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About this book

This book presents a series of new topologies and modulation schemes for soft-switching in isolated DC–DC converters. Providing detailed analyses and design procedures for converters used in a broad range of applications, it offers a wealth of engineering insights for researchers and students in the field of power electronics, as well as stimulating new ideas for future research.

Table of Contents

Frontmatter

Chapter 1. Introduction

Abstract
This chapter presents a brief introduction of the isolated DC–DC converters. The galvanic isolation not only can achieve voltage gain conversion, but also is necessary for the safety consideration in some applications, such as battery chargers, electrical vehicles, and energy storage systems. Soft switching and low current stress can improve the efficiency of wide voltage conversion range. Some typical isolated DC–DC converters, including phase-shift-controlled DC–DC converters, resonant isolated DC–DC converters, voltage-fed bidirectional DC–DC converters, and current-fed bidirectional DC–DC converters, are reviewed in these two aspects to present the prevalent efficiency improvement methods. This chapter also provides a basic foundation for the whole work, and it gives the goal of this book which provides topologies, modulation schemes, and design guidelines for high-frequency isolated DC–DC converters.
Zhiqiang Guo, Deshang Sha

Chapter 2. Hybrid Phase-Shift-Controlled Three-Level and LLC DC–DC Converter with Active Connection at the Secondary Side

Abstract
This chapter presents a hybrid phase-shift-controlled TL and LLC DCDC converter. The TL DCDC converter and LLC DCDC converter have their own transformers, respectively. Compared with conventional half-bridge (HB) TL DCDC converters, the proposed one has no additional switch at the primary side of the transformer, where the TL converter shares the lagging switches with the LLC converter. At the secondary side of the transformers, the TL and LLC converters are connected by an active switch. With the aid of the LLC converter, ZVS of the lagging switches can be achieved easily even at light load conditions. Wide ZVS range for all the switches can be ensured. Both the circulating current at the primary side and the output filter inductance are reduced. Furthermore, the efficiency of the converter is improved. The features of the proposed converter are analyzed, and the design guidelines are given in the chapter. Finally, the performance of the converter is verified by a 1 kW experimental prototype.
Zhiqiang Guo, Deshang Sha

Chapter 3. Hybrid Three-Level and Half-Bridge DC–DC Converter with Reduced Circulating Loss and Output Filter Inductance

Abstract
A hybrid three-level (TL) and half-bridge (HB) DC–DC converter is introduced in this chapter. The TL DC–DC converter and HB converter have their own transformers, respectively. Compared with conventional TL DC–DC converters, the presented one has no additional switch at the primary side of the transformer, where the TL converter shares the lagging switches with the HB converter. In order to reduce the circulating current in the primary side, a blocking capacitor is used to reset the primary winding current of the TL converter. Moreover, the rectifier stage is composed of four diodes in the center-tap rectification, forcing the circulating current at the primary side to stay zero during the freewheeling period. The magnetizing inductor of the HB transformer can extend the ZVS operation range of the lagging switches even at light loads. Furthermore, the presented converter can reduce the output filter inductance. Due to the advantages mentioned above, the efficiency of the converter is improved dramatically. The features and design guidelines of the presented converter are given in the chapter. Finally, the performance of the converter is verified by a 1 kW experimental prototype.
Zhiqiang Guo, Deshang Sha

Chapter 4. Improved ZVS Three-Level DC–DC Converter with Reduced Circulating Loss

Abstract
An improved two-transformer three-level (ITT-TL) DC–DC converter is introduced in this chapter. The converter contains two transformers. Like the conventional TL DC–DC converter, there are no additional switches on the primary side of the transformer. The rectifier stage is composed of four diodes in the center-tapped rectification. On the primary side of the transformer, the two transformers are connected in series. The middle node of the two transformers is connected to the neutral point of the split flying capacitors. Because it cooperates with the four-diode rectifier stage, the circulating current on the primary side of the transformer decays to zero during the freewheeling period. The zero-voltage switching (ZVS) of the leading switches is determined by energy stored in the output filter inductor, which is similar to the conventional TL converter. The ZVS of the lagging switches is determined by the energy stored in the magnetizing inductor of a transformer, rather than the energy stored in the leakage inductor. The ITT-TL converter can reduce the output filter inductance. Because of the advantages given above, the efficiency of the ITT-TL converter is far better than that of traditional methods. Finally, a 1 kW prototype was built to verify the performance of the ITT-TL converter.
Zhiqiang Guo, Deshang Sha

Chapter 5. Analysis and Evaluation of Dual Half-Bridge Cascaded Three-Level DC–DC Converter for Reducing Circulating Current Loss

Abstract
Three-level (TL) DC–DC converters can meet high input voltage requirement. The isolated TL converters with clamping diodes and flying capacitor are the prevalent TL topologies. However, large circulating current in the primary windings may degrade the efficiency. Although many other TL converters with auxiliary components are proposed and investigated for soft switching, the auxiliary components may increase the weight and size of the converters. The dual half-bridge cascaded (DHBC) TL converter splits the one transformer into two, but no clamping diodes, flying capacitors or other auxiliary components in the primary circuit, which meets the compact size requirement. In order to reduce the circulating current further, an improved dual half-bridge cascaded TL (IDHBC-TL) converter with four rectifier diodes and proper sequence of the transformer windings is introduced in this chapter. Because of the proper sequence of the windings, the circulating currents in the primary side of the transformers decay to zero during the freewheeling period. Although the circulating current is reduced, the primary switches still can achieve zero-voltage switching (ZVS) without any other auxiliary circuits. Besides, the presented converter can reduce the current ripple of the filter inductor, leading to a reduction of the output filter inductance. The evaluation of the good performance in the IDHBC-TL converter is investigated, and the IDHBC-TL converter is compared with some other TL converters. It has compact size and higher efficiency. Lastly, a 1 kW prototype is built to verify the performance of the IDHBC-TL converter.
Zhiqiang Guo, Deshang Sha

