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

High step-up DC/DC converters are popular in renewable energy generation systems, like photovoltaic (PV) and fuel cell. To achieve high step-up conversion ratio, in this paper, an input-parallel and output-series hybrid topology of half-bridge LLC resonant converter and phase shifted full-bridge converter (PSFB) is proposed. The following features have been achieved: 1) high voltage conversion ratio: due to the output series connection, the system voltage conversion ratio equals to the sum of LLC resonant converter and PSFB converter. In addition, the voltage doubler rectifier is adopted for the LLC resonant converter to further increase the voltage gain; 2) LLC resonant converter is operating at the series resonant frequency operating point, its voltage conversion ratio is constant and independent of the load, and the high efficiency conversion is achieved; 3) function decoupling idea is adopted for the proposed topology, most of the power is handled by the high efficiency conversion unit-LLC resonant converter, while the PSFB unit is used to regulate the output voltage; 4) since both two converter units are isolated DC/DC converter, fully isolation between the input voltage and output voltage is achieved. Operational principles, design considerations, and experimental validation are presented.

The inductor inductor capacitor (LLC) resonant topology has become more popular for deployment in high power density and high-efficiency power converter applications due to its ability to maintain zero voltage switching (ZVS) over a wider input voltage range. Due to their ease of operation and acceptable accuracy, frequency domain-related analytical methods using fundamental harmonic approximation (FHA) have been frequently utilized for resonant converters. However, when the switching frequency is far from the resonant frequency, the circuit currents contain a large number of harmonics, which cannot be ignored. Therefore, the FHA is incapable of guiding the design when the LLC converter is used to operate in a wide input voltage range applications due to its inaccuracy. As a result, a precise LLC converter model is needed. Time domain analysis is a precise analytical approach for obtaining converter attributes, which supports in the optimal sizing of LLC converters. This work strives to give a precise and an approximation-free time domain analysis for the exact modeling of high-frequency resonant converters. A complete mathematical analysis for an LLC resonant converter operating in discontinuous conduction mode (DCM)—i.e., the boost mode of operation below resonance—is presented in this paper. The proposed technique can confirm that the converter operates in PO mode throughout its working range; in addition, for primary MOSFET switches, it guarantees the ZVS and zero current switching (ZCS) for the secondary rectifier. As a function of frequency, load, and other circuit parameters, closed-form solutions are developed for the converter’s tank root mean square (RMS) current, peak stress, tank capacitor voltage, voltage gain, and zero voltage switching angle. Finally, an 8 KW LLC resonant converter is built in the hardware-in-loop (HIL) testing method on RT-LAB OP-5700 to endorse the theoretical study.

This article presents a two-mode controlled step-up inverter (TMCSI), which is capable of handling single or dual inputs with a wide range of dc input voltage. In comparison to traditional multistage dc/dc power conversion systems, the power conversion stage is reduced and the voltage/current stress of device is significantly alleviated without using high-voltage dc-bus capacitors. As a result, the reliability and overall conversion efficiency are improved accordingly. By combining the two working modes, the proposed inverter achieves uniform distribution of duty ratio under single/dual-input with a wide range of input voltage, and thereby is very suitable for systems having large input voltage variation. Moreover, based on the dual-input TMCSI, a novel power allocation method is also proposed to improve the system load-bearing ability. The power allocation method is controlled separately from two working modes that allows the two-input sources directly supply the ac load simultaneously. In this article, topology derivation, two-mode control strategy, characteristics of steady principle, and design criteria for the key circuit parameters are systematically analyzed, and important conclusions are obtained. Finally, the experimental results from the single/dual-input 500 VA 96-192 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> input and 220 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> /50 Hz output inverter prototype verify the effectiveness of the proposed TMCSI topology and its associated power regulation approach.

This manuscript explain the design of half bridge LLC resonant converter for extensive input voltage range applications with high efficiency in normal mode and high voltage gain during hold-up time. Usually, to minimize the primary circulating current and lower conduction losses, large magnetizing inductance is used. Still, when a large magnetizing inductance is designed, it is difficult to adjust output voltage for a wide input voltage range. Therefore, in this paper, an auxiliary inductor and a switch is used in parallel with a magnetizing inductor to change its value according to the input voltage. During normal operation high magnetizing inductance is used for high efficiency and lower conduction losses, when the input voltage is dropped to its minimum value, smaller magnetizing inductance is used to acquire high voltage gain and increase the hold-up time duration. With the auxiliary inductor and a switch, the converter can achieve high efficiency for Normal mode and high voltage conversion ratio during hold-up time. The proposed method is confirmed through MATLAB simulation

