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ABSTRACT TANEJA, ROHIT. Making Energy-Efficient Lighting more Cost-Effective. (Under the direction of Dr. Alexander Dean and Dr. Subhashish Bhattacharya.) Motivation behind this work is to come up with an energy efficient and a reduced cost LED driver control system. This work encompasses digital control theory and embedded system concepts. LEDs are increasingly finding application in almost every sector, like consumer electronics (TV, phones), space lighting (cluster of LEDs) and indication, which require a durable, inexpensive and an efficient solution. The thesis builds upon an existing closed loop control system developed utilizing the RL-78 microprocessor development board and a boost converter working in the continuous conduction mode (CCM). The primary focus is on cost reduction while maintaining/improving the efficiency, and thus, the discontinuous conduction mode (DCM) of operation is implemented and tested. DCM operation results in cost reduction of the system as only one MOSFET is required in the circuit. Dimming is controlled by exploiting the discontinuous current property of the inductor already present in the circuit, as opposed to a dedicated MOSFET required for dimming, through the PWM signal, in CCM. The efficiency of DCM and CCM are also discussed in this work. Comparison is also drawn in terms of the embedded software and the resources needed in both modes. It is proved that DCM is indeed more energy efficient than CCM. The Thesis attempts to quantify efficiency in terms of Lumens (luminous flux). A comparison is drawn with respect to lumens per input wattage used in CCM and DCM operations. © Copyright 2013 by Rohit Taneja All Rights Reserved Making Energy-Efficient Lighting more Cost-Effective by Taneja Rohit A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Master of Science Computer Engineering Raleigh, North Carolina 2013 APPROVED BY: _______________________________ Dr. Alexander Dean Committee Chair ________________________________ Dr. Subhashish Bhattacharya ________________________________ Dr. James Tuck ii DEDICATION Dedicated to My Parents Sharda Taneja and Dalip Taneja My Brother Varun Taneja And My Beloved Fiancée Rupashree Bhattacharya iii BIOGRAPHY Rohit Taneja was born on January 23, 1987 in New Delhi. He did his primary schooling in New Delhi. He pursued undergraduate studies at Guru Gobind Singh Indraprastha University in New Delhi and received his bachelor’s degree (Bachelor of Technology) in Electronics and Communications, in July 2008. He was an active member of the IEEE chapter at his college, and worked as a technical supervisor for the technical society in his college. After the completion of his undergraduate studies, he joined a startup firm, working on Digital Signage Solutions. Later he went ahead and joined Tata Elxsi in April, 2009 and worked as an Embedded Design Engineer, mainly on the Check Scanner Product for a Client and porting compiler for the ARM architecture. He joined as a graduate student in the Electrical and Computer Engineering Department at North Carolina State University in fall, 2011. He is majoring in Computer Engineering and his focus is on embedded systems. He has been working as a research assistant under the guidance of Dr. Alexander Dean. He is engaged in research in an application of embedded systems requiring software control of switching converters for cost effective LED lighting. His interests span across a wide range of subjects – Embedded Systems Design, Digital Design, Computer Architecture and Power Electronics. iv ACKNOWLEDGMENTS I express my sincere gratitude and thank my advisor, Dr. Alexander Dean for providing me with an opportunity and supporting me at every step during my studies at North Carolina State University. His guidance and mentorship facilitated my professional as well as academic growth. I thank Dr. Subhashish Bhattacharya for his continuous support in my thesis and helping me steer my way through the research. I would like to thank Avik Juneja for being my research peer and helping me in solving technical intricacies. I would also like to mention Shikhar Singh, Michael Plautz and Tharunachalam Pindicura for working with me in accomplishing my research goal. I appreciate the valuable inputs by my seniors, my friends for keeping me motivated and making my stay memorable at North Carolina State University. v TABLE OF CONTENTS LIST OF TABLES .................................................................................................................. vii LIST OF FIGURES ............................................................................................................... viii CHAPTER 1 Introduction......................................................................................................... 1 1.1 Significance of the Study ................................................................................................ 1 1.2 Motivation ....................................................................................................................... 2 1.3 Preview of the Work ....................................................................................................... 2 1.4 Outline for Rest of the document ............................................................................... 3 CHAPTER 2 Previous Work .................................................................................................... 4 2.1 Switching DC-to-DC converters ..................................................................................... 4 2.1.1 Converter Theory ..................................................................................................... 4 2.1.2 Physical Operation and properties of switching converters ..................................... 4 2.1.3 Control via Software ................................................................................................ 