High efficiency and high gain non-isolated bidirectional dc-dc converter with soft switching capability

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HIGH EFFICIENCY AND HIGH GAIN NON -ISOLATED BIDIRECTIONAL DC-DC CONVERTER WITH SOFT SWITCHING

CAPABILITY

RATIL HASNAT ASHIQUE

A thesis submitted in fulfilment of the requirements for the award of the degree of

Doctor of Philosophy

Faculty of Electrical Engineering Universiti Teknologi Malaysia

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DEDICATION

To my beloved parents and my wife, for their endless love, motivation and support.

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ACKNOWLEDGEMENT

All praise is due to Allah (SWT), who granted me the ability to complete this research. Thereby, I am much indebted to His mercy and blessings that he bestowed upon me.

Next, I would like to express my heartiest gratitude to my project supervisor, Prof. Dr. Zainal Salam for his perpetual guidance, scholarly assistance and motivational support throughout this research.

I also sincerely thank all the lecturers from whom I learned a lot. Not to mention, thanks to my fellow postgraduate friends for sharing useful ideas, information, and moral support during this study.

Last but not the least I would like to express my sincere appreciation and gratitude to my parents for their continuous encouragements and especially my wife who endured all the difficulties during my studies happily.

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ABSTRACT

The non-isolated dc-dc power converters are considered as a unique option for flexible voltage control and adaptation in the modern energy conversion systems due to their simple and light configurations. To this date, these converters are primarily investigated to generate high efficiency and high gain with a sustained soft switching capability and a smaller footprint. On that account, this work proposes two effective solutions to address the aforementioned issues. First, a high-efficiency soft switching non-isolated bidirectional dc-dc converter with a simple configuration is proposed. The converter executes the zero voltage zero current switching (ZVZCS) over a wide operating region to ensure high efficiency. For verification, a 150 W experimental prototype is built and tested for soft switching performance by varying the input voltage, switching frequency and the loading. It is observed that the efficiency remains consistently high and has a full-load maximum of 98.2% in the boost mode and 97.5% in the buck mode. The analysis of the Electromagnetic Interference (EMI) performance of the converter also shows the improvement in the noise signature. Second, an improved high gain zero voltage switching (ZVS) non-isolated bidirectional dc-dc converter is proposed. The high gain is realized by using an intermediate energy storage cell with reduced size. Besides, the ZVS is implemented by two integrated auxiliary resonant networks. These networks ensure sustained ZVS operation over the entire duty ratio. A 200 W prototype is built to verify the concept. As a result, a full load efficiency of 97.5% (in boost mode) and 95.5% (in buck mode) is recorded at fs= 30 kHz. Also, these efficiencies are recorded as 97% (boost mode) and 94.5% (buck mode) at fs= 100 kHz. Moreover, it is observed that the efficiency (and so the soft switching) is consistent over the entire gain profile. However, there is a slight additional drop of 1.5% (boost mode) and 1% (buck mode) at extreme duty ratios. Both converters also implement soft switching for auxiliary switches and eliminate the reverse recovery loss.