This paper proposes a single-phase AC-AC solid-state transformer (SST) that eliminates bulky energy storage components. The proposed matrix-type structure comprises a line-frequency (LF) rectifier, a half-bridge (HB) LLC resonant converter, a buck–boost converter, and an LF inverter. The HB LLC resonant converter not only achieves high efficiency at unity voltage gain but also provides high-frequency (HF) isolation as a DC transformer (DCX). Meanwhile, the buck–boost converter ensures precise voltage regulation. The replacement of traditional DC-link electrolytic capacitors with small film capacitors effectively suppresses the second-harmonic power ripple, leading to a significant improvement in both power density and operational reliability. Experimental results from a 1 kW prototype demonstrate high-quality sinusoidal input and output, a wide range of zero-voltage switching (ZVS) operations, and stable output voltage control.

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.

For low-power applications, this paper presents the development and design of a compact and ultra-highly efficient half-bridge LLC resonant converter. By using Galium Nitride (GaN) devices and high-efficient magnetics, the efficiency and power density of resonant converters can be improved. Compared to Silicon MOSFETs, GaN high-electron-mobility transistors (GaN HEMT) have a lower output capacitance and gate charge, resulting in lower driving loss and shorter dead times. Consequently, the proposed LLC converter based on GaN devices has excellent performance characteristics such as ultra-high efficiency, low switching losses, compact size, high voltage endurance, high operating temperature and high operating frequency. Furthermore, the proposed resonant converter features soft switching properties that ensure that the switches and diodes on the primary side are always switched at zero voltage and current. By doing so, LLC resonant converter switching losses are significantly reduced by up to 3.1%, and an overall efficiency of 98.5% is achieved. The LLC resonant converter design with GaN HEMT has great advantages over Si MOSFET solution regarding efficiency, overall losses, switching loose and power factor correction. A 240 W, 240 V to 60 V half-bridge GaN HEMT LLC resonant converter is simulated with a switching frequency of 75 KHz, along with the comparative analysis of the Si metal oxide semiconductor field effect transistor (MOSFET) solution. Moreover, the design and analysis of highly efficient magnetics with a power factor of 0.99 at full load is presented. A 240-Watt single stage LED driver with power factor correction is also designed to verify and compare the performance of proposed LLC resonant converter.

We present an operation scheme for a dc–dc converter consisting of a class-E inverter and a class-E rectifier to always achieve zero-voltage zero- $dv\text{/}dt$ switching at a wide range of input and output voltages. To cope with the high sensitivity due to load variations, previous approaches either forgo the zero- $dv\text{/}dt$ turn- on switching conditions or required additional components and topological changes. Instead, the proposed strategy modulates the duty cycles and switching frequencies depending on the input and output voltages to always achieve zero- $dv\text{/}dt$ turn- on switching and, thus, minimizes switching losses, while preserving the structural simplicity of the converter. Experiments demonstrate the prototype converter maintaining the desired soft-switching condition under 80–200-V input voltage and 5–20-V output voltage variations. In an additional prototype, we implemented a similar design with synchronous rectification that achieves a peak efficiency of 92.3% at an input voltage of 80 V, an output voltage of 12 V, and an output power of 26.9 W.

Photovoltaic (PV) systems with integrated energy storage units traditionally have been complex and required multiple DC-DC converters. In this paper, a novel multiport isolated LLC resonant converter and a modified maximum power point tracking (MMPPT) control method are introduced to reduce complexity. The proposed topology is designed to operate under a wide input voltage range and contains an integrated bidirectional pulse width modulation (PWM) converter and a LLC resonant converter. Frequency modulation control is implemented for regulating the output voltage of the whole system while the PWM converter is controlled by the MMPPT which generates the battery current reference and PI controller. MATLAB/Simulink was used to demonstrate the feasibility of the topology.

Power factor correction (PFC) is widely used in ac–dc converters to improve power quality. The current-source charge pump (CS-CP) PFC converter has the advantages of simple structure, easy control, and low cost, but it can only obtain a high power factor (PF) over a narrow input range. The operation principle of the CS-CP PFC converter and the input voltage range are analyzed in this article. In order to obtain a high PF over a wide input voltage range, in this article, a variable turn ratio CS-CP PFC converter is proposed. By adjusting the turn ratio according to the variation of the input voltage, the proposed converter can obtain a high PF over a wide input voltage range, reduce the voltage stress of the bus capacitor, and solve the problem of insufficient voltage gain at low input voltage. The operating modes and the wide input voltage range of the proposed converter are analyzed, and the relevant parameters are given. A 40-W experimental prototype operating with the input voltage range of 100–242 Vac is built to verify the theoretical analysis. The proposed converter obtains the PF of 0.946–0.991 and its total harmonic distortion of current can satisfy the IEC 61000-3-2-2020 Class-C Standard in the whole input voltage.

