HIGH EFFICIENCY AND DIGITALLY CONTROLLED AC-DC CONVERTER WITH POWER FACTOR CORRECTION AND FAST OUTPUT VOLTAGE REGULATION LIM SHU FAN (B.Eng.(Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2012 i Acknowledgements I would like to express my sincere gratitude to Dr Ashwin M Khambadkone, my research supervisor, for his advice and patient guidance throughout the course of my research studies I am always amazed by how he can make a problem look so simple, his clear grasp of engineering concepts at fundamental level, his energy and enthusiasm while working on research problems with practical usage in mind He has always been a positive role model for me all these years since undergraduate times in the National University of Singapore In addition, I would like to thank Mr Woo Ying Chee and Mr Mukaya Chandra from the Electric Machines and Drives Laboratory for all the help in the ordering and searching of laboratory equipments and components They are always ready to offer technical advice and help, and a listening ear to my problems I would also like to thank Mr Abdul Jalil Bin Din for his advice on PCB design and his help on PCB fabrication I gratefully acknowledge Infineon Technologies Asia Pacific Pte Ltd for the sponsorship of my research studies in the National University of Singapore I would like to express my sincere appreciation to Mr Simon Sim of Infineon Technologies Asia Pacific Pte Ltd for offering this unique opportunity to me During the course of studies in National University of Singapore, the experience has been made pleasant by the many friends surrounding me With special thanks to Ms Zhou Haihua, Ms Yu Xiaoxiao, Ms Wang Huanhuan and Mr Tran Duong for all the encouragement and help in one way or another Lastly, I would not have accomplished the completion of my research studies without the support from my family I would like to thank my parents, Mr Lim Tong Liang and Ms Lee Beh Bee, for taking great care of me while I continued my studies and my sister, Lim Shurong, for her encouragement and company I would like to thank my husband, Richard Ng, who has been by my side all the time, supporting and encouraging me on during the difficult moments, and advising me on digital design iii Contents Acknowledgements Summary i vii List of Tables x List of Figures xi List of Abbreviations Introduction xviii 1.1 Problem Definition 12 1.2 Contribution of the Thesis 17 1.3 Organization of the Thesis 20 Nonlinear Inductor for Improving Efficiency at Light Load in PFC 22 2.1 Meeting Increasing Efficiency Requirement for the Complete Load Range 22 2.2 Causes of Poor Light Load Efficiency in PFC 24 2.3 Nonlinear Inductor for Improving Light Load Efficiency 27 2.4 Nonlinear Inductor Design for PFC 34 2.5 Experimental Results and Analysis 41 2.6 Summary 66 CCM-DCM Digital Control for Improving Efficiency and Power Factor at Light Load 68 3.1 Control of PFC for Complete Load Range 68 3.2 Suitable CCM PFC Control Scheme for Digital Implementation 70 3.2.1 Input Current Control Techniques for CCM PFC Converters 71 3.2.1.1 Average Current Control 71 3.2.1.2 Peak Current Control 75 3.2.1.3 Hysteresis Current Control 77 3.2.1.4 Nonlinear Carrier Control 78 3.2.1.5 One Cycle Control 80 Suitable CCM PFC Control Scheme for Boost PFC with Digital Implementation 81 CCM Control Design 83 3.2.2 3.2.3 3.2.3.1 83 3.2.3.2 Sampling of Variables for Control 93 3.2.3.3 Low Pass Filter Design for Input Voltage Feedforward104 3.2.3.4 Multiplier Design 109 3.2.3.5 Current Controller Design 111 3.2.3.6 3.2.4 Modeling of Boost Converter for Average Current Control Voltage Controller Design 116 Controller Implementation and Simulation Results 119 3.3 Suitable DCM PFC Control Scheme to be Used at Light Load with Digital Implementation 120 3.3.1 Problems in DCM Using CCM Current Controller 120 3.3.2 Inductor Current Sample Correction in DCM 123 3.3.3 DCM Control Techniques for PFC Operating in Both CCM and DCM 125 3.3.4 The Proposed DCM Control Scheme 129 3.3.5 Performance of the Proposed CCM-DCM Control Scheme 3.3.6 Summary 147 132 Multimode Digital Control for Improving Efficiency at Very Light Load 149 4.1 Suitable Control Scheme for Very Light Load Conditions That Ensures Minimum Power Consumption and Output Voltage Regulation 149 4.1.1 4.1.2 Existing Solutions for Reducing Power Consumption Under Very Light Load Conditions of PFC 151 4.1.3 The Proposed Multimode Digital Control Scheme for Improving efficiency and Ensuring Output Voltage Regulation at Very Light Load 159 4.1.4 Control Analysis of the Proposed Multimode Digital Control Scheme 162 4.1.5 Performance of the Proposed Multimode Digital Control Scheme at Very Light Load 170 4.1.6 4.2 Importance of Reducing Power Consumption Under Very Light Load Conditions of PFC 149 Summary 176 Improvement of Efficiency and Power Factor at Light Load with the Proposed Multimode Control Scheme 178 Conclusion and Future Work 189 5.1 Conclusion 189 5.2 Future Work 194 5.2.1 Reducing the Cost of Sensing 196 5.2.1.1 5.2.1.2 Reducing the ADC Requirements 200 5.2.1.3 5.2.2 Reducing the Number of ADCs Required 197 Future Work Required in Reducing the Cost of Sensing 201 Line Frequency Independent Method to Obtain the Average Output Voltage 202 Bibliography 205 List of Publications 217 vii Summary High efficiency and power factor at light load are increasingly desired