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ABSTRAK

Penukar kuasa dc-dc tidak terasing dianggap sebagai pilihan unik untuk pengawal voltan yang fleksibel dan penyesuaian dalam sistem penukaran tenaga moden disebabkan konfigurasinya yang mudah dan ringan. Sehingga kini, penukar ini menjadi keutamaan untuk menjana kecekapan tinggi dan gandaan voltan tinggi dengan keupayaan beralih lembut yang berterusan dan tapak yang lebih kecil. Sehubungan itu, kerja ini mencadangkan penyelesaian yang berkesan untuk menangani isu tersebut. Pertama, penukar dc-dc pensuisan lembut tidak terasing dwiarah berkecekapan tinggi dengan konfigurasi mudah dicadangkan. Penukar melaksanakan mod pensuisan voltan sifar dan arus sifar (ZVZCS) untuk memberikan kecekapan tinggi dalam julat operasi yang luas. Untuk pengesahan, prototaip eksperimen 150 W dibina dan diuji untuk prestasi pensuisan lembut dengan mengubah voltan masukan, frekuensi pensuisan dan bebanan. Diperhatikan bahawa kecekapan secara konsisten tinggi dan mempunyai kecekapan beban penuh 98.2% dalam mod boost dan 97.5% dalam mod buck. Analisis prestasi gangguan elektromagnetik (EMI) penukar jelas menunjukkan penambahbaikan terhadap pengurangan hingar. Kedua, penambahbaikan penukar dc-dc gandaan tinggi pensuisan voltan sifar (ZVS) tidak terasing dwiarah dicadangkan. Pensuisan lembut dilakukan oleh dua rangkaian bersepadu resonan pembantu. Ciri penting bagi penukar ini adalah ZVS dapat dikekalkan dalam keseluruhan kitar tugas (nisbah gandaan). Ini memastikan gandaan voltan tinggi dengan kecekapan operasi yang tinggi. Prototaip 200 W diuji untuk mengesahkan operasi penukar. Hasilnya, kecekapan beban penuh 97.5% (dalam mod boost) dan 95.5% (dalam mod buck) dicatatkan pada fs = 30 kHz. Begitu juga, kecekapan adalah 97% (mod boost) dan 94.5% (mod buck) pada fs = 100 kHz. Tambahan lagi, diperhatikan bahawa kecekapan (dan pensuisan lembut) konsisten di seluruh kawasan operasi. Walau bagaimanapun sedikit penurunan sebanyak 1.5% (mod boost) dan 1% (mod buck) pada nisbah kitaran tugas yang melampau. Kedua-dua penukar memastikan pensuisan lembut bagi suis-suis pembantu dan menghapuskan kehilangan pemulihan terbalik.

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TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xiv

LIST OF FIGURES xv

LIST OF ABBREVIATIONS xx

LIST OF SYMBOLS xxi

LIST OF APPENDICES xxiii

1 INTRODUCTION 1

1.1 Application Overview of the dc-dc Converter 1 1.2 Challenges for High-Performance dc-dc

Converter Operation 3 1.3 Problem Statements 4 1.4 Objectives of Research 5 1.5 Scope of Research 6 1.6 Organization of Thesis 7 2 LITERATURE REVIEW 9 2.1 Introduction 9

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2.2 A Review of the Soft Switching Techniques 11 2.2.1 The Zero Voltage Switching (ZVS) 12 2.2.2 The Zero Current Switching (ZCS) 14 2.2.3 The Zero Voltage Zero Current

Switching (ZVZCS) 16

2.2.4 The True Zero Voltage Zero Current

Switching (true ZVZCS) 16

2.3 A Review of Soft Switching Non-isolated

dc-dc Converters 18

2.3.1 The Snubber Based Converters 21 2.3.1.1 The Simple Lossless Snubber

Based Converter 21

2.3.1.2 The Active Snubber Based

Converter 21

2.3.1.3 The Passive Snubber Based

Converter 22

2.3.2 The Series Resonant Network Based

Converter 24

2.3.3 The Shunt Resonant Network Based

Converter 24

2.3.3.1 The Switched Capacitor

Resonant Converter 25

2.3.3.2 The Switched Inductor Shunt

2.3.3.3 The Coupled Inductor Shunt

2.3.3.4 The Compound Shunt

2.3.4 The Pulse Frequency Modulated

(PFM) Converter 29

2.3.4.1 The PFM Resonant Converter 29 2.3.4.2 The Active Edge Resonant

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2.4 General Comments on the Soft Switching

