LLC Resonant Converter Research Articles

A Comprehensive Overview in Control Algorithms for High Switching-Frequency LLC Resonant Converter

An LLC resonant converter has been widely used in various industrial applications because of its high cost-effectiveness, high power conversion efficiency, simple design methodology, and simple control algorithms using a pulse frequency modulation (PFM). In addition, the soft switching capability of the LLC resonant converter is good to obtain high switching frequency operations, which can get the high-power density of the power converter. Over the past years, several studies have been conducted to improve the performance of a high switching frequency LLC resonant converter with resonant tank design, optimal power stage design, and enhanced control algorithms. This paper is the review paper in terms of the control algorithms for the LLC resonant converter. It focuses on the overview of the high switching-frequency LLC resonant converter in terms of the control algorithms. The advanced control algorithm can improve power conversion efficiency, dynamic performance, tight output voltage regulation, and small electro-magnetic interference. The operational principles of the control algorithms are briefly explained to show their own characteristics and advantages. Thereafter, the research issues for the future works will be discussed in the conclusion.

Optimal Design of Multi-Output LLC Resonant Converter with Independently Regulated Synchronous Single-Switched Power-Regulator

This paper presents a tightly regulated multi-output isolated converter that employs only an independently regulated synchronous Single-Switched Post-Regulator (SSPR). The proposed converter is a highly accurate single-ended secondary side post-regulator based on a Series Resonant Converter (SRC); furthermore, it has a voltage-doubler characteristic. The proposed post-regulator requires only one auxiliary switch, in contrast with a bulky and expensive non-isolated DC–DC converter. Moreover, the added voltage-doubler can tightly regulate the slave output current. In addition, the voltage-doubler can improve electromagnetic interference characteristics and reduce switching losses arising from the Zero Current Switching (ZCS) operation of all power switches. The validity of the proposed converter is verified using experimental results obtained via a prototype converter applicable to an LED 3D TV power supply.

Dual Phase LLC Resonant Converter With Variable Frequency Zero Circulating Current Phase-Shift Modulation for Wide Input Voltage Range Applications

This article proposes a novel dual phase LLC resonant converter with an improved variable frequency-based zero circulating current phase-shift modulation scheme for wide input voltage applications. The presented topology employs a six-switch dual active bridge with two output ports reducing the voltage stress across upper and lower switches to half of the input voltage. The proposed modulation scheme is a three-variable control that enables the converter to operate with increased voltage gain range under all input voltage and loading conditions. Variable frequency control leads to the soft switching operation of the switches connected to the primary side for all load ranges. The phase-shift control is used as power transfer control along with maintaining a constant output voltage under changing input voltage. Moreover, the duty cycle calculation based on load conditions helps in eliminating the reverse power flow especially under light-load conditions. To verify the proposed converter and modulation scheme, a 1.5-kW prototype with 210–400-V input and 80-V output is built and tested with a peak load efficiency of 96.7%.

A Complete Step-by-Step Optimal Design for LLC Resonant Converter

LLC resonant converters have been widely used in many different industrial applications. Analysis and design methodologies have great effect on the converter performance. Accordingly, a complete step-by-step optimal design methodology based on time-domain analysis has been proposed for an LLC resonant converter in this article. The proposed design methodology is implemented under the worst operation condition, and the following considerations are included to obtain the suitable design area: operation mode; voltage stress for resonant capacitor; zero voltage switching operation for primary switches; and resonant tank root-mean-square current. Then, by finding all possible design candidates and comparing them based on the power loss model, the optimized design candidate can be found. Compared with the existing design methodologies, the proposed one has the advantages of high accuracy and small computation requirement, which makes it application in industry possible. Finally, a 192-W experimental prototype was built to validate the effectiveness of the proposed design methodology. In addition, a MATLAB graphical user interference program was built based on the proposed design methodology to visualize and facilitate the design process for engineers.

Equivalent Circuit Modeling of LLC Resonant Converter

LLC resonant converter is widely used in industry. However, up to now, no simple and accurate small-signal equivalent circuit model is available. This article proposes an equivalent circuit model of LLC resonant converter. The simple equivalent circuit model is derived based on modification and simplification of extended describing function method, which has already been successful deriving equivalent circuit model in series resonant converter [15] . The model can well predict the small-signal behaviors observed in pulse-frequency-modulated LLC resonant converter, whenever switching frequency is below, close to, or above the resonant frequency. For the first time, analytical expressions for control to output voltage, input to output voltage, input impedance, and output impedance are provided to aid closed-loop feedback design. SIMPLIS simulation and experimental results are presented to prove the accuracy of the model.

