LLC Resonant Converter Research Articles

Efficiency optimization strategy of LLC resonant converter based on Hybrid PWM and PFM digital control mode

There is a problem that the overall efficiency of LLC resonant converter decreases due to large switching loss under light load, in order to solve it, an efficiency optimization strategy based on hybrid PWM and PFM digital control mode was proposed to improve the efficiency of LLC Resonant Converter in the full load range. In order to adapt to different loads, Boost LLC converter circuit topology was proposed. The former Boost circuit was controlled by pulse width modulation (PWM), and the latter LLC was controlled by pulse frequency modulation (PFM). Pulse width modulation (PWM) control of the front Boost circuit can improve the bus voltage of the rear LLC circuit and adapt to different loads. Under the pulse frequency modulation (PFM), the efficiency of the rear LLC resonant converter under light load can be further improved by adjusting the resonant frequency. The experimental results showed that the proposed digital control efficiency optimization method could enable the Boost LLC converter to achieve high efficiency in the whole load range, and the overall conversion efficiency was not less than 94%.

A Cuk Converter and Resonant LLC Converter Based E-Bike Charger for Wide Output Voltage Variations

In this article, an e-bike charging solution for two wheelers, is presented. This e-bike charger is realized by a Cuk converter followed by a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> resonant converter with an improved power quality (PQ) input current indices. The pulsewidth modulated Cuk converter is operated in discontinuous conduction mode of its output side inductor, which leads to a voltage follower approach for input current shaping. Thus, the control loop is greatly simplified as it consists of a single voltage loop. The e-bike battery charging algorithm is implemented in variable frequency controlled HB <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> resonant converter followed by a diode rectification at its output stage. Furthermore, a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{550}\;\text{W}$</tex-math></inline-formula> , constant current/constant voltage charger with a maximum current rating of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{10}\;\text{A}$</tex-math></inline-formula> is developed to charge a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$20AH$</tex-math></inline-formula> e-bike battery. A variable dc link reference voltage estimator is presented, which varies the dc link voltage and extends the output voltage range for charging e-bike battery of varying voltage rating from 42– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{80}\;\text{V}$</tex-math></inline-formula> . Moreover, a resonant frequency optimizer loop is also proposed, which reduces the turn-off losses during charging a deeply discharged battery. The behavior of this charger is exhaustively analyzed under steady state and transient conditions and the PQ at the input, is found to comply the international standard of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$IEC-61\,000-3-2$</tex-math></inline-formula> .

An Adaptive Synchronous Rectification Driving Strategy for Bidirectional Full-Bridge LLC Resonant Converter

In this study, an adaptive driving method for synchronous rectification in bidirectional full-bridge LLC resonant converters used in railway applications is proposed. The drain to source voltage of the synchronous rectifier is utilized to detect the conduction of the body diode, and a suitable driving signal for synchronous rectification is generated accordingly. The proposed driving scheme is simple and can be realized using a low-cost digital signal processor (DSP). According to the experimental results, which averaged 0.4625% and 1.097%, improvement can be observed under charging and discharging mode, respectively.

PV String-Level Isolated DC–DC Power Optimizer with Wide Voltage Range

This paper proposes a photovoltaic (PV) string-level isolated DC–DC power optimizer with wide voltage range. A hybrid control scheme in which pulse frequency modulation (PFM) control and pulse width modulation (PWM) control are combined with a variable switching frequency is employed to regulate the wide PV voltage range. By adjusting the switching frequency in the above region during the PWM control process, the circulating current period can be eliminated and the turn-on period of the bidirectional switch of the dual-bridge LLC (DBLLC) resonant converter is reduced compared to that with a conventional PWM control scheme with a fixed switching frequency, resulting in better switching and conduction loss. Soft start-up control under a no-load condition is proposed to charge the DC-link electrolytic capacitor from 0 V. A laboratory prototype of a 6.25 kW DBLLC resonant converter with a transformer, including integrated resonant inductance, is built and tested in order to verify the performance and theoretical claims.

Modular Isolated LLC DC/DC Conversion System for Offshore Wind Farm Collection and Integration

Compared with conventional offshore wind farm with ac collection and high-voltage dc (HVDC) transmission system, the offshore wind farm with all dc system shows advantages with characteristics of higher power density, lower costs, and higher efficiency in many analyses. This letter proposed a novel all dc system for the offshore wind farm collection and integration. The presented dc system is composed of several medium-voltage (MV) dc collection systems and a dc transmission system. A novel modular isolated dc/dc converter (MIDC) topology is proposed for the MV dc collection system to raise the medium dc voltage to a higher level, while realizing the maximum power point tracking (MPPT) of the connected wind generators. Thorough comparisons of three dc/dc converter candidates are carried out, and the LLC resonant converter is found to be the optimal submodule (SM) of the MIDC. Furthermore, a two-layer control system for the MIDC is developed to obtain current and voltage sharing while achieving the MPPT of the wind generators in the meantime. A time-domain simulation model consisting of 160 SMs is developed using PSCAD, and a prototype with nine SMs is built for the experiments. The feasibility of the proposed pure dc system is verified by both the simulation and experimental results.