Converters 31

2.4.1 The Snubber Converters 32

2.4.2 The Series Resonant Converters 32 2.4.3 The Shunt Resonant Converters 33 2.4.4 The Pulse Frequency Modulated

(PFM) Converters 34

2.5 A Comparative Evaluation of the Soft

Switching Converters 34

2.5.1 Component Count 35

2.5.2 The Output Voltage and Current

Ripple 36

2.5.3 The Soft Switching Techniques 37

2.5.4 The Soft Switching Range 38

2.5.5 The Power Consumption 39

2.6 Summary 42

3 HIGH EFFICIENCY NON-ISOLATED

BIDIRECTIONAL DC-DC CONVERTER FAMILY WITH SOFT SWITCHING

OPERATION 44

3.1 Introduction 44

3.2 The Efficiency Improvement with Associated

Drawbacks in the Earlier Designs 46

3.3 The Proposed Converter: Configuration and

Operation 49

3.3.1 Boost Mode Operation 50

3.3.1.1 Interval 1 [t0-t1] 51 3.3.1.2 Interval 2 [t1-t2] 52 3.3.1.3 Interval 3[t2-t3] 53 3.3.1.4 Interval 4 [t3-t4] 54 3.3.1.5 Interval 5 [t4-t5] 55 3.3.1.6 Interval 6 [t5-t6] 56 3.3.1.7 Interval 7 [t6-t7] 56 3.3.1.8 Interval 8 [t7-t8] 57

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3.3.2 Buck Mode Operation 58 3.3.2.1 Interval 1 [t0-t1] 59 3.3.2.2 Interval 2 [t1-t2] 60 3.3.2.3 Interval 3 [t2-t3] 61 3.3.2.4 Interval 4 [t3-t4] 63 3.3.2.5 Interval 5 [t4-t5] 63

3.4 Design of the Circuit Parameters 64

3.4.1 Design of Lr1 and Lr2 65

3.4.2 Design of Cr1 66

3.4.3 Design of Lr3 67

3.4.4 Output Capacitors and Main Inductor

Selection 68

3.5 Experimental Verification 69

3.5.1 The DSP TMS320f2812 69

3.5.2 The DC Electronic Load 69

3.5.3 The Experimental Setup 70

3.5.4 The Closed Loop Digital Controller 72 3.5.5 Results and Discussion on Soft

Switching Performance 77

3.5.6 EMI Characterization 85

3.5.7 Other ZVZCS Topologies 90

3.6 Summary 91

4 SOFT SWITCHING NON-ISOLATED

BIDIRECTIONAL DC-DC CONVERTER

WITH HIGH GAINCAPABILITY 92

4.1 Introduction 92

4.2 The Gain Boosting Techniques in

Non-isolated dc-dc Converters 94

4.2.1 The Multilevel High Gain

Non-isolated dc-dc Converters 94

4.2.2 The Coupled Inductor Based High

Gain Non-isolated dc-dc Converters 95 4.2.3 The Switched Capacitor Based High

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4.2.4 Other Hybrid High Gain Topologies 96 4.3 The Proposed Converter: Configuration and

Operation 97 4.3.1 Boost mode 99 4.3.1.1 Interval 1[(t0-t1), resonance mode] 101 4.3.1.2 Interval 2 [(t1-t2), inductor charging mode] 102 4.3.1.3 Interval 3 [(t2-t3), capacitor

charging and resonance mode] 103 4.3.1.4 Interval 4 [(t3-t4), dead time

mode] 104

4.3.1.5 Interval 5 [(t4-t5), inductor and

capacitor discharging mode] 104 4.3.1.6 Interval 6 [(t5-t6), dead time

mode] 105

4.3.2 Buck Mode 106

4.3.2.1 Interval 1[(t0-t1), inductor

charging and resonance mode] 107 4.3.2.2 Interval 2 [(t1-t2), inductor

charging and output capacitor

discharging mode] 107

4.3.2.3 Interval 3 [(t2-t3), resonance

mode] 108

4.3.2.4 Interval 4 [(t3-t4), inductor

discharging mode] 109

4.3.2.5 Interval 5 [(t4-t5), dead time

mode] 110

4.4 Voltage Conversion Ratios 110

4.4.1 The Voltage-Second (vol-sec) Balance

in the Switching Converters 111

4.4.2 Boost Mode 111

4.4.2.1 Derivation of the

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4.4.2.2 Graphical Presentation of the Step-Up Ratio and

