Three-level Dual Active Bridge Research Articles

Comparison of Configurable Modular Two-Level and Three-Level Isolated Bidirectional DC–DC Converters for Super-Capacitor Charging in DC Shore Power Systems

Compared to the AC counterpart, the DC shore power system provides a significant advantage of efficient power supply from renewable sources to ships and onshore loads. Super-capacitors serve as key energy storage units in such a system to buffer the power fluctuations and collect the regenerative energy. However, the ultra-wide voltage range of super-capacitors imposes a significant challenge in the topology selection and efficiency optimization of the interfacing isolated bidirectional DC–DC converter. To tackle this challenge, this paper analyzes and compares two promising converter topologies, which are a configurable modular two-level dual-active bridge (CM-2L-DAB) and a three-level dual-active bridge (3L-DAB). To facilitate an ultra-wide voltage range, extended phase-shift (EPS) modulation in conjunction with the topology reconfiguration is analyzed for the CM-2L-DAB, while a hybrid modulation scheme is proposed for the 3L-DAB. A unified design approach is provided for both topologies, which also yields to the power loss modeling. On this basis, the CM-2L-DAB and 3L-DAB are thoroughly compared in terms of the modulation schemes, current stress, soft-switching operation, power conversion efficiency, material usage, closed-loop control scheme, and reliability. A prominent conclusion can be drawn that the CM-2L-DAB provides a higher efficiency than the 3L-DAB over the whole voltage range, but it relies on additional relays to reconfigure its topology which results in lower reliability and dynamic performance than the 3L-DAB.

Transient DC Bias Suppression for Three-level Dual Active Bridge Converter

Transient DC Bias Suppression for Three-level Dual Active Bridge Converter

Enhanced Efficiency on ANPC-DAB through Adaptive Model Predictive Control

This work studies the DC-DC conversion stage in solid-state transformers (SST). The traditional two- or three-level dual active bridge (DAB) topology faces limitations in microgrid interconnection due to power and voltage limitations. For this reason, the use of multilevel topologies such as active neutral point clamped (ANPC) is a promising alternative. Additionally, the efficiency of the SSTs is a recurring concern, and reducing losses in the DC-DC stage is a subject to be studied. In this context, this work presents a new control technique based on an adaptive model- based predictive control (AMPC) to select the modulation technique of an ANPC-DAB DC-DC converter aimed at reducing losses and increasing efficiency. The single-phase shift (SPS), triangular, and trapezoidal modulation techniques are used according to the converter output power with the aim of maximizing the number of soft-switching points per cycle. The performance of the proposed control technique is demonstrated through real-time simulation and a reduced-scale experimental setup. The findings indicate the effectiveness of the AMPC control technique in mitigating voltage source perturbations. This technique has low output impedance and is robust to converter parameter variations. Prototyping tests revealed that, in steady-state, the AMPC significantly improves converter efficiency without compromising dynamic performance. Despite its advantages, the computational cost of AMPC is not significantly higher than that of traditional model predictive control (MPC), allowing for the allocation of time to other applications.

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.

Loss Balance and Transient DC-Bias Suppression Strategies in Three-Level DAB Converters Modulated With Five DoFs

In the three-level (3 L) neutral-point-clamped (NPC) dual-active-bridge (DAB) converter, both sides of the converter can adapt to higher DC bus voltage, and more control degrees of freedom (DoFs) are provided compared to the conventional DAB converter. In the PWM generating technique with 5 DoFs, loss imbalance may develop in inner switches and outer switches because the leading or lagging arm is introduced, affecting system reliability. In addition, the transient DC-bias issue may also arise in the 3L-NPC-DAB converter modulated with 5 DoFs, leading to the transformer saturation. In this paper, two strategies are proposed to suppress the loss imbalance and the transient DC-bias in the current stress and magnetizing branches. It is also general and applicable for SPS, DPS, and TPS modulations. Furthermore, the loss balance strategy and DC-bias suppression strategy are applied in the PWM generation technique simultaneously without additional components. With the proposed solutions, the loss imbalances of inner switches and outer switches are suppressed and DC-bias in the integral of voltages on both sides of the 3L-NPC-DAB converter is abolished. The effectiveness of the proposed strategies is validated by a series of experiments conducted under varying situations.

