LLC Resonant Converter Research Articles

Non-isolated LLC resonant DC-DC converter with balanced rectifying current and stress

<p><span>In isolated type LLC resonant converters, transformer leakage inductances can be merged with the resonant inductor to extend the ZVS capability of the switches apart from isolation and voltage scaling. However, the transformer presents a resonant imbalance in the secondary side leading to secondary current unbalance, an increase in RMS value of the secondary current and increase thermal stress. This paper proposed a half-bridge non-isolated LLC resonant converter with a balanced rectifying current and stress in the rectifier diodes. The proposed converter can achieve the most advantages of isolated LLC converters, such as ZVS and low MOSFET turn-off loss. By the non-isolation method, secondary current and, transformer loss is significantly reduced. In addition, rectifier diodes operate with zero current switching and balanced rectifying current and stress over the entire operating range. The proposed non-isolated structure is verified by the experimental result with a 60W LLC resonant converter. </span></p>

Hybrid mode‐hopping modulation for LLC resonant converter achieving high efficiency and linear behaviour

Frequency modulation (FM) has been widely used in the inductor–inductor–capacitor (LLC) resonant converter due to excellent soft switching features. However, low power efficiency in light load conditions and complex control due to the nonlinear converter behaviour are common concerns for the FM method. In this study, a hybrid mode-hopping modulation method is proposed for the LLC resonant converter to address these challenges. The LLC resonant converter operates at any two of three power-efficient operating modes for a different amount of time within a single modulation period. By determining the mode hopping time and combining different mode-hopping modulations, the LLC resonant converter can achieve a wide range of output power and maintain high efficiency at the same time. Besides, since the switching frequencies are fixed, the relationships between the duty cycle and the output power are linear, which can significantly simplify the control design. Compared with the existing modulation methods, the proposed hybrid mode-hopping modulation makes the LLC resonant converter behaves linear, simplifies the control design, achieves wide operation range and ensures high efficiency even in light load condition.

A Current-Fed LCL Resonant Converter for Wide Output-Voltage Applications

This article proposed a novel current-fed LCL resonant converter with a simple structure by multiplexing inductors and switches for wide output voltage applications. With three different pulsewidth modulation methods, the proposed converter can work at three voltage gain modes, namely high gain (HG), medium gain (MG), and low gain (LG). In HG mode, it has a higher voltage gain than the conventional LCL converter by an extra boost energy storage stage. In MG mode, it operates similarly as a conventional LCL converter. In LG mode, it is equivalent to a flyback converter in discontinuous conduction mode (DCM), which achieves higher efficiency and is easier to be controlled than an LCL converter. In these three modes, all power switches and rectifier diodes can obtain soft-switching performance, increasing the power conversion efficiency. The operating principle, design methodology, and control scheme are explained and verified by a laboratory prototype with 520 W output power, 30–150 V output voltage.

High-efficiency active cell-to-cell balancing circuit for Lithium-Ion battery modules using LLC resonant converter

A high-efficiency active cell-to-cell balancing circuit for Lithium-Ion battery modules is proposed in this paper. By transferring the charge directly from the highest voltage cell to the lowest voltage cell using an LLC resonant converter designed to achieve zero-voltage switching (ZVS) and nearly zero-current switching (ZCS) for all of the primary switches and zero-voltage zero-current switching (ZVZCS) for the diodes in the secondary, the proposed topology can achieve a high efficiency and a fast balancing speed with a simple control and design. In order to show the improved performance of the proposed method, a prototype circuit was built with a battery module having twelve Lithium-Ion (Li-Ion) batteries. Experimental results are presented to verify the shorter balancing time and higher efficiency of the proposed method when compared with conventional methods. The measured time for the balancing of the twelve cells was 47 min and a maximum efficiency of 94.5% was achieved at 1.5 W.

A Novel Primary-Side Regulation Control Scheme for CCM and DCM LLC LED Driver Based on “Magnetizing Current Cancellation Method”

In this article, the primary-side regulation (PSR) technique to achieve accurate output current in an LLC resonant converter has been studied. A novel PSR control scheme for an LLC light-emitting diode (LED) driver based on the “magnetizing current cancellation method” is proposed. The proposed “magnetizing current cancellation method” can effectively eliminate the effect of magnetizing current in the sampled primary resonant current under both the continuous conduction mode and the discontinuous conduction mode. Thus, the accurate output current can be estimated on the primary side of the LLC resonant converter. The magnetizing current cancellation method for the LLC LED driver with symmetric primary resonant current and asymmetric primary resonant current has been analyzed in detail. Some design considerations have also been presented. Then, an LLC PSR control circuit has been designed based on the theoretical analysis. Finally, a 48-to-78-V/1.3-A laboratory prototype for the LED driver has been built up, and experimental results show that good output current regulation has been obtained.