Comparison with Similar

Converters 114

4.4.3 Buck Mode 115

4.4.3.1 Derivation of the

Voltage-Second Balance Equations 115 4.4.3.2 Graphical Presentation of the

Step-Down Ratio and Comparison with Similar

Converters 117

4.5 Designing the Circuit Parameters 117 4.5.1 Optimization of Resonant Capacitors

(Cr1 and Cr2) 118

4.5.2 Selection of filter capacitors, CL, CH1

and CH2 121

4.5.3 Selection of energy storage capacitor,

C1 122

4.5.4 Determining the Value of Resonant

Inductors, Lr1andLr2 123

4.5.5 Selection of the Main Inductor, LM 123 4.5.6 Selection of Auxiliary

Inductors,La1and La2 124

4.6 Experimental Verification 126

4.6.1 The DSP TMS320f2812 126

4.6.2 The DC Electronic Load 127

4.6.3 The Closed Loop Digital Controller 127

4.6.4 Experimental Setup 129

4.6.5 Results and Discussion 131

4.6.5.1 Voltage Gain 131

4.6.5.2 Inductor Current and

Capacitor Voltages 132

4.6.5.3 Soft Switching Operation 133

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4.7 Summary 142

5 SUMMARY, CONCLUSION AND FUTURE

WORK 143

5.1 Summary of Work 143

5.2 Suggestions for Future Work 145

REFERENCES 147

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LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 The classification of the soft switching 20

2.2 Overview of the converter performances 41

3.1 Circuit parameters and their values 71

3.2 The performance parameters 76

3.3 Value of auxiliary inductors with frequency applied 82

4.1 PID controller and the performance parameters 128

4.2 Specifications for experimental test 129

4.3 Switch and diode ratings 131

4.4 Efficiency measurement at different operating points

in the boost and buck modes

137

4.5 Main features of the proposed converter and its

comparison to relevant high gain topologies reported in the literature

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LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 The hard switching (HS) crossover (a) at the turn on (simulation) (b) at the turn on and turn off

(linearized)

12

2.2 The zero-voltage switching crossover (a) at the turn on (simulation) and (b) at turn on and turn off (linearized)

13

2.3 The exemplary ZVS execution at the main switch (S1) of a boost converter (auxiliary circuit omitted)

14

2.4 The zero-current switching crossover (a) at the turn on (simulation) and (b) at the turn on and turn off (linearization)

15

2.5 Exemplary circuits to achieve the ZCS by applying a current restricting inductor or current diversion circuit in a boost converter

15

2.6 The zero-voltage zero-current crossover at the turn on and turn off

16

2.7 The true zero-voltage zero-current crossover (a) at turn on (simulated) (b) at turn on and turn off (linearized)

17

2.8 The exemplary true ZVZCS execution in a boost converter (the auxiliary circuit is omitted)

18

2.9 The classification of the soft switching converters 19

2.10 Active snubber network (the grey area) based

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converters (a) [52], (b) [66], (c) [72]

2.11 Passive snubber network (the grey area) based converters (a) [39], (b) [60], (c) [47], (d) [44]

23

2.12 The switched capacitor network (the grey area) based converters (a), (b), (c) [49]

25

2.13 The switched inductor network (grey area) based converters (a) [74], (b), (c) [41], (d) [55]

26

2.14 The coupled inductor network (grey area) based converters (a) [73], (b) [50]

27

2.15 The compound shunt resonant network (grey area) based converters (a) [32], (b) [67], (c) [63]

28

2.16 The pulse frequency modulated network (grey area) based converters (a) [68], (b) [36], (c) [33], (d) [35]

30

2.17 AER network (grey area) based converters (a) [45], (b) [46], (c) [59], (d) [49], (e) [52]

31

2.18 Average number of active and passive components in soft switching converters

35

2.19 The output (a) voltage and (b) current ripple in soft switching converters

37

2.20 Average switching loss in the main switches of the soft switching converters

38

2.21 (a) Average loss in percentage share and (b) average efficiency in the soft switching converters