Non‐isolated LLC resonant converter with suppressed circulating current and automatic resonant frequency matching capability

This study proposed a novel non-isolated half-bridge Inductor-Inductor-Capacitor (HB LLC) resonant converter with automatic resonant frequency adjustment ability and a circulating current suppression auxiliary circuit. The proposed converter employs a hybrid modulation technique based on frequency modulation (FM) and pulse width modulation (PWM) to regulate the output voltage. At the nominal operating point, all the excellent attributes of a conventional LLC converter over the entire load range are maintained and output voltage control is achieved with FM. However, when the input voltage decreased to a level that LLC output voltage cannot be maintained with FM, the primary switches will operate at the minimum switching frequency and the auxiliary switch will be turned ON with PWM. The auxiliary switch is turned ON with zero-voltage switching and automatically matching the circuit resonant frequency with the switching frequency for optimal operation. The effective resonant inductor is energised directly from the dc bus storing more energy which is later released to the load to achieving high gain while maintaining the desired output voltage with a narrow frequency span. In addition, the converter exhibits low circulating current under an enhanced gain condition. Experimental results are presented on a 60 W prototype to validate the theoretical prediction. The proposed converter has a peak efficiency of 97%.

An Ultrawide Output Range ${LLC}$ Resonant Converter Based on Adjustable Turns Ratio Transformer and Reconfigurable Bridge

This article proposes an ultrawide output range LLC resonant converter based on adjustable turns ratio transformer and reconfigurable bridge. Due to the reconfiguration of the transformer by two four-quadrant switches, the transformer exhibits three different effective turns ratios, which correspond to three output ranges. Moreover, in case of lower output voltage, the primary side is reconfigured as a half-bridge to further extend the output voltage range. The design of resonant tank parameters is less constrained by the voltage gain requirement. The switching frequency range in each mode can be squeezed close to the resonant frequency, which contributes to an improved soft switch performance. The operational principles, design considerations, and mode transition are analyzed. A 4 A rated prototype that converts 390 V input to a 60-450 V output is designed and tested to validate the concept. A smooth mode transition is achieved without transient issues. The design prototype demonstrates 97.50$\%$ peak efficiency and good overall efficiency performance over this ultrawide output range.

Three-Phase LLC Battery Charger: Wide Regulation and Improved Light-Load Operation

Three-phase LLC resonant converters can handle much higher power compared to half-bridge and full-bridge LLC converters, which makes them suitable for levels 2 and 3 battery charger applications. In addition to the unique features of LLC resonant converters, the three-phase structure provides higher power capacity and higher power density at higher power levels in comparison with single-phase structures. Although single-phase LLC resonant converters were thoroughly investigated in the literature for many different applications, limited work has been done on three-phase LLC converters. Unexplored problems with three-phase LLC resonant converters include issues with their limited gain range and also their poor light-load efficiency. In this article, a modified three-phase LLC resonant converter with a new phase-shedding strategy is proposed. With the proposed modified topology and phase-shedding strategy, the wide gain range needed for covering the recovery zone of charging Li-ion batteries is realized. Moreover, with the proposed phase-shedding strategy, a significant efficiency improvement with light-load absorption charging is achieved. A 3-kW prototype was developed to validate the performance of the proposed converter.

Primary-Side Regulation Scheme for LLC Resonant Converter With Improved Resonant Current Sampling Circuit

A primary-side regulation (PSR) scheme for LLC resonant converter with improved resonant current sampling circuit is presented in this letter. An RC branch together with the auxiliary winding of the transformer is added in parallel with the current sampling resistor. The transformer magnetizing current component in the sampled primary current waveform on the sampling resistor can be offset by the generated voltage waveform on the added capacitor of the RC branch. In this way, PSR control for LLC resonant converter can be realized. Detailed operation principle and deviation analysis on the proposed PSR scheme are presented. Furthermore, a deadtime compensation is introduced to improve the output current accuracy caused by the component tolerance. Finally, a 48-78 V/1.3 A half-bridge LLC prototype has been built up to verify the theoretical analysis.

Design of High Performance Magnetron Power Supply on the Basis of PWM Power Control

This paper proposed a kind of high-performance magnetron power supply circuit, the output power of which could be regulated precisely and stably. Based upon the LLC resonant converter with multiplier, efficient power conversion and stable loop control were realized when the switch frequency was near to the resonant frequency. The driving pulses of the switches were controlled by a low-frequency PWM signal, thus realizing the intermittent operation mode. Therefore, the output voltage was in the form of pulsation, which met the requirement of the magnetron. Furthermore, the mean output power of the microwave could be regulated through the PWM duty ratio. An experimental prototype of 1000 W magnetron power supply was completed. As shown in the experiment, the soft switch of this circuit possessed excellent performance. The converting efficiency was higher than 93%; moreover, its output power could be easily adjusted.

