Secondary Side Research Articles

Light-load performance optimization for LLC resonant converter

PurposeThis study aims to solve the problem that under light-load conditions, the output voltage regulation capability is lost due to the fact that the voltage gain of the LLC resonant converter does not decrease with the increase of the switching frequency.Design/methodology/approachIn this paper, the impedance model considering the parasitic parameters of the primary and secondary sides is calculated under light-load conditions, the limitations of the previous method are explained and a new circuit improvement is proposed.FindingsIn this paper, an improved circuit is proposed, and the impedance Bode plot is used to verify that the circuit can effectively improve the voltage gain problem under light-load conditions. Finally, the experimental results verify the effectiveness of the proposed circuit through comparison with traditional solutions and circuits.Originality/valueIn this paper, the impedance model considering the parasitic parameters of the primary and secondary sides is calculated, the limitations of the previous method are explained and a new circuit improvement is proposed. When compared with the previous method, the proposed circuit improvement can suppress the voltage gain increase that occurs when the switching frequency increases to a certain level.

An active clamped L‐L type ZVS current‐fed front‐end DC–DC converter based solid state transformer in grid connected mode PV applications

AbstractThis paper utilised a solid‐state transformer (SST) with an active clamped L‐L type current fed front‐end converter for maximum power extraction (MPE) and voltage boost operation. The proposed system employed perturb and observe (P&O) based MPE and upheld the ZVS operation of all primary side switches and ZCS operation of all secondary side rectifier diodes for various insolation conditions. The proposed system developed a new fundamental extractor titled multi‐level cascaded dual double fundamental signal extractor (MCDDFSE) and is utilized in the control algorithm for synchronization with the utility grid. The main objective of the proposed system was to deliver a compact size, reduced weight and cheaper magnetic components‐based front‐end converter of SST on the photovoltaic (PV) array side and a fast, secure and trustworthy mitigation technique for the grid current harmonics on the utility grid side. The proposed system was trialled on the platform of OPAL‐RT (RT‐LABv2021.3.2.307) for various grid abnormalities and its behaviour was found well within the IEEE 519 standards.

A Novel Isolated High-Step-Up Interleaved Converter for Renewable Energy Systems

In this paper, an isolated high step-up interleaved converter for renewable energy systems is proposed. The input-parallel configuration introduced by two energy storage inductors, which can disperse the input current, reducing conduction losses and also improve the conversion efficiency. In order to improve the voltage gain and recover the energy stored in the leakage inductance, the auxiliary step-up structure is utilized in the proposed converter, that including an auxiliary inductor, an auxiliary capacitor, a voltage multiplier capacitor and two switches which serve to exchange the energy stored in auxiliary capacitors and inductors. On the secondary side, a voltage-doubler circuit is used to extend the voltage gain. Finally, a 48V-input, 400V-output and the rated power of 500 W prototype is implemented to verified the effectiveness of the proposed converter. The experimental results shows that the maximum efficiency is 95.97%, and the efficiency of full-load is 93.27%.

Discrete Modeling of Passive Rectifiers in LCL Tuned Inductive Power Transfer Receivers

This paper presents a new method of analyzing the performance of the secondary side of an LCL tuned secondary in an Inductive Power Transfer (IPT) system using a passive rectifier. Traditional methods of analyzing such systems simplify the dc load into an equivalent ac load. This approximation skews the phase shifts between the primary and secondary currents and introduces errors in the design stage of the system. The proposed method uses a look up table (LUT) to characterize the performance of the LCL tuned secondary network. The results are presented and validated at three different power levels (48 kW, 9 kW, and 4 kW). The LUT closely agrees to the measured output powers, phase angles, and coil currents. A sensitivity analysis also shows that it can reduce errors in the simulation by taking practical component variations into account. As such, the proposed LUT characterization method is a valuable tool to quickly determine how an IPT system's design impacts the secondary side's performance in an LCL tuned network with a passive rectifier for IPT systems.