41

3.1 The proposed ZVZCS bidirectional converter 50

3.2 The PWM switching and theoretical waveforms in boost mode

51

3.3 Boost operational interval 1(t0- t1) 52

3.4 Boost operational interval 2 (t1- t2) 53

3.6 Boost operational interval 4 (t3- t4) 55

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3.11 The PWM switching and theoretical waveforms in buck mode

59

3.12 Buck operational Interval 1 (t0-t1) 60

3.13 Buck operational Interval 2 (t1-t2) 61

3.16 Buck operational Interval 5 (t4-t5) 64

3.17 Experimental setup for testing the prototype converter

70

3.18 Block diagram of the closed loop system 73

3.19 Schematic block diagram of the closed loop controller embedded in the DSP TMS320f2812

75

3.20 The output voltage response of the converter to the load variations in the (a) boost (b) buck mode

76

3.21 Soft transition of (a) S1 (b) S2 in boost mode; (c) S1 (b) S2 in buck mode

78

3.22 Soft transition of (a) Sr1 (b) Sr2 in boost mode; (c) Sr1 (d) Sr2 in buck mode

80

3.23 The efficiency of the proposed converter (a) in boost mode against hard switching boost and the converters in [32, 76] (b) in buck mode against hard switching buck and the converter in [67] at fs= 100 kHz and

VL= 50 V.

81

3.24 Efficiency in boost mode (a) against fs at VL=50 V (b) against VL at fs=100 kHz

83

3.25 Efficiency in buck mode (a) against fs at VL=50 V (b) against VL at fs=100 kHz

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3.26 The soft switching map for the proposed converter 85

3.27 Boost mode noise level of (a) iH (b) iS1 in the hard switching and the proposed ZVZCS soft switching condition

88

3.28 Buck mode noise level of (a) iL (b) iS1 in the hard switching and the proposed ZVZCS soft switching condition

89

3.29 Other ZVZCS topologies (a) buck/boost (b) Cuk (c) SEPIC

90

4.1 The proposed high gain bidirectional converter with the energy storage cell and two identical auxiliary networks (AN1 and AN2)

98

4.2 The auxiliary resonant networks (AN1 and AN2) in the proposed high gain bidirectional converter

99

4.3 PWM switching and theoretical voltage current waveforms in the boost mode

100

4.4 Boost operational interval 1 (t1-t0) 102

4.5 Boost operational interval 2 (t1 -t2) 103

4.6 Boost operational interval 3 (t2-t3) 103 4.7 Boost operational interval 4 (t3-t4) 104 4.8 Boost operational interval 5 (t4-t5) 105 4.9 Boost operational interval 6 (t5-t6) 105 4.10 PWM switching and theoretical voltage and current

waveforms in the buck mode

106

4.11 Buck mode operational interval 1 (t0-t1) 107 4.12 Buck mode operational interval 2 (t1-t2) 108 4.13 Buck mode operational interval 3 (t2-t3) 109 4.14 Buck mode operational interval 4 (t3-t4) 109 4.15 Buck mode operational interval 5 (t4-t5) 110 4.16 Step up gain vs. duty cycle ratio (d1) against [93] and

[53]

115

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4.18 Maximum inductance value for LM in CCM mode of operation

124

4.19 Block diagram of the closed loop system 128 4.20 The output voltage response of the converter to the

load variations in the (a) boost (b) buck mode

129

4.21 Experimental setup for testing the prototype circuit 130 4.22 Voltage waveforms in boost mode at (a) d1=0.6 (b)

d1=0.8; and in buck mode at (c) d1=0.3 (d) d1= 0.6

132

4.23 Current waveform of iLM (a) in boost mode (b) in buck mode; voltage waveforms of C1 (c) in boost mode (d) in buck mode; (e) voltage waveforms of