Design and Current Unbalance Control Method for a Center-Tapped LLC Resonant Converter on UHD TV

This paper proposes a control algorithm for solving the unbalanced current of the primary side in an LLC converter that uses a center-tap rectifier on the secondary side in ultra-high definition TV applications. The current imbalance in the LLC converter using a center-tap rectifier causes thermal problems, low efficiency, and uncontrollability. The operational principle, primary side control algorithm, and design equation required to solve these problems are presented. The experimental results are presented to verify the efficiency and thermal improvement of proposed control algorithm.

Low Common-Mode Noise LLC Resonant Converter With Static-Point-Connected Transformer

A new structure of the transformer and its winding layout are proposed to reduce the common-mode (CM) noise generated from the planar transformer (PT). In the proposed converter, two half-bridge LLC resonant converters are connected in parallel on the primary side, and the secondary side of them is connected in series. The proposed converter can be utilized for the medium-to-high power applications due to the full-bridge structure of the primary side. For each transformer, one of the terminals is connected to the static point, which has zero dv / dt characteristic. Thus, the primary-side layer can have the same dv / dt characteristics with the secondary-side layer. By adjacently locating the primary and secondary layers, which have the same dv / dt characteristic, the CM current and noise generated from PT can be significantly reduced. Therefore, the proposed converter can achieve a high power density by reducing the size of the electromagnetic interference filter.

Phase-Shift Modulated Interleaved LLC Converter With Ultrawide Output Voltage Range

Conventional frequency modulated LLC topology is unsuitable for ultrawide voltage range applications and suffers from poor voltage regulation performance at light load conditions. To cope with those issues, this article proposes a novel two-phase dual LLC resonant converter. The primary side of the proposed converter is two paralleled half-bridges LLC , and the transformers’ secondary sides are in reverse series with shared full bridge rectifier. Both resonant tanks are driven by two fixed-frequency square waves, whose frequencies are always tuned at the optimal resonant frequency. A phase shift is introduced between those two square waves to regulate the output voltage. The proposed topology demonstrates good voltage regulation performance, which is weakly dependent on the load condition. Zero-voltage switching and zero-current switching are well realized on the power mosfet s and diodes in a wide range. A 1-kW rated laboratory prototype is designed to convert 390 V input to 10–420 V output. The prototype demonstrates a 98.1% peak efficiency and good voltage regulation capability. The experimental results agree well with the theoretical analysis.

Self-Driven Gate Driver for LLC Synchronous Rectification

This letter proposes a self-driven gate driver solely composed of passive components for synchronous rectification (SR). It utilizes the energy of the converter itself, automatically considers component tolerances and temperature variations of critical design parameters, and tunes the SRs on-time. Furthermore, no microcontroller, auxiliary power supply, or current-sensing/voltage-sensing components is required. A great amount of cost is therefore saved. The proposed gate driver is preferable to be used to some resonant converters operating as dc transformers. This letter presents its practical implementation in an LLC resonant converter. Experimental results illustrate that the body diodes' conduction and reverse recovery are eliminated.

Three-Switch LLC Resonant Converter for High-Efficiency Adapter With Universal Input Voltage

The LLC resonant converter is one of the most attractive topologies in many applications, due to its simple structure and zero-voltage-switching capability. However, under wide-input-voltage condition, it has large primary conduction and turn- off switching losses because of small magnetizing inductance. Thus, in this article, the LLC resonant converter using three switches is proposed for high-efficiency under wide universal ac input voltage. These three switches can make two operating modes of LLC resonant converter according to low/high line input conditions. As a result, the proposed converter can be designed with larger magnetizing inductance, resulting in high efficiency. The validity of the proposed converter is verified with a 60-W prototype for high efficiency, almost maintaining the system size, compared with the conventional LLC converter.

Resonant LLC DC–DC Converter Employing Fixed Switching Frequency Based on Dual-Transformer With Wide Input-Voltage Range

In this article, a dual-transformer-based LLC resonant converter is proposed, which is a hybrid combination of a full-bridge and half-bridge LLC circuits. The output voltage is regulated through fixed-frequency phase-shift pulsewidth modulation control scheme, whereas the switching frequency is equal to the resonant frequency, which helps to design the magnetic components. The turns-ratio design of the two transformers is analyzed by the current stress and power distribution of both transformers with two different cases. The proposed converter is capable of realizing soft switching over wide input-voltage and whole load range. Finally, a prototype converter of 1 kW is built and the experiments are conducted under wide input-voltage and full load range. The experimental results show that the proposed converter can achieve high efficiency throughout the full load range.