Control and Study of the Three-Phase Three-Level Dual Active Bridge DC Converter

Multilevel DAB converters can share more effectively the reverse voltage compared to conventional dual active full-bridge converters, reducing the cost of switching tubes at the same power and voltage levels. At the same time, the increased control freedom of multilevel makes the control objectives more flexible, combining optimization objectives such as soft switching and inductor current reduction. In this paper, we propose a three-phase three-level dual active converter, based on the power transfer characteristics of dual phase shift control in the time domain, deriving the effect of inductor current with phase shift angle and variable voltage ratio, and determining the conditions for the switching tubes to work in soft switching, which effectively reduces the DC bus current fluctuation. Finally, the correctness of the theoretical analysis is verified by simulation.

A Bidirectional Fast EV Charger for Wide Voltage Range Using Three-Level DAB Based on Current and Voltage Stress Optimization

Converter cost is the crucial concern while designing off-board electric vehicle (EV) chargers for high-voltage charging applications using advanced high breakover voltage power semiconductor switches. To solve this problem, multilevel topology is used with the typical dual active bridge (DAB) for the development of high-voltage and high-power EV charging. It offers low cost, compact, less complex control, highly efficient, and reliable development. This article presents the design and development of bidirectional off-board fast EV charger with voltage and current stress optimization for wide voltage range. It is designed with two-stage power conversion. Here, active front-end converter is connected to three phase grid and controlled through synchronous reference frame (SRF) d-q control approach for active rectification. The large range of power and voltage variation in off-board EV charger design makes it challenging to develop a controller that allows the converter to operate inside the zero voltage switching (ZVS) operating region. In this context, a detailed circuit analysis of dc–dc stage with dual phase shift (DPS) control under different operating conditions is presented to identify the coordinates of safe operating region with reduced voltage stress. To select the converter control parameters with optimum current stress in conjunction with reduced voltage stress, particle-swarm-optimization technique is applied to the identified operating regions. This provides a unique selection criterion for both control parameters over wide voltage conversion ratio. A coordinated control is designed based on the identified boundaries for easy implementation and less computation burden on microcontroller units (MCUs). A 7.2-kW system is designed and simulated on MATLAB/Simulink. A 3.3-kW laboratory prototype is designed to validate the identified operating points with wide battery voltage range under varying source and loading scenarios.

Optimized Modulation Strategy of NH3L-DAB Converter to Minimize RMS Current for Wide Voltage Range Applications

Dual active bridge (DAB) converter is one of the most popular bidirectional isolated dc/dc converters. Comparing with the two-level DAB converter, the multilevel DAB topologies take advantages of providing more control degrees of freedom to enhance the control flexibility. Therefore, these topologies are promising alternatives for wide voltage range applications. This article proposed an optimizing modulation strategy for neutral-point-clamped hybrid three-level dual active bridge (NH3L-DAB) converter to minimize the rms value of inductor current and the conduction losses. First, the basic operating principle of NH3L-DAB is analyzed comprehensively in the article. Afterward, the concept of global optimal condition is employed to derive the optimal modulation, and the corresponding analytical expressions of the control coordinates for rms current minimization are derived. By applying the optimal modulation strategy, NH3L-DAB facilitates to break the performance barriers of the prior-of-art of two-level DAB converter when the voltage transmission ratio has large variation. Finally, a bidirectional prototype with 200–450 V input and 20–28.8 V output is built to demonstrate the superior performance of the proposed optimal modulation strategy, and the detailed explanation of the experimental results is given as well.