CH1 and CH2 in boost/buck mode

133

4.24 Voltage and current waveforms in boost mode for (a) S1 (b) S3 (c) S4 (d) S5

134

4.25 Voltage and current waveforms in buck mode for (a) S1 (b) S2 (c) S4 (d) S5

135

4.26 Voltage and current waveforms in boost mode for (a) Sr1 (b) Sr2 ;in buck mode for (c) Sr1

136

4.27 Efficiency of the proposed converter against that in [93] for (a) boost mode and (b) buck mode with and without auxiliary network (AN)

138

4.28 Measured efficiency against d1 (a) in boost mode; (b) in buck mode at full load (PO= 200 W)

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LIST OF ABBREVIATIONS

DSP - Digital Signal Processor EV - Electric Vehicle

ESU - Energy Storage Unit

HS - Hard Switching

PV - Photovoltaic

PID - Proportional-Integral-Derivative

RE - Renewable Energy

RES - Renewable Energy Sources ZVS - Zero Voltage Switching ZCS - Zero Current Switching

ZVZCS - Zero Voltage Zero Current Switching ZVT - Zero Voltage Transition

ZCT - Zero Current Transition

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LIST OF SYMBOLS

C - Capacitor

D - Diode

Gstep-up - Step up gain

Gstep-down - Step down gain

I - Current

iH - High side current

iL - Low side current

iSr1 - Current through switch Sr1

iSr2 - Current through switch Sr2

iS1 - Current through switch S1

KP - PID controller proportional coefficient

KI - PID controller integral coefficient

KD - PID controller derivative coefficient

L - Inductor

P - Power

RLr1 - Parasitic resistance of inductor Lr1

V - Voltage

vL - Low side voltage

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vSr1 - Voltage across switch Sr1

vSr2 - Voltage across switch Sr2

vS1 - Voltage across switch S1

vH - High side voltage

vL - Low side voltage

ΔE - Change in efficiency

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LIST OF APPENDICES

APPENDIX TITLE PAGE

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CHAPTER1

INTRODUCTION

1.1 Application Overview of the dc-dc Converter

The sharp increase in the power requirements for modern computers, mobile devices, automotive/spacecraft and renewable energy system (RES) demand more efficient and reliable dc-dc power converters. Notably, as the applications are becoming more complex and diverse, the demand for high-performance, cost-effective converter topologies continue to grow. Consequently, there is an impetus to develop dc-dc converters with higher efficiency, higher gain and a smaller footprint that can function under flexible operating conditions. From this viewpoint, the non-isolated dc-dc power converters are considered as a preferable option, compared to its bulkier and expensive isolated counterparts.

In the communication and computer sector, the power supply can be either an isolated or non-isolated type of dc-dc converter. Regardless of the type, it is required to be efficient, inexpensive and lightweight with low ripple noise to protect the delicate electronics. Besides, it is expected to support a wide range of input voltage, but should not be limited by certain load constraints. For the renewable energy system (RES), these converters are primarily used to convert the dc voltage from one level to another, i.e. from photovoltaic [1], wind [2], fuel cell [3, 4], wave, ocean

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thermal and thermoelectric systems [5, 6]. In the microgrid environment, it also provides seamless integration of the RES to the internal grid by allowing the voltage and current from the RES to be precisely controlled. Additionally, an increasing number of RES deploy energy storage units (ESU) to store and manage the power flow more efficiently. The ESU requires non-isolated dc-dc power converters to interface with, for example, PV arrays or dc loads, such as electric vehicles. Such application illustrates the necessity of a dc conversion system with bidirectional power flow capability.