7 2.2 Closed loop Control of Boost Converter in CCM .......................................................... 7 2.2.1 Continuous Conduction Mode of Boost converter .................................................. 7 2.2.2 Processing requirements and Closed loop Control ................................................ 11 CHAPTER 3 Discontinuous mode of Operation ................................................................... 14 3.1 Design to operate in DCM ............................................................................................ 14 3.2 Circuit behavior in DCM .............................................................................................. 16 3.3 Duty Cycle range Calculation for DCM ....................................................................... 17 3.4 Duty Cycle equations .................................................................................................... 19 3.5 Dimming in DCM mode ............................................................................................... 21 CHAPTER 4 Analysis and results .......................................................................................... 23 4.1 Electrical efficiency ...................................................................................................... 23 4.2 Inference from results ................................................................................................... 27 4.3 Inductor ripple in DCM ................................................................................................ 27 4.4 Trade-offs in DCM operation ....................................................................................... 30 4.5 Lumens Output for a Design .................................................................................... 32 4.5.1 Lumens Output range of Design ....................................................................... 32 4.6 Cost-effectiveness of DCM driver ................................................................................ 34 CHAPTER 5 DCM for Multiple Channels ............................................................................ 35 vi 5.1 Multiphase Operation.................................................................................................... 35 5.1.1 Operating 3 channels in multiphase ....................................................................... 36 5.1.2 Computational Requirements and Software Changes............................................ 37 CHAPTER 6 Software ............................................................................................................ 39 6.1 Software Control ........................................................................................................... 39 6.1.1 Control loop and PWM signaling .......................................................................... 40 6.1.2 ADC and sensing ................................................................................................... 41 6.2 Software Performance and Profiling............................................................................. 41 6.2.1 Memory footprint ................................................................................................... 41 6.2.2 Execution time for Control loop ............................................................................ 43 CHAPTER 7 Conclusions and Future Work ......................................................................... 44 7.1 Conclusion .................................................................................................................... 44 7.2 Future Work .................................................................................................................. 45 REFERENCES ....................................................................................................................... 46 APPENDIX ............................................................................................................................. 47 Appendix A ......................................................................................................................... 48 vii LIST OF TABLES Table 1. Boost converter output voltage equation .................................................................... 6 Table 2. List of Components for CCM ................................................................................... 10 Table 3. List of components for DCM .................................................................................... 15 Table 4. Efficiency for different K values .............................................................................. 30 Table 5. Lumens Range for Our Design ................................................................................. 33 Table 6. Memory footprint ...................................................................................................... 42 Table 7. Memory footprint of Regions of interest in DCM .................................................... 42 Table 8. Bill of materials for three channel HB-LED Driver( Quantity = 1000) ................... 48 viii LIST OF FIGURES Figure 1. Switch regulators Topologies ................................................................................... 5 Figure 2. Boost Converter Transistor ON ................................................................................. 6 Figure 3. Boost Converter Transistor OFF ............................................................................... 6 Figure 4. Converter Board on RL78 Board for CCM ............................................................... 8 Figure 5 CCM Inductor Current ............................................................................................... 9 Figure 6. CCM Output voltage ............................................................................................... 