State‐space modelling of LLC resonant half‐bridge DC–DC converter

Recently, LLC resonant converters have attracted significant research from industry and academia for AC-DC and DC-DC power conversion with high efficiency and remarkable power density. They are appealing candidates for numerous vehicular and renewable energy applications including battery chargers for electric vehicles and drivers of LED lights. This study introduces a mathematical model of LLC resonant half-bridge DC-DC converter, which captures its steady-state behaviours for both continuous conduction mode and discontinuous conduction mode operations. One major advantage of the proposed model lies in accurate estimation of the switching frequency of power switches under a wide range of parametric variations. This benefit is, however, not offered by the prevailing method based on the first harmonic approximation (FHA). The analytical derivations of the system's state-space model, as well as equations for calculating the switching frequency by FHA, are discussed in details. For illustration, a 340 W digitally controlled LLC resonant converter is targeted in this study. The simulation analyses of current and voltage waveforms for light and heavy load conditions are presented. Moreover, the experimental results, along with the comparison of switching frequency estimation for both methods, are demonstrated and discussed, which confirms the validity and effectiveness of the proposed model.

Analysis and Design of the LLC LED Driver Based on State-Space Representation Direct Time-Domain Solution

The LLC resonant converter has been widely used in switched power supply covering several kinds of applications. Although it has been the focus of numerous analysis, their general time-domain (TD) solution is not completely enclosed due to its complexity given by the multiple resonance nature and its variable structure behavior. Consequently, the LLC converter design is also impaired by the lack of an enclosed TD solution. To overcome these issues, this article systematically presents a highly accurate TD solution of the LLC converter supplying a light-emitting diode (LED) load, which is obtained from the state-space representation direct TD solution. Dissimilar to preceding analyses, all the converter states are considered and no current and voltage sinusoidal or average value approximations are taken. Finally, supported by the TD analysis, a new LLC LED driver design procedure is introduced, which is oriented by weighted-average-efficiency concept and constraints that ensure zero voltage switching, enough power gain, and practical switching frequency range over a wide operating window. Experimental results show the feasibility of the proposed design procedure as well as the accurate TD behavior prediction of the LLC converter supplying an LED load. This article is accompanied by active content that exemplifies the proposed TD solution and design procedure.

Hold-Up Time Compensation Circuit of Half-Bridge LLC Resonant Converter for High Light-Load Efficiency

This article proposes a half-bridge LLC resonant converter applying a simple circuit with a variable magnetizing inductance scheme for hold-up time requirement. By changing the magnetizing inductance of the main transformer, the proposed converter can satisfy the hold-up time requirement without sacrificing efficiency, featuring small conduction, and turn-off switching losses of the primary switches in the nominal status. In addition, the proposed converter can relieve the burden of the additional components by positioning them on the signal path, and the proposed circuit is self-powered by adding only one additional winding instead of the isolated power supply. Therefore, those of the size and cost of the proposed circuit can be reduced compared to the previous studies. The feasibility of the proposed method was verified with a 350 W prototype converter (56 V/6.25 A), and the improved circuit for the transient mode was also presented. The experimental results validated the theoretical analysis and showed the effectiveness of the proposed converter for high-efficiency applications, especially in light-load conditions.

Comprehensive comparisons between frequency‐domain analysis and time‐domain analysis for LLC resonant converter

LLC resonant converter has been widely used in many applications due to its characteristics of high efficiency and high frequency characteristics. Analysis method plays great role in converter design and operation performance. For LLC resonant converter, the most frequently used analysis methods are frequency-domain analysis (FDA) and time-domain analysis (TDA). FDA assumes that only fundamental harmonics can deliver energy in the system, while TDA is obtained by solving circuit state equations. In this study, comprehensive and complete comparisons between FDA and TDA in terms of analysis methodology, parameter design, analysis accuracy, conduction loss, other power losses, and system efficiency are implemented. The analysis and comparison results show that under the same system specifications, by adopting TDA, a larger magnetising inductance value can be used when compared with FDA, which will reduce the resonant tank circulating current and increase the system efficiency. The comprehensive analysis can provide guidance for engineers when analysing and designing LLC resonant converter. Finally, a 200 W experimental prototype was built and tested to verify the theoretical analysis.