Analysis and Design of Three-Level Full-Bridge LLC Resonant Converter Based on Pulse Frequency Modulation

Aiming at the problem of serious switching loss in high-power and high-voltage occasions, the traditional LLC resonant network is difficult to adapt, the DC gain is difficult to obtain intuitively, and the voltage stress on the switch is too high, resulting in large switching losses and difficult selection of the switch. This paper uses a three-level full-bridge LLC resonant converter topology, the First Harmonic Approximation (FHA) is used to analyze the LLC resonant converter gain characteristics, and the parameter design ideas are given. A simulation experiment of a three-level full-bridge LLC resonant converter based on frequency modulation control is presented. The simulation results show that each primary side switch transistor can achieve zero voltage switching (ZVS), the secondary side diode realizes zero current switching (ZCS), and can obtain higher working efficiency, which verifies the feasibility of the design.

A New Dead Time Regulation Synchronous Rectification Control Method for High Efficiency LLC Resonant Converters

In low output voltage and high output current LLC resonant converter applications, a synchronous rectification (SR) is essential component for high efficiency and high power density. However, stray inductances in MOSFET package and printed circuit board (PCB) pattern make premature turn-off of SR gate and large SR dead time. To compensate the stray inductance effect and increase system efficiency, a hysteresis band dead time regulation control method is proposed. In the proposed control method, SR dead time is regulated to predetermined dead time target regardless of the stray inductances by using a mixed signal, which includes instantaneous drain voltage information and previous cycle dead time information. As a result, the proposed control method can provide lower conduction losses by the dead time regulation and better transient characteristic by the instantaneous drain voltage information. To verify the validity of the proposed control method, 234-W prototype is built and experimented with the proposed controller, which is implemented with 0.25-μm BCDMOS technology.

Quasi‐constant bus voltage CrCM boost PFC fed LLC resonant converter in high power LED lighting systems

SummaryThis paper deals with a two‐stage LED (light emitting diode) driver on the basis of front‐end boost converter operating in critical conduction mode (CrCM) and second stage LLC resonant converter for driving an LED lighting load. The boost converter operates in the valley switching mode to reduce switching power loss with less component stress. The ZVS (zero voltage switching) operation of both the half‐bridge semiconductor switching devices reduces the switching losses, which improves the system efficiency. Moreover, to ensure the stability of the DC‐bus voltage, state‐space analysis is also carried out for the CrCM boost converter. The first stage provides the power factor correction (PFC) over the universal AC mains and maintains a constant DC‐bus for the next stage. The second‐stage converter maintains a ripple‐free output voltage to an LED load. This circuit has reasonably minimized the design complexity as compared with single‐stage variable DC‐bus PFC integrated resonant converters and also gives improved efficiency in an isolated configuration. In order to validate the system, a 150‐W prototype is investigated, and experimental findings are analyzed in‐line with theoretical results at universal AC mains.

Analysis of a Wide Voltage Hybrid Soft Switching Converter

A hybrid PWM converter is proposed and investigated to realize the benefits of wide zero-voltage switching (ZVS) operation, wide voltage input operation, and low circulating current for direct current (DC) wind power conversion and solar PV power conversion applications. Compared to the drawbacks of high freewheeling current and hard switching operation of active devices at the lagging-leg of conventional full bridge PWM converter, a three-leg PWM converter is studied to have wide input-voltage operation (120–600 V). For low input-voltage condition (120–270 V), two-leg full bridge converter with lower transformer turns ratio is activated to control load voltage. For high input-voltage case (270–600 V), PWM converter with higher transformer turns ratio is operated to regulate load voltage. The LLC resonant converter is connecting to the lagging-leg switches in order to achieve wide load range of soft switching turn-on operation. The high conduction losses at the freewheeling state on conventional full bridge converter are overcome by connecting the output voltage of resonant converter to the output rectified terminal of full bridge converter. Hence, a 5:1 (600–120 V) hybrid converter is realized to have less circulating current loss, wide input-voltage operation and wide soft switching characteristics. An 800 W prototype is set up and tested to validate the converter effectiveness.