Chapter 6. Output-Series-Connected Dual Active Bridge Converters for Zero-Voltage Switching Throughout Full Load Range by Employing Auxiliary LC Networks

Abstract
This chapter presents an output-series-connected dual active bridge (DAB) converter for efficient zero-voltage switching (ZVS) by employing dual auxiliary LC networks in high output voltage applications. The dual auxiliary LC networks are integrated into the converter for zero-voltage switching (ZVS) throughout full load range. The gate signals of the output-side switches can control the current in the LC networks. By analyzing the working modes of the converter, the modulation trajectory is designed in terms of the boundaries of the ZVS range. The conduction loss caused by the auxiliary LC networks is adjusted according to the voltage and load power. The modulation scheme can achieve seamless transition between the adjacent working modes. The conduction loss of the presented converter is compared with the conventional output-series dual active bridge converter. Although the conduction loss is increased under light loads, all the switches can achieve ZVS. The reduced switching loss can improve the overall efficiency. Finally, a 1.3 kW experimental prototype was built to verify the effectiveness of the converter and the modulation scheme, which demonstrates the ZVS performance and efficiency improvement.
Zhiqiang Guo, Deshang Sha

Chapter 7. Dual Active Bridge Converter with Parallel-Connected Full Bridges in Low-Voltage Side for ZVS by Using Auxiliary Coupling Inductor

Abstract
A dual active bridge (DAB) converter with parallel-connected full bridges in low-voltage side is introduced in this chapter. A coupling inductor is integrated into the two low-voltage-side full bridges for zero-voltage switching (ZVS) operation throughout the full load range. By analyzing the working modes of the converter, the ZVS range of the converter is derived. To compromise the ZVS and the conduction loss caused by the coupling inductor, the modulation trajectory is designed in terms of the boundary conditions of the ZVS range. The conduction loss of the converter is compared with the conventional parallel-connected dual active bridge converter, which illustrates that the conduction loss of the presented converter is only increased under light loads. Finally, a 1.2 kW experimental prototype is built to verify the converter and modulation scheme, which demonstrates the remarkable ZVS performance and efficiency improvement.
Zhiqiang Guo, Deshang Sha

Chapter 8. An Isolated Micro-converter Utilizing Fixed-Frequency BCM Control Method for PV Applications

Abstract
This chapter introduces a BCM controlled micro-converter for PV application in order to provide high conversion efficiency over a wide input voltage operating range. Within the wide input range for the PV panel, there is no need to change the operating modes according to the PV panel voltage. The leakage inductor current can work in boundary conduction mode with fixed switching frequency. The operation mode analysis is given first. Then the operating principle of the boundary conduction mode and its implementation are introduced. Loss breakdown of the prototype using GaN devices is analyzed and compared with same voltage rating silicon MOSFETs. A 300 W micro-converter was fabricated and experimental results are provided to verify the effectiveness of the presented BCM control.
Zhiqiang Guo, Deshang Sha

Chapter 9. Modulation Scheme of Dual Active Bridge Converter for Seamless Transitions in Multi-working Modes Compromising ZVS and Conduction Loss

Abstract
A modulation scheme for dual active bridge (DAB) converter is introduced to achieve ZVS and reduce the conduction loss in this chapter. To compromise the ZVS achievement and the conduction loss, optimized working modes are derived from different cases. However, the converter can not achieve seamless transient among the optimized working modes, which may cause the instability. A modulation scheme employing closed-loop control by integrating different working modes is introduced. Furthermore, the seamless transition among the different working modes can be achieved, so the modulation scheme is very suitable to be used for wide battery voltage and wide load requirements. The conduction loss and peak current by using the presented modulation scheme are compared with those of other modulation schemes. With the presented modulation scheme, both the conduction loss and ZVS can be improved. Finally, a 1.2 kW experimental prototype has been built to verify the effectiveness of the converter and the modulation scheme.
Zhiqiang Guo, Deshang Sha

Chapter 10. An Improved Modulation Scheme of Current-fed Bidirectional DC–DC Converters for Loss Reduction

Abstract
Current-fed bidirectional DC–DC converters have attracted much attention in battery energy storage system (BESS) applications due to their zero-voltage-switching (ZVS) performances and greater degree of control freedom. However, high efficiency is difficult to be achieved for both light and heavy load ranges for a current-fed bidirectional DC–DC converter, especially with low battery voltages. In this chapter, a modulation scheme is introduced to improve the conversion efficiency over a wide load range and with variations in the battery voltage. The relationship between the duty cycle of the secondary full-bridge and the phase-shift angle is investigated to lower the power losses with the wide load variations. Based on the operation mode analysis, the control loop with the modified pulse wide modulation (PWM) plus phase-shift modulation scheme is developed. Comparisons of the switch conduction loss and the core loss in the series inductor for the other modulation schemes and the presented modulation scheme are carried out. The results show that lower power losses occur with the improved scheme. The experimental results verify the theoretical analysis and effectiveness of the presented modulation scheme on loss reduction.
Zhiqiang Guo, Deshang Sha
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