Full-bridge LLC Resonant Converter Research Articles

A Novel Polymorphic Topology with Hybrid Control Strategy Based LLC Resonant Converter for Ultra-Wide Input Voltage Range Applications

To realize effective utilization of renewable energy sources, a novel polymorphic topology with hybrid control strategy based LLC resonant converter was analyzed and designed in this paper. By combining the merits of a full bridge LLC resonant converter, three-level half bridge LLC resonant converter, and variable frequency control mode, the converter realizes an intelligent estimation of input voltage by automatically changing its internal circuit topology. Under this control strategy, different input voltages determine different operation modes. This is achieved in full bridge LLC mode when the input voltage is low. If the input voltage rises to a certain level, it operates in three-level half bridge LLC mode. These switches are digital and entirely carried out by the DSP (Digital Signal Processor), which means that an auxiliary circuit is unnecessary, where a simple strategy of software modification can be utilized. Experimental results of a 500W prototype with 100V~600V input voltage and full load efficiency of up to 92% are developed to verify feasibility and practicability. This type of converter is suitable for applications with an ultra-wide input voltage range, such as wind turbines, photovoltaic generators, bioenergy, and other renewable energy sources.

High-Efficiency LLC Resonant Converter With Reconfigurable Voltage Multiplying Rectifier for Wide Output Voltage Applications

This article presents a reconfigurable voltage multiplying rectifier (RVMR) to improve the performance of an LLC resonant converter for wide output voltage applications. The RVMR can operate as voltage-doubler mode or voltage-quadrupler mode according to the primary switching strategy. Two operation modes split a wide output voltage range into two narrow ranges. Therefore, the RVMR enables the LLC resonant tank to be designed in a narrow output voltage range, which results in a narrow switching frequency range, high efficiency, and high power density. A smooth mode transition can be achieved by adopting a phase-shift control to general pulse frequency modulation control without any additional switch. The proposed RVMR LLC converter can improve its efficiency by 2.9% and reduce component volume by 26% compared to the conventional full-bridge LLC resonant converter. A 750-W prototype with 400 V input, 100-300 V output voltage, and 2.5 A max output current has been built and tested to verify the effectiveness of the proposed converter.

Analysis of a Hybrid Variable-Frequency-Duty-Cycle-Modulated Low-$Q$ $LLC$ Resonant Converter for Improving the Light-Load Efficiency for a Wide Input Voltage Range

Light load efficiency and output voltage regulation of alow-Q LLC resonant converter is a critical problem for wide input voltage and load range applications. Parasitic capacitances such as rectifier diode junction capacitance (C3) degrade the soft switching performance. Compact size, high density, and high transformer turns-ratio requirements for microinverter applications add significant distributed capacitance (Cd) of the low-profile transformer, worsening the output regulation and zero-voltage-switching (ZVS) capability at light loads. Wide switching frequency requirement for regulation at light loads, which increases core losses and turnOFF switching losses in power MOSFETs, further degrades the power conversion efficiency. The conventional phase-shift modulation causes a high circulating current and loss of ZVS at light loads. Therefore, a hybrid adjustable switching-frequency-dutycycle modulation technique for improving the light load efficiency is proposed and analyzed for a full-bridge LLC resonant converter. Accurate loss analysis for the proposed modulation scheme, including the effect of parasitic capacitances, is performed using time-domain equations. The proposed methodology precalculates the optimal duty cycle at light load conditions for the required input voltage range such that minimum power losses are incurred. Variation in switching frequency at the preselected duty-cycle value regulates the output voltage. ZVS over a wide range of operating conditions is observed. An experimental prototype for a 20-40 V input, 380-V/300-W output LLC converter is tested for the validation of theoretical analysis.

Wide voltage gain range application for full‐bridge LLC resonant converter with narrow switching frequency range

In wide voltage range applications, traditional LLC converters suffer from the following issues: (i) to cover a wide voltage gain operating range, the required switching frequency range is also large, which challenges the design and optimisation of magnetic components, gate driver circuitry, and electro-magnetic interference filter; (ii) a small magnetising inductance is preferred in wide voltage gain range application, which leads to the increase of circuit circulating current and corresponding conduction loss; (iii) passive elements are determined based on the minimum switching frequency, so bulky passive elements are required to satisfy the required operation range. To solve these problems, in this study, a full-bridge LLC (FBLLC) resonant converter with two operation modes, namely full-bridge operation and frequency doubler operation, is proposed for wide voltage gain range application. The FBLLC converter discussed in this study can be designed in half of the input voltage range when compared with the conventional method, so smaller switching frequency operating range and higher efficiency operation are guaranteed. In addition, the smooth mode transition is achieved by introducing switching frequency feedforward control and small output voltage overshoot or undershoot is observed. A 150 W experimental prototype was built to validate the theoretical analysis.