Analysis of ZVS Realization with Consideration of Secondary Parasitic Capacitance for High Frequency GaN-based LLC Resonant Converter under DCM Mode

LLC resonant converter can achieve zero voltage switching (ZVS) for primary side devices and zero current switching (ZCS) for secondary side rectifiers while operating in discontinuous conduction mode (DCM). However, the parasitic capacitance significantly affects the ZVS realization of primary side switches for high frequency GaN-based LLC converter. In order to reveal the influence, this paper analyses ZVS transition process with consideration of secondary parasitic capacitance in a high frequency GaN-based LLC resonant converter under DCM mode. Then, an accurate model during ZVS transition is derived. To verify the analysis and the derived model, a 250V rating input and 270V/6.6A rating output GaN-based LLC converter prototype operating at 500 kHz is built. The experimental results are consistent with the theoretical analysis.

A mixed–mode analog–digital simulation of LLC resonant power converter with the quality factor limiter

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Hybrid Control Strategy for LLC Converter With Reduced Switching Frequency Range and Circulating Current for Hold-Up Time Operation

Hold-up time operation is a crucial requirement for distributed power systems and server power supplies. LLC resonant converter is widely used in these applications due to its high efficiency and power density characteristics. Conventional pulse frequency modulation (PFM)-controlled inductor-inductor-capacitor (LLC) resonant converter requires a wide switching frequency operation range to satisfy the hold-up requirement. In this article, a novel control scheme is proposed for stacked structure LLC resonant converter. During the normal operation, the frequency doubler modulation strategy is adopted to reduce the switching loss and driving loss, and the output is regulated by PFM. During the hold-up time operation, the pulsewidth modulation is adopted to increase the system voltage gain, where the switching frequency is fixed during this operation stage. For the proposed converter, the resonant tank parameters are designed at the nominal operation point, and the only design consideration for the magnetizing inductor is the zero-voltage switching for primary switches. Therefore, compared with PFM-LLC, both the switching frequency operating range and the circulating current are reduced, and high efficiency feature is obtained. Finally, a 100-W experimental prototype with input voltage range of 160-340 V was built to validate the advantages of the proposed converter.

Thermally-Compensated Magnetic Core Loss Model for Time-Domain Simulations of Electrical Circuits

This article introduces a thermally-compensated magnetic hysteresis model, capable of accurately determining core losses of ferrite materials while accounting for core temperature variations. The employed model is based on permeance-capacitance analogy and it captures the frequency-independent hysteresis effect with the help of the Preisach model. For validation purposes, a 100 kW, 10 kHz realized medium frequency transformer prototype is selected and modeled as a part of a full-bridge LLC resonant converter in a time-domain simulation environment. Conducted simulations show the ability of the thermally-compensated magnetic hysteresis model to impact the generated core losses for core temperatures of up to 120 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C. Finally, a temperature-feedback loop within the transformer is successfully closed inside of a system-level simulation, allowing for a more precise determination of the core and winding steady-state temperatures.

A Self-Protected Single-Stage LLC Resonant Rectifier

This article develops an advanced self-protected single-stage ac-dc LLC resonant converter, in which one diode is introduced to realize overvoltage prevention to achieve self-protection, lower costs, and smaller size. Besides, the proposed converter can also realize power factor correction (PFC), soft switching, and a wide output voltage range, which meets the needs of many applications, such as liquid crystal display (LCD) TV power supply. Detailed analyses are presented to well demonstrate the features of the proposed converter, including the PFC and zero voltage switching turn-on functions. Thereafter, closed-loop control and parameter matching design are conducted in this study. Lastly, prototype experimentation of 220-V ac, 200 W verifies the effectiveness of the proposed converter design.

A Single-Stage High Power Factor Power Supply for Providing an LED Street-Light Lamp Featuring Soft-Switching and Bluetooth Wireless Dimming Capability

Light-emitting diode (LED) has the characteristics of environmental protection and energy saving, having become the lighting source of a new generation of street-light lamps. The traditional two-stage power supply for providing an LED street-light lamp is composed of an AC-DC converter with a power-factor-correction (PFC) function at the front stage and a DC-DC converter at the rear stage. The two-stage power supply for an LED street-light lamp has a large number of electronic components and costs, and the circuit efficiency is not high. Therefore, this paper presents a novel single-stage high power factor AC-DC power supply for providing an LED street-light lamp featuring soft-switching and Bluetooth wireless dimming capability through using smart tablets or smartphones to remote control the output power of the LED street-light lamp for achieving energy-saving benefits. The proposed AC-DC LED power supply integrates an interleaved buck converter circuit with coupled inductors and a half-bridge LLC resonant converter circuit into a single-stage power conversion circuit. Moreover, the coupled inductor of the interleaved buck converter circuit is designed to operate in the discontinuous conduction mode, which can naturally achieve PFC. In addition, the two power switches in the novel LED power supply have zero-voltage switching (ZVS) characteristics, which can reduce the switching losses of the power switches. The two output diodes have the characteristics of zero-current switching (ZCS), which can reduce the conduction losses of the power diodes. This paper developed a single-stage prototype circuit for providing an 144 W (36 V/4 A)-rated LED street-light lamp. According to the experimental results of the prototype circuit with an AC input voltage of 110 volts, the presented single-stage LED power supply offers high power factor (PF &gt; 0.99), low input-current total harmonic distortion factor (THD &lt; 3%), and high efficiency (&gt;89%). In addition, this paper used the built-in Bluetooth wireless communication function of a smart tablet or smart phone to fulfill remote dimming control. By changing the duty ratio of the control signal, we could realize remote dimming control of 20% to 100% of the output LED street-light lamp power.