Accurate Modeling, Design, and Load Estimation of LCC-S Based WPT System With a Wide Range of Load

Modeling of the wireless power transfer (WPT) system is the premise to design and control it. First-harmonic-analysis (FHA) is a simple and effective method to model the WPT system, but its accuracy may decrease with the variation of load in a wide range. In this paper, harmonics are analyzed in the primary side of an LCC-S compensation network based WPT system, depicting a waveform of the output current of the inverter much closer to the real one compared with FHA based model. In the secondary side, the time-domain analysis is conducted both in CCM&DCM of the rectifier, deriving an equivalent input impedance featuring the non-linear characteristics of the system. Based on the proposed model, the parameters of the LCC network are designed, proving to tune the ZVS condition better than the FHA based method. Finally, the load of the system is estimated, achieving a high accuracy (with the estimating error less than 5%) in a wide range with a simple sampling method.

Design and Implementation of High-Misalignment Tolerance WPT System for Underwater Vehicles Based on a Variable Inductor

Wireless charging for autonomous underwater vehicles (AUVs) has been widely explored recently by the merit of safety and convenience. However, due to the misalignment between charging coils in the horizontal and vertical directions, the system may not maintain constant current (CC) or constant voltage (CV) output. Therefore, this article proposes a mistuned IPT system structure and corresponding design methods to improve misalignment tolerance. The proposed IPT system adopts series-series compensation and includes a variable inductor (VI) on the secondary side. A general design method based on the established mathematical model is proposed. The worst scenarios (i.e., the maximum value of misalignment) are used for the optimization of the system parameters. The control strategy for VI without any communication link to the primary side is established. Finally, a 1 kW IPT system for tolerance of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> of 0.15-0.5 and output voltage of 74-150 V variation is set up. The experimental results show that the horizontal and vertical direction misalignment ranges reach ±47% and +140%, respectively, and the maximum system efficiency exceeds 96% for full power output. The proposed system improves the freedom and stability of charging in the harsh underwater environment.

Solid-state Marx generator based on saturable pulse transformer and fast recovery diodes

Solid-state compact Marx generator using saturable pulse transformer (SPT) and fast recovery diodes has been proposed. The primary circuit is switched by three MOSFETs connected in parallel. The SPT functions as a step-up transformer to increase the voltage amplitude and as a closing switch for the secondary circuit. Meanwhile, all the SPTs share the same magnetic core to achieve a compact structure and ensure good synchronization. The energy storage capacitors on the secondary sides are charged through the unsaturated SPT. When the SPT saturates, the capacitors firstly transfer a little energy to the saturated inductors through the diodes reversely during their reverse recovery process. Currents rise quickly in these inductors until diodes totally recover to reverse blocking state. Then capacitors discharge in series to the load and high-voltage pulses are generated over the load. With the currents in the saturated inductors, the front edges of pulses are no longer affected by them but are dominated the turn-off speed of the diodes, which makes high-voltage and high-current pulses with short front edges possible. The regular and cheap fast recovery diodes in the generator act as semiconductor opening switch to sharpen the pulse front edges. Experiments were carried out with a 4-stage Marx generator prototype, 10.8-kV high-voltage pulses with a front edge of 11 ns, a pulse width of 190 ns, were obtained over a 100-Ω resistive load. The total energy efficiency is 49.8%. The proposed Marx generator using regular fast recovery diodes is compact, cheap, and efficient to generate high-voltage pulses with short front edges.

Research on High Power Charging Power Supply Based on LLC and LCL-T Resonant Converter

Electric vehicle charging uses constant current followed by a constant voltage charging method, the MOSFETs operate over a large range of frequencies, which will cause trouble to the design of magnetic components, and the magnetizing current is circulated in the resonant tank, which will lead to more core loss. In this paper, a new resonant converter based on inductive multiplexing of LLC and LCL-T is studied to reduce the switching frequency range by using the inherent output characteristics of the converter. During the constant voltage charging phase, the LLC resonant tank operates as a constant voltage source to provide a stable charging voltage. In the LLC resonant tank, the resonant inductor is equated to the secondary side, and this resonant inductor is reused to develop the LCL-T resonant tank for the constant current charging stage, which is equivalent to a constant current source to provide a stable charging current. The resonant converter in this paper has lower losses and higher efficiency compared to other LLC resonant converters. A prototype with 3.3 kW is created to test the theoretical analysis.