Математическая модель резонансного LLC-преобразователя

The numerical-analytical model of the voltage regulator based on full-bridge LLC resonant converter is proposed in this paper. In the text of the article LLC converters operation algorithm on each working interval is investigated. The proposed mathematical model describes key elements behavior on duty cycle and allows the study of nonlinear dynamic characteristics of these type power converters. Also, the results of mathematical and simulation the modeling main parameters of the LLC converters, which confirm the adequacy of the proposed mathematical model are given.

Low Common-Mode Noise Full-Bridge LLC Resonant Converter With Balanced Resonant Tank

This article proposes a low common-mode (CM) noise full-bridge LLC resonant converter with two balanced resonant tanks. In the proposed converter, each resonant inductor and capacitor is divided into two components with the same value. The CM current flowing through parasitic capacitors of the transformer is reduced due to the balanced resonant tank. Moreover, the CM noise can be significantly reduced by adjacently locating the primary and secondary layers with the same dv/dt characteristic. Therefore, the proposed converter decreases the CM noise generated from the planar transformer (PT) by the balanced resonant tank and proposed layer arrangement of PT and achieves a high power density with a decreased size of the electromagnetic interference filter. The validity of the proposed converter is verified by a 1.5-kW prototype with 400-V input and 270-420-V output.

Zero-Vector-Injection Based Current Sharing Control of Interleaved Full-Bridge LLC Resonant Converters

Interleaved LLC resonant converters are widely used in various fields. However, interleaved LLC converters under Pulse Frequency Modulation (PFM) will lose the regulation of individual phases, causing a load sharing problem. Existing load sharing solutions have limitations; for example, phase shedding and current sharing cannot be realized at the same time. This paper proposed a novel current sharing method for interleaved full-bridge LLC resonant converters. Based on Zero-Vector-Injection, the voltage applied to the resonant tank is controlled to compensate for the difference in gain caused by component tolerance. The modulation strategy is proposed to maintain soft switching after Zero-Vector-Injection, and the phase shedding technique is also used to improve the efficiency at a light load. The detailed theoretical analysis and implementation method are proposed and validated using simulations. Experiments are also carried out to verify the feasibility of the proposed strategy based on a 2-phase 1.8 kW prototype.

Time Domain Simplified Equations and its Iterative Calculation Model for LLC Resonant Converter

LLC resonant converters can achieve soft switching and loss reduction. However, the analysis methods of wide frequency range LLC converters generally have problems with incomplete working condition analysis at non-resonant frequencies and there is always a tradeoff between the accuracy and the simplicity. These problems will affect the loss calculation, synchronous rectification and so on. The research objective of this paper focuses on the full-bridge LLC resonant converter. In this paper, three conditions are analyzed in time-domain including switching frequency lower than the resonant frequency at heavy load, switching frequency lower than the resonant frequency at light load, and switching frequency larger than the resonant frequency. Based on proper assumption and simplification, the approximate equivalent of the trigonometric function is used to simplify the complex time-domain equations. The simplified equations are obtained with the resonant capacitor voltage and resonant inductor current as the key variables. In order to make the simplified equations easy to use, an iterative calculation model is proposed with more simplicity than sophisticated software to solve nonlinear equations. The simplified equations and the model proposed in this paper are verified by comparison with the fundamental analysis methods, the simulation, and the experiments. By using the iterative calculation model, the voltage, current, time and other variables of switching points can be obtained with the relative error less than 3% to theoretic values, which can be used in loss calculation, synchronous rectification and so on. And the iterative calculation model can be realized on DSP or other processors in real-time.

Coordinated Control Strategy for Operation Mode Switching of DC Distribution Networks

Due to the advantages such as low line cost, low transmission loss, and high power supply reliability, DC distribution networks have become the main development trend for future distribution networks. In this paper, a typical DC distribution network with multiple voltage levels is considered as a research object. It is proposed that the interface converters between DC buses with different voltage levels be implemented through the series-parallel combination of full-bridge LLC resonant converters. To realize the decentralized self-discipline control of DC voltage under various working conditions, different slack buses are prepared according to the voltage ranges of the DC buses, and the voltage regulation modes of the DC distribution network are divided into main voltage regulation mode, backup voltage regulation mode, and off-grid voltage droop regulation mode. By introducing a voltage coefficient related to DC voltage deviation as a basis for mode switching, the voltage fluctuations caused by slow switching between control modes in the method of traditional voltage margin control is reduced, facilitating fast and smooth switching between different voltage regulation modes. Finally, a simulation model for DC distribution networks is constructed utilizing MATLAB/Simulink. Simulation results verify the effectiveness and feasibility of the proposed voltage regulation modes and switching methods for DC distribution networks. Finally, an experimental platform is also constructed to verify the feasibility of the mode switching method proposed in this paper.

