LLC Series Resonant Converter Research Articles

Analysis of the time‐domain model of the LLC series resonant converter with frequency and single phase shifted control

AbstractLLC series resonant converter (LLC‐SRC) has attracted much attention due to its simple circuit structure and easy realization of soft‐switching. To get a better efficiency, rectifier stage can be realized by the active switches instead of the diodes. Meanwhile, more control variables can be selected besides the switching frequency, such as single phase shifted (SPS) angle, dual phase shifted (DPS) angle, and extended phase shifted (EPS) angle. Considering the simplicity of the implementation of the control strategy, this paper focused on the converter model with frequency and SPS control. Different from the common modelling method with the Fundamental Harmonic Approximation (FHA), the precise time‐domain mathematic model is built. By a comparison between the two modelling results, it can be proved the FHA model may be failure in some operating condition while the proposed time‐domain model can still show a good accuracy. According to the proposed model, two simple fixed‐frequency SPS control strategies are proposed. The first one can realize infinite soft‐switching margin with no restrictions on the transformer magnetizing inductance. The second one can greatly improve the efficiency of the converter. All the theoretical results are verified by a 1‐kW experiment prototype.

Performance Comparison Analysis of a High Voltage DC LLC Resonant Converter for Telecom and Datacenter Applications

Intermediate Bus Architecture (IBA) is used for power distribution in information communication technology (ICT) equipment due to the flexibility of its power distribution and cost-effectiveness compared to centralized or DC-mediated power. The reason for IBA's good performance in this context is its greater reliability and efficient Intermediate Bus Converter (IBC). Existing IBAs comprise low-voltage systems with multiple power, resulting in power loss. This study outlines the need for a high-voltage DC-to-DC converter to solve the problems associated with conventional 48 VDC converters. A 300 W LLC series resonant converter is utilized for the design and analysis of converter efficiency and losses across various input voltages. The results indicate that the maximum converter efficiency is >92% at full load. MATHCAD is used to analyze the performance of the LLC converter and understand the suitable operating zones for zero-voltage-switching. The LLC converter's efficiency is investigated for multiple input voltage ranges. The LLC converter is designed and developed in MATLAB and the results of the simulation are presented. Trade-offs in the LLC converter's design are also discussed, with the goal of operating the converter in an optimal manner and achieving the maximum possible efficiency.

Half-Bridge LLC Series-Resonant Converter With Hybrid Rectifier for LED Signage Backlighting Systems

Large-area LED signage backlighting systems require power supplies to provide constant voltage to multiple LED modules connected in parallel, each with a built-in LED current regulator to maintain a constant output brightness. LED signage backlighting systems can be divided into 12 V and 24 V, which require different power supply specifications. The development of a universal 2-in-1 LED driver will reduce the number of power supply models and avoid incorrect wiring in the field. A new topology of half-bridge LLC series-resonant converter with hybrid rectifier is proposed in this brief, which automatically switches between full-bridge rectifier and half-wave rectifier by LED current detection. The hybrid rectifier switching mode enables an optimized design with two times the output voltage range, thus reducing the switching frequency range and the circulating current of the LLC series-resonant converter. The performance of the 60 W half-bridge LLC series-resonant converter with hybrid rectifier and the conventional half-bridge LLC series-resonant converter is experimentally compared. The results prove that there is a significant improvement in efficiency based on the 24 V output condition.

Decoupled PI Controllers Based on Pulse-Frequency Modulation for Current Sharing in Multi-Phase LLC Resonant Converters

The LLC series resonant converter has emerged as a solution to applications requiring power conversion with isolation, reduced volume and high efficiency, such as PV systems and EV chargers. However, the LLC resonant converter is limited in power, so it requires a multi-phase configuration in order to provide higher currents. This configuration connects the outputs of two or more LLC converters in parallel, increasing the output current but introducing imbalance and circulating currents due to the mismatch and tolerance values of components in each resonant tank. This paper proposes a simple PI control scheme to compensate the current imbalance and eliminate circulating currents generated when several LLC resonant converters are connected in parallel. Unlike reported current sharing methods, the proposed control scheme is based on multiple current control loops operating independently, using the switching frequency of each parallel-connected unit as a degree of freedom of the overall converter. The proposed control scheme has been successfully validated under simulations and experimental assessment, implementing two resonant tanks with ±5% tolerance of parameters, providing excellent steady-state and transient performance.

