Single-phase vs. three-phase high density power transformers

This paper presents the design of new highfrequency transformer isolated bidirectional dc-dc converter modules connected in module-cascaded solid state transformer (SST). A phase-shift dual active bridge (DAB) converter is employed to achieve high-frequency galvanic isolation, bidirectional power flow, and zero voltage switching (ZVS) of all switching devices, which leads to low switching losses even with high-frequency operation. Bidirectional DC-DC converter is crucial to the power transmission in SST. The proposed DAB converter consists of two three-leg bridges and a high-frequency transformer with winding shunting taps. Furthermore, the commutation inductance connected in series with the transformer in the conventional DAB converter is integrated into the transformer windings, thus, enjoying smaller volume and higher power density. By changing the number of primary winding turns, leakage inductance, which is the key parameter in the energy transfer process of DAB converter, can be adjusted on a large scale, enabling the possibility of reducing loss over wide load range. Additionally, time-sharing circuit topology and operation mode are adopted to adapt to the output power by detecting the output current as the feedback. As a result, the efficiency at both light and heavy load can be significantly improved compared with the conventional DAB converter, and, therefore, high efficiency over wide load range and high power density can be achieved. Besides, the additional bridge arm structure along with a flexible control scheme provides possible high-level fault tolerance. Finally, the simulation results on a 2-kW DAB converter module switching at 10kHz are presented to validate the theoretical analysis.

The dual active bridge (DAB) converter achieves relatively high efficiency and high power density. Therefore, it is widely used in ac–dc–ac converters to provide electrical isolation between the input and output. However, the single-phase converter inherently exhibits significant low-frequency voltage ripple in the DAB topology. Traditional control strategies focus on reducing voltage ripple and optimizing the capacitor size by transferring fluctuating power through the DAB converter. This method leads to higher magnetic component and switch currents, as well as a larger transformer size. To suppress these effects, this article proposes a constant power transmission (CPT) control for the DAB converter. The proposed CPT control dynamically adjusts the modulation width of each operating cycle by tracking the input–output voltage fluctuations in real-time through the control variables. This ensures constant power transfer in the DAB converter. Compared to traditional methods, the proposed control strategy reduces both the RMS and peak-to-peak values of the inductor current, thereby reducing the size of the converter and improving system efficiency. Simulation and experimental results confirm that the CPT control effectively reduces the peak-to-peak and RMS values of the inductor current by 40.7% and 18.8%, respectively, reduces the transformer size by 21.4%, and increases the efficiency by 1.1%.

This paper presents a thorough scope of a wide-used single-phase bidirectional battery charger which composes of a pulse-width modulation (PWM) converter in the front-end and a dual active bridge (DAB) converter for high power applications. The computer simulations of PWM converter and DAB converter are separately shown to support theoretical analyses. A prototype converter was built and tested at different operating conditions. Experiment results at 3.3 kW are given to verify theory and simulation results as well.

The three-phase Solid State Transformer (SST) is one of the key elements in the Future Renewable Electric Energy Delivery and Management (FREEDM) System. The SST consists of an input rectifier, Dual Active Bridge (DAB) bidirectional dc-dc converter and followed by an inverter for ac voltage output. The DAB converter is a bidirectional dc-dc converter using high frequency transformers to step up or down the voltages at high frequency for a reduction in size while maintaining high efficiency and reliability. The single phase high-frequency high-voltage transformer for two-level DAB has been reported by the authors [2]. In this paper, high frequency three-phase transformer for multi-level (3-level) and multi-phase (three-phase) DAB is designed for three-phase multilevel SST. There are two proposed topologies and for each topology the high voltage and high frequency transformer is designed at 3 kHz and 20 kHz. The proposed transformer designs for electromagnetic, thermal and mechanical analysis are validated with the simulation of Finite Element Analysis software.

Dual Active Bridge (DAB) Converter and PushPull Converters are mainly used for Conversion of power with large cycles per second. Dual active bridge converter consists of couple of full bridge converters linked through a transformer. Two way power flow capacity, high power density, high proficiency, soft switching, less number of passive elements and galvanic separation are the important characteristics of DAB pulling in industry. The push-pull converter is utilized as DC-DC converters are reasonable for low and medium power application. High efficiency, low switching loss and high power density are the important characteristics of this converter. By increasing the switching frequency, it results in the reduction of complexity of the circuit. However the switching loss increases as the switching frequency is increased. In this paper, the study is focused on a comparative analysis of DAB converter and Push-Pull converter.

This paper proposes a dual active bridge (DAB) converter constructed with two nine-switch five-level active neutral-point-clamped (9S-5L-ANPC) bridges. The dead time effect in DAB converter with five-level active-neutral-point clamped circuit is analyzed, then an elimination method using the zero state with dual current paths is proposed in 9S-5L-ANPC circuit. On this basis, a modulation method that can effectively balance the loss distribution of power devices and balance the voltage of flying capacitor is proposed. Meanwhile, the soft switching conditions of all nine switches are derived. The performance of the proposed 9S-5L-ANPC DAB converter is verified by a 1.3 kW experimental prototype.