In the automotive industry—one of the main user of dc conversion systems, the non-isolated dc-dc converter is primarily used for internal electronics and vehicle control [7]. There is a wide range of dc voltage levels to be derived from the main battery for various vehicle instrumentation needs. Looking into the future, the electric vehicle (EV) sector is paving the way for the widespread application of high voltage, high power dc-dc converters. The on-board chargers of the EV are required to be bidirectional and lightweight. On the other hand, the off-board chargers must be capable of providing a flexible output voltage range to adapt to a different type of vehicles. With the expected high penetration of EV, the integration of RES into the charging system is foreseen as a viable solution to reduce the burden on the electrical grid [7-11]. Particularly, for the high power dc (fast) charging, the charging efficiency has become a critical issue. From these perspectives, the overall performance of future the power systems is largely determined by these converters.

On a more extreme outlook, the possible replacement of ac distribution system by dc is being widely debated [8-10]. Correspondingly, within the smart grid infrastructure, the dc-dc converter will be the main component that interfaces the dc distribution voltages with the consumer appliances.

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1.2 Challenges for High-Performance dc-dc Converter Operation

In all applications, the efficiency of the dc-dc converters is of utmost concern. Thus, the conduction and switching losses must be reduced. Conventionally, the hard switching PWM converters induce substantial switching losses, particularly at high switching frequency. Nevertheless, the high-frequency operation is highly desirable to reduce the size of the converter. On that account, the soft switching technique is sought to reduce the switching losses and thus to improve the efficiency. The existing soft switching converters apply zero voltage switching (ZVS), zero current switching (ZCS) or zero voltage zero current switching (ZVZCS) techniques to suppress these losses. Evidently, the ZVZCS provides improved loss reduction capability as compared to the former due to manipulation of both the voltage and current waveforms simultaneously.

However, the ZVZCS can only be achieved either at the cost of increased component count (and the conduction losses) and more complex controller. Furthermore, most converters with ZVZCS can only provide unidirectional power flow capability [12] [13] [14-16]. Operational-wise, the change in the input voltage, switching frequency or the loading of the converter disrupts the function of ZVZCS [17-28]. Consequently, the efficiency of the converter is often compromised to allow the converter to work in wider operating range.

Another issue of interest for the non-isolated dc-dc converter is the voltage gain ratio. Unlike its isolated counterpart, which utilizes transformer that can be concurrently used as a step-up mechanism, the nonisolated dc-dc converter has to employ other means to increase its gain. The popular method is to utilize the multilevel configuration, switched capacitor or coupled inductors. However, these topologies cannot provide a competitive gain without significantly increasing their component count and magnetic footprint. In addition, high gain converter comes with a substantial trade-off in terms of efficiency [26, 29-31]. For example, the multilevel topology cascades multiple converters units in series formation. As a result, the

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overall efficiency, which is the multiplication of the efficiency of each converter unit, is reduced. Furthermore, this configuration requires a large number of active and passive components that result in higher conduction losses.

On the other hand, the coupled inductor based topology attains high gain by manipulating the turn‘s ratio of the coupled inductors. On that account, the increase in the turns ratio, requires more conductor, thus increases the I2R losses. In addition, at high power, bigger magnetic circuit is required to accommodate the magnetic flux, in order to achieve the same gain ratio. Thus, the core losses are increased proportionally. As for the switched capacitor designs, multiple switched capacitor cells are required to boost the gain ratio. Besides additional device losses, the residual charges contribute to lower the efficiency.

In general, due to large number of switches in the above-mentioned topologies, it is extremely difficult to integrate the soft switching cells into the circuit. Since they could not exploit the advantages of the soft switching, most of these high gain converters exhibit higher switching losses [25, 32]. Notwithstanding, a small number of converters that integrate the soft switching fail to maintain it consistently over the entire gain profile. This is primarily due to the extreme duty ratio in which high gain converter operates [26, 29-31, 33].