A single‐stage AC‐AC solid‐state transformer with ZVS operation

Solid-state transformer (SST) is an attractive concept, which provides galvanic isolation and voltage scaling by means of a medium-frequency link. In this paper, a single-stage alternative current (AC)-AC SST solution without bulky energy storage elements is presented. The front-end rectifier (FER) and rear-end inverter (REI) of the SST works at line-frequency, while the intermediate LLC series resonant converter (SRC) fulfils the tasks of galvanic isolating and regulating the output voltage. To achieve high efficiency and bidirectional power flow capability of the SST, the zero voltage switching (ZVS) condition of the LLC SRC is comprehensively analysed, and the parameters design method of the LLC SRC is developed, where the resonant currents are constructed by applying the same gating signals to the primary-side and secondary-side bridges and the dead-time is properly selected. Consequently, the presented SST has the merits of high conversion efficiency, naturally bidirectional power flow capability, potentially high power density and high reliability. The experimental results on a 1.5 kW SST prototype show that sinusoidal input and output current, ZVS operation over the full load range are achieved, and the peak efficiency is 97.63%. The experimental results verify the functionality and effectiveness of the developed methods.

Solid-State Transformer for Power Distribution Grid Based on a Hybrid Switched-Capacitor LLC-SRC Converter: Analysis, Design, and Experimentation

A solid-state transformer (SST) is being proposed for a distribution grid of 13.8 kV/380 V. The SST is based on the input-series output-parallel (ISOP) arrangement of twelve modules using the 1.2 kV SiC switches. The cost and losses analysis are discussed through a case study regarding the number of modules using the 1.2 kV and 1.7 kV SiC switches. The modules are composed of two stages: the first one is an AC-|AC| low-frequency rectifier and the second one is an |AC|-|AC| medium frequency Hybrid Switched-Capacitor LLC Series-Resonant Converter (HSCSRC). A single back-end |AC|-AC low-frequency inverter is employed to generate the low voltage AC output port of the SST. The switched capacitor ladder cell in the HSCSRC converter enables the reduction of the number of modules employed on the ISOP arrangement because the voltage stresses on the MV side’s switches are halved, while the resonant stage increases the efficiency of the structure due to soft-switching on all the switches. Moreover, the front-end AC-|AC| and back-end |AC|-AC low-frequency converters enable the use of two-quadrant switches on the |AC|-|AC| HSCSRC converter, reducing the switch count on the medium frequency stage. The operating behavior and design methodology of the SST’s modules are presented. A single module reduced scale prototype with the rated power of 1.67 kVA and an input voltage of 1.15 kV and an output voltage of 220 V is experimentally verified. The maximum efficiency is 97%.

Open-Loop Power Sharing Characteristic of a Three-Port Resonant LLC Converter

The Solid State Transformer (SST) is an attractive solution for highly flexible, cost-effective, compact and efficient power transfer among different grids. Furthermore, a three-port topology is proven as a suitable solution to integrate energy storage resources, the key functionality of emerging SST concept. Among other alternatives, the resonant LLC series resonant converter (SRC) is the cost-effective solution to implement the DC-transformer functionality, which is a core part of the SST. This paper addresses the power sharing characteristics and the zero-voltage switching (ZVS) conditions of a galvanically isolated three-port SRC, operated in DC-transformer mode. A mathematical model, which effectively decouples principal from circulating currents and power flows, is proposed and developed. This new mathematical framework eases the analysis; and reveals a constant power sharing characteristic tightly dominated by the resonant tank parameters even though some degrees of freedom are allowed thanks to the introduction of a differential voltage at the input terminals. Subsequently, design aspects and assessments of working operation conditions are also reported. The accuracy of the proposed model is verified by experimental validation on a labscale prototype.