in desktop computer power supplies for energy saving initiative and product differentiation with the certification of energy saving programs However, the efficiency and power factor of power factor correctors (PFCs) in desktop computer power supplies are poor at light load The constant frequency PFC controller designed for continuous conduction mode (CCM) is unable to ensure good input current shaping in discontinuous conduction mode (DCM) due to nonlinear converter characteristics and incorrect average current samples obtained if digital control is used Poor input current shaping in DCM causes higher current distortion and larger RMS current drawn from the AC mains, resulting in poor efficiency and power factor at light load At very light load, the load independent constant losses become dominant and cause a steep fall in efficiency A nonlinear inductor that has a higher inductance at low average inductor current and under light load conditions is proposed to improve light load efficiency of PFC by reducing the constant losses contributed by inductors in the system Efficiency of a 300W CCM boost PFC is improved at 0.02p.u (per unit) load with rated load as base by 4.22% and 3.42% under an input voltage of 85VAC and 265VAC respectively The nonlinear inductor achieves efficiency improvement at light load without additional external components or complex control as compared to other efficiency improvement efforts It is a simple idea that does not require any advance tool for its design and is applicable to any topology or system with inductors A CCM-DCM digital control scheme that improves power factor and efficiency at light load by ensuring good input current shaping in both CCM and DCM is proposed for boost PFC At a light load of 0.1p.u and an input voltage of 230VAC, the total harmonic distortion of the input current is significantly reduced by 87.85%, the power factor is improved from 0.63 to 0.77, and the efficiency is increased by 1.1% for a 300W boost PFC The proposed CCM-DCM digital control scheme is mathematically and computationally simple The result of all arithmetic operations in the current control loop is achievable in one clock cycle, whereas other DCM control schemes require multiple clock cycles There is a smooth transition between CCM and DCM operations of the boost converter in each AC half cycle and between heavy and light loads with the proposed CCM-DCM digital control scheme Since constant losses are frequency dependent, they can be reduced as a whole by reducing switching in the PFC A multimode digital control scheme that improves efficiency and ensures output voltage regulation at very light load in PFC is proposed The proposed multimode digital control scheme consists of the proposed CCM-DCM digital control scheme and a no load digital control scheme The proposed no load digital control scheme that is based on on-off control of the PFC is primarily responsible for reducing constant losses with reduced switching in the PFC and for ensuring output voltage regulation at very light load It can be added easily to the CCM-DCM digital control scheme without additional and costly external components Compared to other on-off control schemes, a small load jump is sufficient to exit the no load control scheme, and this allows a smooth transition between the no load control and the CCM-DCM active mode control Efficiency of a 300W boost PFC is improved at 0.007p.u load by 11.53% and 2.19% with the proposed multimode digital control scheme 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1527–1531, 2007 218 List of Publications Published Conference Paper Shu Fan Lim and Ashwin M Khambadkone, “Non Linear Inductor Design for Improving Light Load Efficiency of Boost PFC”, IEEE Energy Conversion Congress and Exposition, ECCE 2009, pp 1339-1346, September 2009 Shu Fan Lim and Ashwin M Khambadkone, “A Simple Digital DCM Control Scheme for Boost PFC Operating in Both CCM and DCM”, IEEE Energy Conversion Congress and Exposition, ECCE 2010, pp 1218-1225, September 2010 Shu Fan Lim and Ashwin M Khambadkone, “A Multimode Digital Control Scheme for Boost PFC with Higher Efficiency and Power Factor at Light Load”, IEEE The Applied Power Electronics Conference and Exposition, APEC 2012, pp.291-298, February 2012 219 Published Journal Paper Shu Fan Lim and Ashwin M Khambadkone, “A Simple Digital DCM Control Scheme for Boost PFC Operating in Both CCM and DCM”, IEEE Transactions on Industry Applications, Vol 47, No 4, pp 1802-1812, July/August 2011 Provisional Patent Shu Fan Lim and Ashwin M Khambadkone, “A Digital Control Scheme For Reduced Input Power Consumption While Maintaining Regulated Output Voltage Under No Load Condition of Power Factor Corrector”, US Provisional Patent Application No 61/477,236 ... results at 230VAC and 0.4p.u load with sample correction and with CCM-DCM control 141 3.49 Experimental results at 230VAC and 0.1p.u load without sample correction and with CCM control... load efficiency and poor light load power factor are the two major problems hindering the achievement of high energy efficiency and high power factor across the complete load range With the increasing... results at 230VAC, full load with sample correction and with CCM-DCM control 140 3.47 Experimental results at 230VAC and 0.4p.u load without sample correction and with CCM control