11 Figure 7. CCM Circuit with 2 MOSFETs ............................................................................... 11 Figure 8. Boost converter board for DCM .............................................................................. 14 Figure 9 Circuit behavior in DCM .......................................................................................... 16 Figure 10. DCM Inductor Waveform ..................................................................................... 17 Figure 11. Range calculation depending upon K .................................................................... 18 Figure 12. M (Vo/Vi) with respect to Duty Cycle for DCM, K=0.1 ...................................... 20 Figure 13. Circuit in DCM with one MOSFET ...................................................................... 21 Figure 14 DCM input and output current ............................................................................... 22 Figure 15 DCM input and output current ............................................................................... 22 Figure 16. CCM Output voltage ............................................................................................. 23 Figure 17. CCM Input current ................................................................................................ 24 Figure 18. CCM Output current .............................................................................................. 24 Figure 19. DCM Output voltage ............................................................................................. 25 Figure 20. DCM Input Current ............................................................................................... 25 Figure 21. DCM Output Current ............................................................................................. 26 Figure 22. Indcutor current variation with respect to Switching Frequency .......................... 28 Figure 23. Inductor value for different Switching frequencies ............................................... 29 Figure 24. Vout and Inductor ripple vs Duty Cycle................................................................ 29 Figure 25. Efficiency for different K values ........................................................................... 31 Figure 26. Development Board with HB-LED as load ........................................................... 33 Figure 27. Cost Analysis ......................................................................................................... 34 Figure 28. Multiphase Circuit in DCM ................................................................................... 35 Figure 29. Overlapping of PWM signals resulting in shoot-up in Overall Load Current ...... 36 Figure 30. Interleaved PWM signals resulting in uniform Overall Load Current .................. 36 Figure 31. 3 channels in DCM ................................................................................................ 37 Figure 32. PWM signals with different duty cycles ............................................................... 38 Figure 33. Software state machine .......................................................................................... 39 Figure 34. Execution time DCM ............................................................................................. 43 Figure 35. Execution time CCM ............................................................................................. 43 1 CHAPTER 1 Introduction 1.1 Significance of the Study Popularity of lighting applications based on LEDs is increasing day by day. Providing high efficiency (Lumens/Watt), superior longevity and low maintenance requirements are strong reasons for the industry to shift towards LEDs. For example, a 50W halogen based car spot light can be replaced by a LED array of 8-12 W. LCD backlighting, automobiles, traffic lights and general-purpose lighting are major areas where LED technology is increasingly finding applications. LED drivers can be broadly classified as linear regulators or switched regulators. Linear regulation is a low cost solution, but suffers from poor operating efficiency as the voltage drop across the linear regulator cannot be minimized under all operating conditions. Switchmode operation has a higher component cost but offers a more efficient solution and a prolonged life of LED operation. LED driver designed in my work is a DC-DC switching regulator studied in two conduction modes i.e. the continuous conduction mode (CCM) and the discontinuous conduction mode (DCM). A closed loop control system is implemented using a microcontroller, which provides PWM signals for control and a sensing environment for over current and over temperature protection. 2 1.2 Motivation The thesis discusses operation of HB-LEDs in the discontinuous inductor current conduction mode of operation and compares the efficiency with earlier designed continuous inductor current conduction mode of operation. Increasing demand for LEDs in a variety of applications mandates a low cost solution of drivers, reducing the cost of operation, thus, benefitting not only the commercial sectors but also the end consumer. An embedded solution involves controlling the system through software. Hence, its impact on the code size, the computational requirements as well as the resources is studied and analyzed for CCM and DCM operation. Using a microcontroller allows multitasking to be achieved with other application software along with digital closed loop control for this application. 1.3 Preview of the Work The thesis builds up on the previous work in which a closed loop control of boost converter is implemented in the continuous conduction mode by Tharunachalam Pindicura [1]. The HB-LED driver operating in DCM is built on a boost converter topology. The closed loop control is implemented in RL-78’s development environment. Efficiency of the new system i.e. the DCM operation is analyzed and compared with the CCM operation in terms of electrical efficiency as well as Lumens per Watt. Dimming is achieved by exploiting the property of an inductor operating in Discontinuous Conduction mode as a replacement to the dimming MOSFET. This further reduces the computational requirements for providing PWM signals to the dimming MOSFET. 