||LED Current-Reduction Drive and Modeling
||Department of Electrical Engineering
This thesis presents the LED current-reduction drive and modeling. With the LED current-reduction drive, the conduction losses on MOSFET and diode are reduced to improve the efficiency of LED driver systems.
The LED power factor and quality factor are proposed to evaluate the LED power conversion efficiency. Besides, the state-space average approach is demonstrated to model the CCM Buck-Boost, Boost and Buck type LED drivers. The expanded three-terminal PWM switch model incorporates the MOSFET on-resistance, the equivalent series resistance (ESR) and forward voltage drop of diode is proposed to model the CCM DC-DC converters and LED drivers.
With the same LED number, compared with the LEDs in parallel connection, the LEDs in series connection flow less current to achieve high power conversion efficiency of LED driver systems. However, the LEDs in series connection requires high-voltage MOSFET and diode for LED drivers. The high-voltage MOSFET and diode has higher on-resistance and forward voltage drop. Therefore, in order to enhance the modeling accuracy, besides the on-resistance of the high-voltage MOSFET, the ESR and forward voltage drop of high-voltage diode have to be included into the equivalent circuit models of LED drivers.
For the demanded specifications and current-reduction drive, the required number of LEDs is specified for maximum power conversion efficiency by the graphical approach. For the CCM converters and LED drivers of Buck-Boost, Boost and Buck types, by using the proposed expanded PWM switch model, the transfer functions of control-to-output are obtained and validated with SIMPLIS® circuit simulation results.
Chapter 1. Introduction 1
1.1. Background 1
1.2. Motivation 3
1.3. Thesis Outline 4
Chapter 2. LED Array Design for Current-Reduction Drive 5
2.1. Introduction 5
2.2. Three-Parameter Approach for Single LED Characterization 5
2.2.1. Parameters Characterization 5
2.2.2. DC and AC Equivalent Resistances 8
2.2.3. Modified Approximate Linear Model 9
2.3. LED Power Factor and Quality Factor 10
2.3.1. Alternative Approximate Linear Model 10
2.3.2. Bandgap, Resistive and Dissipated Powers 11
2.3.3. LED Power Factor and Quality Factor 12
2.3.4. LED Current-Reduction Drive 13
2.4. Design Guideline of LED Array for Current-Reduction Drive 14
2.4.1. Relationship of LED Luminous Flux and Current 14
2.4.2. Graphical Approach for LED Current-Reduction Drive Design 16
2.4.3. Demonstration Example 22
2.4.4. Parameterization of Approximate Linear Model for LED Array 26
2.5. Summary 27
Chapter 3. State-Space Average Model for CCM LED Drivers 28
3.1. Introduction 28
3.2. State-Space Equations for LED Drivers 28
3.2.1. Buck-Boost Type LED Driver 28
3.2.2. Boost-Type and Buck-Type LED Drivers 34
3.3. State-Space Average Model for LED Drivers 40
3.3.1. Buck-Boost Type LED Driver 40
3.3.2. Boost-Type and Buck-Type LED Drivers 44
3.4. DC Output Voltages and Currents of LED Drivers 47
3.5. Decoupling of Equivalent Circuit Models for LED Drivers 48
3.6. Validation of State-Space Average Model for LED Drivers 51
3.7. Summary 60
Chapter 4. Expanded Three-Terminal PWM Switch Model for CCM Converters 62
4.1. Introduction 62
4.2. CCM Three-Terminal PWM Switch Model 62
4.3. Expanded CCM Three-Terminal PWM Switch Model 65
4.4. Equivalent Circuit Models of CCM DC-DC Converters 69
4.4.1. Buck-Boost Converter 69
4.4.2. Boost Converter 72
4.4.3. Buck Converter 74
4.5. Decoupling of Equivalent Circuit Models for CCM DC-DC Converters 76
4.5.1. Decoupling of DC Equivalent Circuits 77
4.5.2. Decoupling of Small-Signal Equivalent Circuits 80
4.6. Validation of Small-Signal Circuit Models for CCM DC-DC Converters 87
4.7. Summary 97
Chapter 5. Expanded Three-Terminal PWM Switch Model for CCM LED Drivers 98
5.1. Introduction 98
5.2. DC Equivalent Circuit Using Expended PWM Switch Model 98
5.2.1. Buck-Boost Type LED Driver 98
5.2.2. Boost-Type LED Driver 102
5.2.3. Buck-Type LED Driver 105
5.3. Small-Signal Equivalent Circuit Using Expended PWM Switch Model 108
5.3.1. Buck-Boost Type LED Driver 108
5.3.2. Boost-Type LED Driver 110
5.3.3. Buck-Type LED Driver 113
5.4. Validations of Small-Signal Equivalent Circuits for LED Drivers 116
5.5. Summary 121
Chapter 6. Conclusions 123
Appendix A Transfer Functions of Control-to-Output for CCM DC-DC Converters and LED Drivers 129
A.1 Transfer Functions of Control-to-Output for CCM Buck-Boost, Boost and Buck LED Drivers Derived by Using State-Space Average Model 129
A.2 Transfer Functions of Control-to-Output for CCM Buck-Boost, Boost and Buck Converters Derived by Using Expanded PWM Switch Model 131
A.3 Transfer Functions of Control-to-Output for CCM Buck-Boost, Boost and Buck LED Drivers Derived by Using Expanded PWM Switch Model 132
Appendix B Characterization of MOSFET On-Resistance, Diode ESR and Forward Voltage Drop by SIMPLIS® Simulations 134
B.1 Characterization of MOSFET On-Resistance 134
B.2 Characterization of Diode ESR and Forward Voltage Drop 137
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