Common-Mode Voltage Cancellation for Reducing the Common-Mode Noise in DC–DC Converters

Common-mode (CM) noise cancellation method is an attractive approach to reduce the original CM noise in power converters, which helps to shrink the size of electromagnetic interference filter. Traditional CM noise cancellation method cancels the CM noise current by adding a branch consisted of a compensation voltage source and a compensation capacitor, which generates a cancellation current and nullifies the noise current. As the duality, the voltage cancellation method is also feasible for reducing the CM noise, which has not been investigated in dc–dc converters. In this article, a common-mode voltage cancellation (CMVC) method is proposed, which cancels the CM voltage of the converter by inserting compensation voltage source into the input power cord. The requirement for achieving CMVC is that the cancellation voltage and the parasitic capacitances in the circuit are matched. The applications of CMVC method in basic nonisolated and isolated dc–dc converters are presented, and the considerations for practical applications of this method are discussed. Finally, a buck converter and a half-bridge LLC resonant converter prototype are built, and the proposed method is verified by experiment, respectively.

Optimal peak flux density model (OPFDM) for non‐iterative design of high‐frequency gapped transformer (HFGT) in LLC resonant converters

In LLC resonant converters, litz-wire high-frequency gapped transformer (HFGT) is one of the most critical components, responsible for efficient operation. However, it is also a major contributor to overall weight and volume. Therefore, designers perform multi-objective optimisation for minimising its losses and size, which is itself complex and cumbersome. This usually requires many iterations because the losses vary non-linearly with the decrease in size. The selection of initial-setup parameter (ISP) values, plays a vital role in overall performance improvement of the optimisation. The closer the ISPs to optimal values, the smaller are the iterations required for convergence. Therefore, this study proposes an optimal peak flux density model (OPFDM), considering the voltage excitation waveform, duty cycle, core material parameters, and the response of litz-wire to high-frequency. The aim is to obtain an optimised HFGT design through a non-iterative approach by directly estimating optimal values of ISPs through OPFDM. The designed HFGT results in minimised losses and size, while keeping the thermal constraints within limits. Moreover, the impact of frequency and duty cycle on OPFDM is also presented. The proposed model is implemented to design HFGT for 380 VDC/12 VDC LLC converter. Comparison-based analytical, finite-elements method and experimental results validate the model.

다출력 LLC 공진형 컨버터의 변압기 누설 인덕턴스를 고려한 Cross-regulation 특성 분석

This paper presents analysis and experimental results of cross-regulation characteristics in multi-output LLC resonant converter focus on transformer leakage inductance. The detail leakage inductance is extracted using the multi-winding transformer equivalent model, and the effects of leakage inductance on the cross-regulation characteristics under two load conditions are investigated. The analytical results are verified through an experiment results using 240W 4-output LLC resonant converter. Furthermore, the solution for reducing the cross-regulation error is discussed.

A Phase-Shift-Modulated LLC-Resonant Micro-Inverter Based on Fixed Frequency Predictive-MPPT

Maximum Power Point Tracking (MPPT) control is an essential part of every photovoltaic (PV) system, in order to overcome any change in ambient environmental conditions and ensure operation at maximum power.. Recently, micro-inverters have gained a lot of attention due to their ability to track the true MPP for each individual PV module, which is considered a powerful solution to overcome the partial shading and power mismatch problems which exist in series-connected panels. Although the LLC resonant converter has high efficiency and high boosting ability, traditional MPPT techniques based on Pulse Width Modulation (PWM) do not work well with it. In this paper, a fixed frequency predictive MPPT technique is presented for the LLC resonant converter to be used as the first-stage in a PV micro-inverter. Using predictive control enhances the tracking efficiency and reduces the steady state oscillation. Operation with fixed switching frequency for the LLC resonant converter improves the total harmonic distortion profile of the system and ease the selection of circuit magnetic component. To demonstrate the effectiveness of the proposed MPPT technique, the system is simulated using MATLAB/Simulink platform. Furthermore, a 150 W hardware prototype is developed and tested. Both simulation and experimental results are consistent and validate the proper operation of the developed system.

Hold-Up Time Extension Method for LLC Resonant Converter by Detecting Operation Region

An LLC resonant converter is widely used due to the low-voltage stress and zero-voltage-switching (ZVS) capability. The LLC resonant converter should operate in the inductive region due to the ZVS operation. For the stable ZVS operation, a sufficient margin of the maximum operation gain should be guaranteed because the theoretical peak voltage gain analysis is not well-matched with the experimental peak gain. Therefore, it is hard to use the peak gain of the LLC resonant converter. As a result, to satisfy enough hold-up time, there are many side-effects such as a large link capacitor and an unoptimized design for high efficiency. This letter proposes a new hold-up time extension method for the LLC resonant converter, which can use the peak gain without margin by detecting the accurate peak gain point. By using the proposed method, the hold-up time can be extended, and the link capacitor can be reduced for high power density. The feasibility of the proposed structure is confirmed with a 500 W prototype.