Active Magnetizing Current Splitting ZVS Modulation of a 7 kV/400 V DC Transformer

The LLC series resonant converter (SRC) is one of the most popular galvanically isolated dc–dc converters since it provides zero voltage switching (ZVS), reduces rms currents, and tightly couples the input and output voltages, when it is operated at (or below) the resonance frequency, and, therefore, acts as a dc transformer (DCX) without requiring closed-loop voltage control. Hence, this topology is of particular importance for the dc–dc converter stage of high power medium voltage to low voltage solid-state transformers (SSTs). This paper first highlights the limitations of passive and synchronous rectification (e.g., oscillations, current distortion, load-dependent voltage transfer ratio) for bridges employing semiconductors with large output capacitances. Afterward, a magnetizing current splitting ZVS (MCS-ZVS) modulation scheme, which allows an active sharing of the magnetizing current between the primary side and secondary side metal oxide semiconductor field effect transistor ( mosfet )-based bridges, is analyzed. It is shown that the ZVS mechanism is acting equivalent to a controller, allowing for a robust open-loop operation of the converter. The proposed modulation scheme features a load-independent voltage transfer ratio, load-independent ZVS for both bridges, and quasi-sinusoidal currents. Finally, the phase shift modulation scheme is experimentally verified for the SiC mosfet -based dc–dc converter of a ${\text{25}}$ kW ac–dc SST, which operates at ${\text{48}}$ kHz between a ${\text{7}}$ kV and a ${\text{400}}$ V dc bus with an efficiency of ${\text{99.0}}{\%}$ .

Architecture of urban rail transit power supply based on PET braking energy feedback

In recent years, the urban rail transit has a great development in many cities of China. It is important to keep the DC transit grid strong and the power transmitted/power transmission high quality. During the process of rail operation, braking energy leads to a wide range of load oscillation for the traction and city grid, which cannot be absorbed inside the rail transit system. In this study, a novel architecture of urban rail transit power supply based on power electronics transformer braking energy feedback is put forward. Based on the circle grid, the topology adds an inverter to LLC series resonant converter to carry the power from 1500 V DC traction grid to 1180 V AC. Analysis together with simulation on the topology and control strategy is made on the basis of MATLAB/Simulink. Process of braking energy feedback and transmission of energy are illustrated with simulation. By paralleling the three-phase H bridge and braking energy feedback device, double direction of the power flow is ensured. As a consequence, the power quality increases and harmonics reduce. The energy can be used more efficiently to achieve better effects on energy conservation.

A Frequency Adaptive Phase Shift Modulation Control Based LLC Series Resonant Converter for Wide Input Voltage Applications

This paper presents an isolated LLC series resonant DC/DC converter with novel frequency adaptive phase shift modulation control, which suitable for wide input voltage (200–400 V) applications. The proposed topology integrates two half-bridge in series on the primary side to reduce the switching stress to half of the input voltage. Unlike the conventional converter, this control strategy increases the voltage gain range with zero-voltage-switching (ZVS) to all switches under all operating voltage and load variations. Adaptive frequency control is used to secure ZVS in the primary bridge with regards to load change. To do so, the voltage gain becomes independent of the loaded quality factor. In addition, the phase shift control is used to regulate the output voltage as constant under all possible inputs. The control of these two variables also significantly minimizes the circulating current, especially from the low-voltage side, which increases the efficiency as compared to a conventional converter. Experimental results of a 1-Kw prototype converter with 200–400-V input and 48-V output are presented to verify all theoretical analysis and characteristics.

Improving the Light-Load Regulation Capability of LLC Series Resonant Converter Using Impedance Analysis

Generally, an LLC series resonant converter ( LLC SRC) is an attractive topology for applications, which require wide input variation and high conversion efficiency because of its wide gain capability and soft-switching capability. However, there is a regulation problem in which the output voltage increases as the load current decreases. In this paper, Bode plot and impedance asymptote analysis were conducted to obtain an intuitive sense of the regulation characteristic of LLC SRC under the light-load condition. Moreover, to improve the regulation capability, a new resonant tank with an additional capacitor is proposed. Its design guidelines were determined by Bode plot and impedance asymptote analysis. Therefore, the proposed LLC SRC achieves very light load regulation, while it maintains the advantages of typical LLC SRCs.