MATLAB GUI Based Steady State Open-Loop and Closed-Loop Simulation Tools for Different LLC Converters With all Operation Modes

LLC resonant converters have been widely adopted in many different industrial applications due to their characteristics of high efficiency and high power density. Simulation tools are of great importance for the analysis and design of LLC resonant converters. For example, many LLC design methods require massive simulations to finalize the circuit parameters, iterations are necessary, which can be tedious and time-consuming. Therefore, in this work, accurate and complete steady state simulation tools based on MATLAB graphical user interface (GUI) are designed for LLC resonant converters. All common and possible operation modes and LLC converter topologies are included in the designed simulation tools, which makes them practical for different applications and operation conditions. An improved mode judgement method and guidelines for initial point setting are proposed to ensure fast simulation speed. More than that, the key circuit voltage and current values are summarized for the user to evaluate the converter performance, which is more convenient than commercial simulation software. Moreover, the designed simulation tools are suitable and convenient for automatic converter parameter design.

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.

Analysis and Design of an Integrated Magnetics Planar Transformer for High Power Density LLC Resonant Converter

Recent studies on compact and lightweight electronic devices have demonstrated that the LLC resonant converter (LRC) can facilitate achieving high efficiency and high power density. Moreover, employing a planar transformer can further improve the overall system power density. Although the planar transformer can assist in reducing the size of the converter, its high magnetic coupling makes the leakage inductance too small for a resonant inductor in the LRC. Therefore, an extra inductor must be employed separately, leading a considerable decrease in the power density. From this reason, significant research on integrated magnetics has been conducted to combine a transformer and external inductor into a single core. However, they require additional wires or magnetic sheets, and their structures are complex and costly. To overcome these limitations, the integrated magnetics planar transformer (IMPT) for a high power density LRC is proposed in this paper. In the proposed approach, since the primary wire is split into each side leg of the EE-type magnetic core and each operates as a transformer and a resonant inductor alternatively, an external inductor or additional wires are unnecessary. In addition, since the magnetic flux density of the IMPT is approximately equivalent to that of conventional transformer, the core size and the number of turns are almost the same. Therefore, the proposed IMPT features higher efficiency and power density without additional size and costs. To confirm the validity of the proposed IMPT, the operational principles, theoretical analysis, design considerations, and experimental results from a 350 W prototype are presented.

A Single Phase, Single Stage AC-DC Multilevel LLC Resonant Converter With Power Factor Correction

Single stage LLC resonant converters with inherent power factor correction are getting popularity in AC-DC converters due to its reduced size and weight. However, single stage topologies are usually less efficient in regulating the dc bus capacitor voltage pertaining to line and load transients. This paper proposes a multi-level flying capacitor based single stage AC-DC LLC topology to address the issue of voltage balancing of dc-bus capacitor and to reduce the voltage stress of the switching devices. The proposed three-level inverter topology guarantees zero voltage switching, less circulating currents, reduced switching stress and losses. The converter uses bridgeless rectification scheme for better efficiency and the power factor is made nearly unity by operating the source-side inductor in discontinuous current conduction. Variable switching frequency control is used to regulate the output voltage of the converter and pulse width modulation is used to control the dc-bus voltage. This dual control scheme is very effective to keep the dc-bus voltage nearly constant over a wide range of line and load variations. The proposed topology and control scheme have been validated by hardware results on a 250W resistive load.

Optimization of LLC Resonant Converter With Two Degrees of Freedom Based on Operation Stage Trajectory Analysis

LLC resonant DC-DC converter has been widely used in high power density and high efficiency applications. Pulse frequency modulation (PFM) is commonly applied due to its wide soft-switching range. In some special cases, phase shift modulation (PSM) is applied to widen voltage regulation range, improve light-load efficiency, or implement a soft start-up process. Because LLC is with 2 control degrees of freedom (2DOFs), switching frequency and duty ratio, a unified analysis and optimization for LLC with 2DOFs is desired. This paper makes an effort to present a unified time domain operation stage trajectories (OSTs) analysis of LLC. On this basis, a unified numerical optimization method is proposed with arbitrarily designed optimization objectives. With the proposed optimization method, the optimized LLC with 2DOFs is with better performance compared to conventional PFM and PSM, which is verified by the experimental results.