-phase inverter, and the three-phase rectifier. LLC converters that can increase the voltage from 25–40 V to 330 V cannot be bought directly from manufacturers. Therefore, a custom LLC converter was made for the research. It was necessary to build a custom converter to avoid the use of solar panel strings. This way, solar panels connected in parallel can be used. A low-voltage (25–40 V) supply was implemented from the solar side, while the induction motor requires 230 V AC three-phase voltage in delta connection. For this reason, a voltage boost is required from the low voltage side. The grid feeds the universal DC link through the three-phase rectifier. This allows the motor to consume varying amounts of electricity from the grid or the solar panel. The study also presents in detail the LLC converter that performs the voltage boost. Measuring the entire motor drive, switching transients and efficiencies can be observed at different input voltages for different supplies as well as loads.

High-Flexibility MPPT Techniques with Communication Scan Network for PV Micro-Grid System

This paper proposes the design and implementation of inductor-inductor-capacitor (LLC) converters with modules connected in series with the power scan method and communication scan network (CSN) to achieve MPPT and regulate the output voltage for the PV micro-grid system. The Dc/Dc converters includes six isolated LLC modules in series to supply ±380 V output voltage and track the maximum power point of the PV system. The series LLC converters are adopted to achieve high efficiency and high flexibility for the PV micro-grid system. The proposed global maximum power scan technique is implemented to achieve global maximum power tracking by adjusting the switching frequency of the LLC converter. To improve the system flexibility and achieve system redundancy, module failure can be detected in real time with a communication scan network, and then the output voltage of other modules will be changed by adjusting the switching frequency to maintain the same voltage as before the failure. Additionally, the proposed communication scan network includes the RS-485 interface of the MPPT series module and the CAN BUS communication interface with other subsystems’ communication for the PV micro-grid application system. Finally, a 6 kW MPPT prototype with a communication scan network is implemented and the proposed control method is verified for the PV system.

Comprehensive Analysis and Gain Derivation of Phase-Shifted Dual-Input LLC Converter

This paper presents a comprehensive time-domain analysis of the dual-input LLC converter. By utilizing PSM and FSM, the converter power stage interfaces two photovoltaic (PV) panels to achieve maximum power point tracking (MPPT). The detailed mode-by-mode operation and steady-state analysis are derived using three parameters: the switching frequency, phase shift, and load weight. Furthermore, the DC gain equations of each PV panel and their characteristic curves are derived. In addition, the waveform boundaries and ZVS analysis are discussed as well. Moreover, an extended perturbation & observation (P&O) algorithm that guarantees achieving MPPT for each panel is proposed. Finally, the theoretical analysis has been tested and verified by a 450-W prototype of dual-input LLC converter.

600W DC-DC Converter Design Using Flyback, Half Bridge, Full Bridge LLC Topologies and Comparison of Simulation Results

This paper presents design and simulation of typical DC-DC converter topologies such as Flyback, half bridge, full bridge LLC, using the PLECS simulation tool. Requirements are chosen to be 600 W power rating and 250 kHz switching frequency with high efficiency and reduced built size. MOSFET is used as the switching element. According to the simulation results, optimum design with the lowest fluctuations in output voltage and current are achieved using the full bridge LLC converter topology. Thanks to the resonant coil and resonant capacitor in its structure, reduced switching losses have been observed in this converter design, which provides soft switching and high switching frequency.

LLC DC-DC Converter Performances Improvement for Bidirectional Electric Vehicle Charger Application

Electric Vehicle (EV) bidirectional charger technology is growing in importance. It defines the fact of returning the electricity stored in the batteries of EV to Grid (V2G), to Home (V2H), to Load (V2L), or in one word V2X mode. The EV onboard charger is divided into two parts: AC-DC and DC-DC converters. The isolated bidirectional DC-DC LLC resonant converter is used to improve the charger efficiency within both battery power and voltage ranges. It is controlled by varying the switching frequency based on a small signal modeling approach using the gain transfer function inversion method. The dimensions of the DC-DC LLC converter directly affect the charger cost. Moreover, they cause an important control frequency saturation zone, especially in V2X mode, where the switching frequency is out of the feasibility zone. The new challenge in this paper is to design an optimization strategy to minimize the LLC converter cost and improve the control frequency feasibility zone, for a wide variation of battery voltage and converter power, in the charging (G2V) and discharging (V2X) modes simultaneously. For our best knowledge, this optimization problem, in the case of a bidirectional (G2V and V2X) charger, is not yet considered in the literature. An optimal design that considers the control stability equations in the optimization algorithm is elaborated. The obtained results show a significant converter cost decrease and important expansion of control frequency feasibility zones. A comparative study between initial and optimized values, in G2V and V2X modes, is generated according to the converter efficiency.