A Multilevel Current-Fed DAB Converter With Direct Power Transfer

SummaryIn battery energy storage system (BESS) applications, the life of the battery depends on the quality of the charging/discharging current. However, the dual active bridge (DAB) converter, which interfaces BESS with the DC link, suffers from a backflow current in all modulation schemes, degrading the battery's performance. Hence, this paper proposes a particle swarm optimization (PSO) based universal phase shift (UPS) modulation scheme to improve the battery's life. In the proposed modulation scheme, the phase shift ratio of DAB to eliminate the backflow power is estimated using PSO. Also, it is observed that there is an improvement in the zero voltage switching (ZVS) range of the DAB. Hence, the overall efficiency of the DAB converter is increased over a wide operating region. The operating principle of the proposed modulation scheme, steady state analysis of power transmission, and backflow power are reported in this paper. The theoretical performance of the proposed modulation scheme is validated in a 2 kW experimental setup.

The increasing popularity of power electronic (PE) converters in different applications has led the research work to obtain higher efficiency and power density of the system. The dual active bridge (DAB) converter becomes very important in the domain of DC-DC isolated converters, due to its several advantages. Multiple research articles have studied the operation procedure of DAB converter, and proposed various modulation techniques for the same. Several phase shift modulation techniques have improved the operation of DAB for wide voltage and power range. However, at low power and wide voltage variation of converter, some limitations of those phase sift modulation techniques can be identified such as, low efficiency, hard switching operation, high peak and rms current of transformer, high computational effort etc. Targeting those issues, a hybrid modulation technique is proposed combining single phase shift and variable phase shift modulation techniques. The proposed modulation scheme a) optimizes the transformer peak current at its minimum value for full range of power, b) provides zero voltage switching (ZVS) operation for all operating conditions, c) ensures minimum voltage stress on switches, d) does not use any current sensor for the optimization process and e) operates with simplified control strategy which results low computational efforts. The optimization of peak current is achieved using two different methods, which are differential method and Lagrangian multiplier method. The optimization process is carried out for full range of voltage and power to ensure the ZVS operation for all operating conditions. All the claimed characteristics of the proposed modulation technique are verified through simulation and the experimental results derived from a hardware prototype.

In this article, a band-limited three-level modulation scheme is proposed for a three-phase neutral-point-clamped (NPC) converter with an aim to balance the average capacitor voltages under all operating conditions. When conventional carrier-based pulsewidth modulation is applied, the amplitude of neutral point (NP)-voltage ripple tends to increase under linear imbalances, while the NP voltage can shift, leading to instability in the presence of nonlinear loads. A previously proposed three-level modulation scheme was able to eliminate all low-frequency capacitor voltage ripple by switching a phase between three levels in each switching period. This approach significantly increased the switching loss. The proposed modulation scheme limits the increase in switching loss by employing two operational bands, one fixed band applied to the capacitor voltages and another variable band applied to the phase currents. If the instantaneous capacitor voltages exceed the voltage band, then an appropriate phase-leg is switched in three levels to guarantee that the capacitor voltages cannot diverge further. If the instantaneous capacitor voltages reside within the voltage band, a variable current band is used to regulate switching in three voltage levels in order to obtain lower switching losses as compared to the conventional three-level modulation scheme. Therefore, the proposed modulation scheme presents a tradeoff between switching losses and capacitor voltage ripple. The experimental results are obtained under different loading conditions for validating the performance of the proposed modulation scheme.

Hard-switching-induced switching loss can contribute significantly to the power loss of an isolated bidirectional dual-active-bridge (DAB) dc–dc converter operating at high frequency. An $LC$ -type series resonant DAB converter based on a switch-controlled-inductor (SCI) is proposed to mitigate the loss arising from hard switching under wide-range variations in output voltage and current. Zero-voltage switching is achieved at the primary side (high voltage), while at the secondary side (low voltage), zero-current switching is preferred to reduce excessive ringing due to circulating current and switching loss. In order to achieve reduced conduction loss, a nominal operating point is chosen where the root-mean-square resonant tank current is the minimum. To validate the proposed topology and modulation scheme, an $LC$ -type series resonant DAB converter based on SCI operating at 100 kHz is designed to interface a 400-V dc bus to a supercapacitor-based energy storage. Simulation and experimental results validate the effectiveness of the proposed topology for charging/discharging a supercapacitor with an output voltage variation of between 10 and 48 V and a maximum rated power of 480 W. A maximum efficiency of 94.6% is achieved using the proposed topology and modulation scheme.

