High Efficiency DC/AC Power Converter for Photovoltaic Applications

218 Pages·2010·14.6 MB·English

High Efficiency DC/AC Power Converter
for Photovoltaic Applications
by
Aleksey Trubitsyn
B.S., Boston University (2008)
Submitted to the Department of Electrical Engineering and Computer
Science
in partial fulfillment of the requirements for the degree of
Master of Science
at the
Massachusetts Institute of Technology
June 2010
(cid:13)c Massachusetts Institute of Technology, MMX. All rights reserved.
Author
Department of Electrical Engineering and Computer Science
May 21, 2010
Certified by
David J. Perreault
Associate Professor of Electrical Engineering
Thesis Supervisor
Accepted by
Terry P. Orlando
Chairman, Department Committee on Graduate Students

High Efficiency DC/AC Power Converter for Photovoltaic
Applications
by
Aleksey Trubitsyn
Submitted to the Department of Electrical Engineering and Computer Science
on May 21, 2010, in partial fulfillment of the
requirements for the degree of
Master of Science
Abstract
This thesis presents the development of a microinverter for single-phase photovol-
taic applications that is suitable for conversion from low-voltage (25-40V) DC to
high voltage AC (e.g. 240V ). The circuit topology is based on a full-bridge
AC,RMS
series resonant inverter, a high-frequency transformer, and a novel half-wave cyclo-
converter. The operational characteristics are analyzed, and a multidimensional con-
trol technique is utilized to achieve high efficiency, encompassing frequency control
and inverter and cycloconverter phase shift control. An experimental prototype is
demonstrated in DC/DC conversion mode for a wide range of output voltages. The
proposed control strategy is shown to allow for accurate power delivery with minimal
steps taken towards correction. The prototype achieves a CEC averaged efficiency of
approximately 95.1%. Guidelines for optimization are presented along with experi-
mental results which validate the method.
Thesis Supervisor: David J. Perreault
Title: Associate Professor of Electrical Engineering

