This paper proposes a four-degrees-of-freedom modulation scheme to mitigate the conduction and switching losses in a dual-active-bridge (DAB) series-resonant converter. Under wide-range variations in the voltage gain and output current, the increased reactive power and root-mean-square tank current can contribute significantly to the conduction loss in a DAB converter, while the occurrence of hard switching leads to a switching loss and an increased device stress. The proposed modulation scheme utilizes internal, external phase shifts, and switching frequency as modulation parameters to achieve zero reactive power, minimum-tank-current, and complete soft-switching operation. Analysis of the proposed modulation scheme is given for both buck- and boost-mode operations. The proposed modulation scheme is validated by means of a 1-kW experimental prototype of a DAB series-resonant converter operating at 100 kHz, designed to interface a supercapacitor with a rated output voltage of 48 V to a 250-V dc bus. The effectiveness of the proposed topology for charging/discharging a supercapacitor under wide-range variations in the voltage gain and output current is verified by simulations and experimental results. A maximum efficiency of 97.7% is recorded from the experimental prototype (The experimental setup used for measuring converter's efficiency is shown in the supplementary material).

A Four-Degrees-of-Freedom Modulation Strategy for Dual-Active-Bridge Series-Resonant Converter Designed for Total Loss Minimization

A transient dc-bias current due to the voltage-second imbalance of isolated bidirectional dual-active-bridge (DAB) converters for the disturbance in line or load may result in the transformer saturation and oscillations in both sides dc currents. This article focuses on the transient dc-bias current elimination by using an improved triple-phase-shift (ITPS) control for DAB converters. The inductor peak current stress optimization is adopted in the proposed ITPS to determine the steady-state phase-shift variables. Originated from the dc-bias current model of DAB converters with the TPS control, the transient phase-shift adjustment strategy can be determined, which has the ability to improve the inductor current changing slope and shorten the settling time. Both simulation and experiments for different conditions are provided to evaluate main dynamic indexes such as the transient period, dc-bias current, and inductor current stress for three different transition cases. The proposed ITPS is proved as a promising solution in eliminating the dc-bias current, minimizing the transient current stress.

Transient DC Bias Elimination of Dual-Active-Bridge DC–DC Converter With Improved Triple-Phase-Shift Control

The LLC series resonant converter (SRC) is one of the most popular galvanically isolated dc–dc converters since it provides zero voltage switching (ZVS), reduces rms currents, and tightly couples the input and output voltages, when it is operated at (or below) the resonance frequency, and, therefore, acts as a dc transformer (DCX) without requiring closed-loop voltage control. Hence, this topology is of particular importance for the dc–dc converter stage of high power medium voltage to low voltage solid-state transformers (SSTs). This paper first highlights the limitations of passive and synchronous rectification (e.g., oscillations, current distortion, load-dependent voltage transfer ratio) for bridges employing semiconductors with large output capacitances. Afterward, a magnetizing current splitting ZVS (MCS-ZVS) modulation scheme, which allows an active sharing of the magnetizing current between the primary side and secondary side metal oxide semiconductor field effect transistor ( mosfet )-based bridges, is analyzed. It is shown that the ZVS mechanism is acting equivalent to a controller, allowing for a robust open-loop operation of the converter. The proposed modulation scheme features a load-independent voltage transfer ratio, load-independent ZVS for both bridges, and quasi-sinusoidal currents. Finally, the phase shift modulation scheme is experimentally verified for the SiC mosfet -based dc–dc converter of a ${\text{25}}$ kW ac–dc SST, which operates at ${\text{48}}$  kHz between a ${\text{7}}$ kV and a ${\text{400}}$ V dc bus with an efficiency of ${\text{99.0}}{\%}$ .

Active Magnetizing Current Splitting ZVS Modulation of a 7 kV/400 V DC Transformer

In the past ten years, LLC resonant converters have become a mainstream topology for dc/dc power conversion, and multiple design tools have been developed for this topology, including controllers, regulators, soft-switching techniques, etc. While many tools are available for designing this converter, techniques for accurately determining power losses in the inverter mosfet s of the topology based on time-domain analysis have not been fully explored yet. Precise power loss estimation is fundamental to determine the thermal behavior of the switches before the converter is built, which accelerates and optimizes the thermal management design process. In addition, accurate methods of estimating conduction losses, which are dominant in this topology, switching losses, and body diode losses are lacking in the literature. This paper proposes a method for enhancing power loss estimation in LLC inverter mosfet s based on time-domain analysis of the converter. Moreover, a detailed characterization of mosfet 's conduction losses ( $P_{\text{cond}}$ ), switching losses ( $P_{\text{sw}}$ ), and body diode losses ( $P_{\text{diode}}$ ), including the effects of different parameters such as gate-source voltage ( $V_{\text{GS}}$ ), junction temperature ( $T_{j}$ ), drain current ( $\displaystyle I_{D}$ ), and drain-source voltage ( $V_{\text{DS}}$ ), is presented, which can further improve power loss assessment in this topology. The developed method based on time interval analysis replaces the simplistic first-harmonic approximation (FHA), which allows for improved power loss calculations. Further improvement is obtained with the detailed characterization of the switching device. As verified by simulation and experimental results, the proposed estimation tool provides a significant boost in accuracy for power loss determination when compared to the existing method for power loss estimation using FHA.