A New Phase Shift Full Bridge LLC Resonant Converter with Auxiliary Circuit

A New Phase Shift Full Bridge LLC Resonant Converter with Auxiliary Circuit

Hybrid modulation method combining variable frequency and double phase‐shift for a 10 kW LLC resonant converter

Features of the wide input voltage, high efficiency, and high power density are eagerly required in electric vehicle charging pile applications (EVCPA). To realise these features, this study devises a novel variable frequency and double phase-shift hybrid modulation method into the full-bridge three-level LLC resonant converter for EVCPA. Therein, this study deeply analyses the characteristics and conditions of soft switching in dual phase-shift operations, and wide voltage gain range realisations under technique of variable frequency. A 10 kW/400 V/25 A prototype with 95.5% efficiency is built for verification, whose experimental results show that the proposed converter can well achieve the aforementioned features to further drive its wide applications.

A Novel Dual Full-Bridge LLC Resonant Converter for CC and CV Charges of Batteries for Electric Vehicles

This paper introduces a novel concept for implementing constant current (CC) and constant voltage (CV) charges of batteries for electric vehicles using a dual full-bridge LLC (FBLLC) resonant converter, which shares its primary switches. In the proposed concept, the CC and CV charges are implemented using the inherent characteristics of an LLC converter as a current source and a voltage source. One FBLLC resonant converter operates with a fixed-resonant network whereas the other operates with a variable resonant network for the CC and CV charge operations. The proposed converter can achieve zero voltage switching (ZVS) and nearly zero current switching (ZCS) for all of the primary switches in the CC charge operation, and ZVS for all of the primary switches in the CV charge operation, which lead to a high efficiency. In addition, fixed switching frequency operation is possible in both the CC and CV charges thanks to the variable resonant network. A 3.3-kW prototype converter is implemented to verify the feasibility and validity of the proposed converter and a maximum efficiency of 98% was achieved.

Full-Bridge LLC Resonant Converter With Series-Parallel Connected Transformers for Electric Vehicle On-Board Charger

A full-bridge LLC resonant converter with series-parallel connected transformers for an onboard battery charger of electric vehicles is proposed, which can realize zero voltage switching turn-on of power switches and zero current switching turn-off of rectifier diodes. In this converter, two same small transformers are employed instead of the single transformer in the traditional LLC resonant converter. The primary windings of these two transformers are series-connected to obtain equal primary current, while the secondary windings are parallel-connected to be provided with the same secondary voltage, so the power can be automatically balanced. Series-connection can reduce the turns of primary windings. Parallel-connection can reduce the current stress of the secondary windings and the conduction loss of rectifier diodes. Compared with the traditional LLC resonant converter with single transformer under same power level, the smaller low-profile cores can be used to reduce the transformers loss and improve heat dissipation. In this paper, the operating principle, steady state analysis, and design of the proposed converter are described, simulation and experimental prototype of the proposed LLC converter is established to verify the effectiveness of the proposed converter.

Modulation Method of a Full-Bridge Three-Level LLC Resonant Converter for Battery Charger of Electrical Vehicles

This paper proposes a modulation method for a full-bridge three-level LLC resonant converter. The target converter is known to enable fixed frequency operation, small resonant inductance ratio, and zero-current switching operation of rectifier diodes under wide output-voltage variation. Therefore, it is suitable for an application such as in battery chargers. A new parameter named “master duty,” which determines each voltage-level duty, is introduced to this modulation method. It simplifies the modulation method; therefore, a modification of the modulation is easy. It also makes possible seamless handling of two-level, three-level, and mixed modes. The modulation method is represented by the relationship between the master duty and each gate-pulse edge position. This representation can clearly depict a complex modulation such as a combination of pulse-width and phase-shift modulations. The voltage gain of the converter is analyzed using the first-harmonic approximation technique. The result of the analysis is verified by the circuit simulation. The voltage gain is also analyzed using lossy components, which is verified by the experimental result. A 385-V input 225- to 378-V/6.6-kW output prototype converter is fabricated for this verification. It shows 98.14% peak efficiency, and its power density is 1036 W/L.