Dual-bridge LLC-SRC with extended voltage range for deeply depleted PEV battery charging

ABSTRACTThis paper proposes a dual-bridge LLC series resonant converter with hybrid-rectifier for achieving extended charging voltage range of 50–420 V for on-board battery charger of plug-in electric vehicle for normal and deeply depleted battery charging. Depending upon the configuration of primary switching network and secondary rectifier, the proposed topology has three operating modes as half-bridge with bridge rectifier (HBBR), full-bridge with bridge rectifier (FBBR) and full-bridge with voltage doubler (FBVD). HBBR, FBBR and FBVD operating modes of converter achieve 50–125, 125–250 and 250–420 V voltage ranges, respectively. For voltage above 62 V, the converter operates below resonance frequency zero voltage switching region with narrow switching frequency range for soft commutation of secondary diodes and low turn-off current of MOSFETs to reduce switching losses. The proposed converter is simulated using MATLAB Simulink and a 1.5 kW laboratory prototype is also built to validate the operation of proposed topology. Simulation and experimental results show that the converter meets all the charging requirements for deeply depleted to fully charged battery using constant current-constant voltage charging method with fixed 400 V DC input and achieves 96.22% peak efficiency.

Analysis and Design of a Hybrid-Type Converter for Optimal Conversion Efficiency in Electric Vehicle Chargers

In this paper, a hybrid-type converter, which can overcome the drawbacks of the conventional phase-shifted full-bridge (PSFB) converter, such as narrow zero-voltage-switching range, conduction loss by circulating current, and large output filter inductor, is analyzed and evaluated. The hybrid-type converter discussed in this paper consists of the conventional PSFB converter with a full-bridge rectifier and a half-bridge LLC series-resonant converter (SRC) with a voltage-doubler rectifier (VDR), sharing the lagging-leg switches and integrating the VDR of LLC SRC into the node of the secondary-side rectifier in the PSFB converter. This rectifier structure allows the targeted hybrid-type converter to have the advantages of high efficiency and high power density by the modification of the output voltage of LLC SRC. This paper analyzes the operation principle and power distribution of the targeted hybrid-type converter. Based on the analysis, the optimal design of transformer and the control strategy with frequency and pulse-width modulation are presented. With the precise design and control, the conversion efficiency and power density can be increased. Its validity is confirmed by experimental results involving a 3.3-kW (250–450-V/7.33-A) prototype converter.

A Variable Duty Cycle Soft Startup Strategy for <italic>LLC</italic> Series Resonant Converter Based on Optimal Current-Limiting Curve

For the LLC series resonant converter (LLC-SRC), the resonant current inrush during the startup is a threat to the safe operation of the main switches, and frequency-decreasing startup method is usually adopted. The very high initial start-up frequency can suppress this inrush effectively, but increases the output current demand of drive ICs. In this paper, a variable duty cycle soft startup strategy based on optimal current-limiting curve is presented, which can realize the suppression of the resonant current inrush under low startup frequency. Due to the nonlinearity of the LLC-SRC, the mode transition conditions are illustrated and a generalized numerical model of the LLC-SRC is established, providing a highly accurate prediction on resonant variables and output during the startup process. Based on the numerical model, the relationship between the startup current, initial startup frequency, and the duty cycle is revealed to quantify the startup process and achieve optimal initial startup parameters. A numerical algorithm is presented to obtain the optimal current-limiting curve during the startup. Two variable duty cycle startup methods are applied to verify the effectiveness of the proposed strategy, namely the PWM startup method and the phase-shift startup method. The experimental results are presented to verify the proposed soft startup strategy.

Flying-Capacitor-Based Hybrid LLC Converters With Input Voltage Autobalance Ability for High Voltage Applications

An advanced hybrid LLC series resonant converter with integrated flying-capacitor cell is proposed in this paper to enable the high step-down conversion in the high input voltage applications. The inherent flying-capacitor branch in the primary side can effectively halve the primary switch voltage stress compared with the half-bridge LLC converters. And the input voltage can be shared equally and automatically between the two series half-bridge modules without additional balance circuit or control strategies due to the built-in flying-capacitor cell. Moreover, the inherent soft switching performance during wide load range that exists in the LLC converters is still kept to reduce the switching losses, which ensures the high efficiency. Besides, the proposed converter can be extended to further decrease the switch voltage stress by employing stacked connection. Finally, a 500 ∼ 640 V-input 48 V-output 1 kW prototype is built and tested to verify the effectiveness of the proposed converter. The results prove that the proposed converter is an excellent candidate for the high input voltage and high step-down dc/dc conversion systems.

