Design of High-density Transformers for High-frequency High-power
Converters
by
Wei Shen
Dissertation submitted to the faculty of the Virginia Polytechnic Institute and
State University in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
In
Electrical Engineering
Dr. Dushan Boroyevich Committee Co-Chair
Dr. Fred Wang Committee Co-Chair
Dr. Jacobus Daniel van Wyk Committee Member
Dr. Guo-Quan Lu Committee Member
Dr. Yilu Liu Committee Member
July, 2006
Blacksburg, Virginia
Keywords: High-frequency Transformer, High power density, Core loss calculation,
Leakage inductance calculation, Transformer optimal design
Design of High-density Transformers for High-frequency High-power
Converters
Wei Shen
ABSTRACT
Moore’s Law has been used to describe and predict the blossom of IC industries,
so increasing the data density is clearly the ultimate goal of all technological
development. If the power density of power electronics converters can be analogized to
the data density of IC’s, then power density is a critical indicator and inherent driving
force to the development of power electronics. Increasing the power density while
reducing or keeping the cost would allow power electronics to be used in more
applications.
One of the design challenges of the high-density power converter design is to
have high-density magnetic components which are usually the most bulky parts in a
converter. Increasing the switching frequency to shrink the passive component size is the
biggest contribution towards increasing power density. However, two factors, losses and
parasitics, loom and compromise the effect. Losses of high-frequency magnetic
components are complicated due to the eddy current effect in magnetic cores and copper
windings. Parasitics of magnetic components, including leakage inductances and winding
capacitances, can significantly change converter behavior. Therefore, modeling loss and
parasitic mechanism and control them for certain design are major challenges and need to
be explored extensively.
In this dissertation, the abovementioned issues of high-frequency transformers are
explored, particularly in regards to high-power converter applications. Loss calculations
accommodating resonant operating waveform and Litz wire windings are explored.
Leakage inductance modeling for large-number-of-stand Litz wire windings is proposed.
The optimal design procedure based on the models is developed.
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Acknowledgements
Acknowledgements
I owe an enormous debt of gratitude to my advisor, Dr. Dushan Boroyevich, for
his support and guidance during my study. His profound knowledge, masterly creative
thinking, and sense of humor have been my source of inspiration through out this work.
To Dr. Fred Wang, my co-advisor, I want to express my sincere appreciation to
him for his instruction, time, and patience. His gentle personality and rigorous attitude
toward research will benefit my career as well as my personal life. Most importantly, I
have learned motivation and confidence from them. I am very lucky to have both
professors as mentors during my time in CPES.
I would like to express my appreciation to my committee member, Dr. van Wyk,
who is such an elegant and admirable professor. I enjoyed each of our meetings and
always learned more from him. I would also like to thank my other committee members
Dr. Yilu Liu and Dr. Guo-Quan Lu for always helping and encouraging me.
I would also like to thank all my colleagues in CPES for their help, mentorship,
and friendship. I cherish the wonderful time that we worked together. Although this is not
a complete list, I must mention some of those who made valuable input to my work. They
are Dr. Bing Lu, Dr. Qian Liu, Dr. Gang Chen, Dr. Lingyin Zhao, Dr. Rengang Chen, Dr.
Wei Dong, Dr. Shuo Wang, Dr. Ming Xu, Jerry Francis, Tim Thacker, Arnedo Luis,
Dianbo Fu, Chuanyun Wang, Jinggen Qian, Liyu Yang, Manjin Xie, Yu Meng,
Chucheng Xiao, Dr. Wenduo Liu, Michele Lim, Jing Xu, Yang Liang, Yan Jiang,
Sebastian Rosado, Xiangfei Ma, Dr. Jinghong Guo, Dr. Zhenxian Liang, Dr. Yingfeng
Pang, Dr. Luisa Coppola, and so many others. The last but not the least, I want to thank
group members of the ARL project: Hongfang Wang, Honggang Sheng, Dr. Xigen Zhou,
Dr. Xu Yang, Yonghan Kang, Brayn Charboneau, Dr. Yunqing Pei, and Dr. Ning Zhu.
I would like to thank the administrative staff members, Marianne Hawthorne,
Robert Martin, Teresa Shaw, Trish Rose, Elizabeth Tranter, Michelle Czamanske, Dan
Huff, who always smiled at me and helped me to get things done smoothly.
This work made use of ERC Shared Facilities supported by the National Science
Foundation under Award Number EEC-9731677.
iii
Acknowledgements
I dedicate this achievement to my wife Shen Wang
It would not have been possible without your support, encouragement and love. Thank
you for being with me for the whole five years of study.
Also to my parents
Mr. Hancai Shen and Ms. Xiangdai Yang
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Table of Contents
Table of Contents
ABSTRACT........................................................................................................................ ii
Acknowledgements............................................................................................................ iii
Chapter 1 Introduction.................................................................................................... 1
1.1. Background ....................................................................................................... 1
1.2. Literature Review .............................................................................................. 3
1.2.1. Low power & Ultra-high frequency applications ................................... 4
1.2.2. High power & mid-frequency applications............................................. 5
1.2.3. Mid-power & High-frequency applications............................................ 6
1.2.4. Summaries............................................................................................... 7
1.3. Research Scope and Challenges ........................................................................ 8
1.3.1. Research scope........................................................................................ 8
1.3.2. Research challenges................................................................................ 9
1.4. Dissertation Organization................................................................................ 10
Chapter 2 Nanocrystalline Material Characterization .................................................. 12
2.1. Conventional high-frequency magnetic materials........................................... 13
2.1.1. Magnetic material introduction............................................................. 13
2.1.2. Characteristics of conventional ferri- and ferro-materials .................... 15
2.1.3. Ferrites .................................................................................................. 15
2.1.4. Amorphous metals ................................................................................ 18
2.1.5. Supermalloy .......................................................................................... 20
2.2. Characteristics of nanocrystalline materials.................................................... 21
2.2.1. B/H curve .............................................................................................. 23
2.2.2. Loss performance.................................................................................. 26
2.2.3. Temperature dependence performance ................................................. 28
2.2.4. Cut core issues ...................................................................................... 29
2.3. Summaries ....................................................................................................... 34
Chapter 3 Loss Calculation and Verification ............................................................... 37
3.1. Core loss calculation ....................................................................................... 38
3.1.1. Calculation method survey ................................................................... 38
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