Mode analysis and fault-tolerant method of open-circuit fault for a three-level dual active bridge DC-DC converter

The three-level dual-active-bridge (TL-DAB) DC-DC converter is a promising dc-dc converter for high-voltage and high-power systems. Open-circuit faults on power switches are crucial issues for the TL-DAB dc-dc converters, resulting in various performance degradations such as dc bias and divided capacitor voltage imbalance. The mode analysis after fault is necessary for fault detection and fault-tolerant control. Subsequently, fault-tolerant control is essential to reduce or overcome these issues. In the proposed fault-tolerant method, when the primary side inner switch or the secondary side switch breaks down, the gate-driving signal of the complementary switch is recommended to block and the effects can be removed. Finally, experiment results demonstrate that the issues caused by open-circuit faults can be reduced significantly with the proposed fault-tolerant method.

High-Efficiency Multioutput LLC Resonant Converter With Multiwinding Transformer and Cost-Effective Analog Control Circuit

A new multi-output LLC resonant converter with a multi-winding transformer is proposed. In the proposed converter, the main output is controlled by the pulse frequency modulation of the primary switches, and the additional outputs are regulated by the pulse width modulation of the secondary synchronous rectifiers (SRs). Synchronization between the primary switches and the SRs can be easily achieved by sensing the voltage across the secondary side of the transformer at the main output with simple and cost-effective analog circuitry. The proposed converter features high efficiency because of the reduced transformer core loss and conduction losses in the primary switches. It also achieves high power density because of the small size of the transformer, resonant capacitor, and primary switches. The validity of the proposed converter is verified through experiments on a prototype for a 55-inch TV power supply converting 390 VDC input to 13 V/6.3 A and 65-83 V/ 2.25 A outputs.

Research on Tuning Control Technology for Wireless Power Transfer Systems for Concrete Embedded Sensors

Concrete embedded sensors play a very important role in structural health monitoring. However, the time of endurance of sensors remains a performance bottleneck and sensors need to be charged without damaging the structure as well. Wireless power transfer (WPT) technology is a promising approach to solving this problem. However, the electromagnetic characteristics of concrete medium can cause WPT systems to be untuned and can reduce the energy transmission efficiency of the system. In this paper, the induced medium loss and eddy current loss of a WPT system in concrete are calculated using analytical equations and finite element analysis method. The equivalent circuit model of a concrete–air transmedia WPT system is established according to the calculated losses and a composite tuning control technology is proposed based on the above analysis. In addition, the composite tuning control technology combines the advantages of frequency-modulation tuning and dynamic compensation tuning to ensure the overall resonance of the WPT system. The tuning control technology can ensure the whole resonance of the WPT system and make the natural resonant frequencies of primary and secondary sides consistent. The experimental results show that compared with the untuned control technology, the output power and efficiency of the tuned control system increased by 73% and 11.05%, respectively. The proposed tuning control technology provides direction for future charging of concrete-embedded sensors.

Research on coil identification algorithm of wireless power transmission system based on deep learning

In the practical application of wireless power transfer (WPT) technology, the identification of uncertain secondary side coil is of great significance to the interoperability of the WPT system. Whether the secondary coil can be accurately identified to realize the ideal match of the primary coil and the secondary coil is one of the keys to achieve efficient power transmission. Based on the characteristics of magnetic field image, this paper combines the convolutional neural network (CNN) to realize the recognition of the coil type, and establishes a 4-layer and 6-layer neural network respectively, and the recognition accuracy is as high as 98%. Compared with the existing visual geometry group (VGG) 16 neural network algorithm, the proposed algorithm has lower complexity and is more suitable for the WPT application field.

Linear and Nonlinear Stability Analysis of Molten Salt Natural Circulation Loop

Abstract A molten salt natural circulation loop (MSNCL) has been setup to study the steady-state, transient, and stability characteristics of molten nitrate salt, mixture of sodium nitrate, and potassium nitrate in 60:40 ratios by weight. Natural circulation experiments in a temperature range of 290 °C–600 °C have been performed. A semi-analytical linear model was derived for stability analysis of rectangular natural circulation loops with conventional localized surface heating and cooling. The developed model includes the effect of wall thermal inertia, variable internal heat transfer coefficient, finite secondary side heat transfer coefficient at cooler, and heat losses for predicting the stability map. These effects are incorporated considering their significant role in modeling high-temperature molten salt-based natural circulation systems. The developed model has been first validated with the experimental data of water loop available in literature. The developed model is then validated with the experimental data generated in MSNCL. Validation of in-house developed nonlinear model has also been performed against the same experimental data. The comparison of both linear and nonlinear stability analysis with the experimental data shows good agreement and articulates the importance of various parameters which have been included in the developed model.