MOSFET Power Loss Estimation in LLC Resonant Converters: Time Interval Analysis

Conventional voltage mode control only offers limited performance for LLC series resonant DC-to-DC converters experiencing wide variations in operational conditions. When the existing voltage mode control is employed, the closed-loop performance of the converter is directly affected by unavoidable changes in power stage dynamics. Thus, a specific control design optimized at one particular operating point could become unacceptable when the operational condition is varied. This paper presents a new current mode control scheme which could consistently provide good closed-loop performance for LLC resonant converters for the entire operational range. The proposed control scheme employs an additional feedback from the current of the resonant tank network to overcome the limitation of the existing voltage mode control. The superiority of the proposed current mode control over the conventional voltage mode control is verified using an experimental 150 W LLC series resonant DC-to-DC converter.

https://www.mdpi.com/1996-1073/8/6/6098/pdf

Current Mode Control for LLC Series Resonant DC-to-DC Converters

Due to wide soft-switching operation range, high-frequency isolated resonant converters are featured with low switching losses. Therefore, it is meaningful to minimize the resonant tank current in an isolated resonant converter in order to enhance the overall system efficiency. In this work, a bidirectional series-resonant converter under phase-shifted modulation combined with a modified pulsewidth modulation gating scheme is addressed. The modified pulsewidth modulation gating scheme is helpful to keep more switches operating in zero-voltage switching (ZVS) at light load. According to the steady-state analysis with the fundamental harmonic approximation approach, a specified control strategy with minimum rms tank current based on three control dimensions is proposed in closed form. The validity of the proposed control strategy is verified through experimental tests. The pulsewidth of modulated voltage using the modified gating scheme is larger than that of the conventional pulsewidth modulation voltage, which compensates for the effects of more high-order harmonics on rms current. Therefore, compared with existing control strategies for minimum current operation, the proposed control strategy demonstrates comparable performance in depressing conduction loss and wider soft-switching range in high load range. And it is able to achieve higher efficiency at light load, thanks to more switches working in ZVS.

Operation of a Bidirectional Series-Resonant Converter With Minimized Tank Current and Wide ZVS Range

In this work, a unidirectional series-resonant dc/dc converter with a semi-dual-bridge structure is evaluated comprehensively. This converter can realize buck–boost conversion by means of the simple secondary phase-shifted control. With the variations of load level and converter gain, the converter may enter into different operation modes. Features of all possible operation modes and the boundary conditions among them are identified explicitly. Despite the differences between the modes, the steady-state solution for all modes with continuous resonant current could be obtained uniformly by using the fundamental harmonics approximation approach. Verifications of each operation modes are exemplified through experimental tests on a 300-W prototype converter. The measured efficiency can be maintained over 90% at low-load level when the input voltage is varied. Through comparison with reported topologies having similar structures, it is shown that this converter has lower circulation energy than a dual-bridge series-resonant converter. And the secondary arrangement of this converter is advantageous in terms of simpler driver circuit, no-risk of shoot-through fault, and low conduction loss.

Performance Evaluation of a Semi-Dual-Bridge Resonant DC/DC Converter With Secondary Phase-Shifted Control

To eliminate the dc bias current generated during load-change transition, the asymmetric-double-sided modulation (ADSM) control is applied to a semi-dual-active-bridge converter. Although the converter has three main different operation modes, the ADSM control can effectively remove the dc bias current during load-change transition among different modes in one or two high-frequency cycles. Theoretical analyses for different cases of transition are presented. Verifications through experimental test are also included. Besides the effectiveness, the involved calculation in ADSM control is quite simple and the only needed parameter is the converter voltage gain, which makes it easy for actual implementation.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-pel.2016.0932

Asymmetric-double-sided modulation for fast load transition in a semi-dual-active-bridge converter

In this work, a unidirectional full-bridge resonant converter with secondary-side phase-shifted control is addressed. With the variation of voltage gain and power level, this converter may enter into different operational modes, including three continuous current modes and one discontinuous current mode. The details in each operation mode are discussed one by one. The boundary conditions between these modes have been identified. Additionally, the steady-state analysis of all continuous current modes is performed uniformly by using the fundamental harmonics approximation approach. Experimental results based on a lab prototype converter are also included to verify the operation modes predicted by the theoretical analysis.

Operation Modes of a Secondary-Side Phase-Shifted Resonant Converter

To enhance the overall efficiency of a dual-bridge resonant converter, a new asymmetric pulsewidth modulation (PWM) control named extended pulsewidth modulation (EPWM) with both variable pulsewidth and variable symmetry is introduced in this article, which is applied to the high-voltage-side bridge of the converter and is combined with phase-shift control to form a 3-D gating scheme. A specified control strategy based on the 3-D gating scheme is then derived for the purpose of minimum tank current operation. Thanks to the flexibility brought by EPWM, full zero-voltage switching (ZVS) operation range is maintained as large as that of a conventional PWM-based minimum current trajectory, and only one switch loses ZVS at light load. The modulated voltage pulsewidth is kept at high level in a wide load range, which compromises the by-effect of high-order harmonics due to asymmetric modulation. Besides, the backflow power on both sides is almost minimized naturally. Therefore, the proposed control trajectory can have better performance in a light-load region than previous reported solutions. Experimental results prove the accuracy and effectiveness of the proposed minimum current operation with wide ZVS range and show the improvement of overall conversion efficiency.

Song Hu

Papers

Operation of a Bidirectional Series-Resonant Converter With Minimized Tank Current and Wide ZVS Range

Performance Evaluation of a Semi-Dual-Bridge Resonant DC/DC Converter With Secondary Phase-Shifted Control

Asymmetric-double-sided modulation for fast load transition in a semi-dual-active-bridge converter

Operation Modes of a Secondary-Side Phase-Shifted Resonant Converter

A Dual-Bridge DC–DC Resonant Converter Using Extended PWM and Phase-Shift Control