Analysis of Intrinsic Switching Losses in Superjunction MOSFETs Under Zero Voltage Switching

Switching losses of power transistors usually are the most relevant energy losses in high-frequency power converters. Soft-switching techniques allow a reduction of these losses, but even under soft-switching conditions, these losses can be significant, especially at light load and very high switching frequency. In this paper, hysteresis and energy losses are shown during the charge and discharge of the output capacitance (COSS) of commercial high voltage Superjunction MOSFETs. Moreover, a simple methodology to include information about these two phenomena in datasheets using a commercial system is suggested to manufacturers. Simulation models including COSS hysteresis and a figure of merit considering these intrinsic energy losses are also proposed. Simulation and experimental measurements using an LLC resonant converter have been performed to validate the proposed mechanism and the usefulness of the proposed simulation models.

High-Gain High-Efficiency IPOS LLC Converter With Coupled Transformer and Current Sharing Capability

In this paper, an input-parallel output-series (IPOS) LLC resonant converter with a coupled transformer and current sharing capability is proposed for high-gain high-efficiency applications such as the renewable energy grid connection. The coupled transformer is utilized to reduce the core size and core loss by implementing the magnetic flux cancellation method, which achieves a natural balance of output voltage simultaneously. The current sharing between the converter modules is achieved by a bridge wire that interconnects the resonant tanks of different modules. Furthermore, the current sharing inside a module is also achieved by the proposed compensation current based on the coordination of compensation inductor and delay control. A 200 kHz 100 V/1.2 kV 3.6 kW two-module IPOS LLC resonant converter prototype is built to verify the proposed method. The peak efficiency can reach as high as 97.5%, and the current sharing between modules and inside a module are well presented.

Overview of Modulation Strategies for LLC Resonant Converter

In this article, based on the LLC resonant converter fundamental harmonic analysis model, the modulation strategies for LLC resonant converter can be categorized into the following three groups: 1) input voltage fundamental harmonic modulation; 2) resonant tank elements modulation; and 3) secondary equivalent impedance modulation. Operational principles and control diagrams for different modulation strategies are given. Comprehensive comparisons between these modulation strategies in terms of topology complexity, control complexity, and voltage gain range are presented with respect to the same system specifications. The advantages and disadvantages for each modulation strategy are summarized to provide guidance for engineers when analyzing and designing an LLC resonant converter. The hybrid modulation strategies are categorized into different groups based on the specific applications. Brief introductions of these hybrid modulation strategies are presented. Future trends regarding the modulation strategies of LLC resonant converters are presented.

Wide Voltage Resonant Converter Using a Variable Winding Turns Ratio

This paper presents a inductor–inductor–capacitor (LLC) resonant converter with variable winding turns to achieve wide voltage operation (100–400 V) and realize soft switching operation over the entire load range. Resonant converters have been developed for consumer power units in computers, power servers, medical equipment, and adaptors due to the advantages of less switching loss and better circuit efficiency. The main disadvantages of the LLC resonant converter are narrow voltage range operation owing to wide switching frequency variation and limited voltage gain. For computer power supplies with hold-up time function, electric vehicle battery chargers, and for power conversion in solar panels, wide input voltage or wide output voltage operation capability is normally demanded for powered electronics. To meet these requirements, the variable winding turns are used in the presented circuit to achieve high- or low-voltage gain when Vin is at low- or high-voltage, respectively. Therefore, the wide voltage operation capability can be implemented in the presented resonant circuit. The variable winding turns are controlled by an alternating current (AC) power switch with two back-to-back metal-oxide-semiconductor field-effect transistors (MOSFETs). A 500-W prototype is implemented and test results are presented to confirm the converter performance.

LLC Resonant Converter With Damping Split Inductor Improving Light-Load Regulation Ability

LLC resonant converters are widely used in On-board Power Converter but the pumping voltage of traditional LLC converter is unavoidable under light-load conditions due to the existence of parasitic parameter. In this paper, an LLC resonant converter with damping split inductor is proposed to improve the light-load regulation capability. The original resonant inductor is divided into two parts in series. Then, a simple damping resistor connected on one part in parallel and the other part can be integrated into the transformer. Using damping split inductor, significant regulation improvements could be achieved under light-load conditions with not any change in control strategy and almost zero hardware cost. In addition, based on impedance analysis of the resonant tank, the effects of the proposed method are analyzed including voltage gain, soft switching characteristics, suppression of high frequency interference components, and system power loss. Moreover, the detailed design process and optimization conditions of the damping split inductor parameters are carefully illustrated. Eventually, the experimental results demonstrate the characteristics mentioned above for the 96 W prototype at the expense of a maximum increase in device loss of 0.85 W.