A Hybrid PWM-Resonant DC-DC Converter for Electric Vehicle Battery Charger Applications

In this paper, a new hybrid DC-DC converter is proposed for electric vehicle 3.3 kW on-board battery charger applications, which can be modulated in a phase-shift manner under a fixed frequency or frequency variation. By integrating a half-bridge (HB) LLC series resonant converter (SRC) into the conventional phase-shift full-bridge (PSFB) converter with a full-bridge rectifier, the proposed converter has many advantages such as a full soft-switching range without duty-cycle loss, zero-current-switching operation of the rectifier diodes, minimized circulating current, reduced filter inductor size, and better utilization of transformers than other hybrid dc-dc converters. The feasibility of the proposed converter has been verified by experimental results under an output voltage range of 250-420V dc at 3.3 kW.

Step-Up DC/DC Transformer Based on LLC Resonant Full Bridge

In view of the LLC series resonant converter topology, it is well known as suitable for small and medium power application, the stabilization of output voltage and the environment of step-dowm. The scheme is proposed for a kind of the step-up DC/DC transformer based on LLC resonant full bridge. The detailed design method is given. The 3KW prototype is built. The experimental results show it has the characteristics of the constant current discharge, wide working frequency and high power desity, as well, the feasibility is verified.

On Automatic Resonant Frequency Tracking in LLC Series Resonant Converter Based on Zero-Current Duration Time of Secondary Diode

In order to achieve higher efficiency for the unregulated LLC series resonant converter, the switching frequency must be equal to the resonant frequency. However, since the converter works in open-loop condition, the switching frequency usually deviates from the resonant frequency in real converter. This paper proposed a new control method to track the resonant frequency of the unregulated LLC series resonant converter. Theoretically, the time of zero diode current in secondary is zero when the working frequency is equal or greater than the resonance frequency, and it is changed as the frequency varies when the working frequency is less than the resonance frequency. The proposed control algorithm is based on the time measurement of zero diode current to realize the resonance frequency tracking. A closed-loop digital controller is designed to realize the proposed tracking approach. The simulation and experiments are conducted to verify the proposed tracking approach. The simulation and experimental results show that (1) the switching frequency can be well controlled to track the resonant frequency, and (2) the tracking accuracy can be at least 96%.

Coupled Inductor를 활용한 배전류 정류 회로를 적용한 LLC 직렬 공진 컨버터의 수식화 해석

Coupled Inductor를 활용한 배전류 정류 회로를 적용한 LLC 직렬 공진 컨버터의 수식화 해석

A New LLC Series Resonant Converter with a Narrow Switching Frequency Variation and Reduced Conduction Losses

A new LLC series resonant converter that has a narrow switching frequency variation and reduced conduction losses is proposed in this paper. In the proposed converter, one leg of the full-bridge diode rectifier is replaced with synchronous rectifier (SR) switches. In the nominal state, the proposed converter is controlled using frequency modulation, and the SR switches are controlled using in-phase switching signals with primary side switches in order that the proposed converter can obtain a reduced conduction loss in its secondary side rectifiers. During the hold-up time which is the condition where the input voltage of the converter decreases, the SR switches are controlled using phase-shifted switching signals in order that the proposed converter can obtain a higher gain without decreasing the switching frequency. Because the proposed converter operates with a narrow switching frequency variation, a small transformer core can be selected, which results in a decreased core size and loss. Furthermore, it is possible for the proposed converter to be designed optimally in the nominal state and this results in the proposed converter exhibiting a maximized efficiency over the entire load condition. The feasibility of the proposed converter is verified with a 400-250 V input and 200 V/1.5 A output prototype.