Three-dimensional numerical research on coupling heat transfer characteristics of PRHR HX under different water levels based on the experimental model

The passive residual heat removal system (PRHRs) is an important heat transfer system to ensure the removal of residual heat in non-LOCA conditions occurring in advanced Generation III nuclear reactors. The water level of the In-containment Refueling Water Storage Tank (IRWST) decreases due to long-term water evaporation under accident conditions and the later stages of PRHRs operation. Additionally, the water level also decreases during the water leakage accidents. The reduction of water level contributes to a decrease in the heat transfer area and the passive residual heat removal ability of the core. In order to investigate the coupling heat transfer characteristics of both primary and secondary side in the heat exchanger under different water levels, Three-dimensional numerical simulations of the PRHR HX's operation under different water levels was carried out based on the scaled-down IRWST&PRHR HX experimental model. The fluid flow and heat transfer processes inside and outside the C-type tubes were analyzed in detail. The results indicate that when heat transfer area is reduced by 50%, the overall heat transfer rate is reduced by about 41%, meanwhile the average heat transfer coefficient is about 37.4% higher than that under the full water level condition. It indicates that PRHR HX can maintain high passive residual heat removal capacity even under half water level, which provides an important reference for the design and safe operation of the PRHR HX.

Simulation and Optimization of the Operation Efficiency of Heat Exchange Units in Intelligent Heating Systems Based on Entransy Dissipation Thermal Resistance

So as to investigate heat exchange efficiency of heat exchange units under different operating conditions, this paper builds a simulation analysis model for heat exchange units in the framework of intelligent district heating systems based on the Modelica unified modeling language and the MWorks platform. Analyze the real-time operating efficiency of heat exchange units using entransy dissipation thermal resistance as an evaluation indicator of heat exchange efficiency. This paper analyzes the trend of heat exchange efficiency changes of heat exchange units when the primary side flow rate, secondary side flow rate, primary side inlet temperature, secondary side inlet temperature, and secondary side bypass pipeline valve opening change. The simulation results verify that when the primary side and the secondary side flow match, the entransy dissipation thermal resistance is the smallest, and its heat transfer efficiency is the highest. Under the simulation condition where the convective heat transfer coefficient is constant, the entransy dissipation thermal resistance is independent of the inlet temperature changes on the primary and secondary sides. The minimum entransy dissipation thermal resistance could be achieved, which represents the optimal real-time heat exchange efficiency state, when the valve opening of the mixed water heat exchange unit is 45% under the specific flow conditions.

ZVS Design in Full-SiC Three-Level Neutral-Point-Clamped DC–DC Converter Considering Quasi-Linear Output Capacitance $C_{oss}$

The Three-level Neutral-point-clamped (TLNPC) dc-dc converter is a popular topology for applications with high input voltage due to its simplicity of modulation, capability of withstanding high input voltage, and Zero-voltage-switching (ZVS) operation. Regarding mitigating the voltage ringing on secondary side, energy recovery approach is preferred to RCD approach for the sake of high efficiency and small component counts. This paper demonstrates that, using recovery approach in a Silicon Carbide (SiC) based TLNPC dc-dc converter causes hard-switching of the outer switches due to the quasi-linear output capacitance <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_oss</i> of SiC MOSFETs. The ZVS design of a full-SiC TLNPC converter must not follow conventional methods for Silicon (Si) based converters. To address this issue, a modified design approach, which can guarantee ZVS of all switches in a full-SiC TLNPC dc-dc converter, is introduced and discussed. All of the analysis is verified on a 3 kW full-SiC TLNPC prototype, which has rated input voltage of 1.2 kV, output voltage of 600 V, and achieves ZVS in the output power range of 1.2 kW <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 3 kW.