Optimal phase‐shift control of three‐level dual‐active bridge DC–DC converter with minimum reflux power for wide voltage conversion ratio range application

Three-level dual-active bridge (3L-DAB) DC–DC converter under conventional phase-shift (CPS) control would produce large reflux power and current stress, especially when the voltage conversion ratio is varied in a wider range, which will lead to high power loss and low system efficiency. An optimised phase-shift (OPS) control strategy is proposed to control the 3L-DAB operating at the point with minimum reflux power. The power characteristics of 3L-DAB under CPS control are analysed. Then the mathematical model of the reflux power is established, and the relationship equations among the reflux power, phase-shift (PS) ratios, and voltage conversion ratio are deduced. The minimum reflux power and optimal PS ratio are calculated by using the segmentation optimisation algorithm in different ranges of transmission power and voltage conversion ratio. Performance comparative analysis between CPS and OPS controls is carried out later. Adopting the proposed OPS control strategy, the reflux power of 3L-DAB is minimised, as well as reducing the current stress and increasing the efficiency in a wide voltage conversion ratio range. An experimental prototype was developed to verify the effectiveness of the OPS control strategy and the correctness of the theoretical analysis.

ZVS Soft Switching Operation Range Analysis of Three-Level Dual-Active Bridge DC–DC Converter Under Phase Shift Control Strategy

The switches of three-level dual active bridge (3L-DAB) dc–dc converter under phase shift control may operate with hard working especially when the voltage conversion ratio is not equal to one, which will lead to high power loss and low system efficiency. A comprehensive analysis of the ZVS soft switching operation range of 3L-DAB is presented for the first time. The power characteristics of 3L-DAB operating under phase shift control is analyzed. Then, the ZVS operation range of the switches in the primary and secondary sides varied with phase shift ratios and voltage conversion ratio is deduced. The ZVS operation range is obtained by using steady-state time domain analysis under two conditions, without and with considering the junction capacitance of the semiconductor, and the comparative analysis are carried out later. Finally, simulation and experimental results obtained from a 3L-DAB prototype verify the discussed theoretical analysis.

An Optimized Modulation Strategy for the Three-Level DAB Converter With Five Control Degrees of Freedom

Compared with the conventional two-level dual-active-bridge (DAB) dc–dc converter, the three-level DAB converter could operate with lower voltage rated switching devices and provide more control degrees of freedom, which enhances the control flexibility. This paper proposes a modulation strategy with five control degrees of freedom. In the previous work with multiple control degrees, the optimization strategies for efficiency or inductor current minimization are investigated; however, these strategies all need detailed mode analysis and complex mathematical optimization tools to obtain the optimal duty cycles. In this paper, an optimization strategy for the root-mean-square value of the inductor current is proposed. This method extracts the fundamental and third-harmonic components from the high-frequency voltages on both sides and adjusts the amplitude of the bridge voltages by the vector diagram to obtain the optimal duty cycles. The inductor current can be reduced significantly at light loads with the proposed strategy, so the efficiency of the converter can be improved. Moreover, the optimization process does not need the detailed operation analysis of the converter, nor does it require the complex mathematical optimization tools. Finally, the performance of the three-level DAB and the optimization strategy are verified by the experimental results.

A Three-Level Dual-Active-Bridge Converter With Blocking Capacitors for Bidirectional Electric Vehicle Charger

In order to solve the considerable issues of air pollution, climate warming and fossil energy shortage, many governments around the world have formulated policies to encourage the development of electric vehicles over the past decade. This paper proposes a three-level dual-active-bridge (DAB) converter with blocking capacitors for bidirectional electric vehicle charger. In the proposed three-level DAB converter, two blocking capacitors are placed in series with the two windings of the transformer, respectively. Both of the two three-level bridges can operate in four modes, and generate square waves with four kinds of amplitude. Therefore, the proposed three-level DAB converter can adapt to the wide voltage range of electric vehicle. A working mode selection algorithm is developed to minimize the transformer RMS current of the three-level DAB converter. The optimal working mode can be selected in real time according to the voltage ratio and the active power. Finally, experimental results are presented to verify the feasibility and the advantage of the proposed three-level DAB converter.