Boost‐integrated LCL resonant converter with high voltage gain

A boost-integrated LCL resonant DC–DC converter is proposed, which is constituted by a full-bridge inverter module, the main transformer and a flyback transformer, resonant unit, and a rectifier module. The primary winding of the flyback transformer is also used as a resonant inductor and a boost inductor to improve the power density and the voltage gain. In order to meet different output environments, the converter can operate in low voltage-gain mode or high voltage gain mode. In low voltage-gain mode, the converter operates in PWM with LCL resonance. In high voltage-gain mode, the converter operates in PWM with boost and LCL resonance, in which boost modulation plays a role before the LCL resonance to increase the output voltage. Compared with the conventional LCL resonant converter, the new converter has higher voltage gain and is more suitable for a wide output range with the same resonant parameters. The operation principles of the proposed converter are clarified. A prototype with 80–140 V output and maximum output current 2.9 A is built, and experimental data shows that the maximum efficiency of the prototype can be 94.5%, verifying the correctness of the theoretical analysis.

Family of DTMRC‐based DC–DC converters with an RZP

This study presents a novel family of dual-transformer multi-resonant DC–DC converters (DTMRCs) with a resonant zero point (RZP). The limitations of the conventional LLC resonant converter and its fundamental harmonic approximation (FHA) method are firstly discussed. On this basis, a unique RZP is combined with the dual-transformer architecture. As a result, both of the output voltage regulation capability and the over-current protection feature are promoted. Then, an extended describing modelling method is presented to improve the calculative accuracy of the FHA method. Moreover, by means of the appropriate placement of all three resonant frequencies, the fundamental and the third-order harmonic components are employed to transmit active power to the load simultaneously. The zero-voltage soft-switching characteristic is acquired for all the primary-side switches. Meanwhile, the preferable zero-current soft-switching feature is obtained for diodes both at the turn-on and turn-off moments. Therefore, the switching losses are significantly reduced for the proposed converters. Finally, with the employed SiC metal–oxide–semiconductor field-effect transistors, a 500-W half-bridge DTMRC prototype is built as an example, based on the P-type multi-resonant network. The experimental results demonstrate the validity and correctness of the theoretical analyses.

A Magnetically Controlled Single-Stage AC–DC Converter

In this letter, a magnetically controlled single-stage ac-dc converter is proposed for low-power application which has the following features. First, single-stage operation is achieved by sharing switches between LLC resonant converter and totem-pole bridgeless power factor corrector (PFC) converter. Second, the power factor correction function is inherently achieved by designing the PFC converter to operate in discontinuous conduction mode. Third, the dc-link voltage is almost constant. Fourth, fixed switching frequency and duty cycle can be implemented for the switches, which simplifies the magnetic component and driver circuit design. Fifth, the LLC resonant converter is designed to operate at series resonant frequency, the highest efficiency operating point, so the converter can always achieve the maximum efficiency operation. Meanwhile, at this operating point, the converter output voltage is independent of the load so that it is functioning as a dc transformer; and finally, soft switching for all semiconductors is achieved. A 75-W experimental prototype is built to validate the proposed magnetically controlled single-stage ac-dc converter.

철도 차량용 배터리 백업 시스템을 위한 다중 LLC 공진형 컨버터의 병렬 입력-직렬 출력 구조

철도 차량용 배터리 백업 시스템을 위한 다중 LLC 공진형 컨버터의 병렬 입력-직렬 출력 구조

High-Efficiency Hybrid LLC Resonant Converter for On-Board Chargers of Plug-In Electric Vehicles

This article proposes a hybrid LLC resonant converter with three modes of operation for on-board chargers of electric vehicles. The converter can operate as a full-bridge converter, dual-phase half-bridge LLC converter, or single-phase half-bridge LLC converter to improve efficiency at low-voltage and light-load conditions. Without using additional switches, the operation mode of the proposed converter is switched by different modulation schemes. Moreover, the resonant inductance for LLC operation can be reduced so as to be integrated into the transformers, resulting in an increase in the power density. During the operation of three modes, all the primary switches achieve zero-voltage switching, thus minimizing the switching losses. A 3.2-kW-rated prototype using GaN devices with a power density of 65 W/in3 has been designed and tested to verify the effectiveness of the converter. Experimental results have shown smooth and stable transitions among three modes. A flat efficiency curve with a peak efficiency of 98.5% at full load condition has been achieved.