Acknowledgements
IwanttothankBrandonPierquetforansweringallofmyquestionsaboutthisproject,
at absolutely any time, and no matter how trivial or off-point they were. I sincerely
appreciate his endless willingness to help me understand the concepts of this project
and I certainly would not have been able to get through this thesis without his help. I
want to thank Alexander Hayman for all the practical information that he taught me
in the several months that we worked together. I learned most of what I know about
dealing with hardware from him and this was a crucial skill to have for advancing
the prototype. I want to thank Professor David Perreault for bringing me onto this
project and being very guiding, knowledgeable and extremely supportive during the
last two years. Also very noteworthy is his willingness to stay late in lab, beyond
regular hours, and helping us debug our projects. Many thanks go out to Enphase
Energy and Dr. Landsman for their funding of my education. I want to thank the
many members of LEES that made the lab a fun, coherent and productive work
environment: George Hwang, Grace Cheung, Jackie Hu, Makiko Wada, Ben Cannon,
Ian Smith, and Justin Burkhart. I also appreciate the numerous times that Anthony
Sagneri and Robert Pilawa offered very helpful advice on my work.
Many thanks to Allan Hutchison-Maxwell for proofreading a large portion of this
thesis!
Finally, I want to extend special thanks to my girlfriend, Maria, and my family,
Anton, Mariam and Yuriy for their continued love and support throughout my college
career!!
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Contents
1 Introduction 15
1.1 Solar inverters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.2 Thesis scope and organization . . . . . . . . . . . . . . . . . . . . . . 17
2 System Overview 19
2.1 Soft switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2 Design targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.1 Grid tie power delivery . . . . . . . . . . . . . . . . . . . . . . 22
2.2.2 CEC efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2.3 Switching cycle averaged quantities . . . . . . . . . . . . . . . 24
2.3 Circuit topology and overview . . . . . . . . . . . . . . . . . . . . . . 25
2.4 Operating a half-bridge under ZVS . . . . . . . . . . . . . . . . . . . 26
3 Design 33
3.1 Component description . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.1.1 Full-bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.1.1.1 Full-bridge losses . . . . . . . . . . . . . . . . . . . . 37
3.1.1.2 Component selection . . . . . . . . . . . . . . . . . . 39
3.1.2 Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.1.2.1 Transformer losses . . . . . . . . . . . . . . . . . . . 40
3.1.3 Resonant tank . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.1.3.1 Resonant tank losses . . . . . . . . . . . . . . . . . . 45
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Contents
3.1.4 Cycloconverter . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.1.4.1 Cycloconverter losses . . . . . . . . . . . . . . . . . . 50
3.1.5 Component Selection . . . . . . . . . . . . . . . . . . . . . . . 51
3.1.6 Capacitor banks . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.1.6.1 Input . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.1.6.2 Blocking . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.1.6.3 Output . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.1.6.4 Capacitor losses . . . . . . . . . . . . . . . . . . . . . 53
4 Control and Optimization 55
4.1 Inverter control inputs . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.1.1 Full bridge phase shift . . . . . . . . . . . . . . . . . . . . . . 57
4.1.2 Switching frequency . . . . . . . . . . . . . . . . . . . . . . . 57
4.1.3 Cycloconverter phase shift . . . . . . . . . . . . . . . . . . . . 58
4.1.4 Burst mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.2 Optimal control strategy . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.3 Equivalent circuit model . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.3.1 Governing equations . . . . . . . . . . . . . . . . . . . . . . . 62
4.3.2 Ideal diode rectifier . . . . . . . . . . . . . . . . . . . . . . . . 64
4.3.3 Diode rectifier with non-zero capacitance . . . . . . . . . . . . 67
4.3.4 Extension to the real inverter . . . . . . . . . . . . . . . . . . 70
4.3.4.1 Equivalent charge . . . . . . . . . . . . . . . . . . . . 72
4.3.4.2 Critical angles for ZVS . . . . . . . . . . . . . . . . . 73
4.3.4.3 Deadtime requirements . . . . . . . . . . . . . . . . . 75
4.3.4.4 Cycloconverter turn on time . . . . . . . . . . . . . . 76
4.3.5 Other considerations . . . . . . . . . . . . . . . . . . . . . . . 76
4.3.6 Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.4 Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
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Contents
4.4.1 Resonant components . . . . . . . . . . . . . . . . . . . . . . . 79
4.4.2 Transformer turns ratio . . . . . . . . . . . . . . . . . . . . . . 83
4.4.3 MOSFET selection . . . . . . . . . . . . . . . . . . . . . . . . 84
4.4.4 Loss breakdown and efficiency . . . . . . . . . . . . . . . . . . 86
4.4.5 Sloshing boundary . . . . . . . . . . . . . . . . . . . . . . . . 86
5 Verification 93
5.1 Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
5.1.1 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . 94
5.1.2 Selection of operating points . . . . . . . . . . . . . . . . . . . 98
5.2 Results I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
5.3 Results II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
5.3.1 Resonant tank placement . . . . . . . . . . . . . . . . . . . . . 107
5.4 Loss estimates using thermal measurements . . . . . . . . . . . . . . 107
6 Conclusion 111
6.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
6.2 Next steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
6.3 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
A Converter models: MATLAB 115
A.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
B Converter models: LTspice 127
B.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
C Optimization scripts 141
C.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
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Contents
D Experimental setup scripts 175
D.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
D.2 FPGA software description . . . . . . . . . . . . . . . . . . . . . . . . 175
D.3 MATLAB scripts for FPGA . . . . . . . . . . . . . . . . . . . . . . . 177
D.4 MATLAB scripts for efficiency measurements . . . . . . . . . . . . . 189
D.5 Verilog code for FPGA . . . . . . . . . . . . . . . . . . . . . . . . . . 193
E Thermal measurements 207
E.1 Thermal characterization . . . . . . . . . . . . . . . . . . . . . . . . . 207
E.2 Thermal impedance matrices . . . . . . . . . . . . . . . . . . . . . . . 209
F Prototype hardware 211
F.1 Parts list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
F.2 PCB Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
Bibliography 217
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