A Detuned S-S Compensated IPT System With Two Discrete Frequencies for Maintaining Stable Power Transfer Versus Wide Coupling Variation

Stable power transmission is one of the key factors in the inductive power transfer (IPT) system. However, misalignment between the primary and secondary sides is almost inevitable in practice, affecting the system performance due to the coupling variation. With the widespread use of IPT technology, it is desired to transfer power from the primary side to the secondary side with a wide misalignment range as large as possible. To address this issue, the design method of the detuned circuit is widely applied in the IPT system. This paper analyzes the characteristics of the transfer power of the detuned series–series (S-S) topology with frequency variations and proposes a maintaining stable power transfer method versus wide coupling variation by adopting two discrete frequencies. Further, a design step is given to obtain the system parameters. Theoretical and experimental results are provided to demonstrate the misalignment performance of the proposed method. The results show that the coupling range is extended from [0.115-0.2] to [0.115-0.27] with the 5.6% fluctuation of the output power, and the corresponding efficiency varies from 91.46% to 95.52%.

A receiver-side power control method for series-series magnetic topology in inductive contactless electric vehicles battery charger application

Wireless power transfer (WPT) can be used to charge the battery conveniently and efficiently. In this paper, the investigation of high-efficiency S/S resonant magnetic topology in inductive wireless battery charging of electric vehicles (EVs) is analyzed, designed, and controlled. To regulate the output power efficiently rather than controlling the supply voltage, novel bidirectional switches are introduced to control the output power by using the duty cycle control method. The output power of the secondary side is derived and discussed based on the fundamental harmonic approximation (FHA) approach. A 1.5 kW, 120 mm distance, and 85 kHz resonance frequency are verified in MATLAB/Simulink.

Starting Mandibular Advancement Device Therapy in Patients with Good Protrusive Capacity: A Randomized Pilot Study.

Discomfort has been related to the poor acceptance of a mandibular advancement device (MAD) in patients with obstructive sleep apnea. The present study compared severe initial side effects between a smaller and a larger degree of mandibular advancement in patients with a good protrusive capacity. Consecutive patients with obstructive sleep apnea and a good protrusive capacity (≥8 mm) were randomized to start treatment with the mandible advanced by either 70% of maximum protrusion (Adv70%) or by 4 mm (Adv4mm) in a pilot study with a parallel design. The main outcome was tenderness or pain in the teeth or jaws using a 0-10 visual analogue scale (VAS) (from "not at all" to "very extensive") or excluded use because of side effects during the first week of treatment. Secondary outcomes included salivation problems and bite changes. Eighteen patients were randomly selected and 17 patients fulfilled the study protocol. Four patients in the Adv70% group and none in the Adv4mm group reported severe tenderness or pain (VAS ≥7) on five or more of the seven days (p=0.03). The degree of mandibular advancement measured in millimeters correlated with the number of days with severe side effects, r=0.64 (p=0.006). The secondary side effects were minor. Starting MAD treatment with 70% mandibular advancement was related to more severe side effects during the first week of treatment compared with a smaller fixed millimeter value in patients with a good protrusive capacity in this pilot study.

A Novel Hybrid Control Strategy and Dynamic Performance Enhancement of a 3.3 kW GaN–HEMT-Based iL2C Resonant Full-Bridge DC–DC Power Converter Methodology for Electric Vehicle Charging Systems

The conventional resonant inductor–inductor–capacitor (L2C) DC–DC converters have the major drawbacks of poor regulation, improper current sharing, load current ripples, conduction losses, and limiting the power levels to operate at higher loads for electric vehicle (EV) charging systems. To address the issues of the L2C converter, this paper proposes an interleaved inductor–inductor–capacitor (iL2C) full-bridge (FB) DC–DC converter as an EV charger with wide input voltage conditions. It comprises two L2C converters operating in parallel on the primary side with 8-GaN switches and maintains the single rectifier circuit on the secondary side as common. Further, it introduces the hybrid control strategy called variable frequency + phase shift modulation (VFPSM) technique for iL2C with a constant voltage charging mode operation. The design requirements, modeling, dynamic responses, and operation of an iL2C converter with a controller are discussed. The analysis of the proposed concept designed and simulated with an input voltage of 400 Vin at a load voltage of 48 V0 presented at different load conditions, i.e., full load (3.3 kW), half load (1.65 kW), and light load (330 W). The dynamic performances of the converter during line and load regulations are presented at assorted input voltages. In addition, to analyze the controller and converter performance, the concept was validated experimentally for wide input voltage applications of 300–500 Vin with a desired output of 48 V0 at full load condition, i.e., 3.3 kW and the practical efficiency of the iL2C converter was 98.2% at full load.