This article proposes a control strategy for a three-phase Z-source inverter (ZSI) using a modified discontinuous space vector modulation. The main advantages of the proposed control strategy are the enhanced output power quality and the compromise in the reduction between the conduction and switching losses. Furthermore, the reduced switching losses are achieved by the elimination of one switching transition in each half sector with a proper selection of the shoot-through (ST) state distribution. In addition, the reduced conduction losses are achieved by dividing all sectors equally into 30°. The MATLAB/SIMULINK simulation is carried out to verify the effectiveness of the proposed control strategy and to compare it with the existing space vector modulation techniques that utilize distributions of four and six ST vectors. PLECS software is used for the calculation of the conduction and switching losses. Compared to the other modulation techniques, the key features of the proposed control strategy are the improved boosting capability of the output voltage, the low total harmonic distortion, the reduced conduction and switching losses, and the reduced inductance current ripple. An experimental test bench that comprises a three-phase ZSI feeding an R load, controlled by a dSPACE MicroLabBOX, is used to verify experimentally the effectiveness of the proposed control strategy. It is envisaged that the proposed control strategy can be very useful for electrical drive applications integrating a three-phase ZSI due to its capability to improve the output waveform and reduce the harmonic distortions, switching, and conduction losses.

Multilevel dual-active-bridge (DAB) converters utilize low-voltage-rating power switches with reduced channel resistance, provide additional degrees of freedoms for control, and exhibit improved efficiency. This article proposes a four-level modulation strategy for a neutral-point-clamped (NPC) DAB converter to address the performance limitations of NPC-DAB topologies operating under conventional modulation schemes—particularly in applications with wide voltage gain variations, such as renewable energy and battery energy storage systems. The proposed modulation scheme preserves compatibility with passive flying capacitor voltage balancing, while extending the zero-voltage switching (ZVS) range and mitigating over-voltage stress across NPC switches through adjusting two independent control parameters: duty-cycle and phase shift, without the need for complicated calculations or prestored lookup table of optimized control trajectories. The four-level voltage waveform significantly reduces inductor peak and rms currents under extreme voltage gain conditions, alleviating current stress and conduction losses in both the inductor and the switches. Unlike conventional modulation strategies where unequal voltage distribution is observed across the switches under varying voltage gains, the converter ensures safe operation by maintaining voltage balancing across switches and split-capacitors even under high voltage gains without requiring any additional active switches or complicated control methods. Theoretical analysis, validated through experimental results, demonstrates reduced turn-<sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> and conduction losses, reduced transformer turns-ratio, and enhanced efficiency compared to conventional modulation techniques, particularly with significant voltage gain deviations.

This paper presents loss analysis of SiC MOSFET based bidirectional DAB (Dual Active Bridge) DC-DC converter for EV application. This converter benefits the efficiency of the EV vehicle by reducing power loss and voltage drop across the converter. The DAB uses WBG (Wide-Bandgap) semiconductor devices such as SiC MOSFET to optimize the efficiency and extend soft-switching over whole operating range. The SiC MOSFET switching devices rated at 1200 V with different drain- source on-state resistance Rds(on) of 21 mΩ/32 mΩ is considered. Therefore, a reliable converter rated at 10 kW which works under normal operating condition at high-switching frequency of 100 kHz. The switching and conduction losses are taken under two different junction temperature such as 25 °C and 100 °C with various operating load current between 10 A to 40 A. Based on an analysis of SiC MOSFET based DAB attains 98.8% efficiency. The simulation results are taken as voltage and current waveforms at various points of converter are reported. This SiC based DAB converter would be used in next generation of V2G (Vehicle-to-Grid) and G2V (Grid-to-Vehicle) technology.

It is becoming a great interest to employ SiC based power devices in dual active bridge (DAB) converter as an alternative to conventional Si-IGBT, due to its higher switching frequency potential, smaller switching losses as well as the capability to operate at synchronous rectification (SR) condition. This paper introduces the device loss model of a SiC MOSFET power module based DAB converter considering the effect of synchronous rectification, and the dead-time effect is also discussed. The calculated device loss for both SiC-MOSFET and Si-IGBT are discussed. The results show that the overall device loss is reduced by 40%, where the conduction loss is reduced by 38% because of SR capability of SiC-MOSFET, and the switching loss is reduced by 48% due to the faster transient of SiC-MOSFET during dead-time. On the other hand, the device losses are not even between the primary bridge and the secondary bridge of the DAB converter, and it is more significant for SiC-MOSFET based DAB due to the effect of SR with a maximum of 20%. At last, the dead-time range is given based on the device properties.