Accurate ZVS Analysis for Magnetic Design and Efficiency Improvement of Full-Bridge LLC Resonant Converter

This letter proposes a comprehensive zero-voltage switching analysis (ZVSA) to illustrate the commutation mechanism of full-bridge (FB) mosfet s in resonant converters. Inherent contradiction between two conventional ZVSA techniques is discussed with specific consideration of a full-bridge LLC (FBLLC) resonant converter. An analytical approach is proposed to determine the accurate zero-voltage switching (ZVS) criterion for both zero- and finite-phase overlap operations of an FB network. Impact of the proposed ZVSA on power losses of FBLLC is explained analytically. Experimental results are presented not only to validate the proposed ZVSA, but also to validate its effect on FBLLC. Low load efficiency is improved by 6% with the proposed approach.

A new variable-mode control strategy for LLC resonant converters operating in a wide input voltage range

This paper proposes a new variable-mode control strategy that is applicable for LLC resonant converters operating in a wide input voltage range. This control strategy incorporates advantages from full-bridge LLC resonant converters, half-bridge LLC resonant converters, variable-frequency control mode, and phase-shift control mode. Under this control strategy, different input voltages determine the different operating modes of the circuit. When the input voltage is very low, it works in a full-bridge circuit and variable frequency mode (FB_VF mode). When the input voltage rises to a certain level, it shifts to a full-bridge circuit and phase-shifting control mode (FB_PS mode). When the input voltage further increases, it shifts into a half-bridge circuit and variable frequency mode (HB_VF mode). Such shifts are enabled by the digital signal processor (DSP), which means that no auxiliary circuit is needed, just a modification of the software. From light load to heavy load, the primary MOSFET for the LLC resonant converter can realize zero-voltage switching (ZVS), and the secondary rectifier diode can realize zero-current switching (ZCS). With an LLC resonant converter prototype with a 300 W rated power and a 450 V output voltage, as well as a resonant converter with 20–120 V input voltage, the experiments verified the proposed control strategy. Experimental results showed that under this control strategy, the maximum converter efficiency reaches 95.7% and the range of the input voltage expands threefold.

A Cost-Effective Design and Analysis of Full Bridge LLC Resonant Converter

A Cost-Effective Design and Analysis of Full Bridge LLC Resonant Converter

Analysis on Load-Adaptive Phase-Shift Control for High Efficiency Full-Bridge LLC Resonant Converter Under Light-Load Conditions

Recently, the full-bridge (FB) LLC resonant converter is getting more attention due to its zero-voltage switching of the primary switches, zero-current switching of rectifier diodes, and high power capability. However, the conversion efficiency is degraded under light-load conditions due to the core loss in magnetic components and the switching loss in semiconductor devices. In this paper, a new control method is proposed for the FB LLC resonant converter. In the proposed method, the frequency control is basically used to regulate the output voltage over entire load conditions. Moreover, the proposed method utilizes phase-shifted gate signals between switch legs, based on predetermined optimal duty ratio under light-load conditions. At this point, the optimal duty ratio is determined to get minimum power loss under each light-load condition through the loss analysis of all components. Therefore, the proposed method makes high efficiency under light-load conditions. To confirm the validity of this paper, the prototype of the network power supply with 320-385 V dc input and 48 V/720 W dc output is tested.

A Digital Self-Sustained Phase Shift Modulation Control Strategy for Full-Bridge LLC Resonant Converters

A digital self-sustained phase shift modulation (DSSPSM) strategy that allows for good soft switching and dynamic response performance in the presence of step variations is presented in this paper. The working principle, soft switching characteristics, and voltage gain formulae of a LLC converter with DSSPSM have been provided separately. Furthermore, the method for realizing DSSPSM is proposed. Specifically, some key components of the proposed DSSPSM are carefully investigated, including a parameter variation analysis, the start-up process, and the zero-crossing capture of the resonant current. The simulation and experiment results verify the feasibility of the proposed control method. It is observed that the zero voltage switching of the switches and the zero current switching of the rectifier diodes can be easily realized in presence of step load variations.

Traveling wave tube LLC resonant converter operating with self-sustained phase shift modulation

In order to improve the soft switching and dynamic response characteristics of the traveling wave tube high voltage converter, the load characteristic of a traveling wave tube is analyzed, and a full bridge LLC resonant converter with a multiplier rectifier is designed. The working principle, modeling and designing methods of the LLC converter are given based on self-sustained phase shift modulation, especially the soft switching in transient condition and the designing guide of control parameter are analyzed. By simulation, the zero voltage switching in the load mutation condition is proved, and the dynamic response characteristic can be improved by changing the control parameter adaptively. By experiment, the typical wave form of LLC converter is further analyzed based on self-sustained phase shift modulation, and the experiment results show that the soft switching can be implemented.