1.3 Problem Statements

From the brief overview above, it can be inferred that the primary challenges that the non-isolated dc-dc power converters are required to address are: 1) High-efficiency operation in a wide operating range, 2) High step-up/step-down gain with sustained soft switching capability, and 3) Simple configuration with lower component count. In the recent literature, the nonisolated dc-dc converters are widely investigated to maintain low switching losses and to achieve high-efficiency

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operation. However, the high efficiency operation is achieved at the expense of larger component count, higher control complexity or limited operational range. The latter is formulated in the form of soft switching that is limited by the input voltage, switching frequency or loading. In other cases, the converter only allows unidirectional power flow, thus limiting the scope of their applications. On the other hand, in many circumstances (EV, spacecraft), the high gain is required with substantial constraint imposed on the size and weight of the converter. From this viewpoint, the gain heightening in a (low-weight) transformer-less topology is a big challenge by itself. By surveying the relevant literature, it is observed that the existing group of high gain non-isolated dc-dc power converters cannot sustain a high-efficiency operation over the entire operating range of gain ratio and power. Also, most of these converters are not equipped with the soft switching feature so that the switching losses could be suppressed consistently.

1.4 Objectives of Research

Given these drawbacks of the current state-of the art of non-isolated bidirectional dc-dc converter argued in Section 1.2, this work sets two primary objectives. They are summarized as follows.

(i) To design a high-efficiency non-isolated bidirectional dc-dc power converter with soft switching (ZVZCS) capability. The latter is to be implemented by an auxiliary network to reduce the switching losses in the main switches. Most importantly, the ZVZCS must operate in a wide range of input voltage, switching frequencyand loading, while maintaining high efficiency. Besides, the converter is expected to be built using low number of components.

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converter that can provide high gain with high efficiency. The high gain is realized by using an intermediate energy storage cell, with reduced size and weight. In addition, the soft switching (ZVS) is to be implemented over the entire operational duty cycle ratios.

Objectives (i) and (ii) are realized using two different circuits.

1.5 Scope of Research

To achieve the objectives of the research, this work is limited by the following scopes:

(i) The topologies of non-isolated soft switching dc-dc converters that are covered in the literature review in Chapter 2 are not exhaustive. However, it provides a comprehensive classification so that the existing circuits should fall under any of these categories.

(ii) The test results of the experimental prototype of the high-efficiency converter in Chapter 3 are recorded at 150 W. Mainly, the low power region allows analyzing the switching losses more effectively. This is because the latter loss is more dominating than the conduction losses in this region.

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1.6 Organization of Thesis

This thesis is organized into five chapters. Their contents are outlined as follows:

(i) Chapter 2 provides an extensive review of the soft switching techniques, the soft switching converters and the gain boosting techniques employed in non-isolated bidirectional dc-dc converters. The soft switching techniques are divided into the ZVS, the ZCS, the ZVZCS and the true ZVZCS. On the other hand, the soft switching converters are categorized into the active and passive snubber, the series and shunt resonant and the pulse width modulated converters. The merits and drawbacks of these soft switching configurations are highlighted. Besides, the gain boosting techniques are discussed along with their advantages and disadvantages.

(ii) Chapter 3 introduces the proposed non-isolated bidirectional dc-dc converter family. The proposed converter employs a ZVZCS network to improve the efficiency. Despite having a low component count and a simple configuration, it is capable of maintaining the soft switching condition for a wide operating range. Furthermore, it can be integrated to other basic converter platforms to improve the efficiency and extend the soft switching range.

(iii) Chapter 4 presents the proposed high gain and high efficiency non-isolated bidirectional dc-dc converter. The converter is integrated with the soft switching capability implemented by two identical auxiliary resonant networks. The dedicated networks ensure the soft switching over the whole operating condition. Resultantly, the high gain ratio is achievable with high operating efficiency. Besides, the reverse recovery loss is also eliminated.

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contributions of this research. Several suggestions are provided for possible directions of future work.

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