A comprehensive procedure for the derivation of optimal, full-operating-range zero voltage switching (ZVS) modulation schemes for single-phase, single-stage, bidirectional and isolated dual active bridge (DAB) ac-dc converters is presented. The converter topology consists of a DAB dc-dc converter, receiving a rectified ac line voltage via a synchronous rectifier. The DAB comprises primary and secondary side full bridges, linked by a high-frequency isolation transformer and a series inductor. ZVS modulation schemes previously proposed in the literature are either based on current-based or energy-based ZVS analyses. The procedure outlined in this paper for the calculation of optimal DAB modulation schemes (i.e., combined phase-shift, duty-cycle, and switching frequency modulation) relies on a novel, more accurate, current-dependent charge-based ZVS analysis, taking into account the amount of charge that is required to charge the nonlinear parasitic output capacitances of the switches during commutation. Thereby, the concept of “commutation inductance(s)” is shown to be an essential element in achieving full-operating-range ZVS. The proposed methods are applied to a 3.7 kW, bidirectional, and unity power factor electric vehicle battery charger which interfaces a 400 V dc-bus with the 230 Vac, 50-Hz utility grid. Experimental results obtained from a high-power-density, high-efficiency converter prototype are given to validate the theoretical analysis and practical feasibility of the proposed strategy.

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.

This study focuses on the possibility of modular multilevel converter (MMC) loss optimisation by modulation without any additional circuit, where a common‐mode voltage is injected into a nearest level modulator. Owing to the common‐mode voltage injection, the proposed modulation strategy increases the fundamental voltage. Therefore, for a given voltage, operation is possible with the submodule (SM) voltage of at least one SM inserted each arm. This reduces the number of SMs required, resulting in reduction of conduction losses and leading to significant cost savings and/or returns on investment for facilities. Furthermore, the injection of common‐mode voltage also decreases the total SM transitions, resulting in reduction of switching losses. The proposed modulation strategy is valid under both normal and unbalanced ac fault conditions for MMCs. The common‐mode voltage extraction method is provided and its harmonic components are extracted. Then, the SM saving capability of the injection‐based nearest level modulation is discussed and the SM saving limit is presented in relation with modulation indexes. The optimisation of both conduction losses and switching losses is analysed. The effectiveness of the proposed modulation strategy under both normal and unbalanced ac fault condition is verified by simulations of a 101‐level three‐phase MMC in MATLAB/Simulink.

One promising topology for solid state transformer (SST) is a modular multilevel cascaded converter, in which submodule is composed of dual active bridge (DAB) and H-bridge. For SST application in PV system, the efficiency could be severely affected especially for DAB due to the wide voltage and power range of PV panels. Thus, the motivation of this paper is to deal with the control strategy to improve DAB efficiency inside SST for PV application. Instead of utilizing time-domain based analysis method, which requires complex modeling process, this paper models DAB under frequency domain by fully considering the effect of both fundamental and harmonic frequency component. To achieve high efficiency operation, a multiorder reactive-current suppression (MRS) strategy is proposed by controlling phase shift angle as well as the duty cycles of primary and secondary side H-bridges. In terms of DAB controller design inside SST, the small signal model under MRS is established and a notch filter is implemented to suppress the second order line frequency fluctuation in the control loop. Finally, a 10 kV/1 MW SST prototype is introduced along with the system control structure and implementation method. The experiment of the submodule and SST confirms the effectiveness of proposed method.

This study extends the usage of minimum separable switching configuration (MISSCO) in generating circuit-oriented average model to more complex converter structures, which contain more than one switching cell, for example, single-stage AC/DC converters, in systematic and efficient manner. The model is capable of both large and AC small signal analyses with high accuracy. Also, the model gives electrical information of input and output ports as well as the intermediate bus voltage. Unlike the state-space averaging technique, the model is fully compatible with general-purpose circuit simulators such as SPICE. This greatly facilitates the analysis and design of the converters. In this study, a generalised MISSCO model for non-isolated AC/DC converters is presented and applied to a recently reported AC/DC converter as an example. Bode plots of transfer functions are also given for designing the feedback controller. Both its steady-state and frequency performances will be verified by simulation, experimental results and compared with state-space averaging counterpart.

This article addresses the phenomenon of backflow power (BFP) in traditional phase shift control of novel bidirectional dc-dc converters (BDCs) with direct power transfer (DPT). BFP typically results in lower efficiency and higher current stress, particularly in applications with a wide range of input/output voltages. By employing extended phase shift (EPS) control, this study eliminates BFP while extending the range of zero voltage switching (ZVS), even under light load conditions. Furthermore, the adoption of coupled inductors, which facilitate DPT and current slope adjustment within the topology, reduces the total conduction losses and current stress. Detailed mathematical analysis, simulation results, and experimental findings validate the superior performance of the proposed converter in battery charging applications.

In this paper, an improved Space Vector Modulation (SVM) scheme is developed for matrix converter based rectifier. The conditions of using traditional SVM include small current ripple and continuous current operation mode (CCM). Thus the converter requires a very large inductor on DC side. To further reduce the size of converter, Discontinues current mode (DCM) is adopted for high efficiency compact design. The developed the modulation scheme works well with both CCM and DCM. The simulation results show that the line side current THD is further improved with proposed modulation scheme, especially for DCM operation.