A novel Space Vector Pulse Width Modulation (SVPWM) control strategy added feedforward link used for wide input voltage range inverter is proposed. The input voltage is sampled in real time and the sampled values are taken into account, and thus a quick response to the change of input voltage can be achieved. Different form previous researches, the modified SVPWM control strategy presented in this paper is simple and easy to implement without any extra complexity of inverter topology and without switching between different control modes. This method has been applied in the five-level Neutral Point Clamped (NPC) inverter with high frequency link, and the simulation model with 80~780VDC input, 320VAC output, and the THD of output voltage less than 2% is established using the software Power Simulation (PSIM). The THD of output voltage are all less than 2% no matter the input voltage varies or stays at anywhere among this range. The feasibility and effectiveness of the modified SVPWM control strategies proposed are well verified by the simulation results.

This paper proposes a novel hybrid full-bridge (H-FB) three-level (TL) LLC resonant converter. It integrates the advantages of the H-FB TL converter and the LLC resonant converter. It can operate under both three-level mode and two-level mode, so it is very suitable for wide-input-voltage-range applications, such as fuel-cell power systems. Compared with the traditional full-bridge converter, the input current ripple and output filter can be reduced. In addition, all the switches can realize zero-voltage switching from nearly zero to full load, and the switches of the TL leg sustain only half of the input voltage. Moreover, the rectifier diodes can achieve zero-current switching, and the voltage stress across them can be minimized to the output voltage. A prototype of 200-400-V input and 360-V/4-A output is built in our laboratory to verify the operation principle of the proposed converter

A new series resonant converter composed of two resonant circuits with the same converter leg is presented to achieve the zero-voltage switching (ZVS) turn-on of power switches at a wide range of input voltage and load conditions and zero-current switching (ZCS) turn-off of rectifier diodes in a low input voltage case. Thus, the switching losses of power switches and reverse recovery current on rectifier diodes are improved. If the minimum dc voltage gain of the proposed converter is properly selected, the output voltage can be regulated at no-load condition. Two resonant converters using the same power switches are operated by the phase shift half-switching cycle. Thus, the current ripple at the input terminal capacitor can be reduced. In order to reduce the current stresses of output diodes, the output sides of two resonant converters are connected in parallel. The root mean square currents on transformer secondary windings, transformer copper losses and thermal losses can be further reduced. The output voltage doubler topology is adopted in the output side such that the voltage stress of rectifier diodes is equal to the output voltage instead of two times the output voltage in the conventional centre-tapped rectifier topology. Based on the resonant behaviour, all switches are turned on at ZVS and rectifier diodes are turned off at ZCS if the switching frequency is less than the series resonant frequency. The laboratory experiments with a 1000 W prototype are provided to verify the effectiveness of the proposed converter.

The four-switch buck-boost (FSBB) converter effectively reduces the average inductance current of FSBB when the input and output voltage is close to each other by using the four-mode operation, which alleviates the problem of limited duty cycle and low efficiency in the traditional control modes. However, the steady-state duty cycle mutation of FSBB with four-mode control will occur during mode switching, resulting in output voltage fluctuation. We analyzed the mechanism of duty cycle sudden change of four-mode control FSBB during mode switching, explored its influencing factors and governing laws, and proposed to use input voltage feedforward control to improve the smoothness of four-mode switching, so as to improve the response speed of output voltage when input voltage changes suddenly. The simulation results showed that after the input voltage feedforward control method was adopted, the four working modes can be switched smoothly in a wide input voltage range, the output voltage can be adjusted quickly, and the full load peak efficiency can reach 98%.

This article presents a novel multi-level current-driven DC-DC converter along with a modulation scheme that is able to maintain a consistently high efficiency across a wide range of input voltages. This proposed circuit is targeted to applications with a wide range of operating conditions, e.g., PV microinverters, energy storage, etc. Industry standard converters such as the flyback and the LLC resonant converter struggle to accommodate the wide range of voltages, and their efficiency suffers when operating points deviate from the nominal. The proposed approach takes advantage of the multi-level structure and flexibility of the modulation scheme to achieve high performance for a wide operating range. In addition to theoretical analysis of the operation of the converter, this paper demonstrates the functionality and performance of a laboratory prototype in direct comparison to resonant converters to show its superior performance across a wide range. The results validate the benefits of the proposed topology and show a significantly flattened efficiency curve over a wide range of input voltages (0.56% variation over an input voltage range of 32 V to 48 V).