Optimised phase shift control to minimise current stress of three‐level dual active bridge DC–DC converter

The current stress of three-level dual active bridge (3L-DAB) converter under the conventional phase shift (CPS) control becomes much high, especially when the voltage conversion ratio is unequal to one, which may bring low efficiency and damage of power devices. An optimised phase shift (OPS) control strategy is presented to address the issue. The OPS control is realised by calculating the optimal operation point with real-time detection of voltages and current. The principle of 3L-DAB under CPS control is presented. The mathematical models of transmission power and current stress are then established. The minimum current stress and the corresponding optimal phase shift ratios are deduced within different ranges of transmission power and voltage conversion ratio, the comparative analysis of current stress and reflux power under OPS and CPS controls have been carried out. The current stress is minimised with any given transmission power and voltage conversion ratio in the whole power range after adopting the OPS control strategy. Moreover, the reflux power is reduced as well within the boundary of voltage conversion range. An experimental prototype was developed to verify the effectiveness of the proposed control strategy and the correctness of the theoretical analysis.

Comparative analysis of three‐level dual active bridge DC–DC converter between reflux‐power‐optimised and current‐stress‐optimised phase shift control

The reflux power and current stress of three-level dual active bridge (3L-DAB) become very large under the conventional phase shift (CPS) control, especially when the voltage conversion ratio is unequal to one or working with light load. To improve the performance, reflux-power-optimised (RPO) and current-stress-optimised (CSO) have been investigated in many literatures. However, the current stress variation with RPO control, the reflux power variation with CSO control, and the relations between the two optimal strategies have not been decided yet. Thus, a detailed comparative analysis is presented in this study. The operation principle of 3L-DAB under CPS, RPO and CSO control strategies is briefly described. The reflux power and current stress comparison among the three strategies is performed within different ranges of transmission power and voltage conversion ratio. The RPO and CSO control strategies are approximately equivalent when the voltage conversion ratio is less than 1/4, and they can minimise the reflux power as well as the current stress in the whole power range. An experimental prototype was developed to verify the correctness of the theoretical analysis.

Dual Phase-Shifted Modulation Strategy for the Three-Level Dual Active Bridge DC–DC Converter

In high-voltage dc–dc applications, the switches in the conventional two-level dual active bridge (DAB) dc–dc converter have to bear the whole port voltage, so high voltage switches should be selected. The voltage stress of the switches in the three-level DAB converter can be reduced to half of the port voltage, so it is suitable for high-voltage applications. However, part of the switches still has to bear the entire port voltage under some operation conditions with the single phase-shifted (SPS) modulation strategy. A dual phase-shifted (DPS) modulation strategy is proposed for the converter, compared with SPS modulation strategy, it expands regulating range of transmission power and enhances control flexibility. Moreover, a control algorithm can be developed to ensure that the voltage stress of each switch is half of the port voltage. The detailed operation principle of the three-level DAB converter with DPS control is analyzed in the paper. The characteristics of each operation region and the critical conditions between different operating regions are derived. Finally, the performance of the three-level DAB converter and DPS modulation strategy are verified by the simulation and experimental results of a 125 kW prototype.

Comparative $\eta$– $\rho$– $\sigma$ Pareto Optimization of Si and SiC Multilevel Dual-Active-Bridge Topologies With Wide Input Voltage Range

This work presents a comprehensive cost-aware comparison of isolated bidirectional Si and SiC dual-active-bridge (DAB) concepts for a 5-kW 100-700-V input voltage range dc microgrid application. A conventional three-level DAB (3LDAB) is compared to an advanced five-level DAB (5LDAB) topology, where the latter enables reduced rms currents within the given voltage range. Both concepts employ a loss-optimized modulation scheme enabling zero-voltage switching. A multiobjective optimization routine is proposed to systematically assess the concepts with respect to the efficiency, power density, and the costs. A novel waveform model and advanced component models are considered, which are verified using a hardware prototype. The calculated Pareto fronts show that SiC MOSFETs enable significantly higher efficiencies and power densities than Si IGBTs, while similar costs can be achieved. The performance comparison between the SiC MOSFET-based 3LDAB and 5LDAB reveals a fundamental superiority of the 3LDAB, which is mainly due to the higher chip area utilization and the lower component count of this concept. Finally, the calculations and the hardware prototype prove that despite the galvanic isolation and wide voltage range, efficiencies above 98 % in a wide operating range are possible, which was previously not seen in the literature.