This paper presents a ripple-current compensation method for a dual active bridge (DAB) converter with a matching transformerless series DC active filter (DCAF) in a DC distribution feeder. The input and output currents of the DAB converter contain large ripples, which can negatively affect connected devices such as batteries and solar cells. A matching transformerless DCAF is connected in series between the DC source and DAB converter. The DCAF compensates for the ripple voltage and current of the DAB converter. Experimental results indicate that the proposed method lowers the ripple voltage and current of the DAB converter to 44.9% and 65.0%, respectively.

The dual active bridge (DAB) converter attracts more and more attentions thanks to its advantages over the other isolated dc-dc converters, such as soft-switching, immunity to parasitic inductance, less circuit components and less component stress. Rectangular (also named as phase-shifted) modulation method on DAB has been widely studied and but other two proposed modulations (trapezoidal and triangular methods) have much less applications. Due to the characteristics of the modulation principles, one of these three methods outperforms the other two in certain operating ranges from system efficiency point of view. This paper summarizes the properties of three modulation methods and optimizes the combined use of three modulations on a DAB converter (12V/360V, 1 kW and 25 kHz) in order to obtain the best system efficiencies over full converter operating range. A validated loss model is utilized as the analyzing platform, based on which the optimal operating ranges of three modulations are determined.

The high-frequency transformer (HFT) is an important component of dual active bridge (DAB) converters to achieve galvanic isolation and bidirectional power transmission, which is widely utilized in renewable energy, distribution grids, railway traction and electric ships. However, the real characteristics of the HFT such as magnetic inductance, leakage inductance and stray capacitance would influence the operation conditions of power converters. This paper analyzes the inductance characteristics of the HFT, focusing on the variation of the leakage inductance. To evaluate the magnitude of the leakage inductance, HFT samples wounded by nanocrystalline cores under 600 V/10 kW with various winding structures and air gap positions are simulated by finite element analysis (FEA) and tested by experimental measurements. The results show that the leakage inductance with the concentric windings is negligibly small, whereas the leakage inductance with the separate windings is so much larger that is even within the range of the phase-shift inductance required in DAB converters. On the other hand, the influence of air gaps on the leakage inductance is not evident not only in the concentric windings but also in the separate windings. A DAB prototype of 600 V/10 kW is designed and established, where the HFT with the separate windings has a leakage inductance adjusted to the magnitude of the phase-shift inductance required. The measurement results validate the feasibility that the phase-shift inductance could be substituted by the leakage inductance of the HFT.

A Dual active bridge (DAB) converter can achieve a wide high-efficiency range when its input and output voltages are equal, assuming a 1:1 turns ratio for its isolation transformer. If its input or output voltage is doubled, efficiency of the DAB will drop significantly, because of the introduction of the hard switching and high circulating power. Thus, a new modulation scheme has been proposed, whose main idea is to introduce a voltage offset across the dc blocking capacitor connected in series with the transformer. Operational principle of the proposed modulation has been introduced, before analyzing its soft-switching area and circulating power mathematically. The final modulation scheme is not difficult to implement, but can help the DAB achieve soft switching, low circulating power, and thereby high efficiency, even with its input or output voltage doubled. These features have been verified by experimental results obtained with a 1.2-kW prototype.

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.

Dc-dc converters can be found in different kinds of electric vehicles (EVs), Their main function is to accommodate voltages and currents to the motor or other EV systems requirements. The use of wide-bandgap (WBG) devices can improve the efficiency of silicon-based power converters, qualifying also for higher switching frequencies. In this article the features of a dual active bridge (DAB) converter are studied. The high voltage side of the DAB is implemented with Silicon Carbide (SiC) MOSFETs. For the low voltage side two types of devices are used: either Gallium Nitride (GaN) enhancement high-electron-mobility transistors (e-HEMTs) or SiC MOSFETs. The influence of switching frequency and output power on the efficiency are evaluated. The parallel connection of GaN devices is proposed to overcome the device current limits and thus increase the overall DAB converter output power. A feedback controller has been designed to reduce the effects on the output voltage of load changes. The DAB converter evaluation has been realized by using MATLAB/Simulink and PLECS software.

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.

Alahakoon, S ORCiD: 0000-0002-5802-7869; Haque, M ORCiD: 0000-0002-7615-7982; Wolfs, PJ ORCiD: 0000-0001-7048-1231

A concise, analytical method for incorporating the effects of zero voltage switching (ZVS) interval dynamics in small-signal discrete-time models of the dual-active bridge (DAB) converter is detailed. The method allows the influence of the resonant transition between tank inductor and switching device output capacitance to be examined. Importantly, the method does not require the inclusion of an additional state to account for these dynamics, which greatly simplifies the resulting models of converter behavior. The calculations are applicable to any alternate topology where ZVS transition behaviors contribute significantly to converter dynamics, as in many high frequency converters. The method is verified through experimental results on a 1 MHz DAB converter.