3 1.4 Outline for Rest of the document Rest of the document is organized as follows: Chapter 2 discusses the switching converter theory and previously designed HB-LED driver in the continuous conduction mode with its computational requirements. Chapter 3 focusses on the discontinuous conduction mode and the operating range. Chapter 4 analyzes and compares the electrical efficiency for both, CCM and DCM. It also introduces lumens per watt as an efficiency parameter and the requirements for both modes to maintain same lumens output. Chapter 5 covers the multiphase operation of 3 channels in DCM. Chapter 6 covers the software aspect of HB-LED drivers and computational requirements for the CCM and the DCM operations. Chapter 7 contains the conclusions drawn from this work and the prospects for future work. 4 CHAPTER 2 Previous Work 2.1 Switching DC-to-DC converters 2.1.1 Converter Theory Smaller size, light weight and most of all, higher efficiency than its linear counterpart makes switching converters ideal for voltage or current regulation. Switch (MOSFET) also called the boost switch transistor, continually switches between the full ON and the full OFF states. Ratio of the ON time and the time period performs the regulation. This is in contrast to the linear regulators which provide a desired output voltage by dissipating extra power as ohmic losses. Switch-mode power supply utilizes the storage elements (inductor, capacitor) in different electrical configurations (topologies) for voltage and/or current regulation. 2.1.2 Physical Operation and properties of switching converters Three common switching topologies are shown in Figure 1. Reactive storage elements (inductor and capacitor) along with the duty cycle (ON-to-PERIOD ratio) of a PWM signal determines the proportion of the output voltage change with respect to the input voltage. Figure 2 shows a boost converter in the ON state. The ON state means the pass transistor is currently switched ON and the current flows through the inductor while storing energy in the inductor. 5 Inductor Diode V Load MOSFET Cap Boost Converter Inductor MOSFET Load Diode V Cap Buck Converter Diode MOSFET V Load Inductor Buck-Boost Converter Figure 1. Switch regulators Topologies Cap 6 During the OFF state, the switch is open and the inductor will resist any change in the flow of current and act as a current source in series with the supply, thus charging the capacitor with a higher voltage as shown in Figure 3. Figure 2. Boost Converter Transistor ON Inductor Diode Load V Cap Figure 3. Boost Converter Transistor OFF Table 1. Boost converter output voltage equation Output Buck Vo D*Vin Boost (1/1-D)*Vin Buck-Boost (D/1-D)*Vin 7 2.1.3 Control via Software A significant feature of a switching converter is the degree of control achieved over the output voltage depending on the duty ratio (D) of the PWM signal. This factor being controlled through software allows direct control and helps a programmer in implementing other functionalities using software, like a closed loop control by sensing the currents and the voltages. A pre-existing system with a microprocessor can be utilized to achieve software control of switching converters. 2.2 Closed loop Control of Boost Converter in CCM There is a significant amount of work done by Tharunachalam Pindicura in his thesis [1] on the development of a closed loop control of a boost converter operating in CCM. 2.2.1 Continuous Conduction Mode of Boost converter A boost converter board was designed by Tharunachalam Pindicura and Mihir Shah on a separate daughter board as an extension to the RL78 board for easier control. Figure 4 shows the RL-78 and the converter board connected for driving a HB-LED in CCM. 8 Figure 4. Converter Board on RL78 Board for CCM The converter operates in a particular mode depending primarily on the switching frequency for the boost switch (MOSFET) and the inductor value. For a particular range of these two parameters, if the inductor current never drops to zero, the converter is defined to be operating in the continuous conduction mode (CCM). Figure 5 below shows the inductor current corresponding to the switching frequency in CCM. 9 Figure 5 CCM Inductor Current Table 2 lists the component values used for the boost converter board. An inductor value is chosen to keep the inductor current ripples to a minimum. An output capacitor of 47uF is connected across the load to smoothen out the output current and at the same time have a fast output voltage response time. 10 Table 2. List of Components for CCM Component Value/Type Inductor 100uH / Ferrite core Capacitor 47uF / Electrolytic Resistor 1 ohm MCU RL78G13 / 16 bit Transistor ( Boost) NMOS Transistor ( Dimming) NMOS The continuous conduction mode uses 2 MOSFETs in the boost converter circuit; one for the boost switching frequency which is kept to 62.5 KHz, whereas the second MOSFET is used as a dimming transistor. The average current through the load depends on the ON-toPERIOD ratio of this MOSFET. Figure 6 shows the current through a string of LEDs with the dimming transistor switching at regular intervals. 11 Figure 6. CCM Output voltage 2.2.2 Processing requirements and Closed loop Control Software control and PWM signaling is achieved through the code running on the RL-78 board. 100uH Diode Vin PWM HB-LED Vout Boost MOS 47uF PWM Dimming MOS 1 ohm 1 ohm1 ADC_IN (Over Current Protection) ADC_IN (Output Current) Figure 7. CCM Circuit with 2 MOSFETs 12 Software Design: An interrupt driven system is coded in C language which runs tasks of fixed priority using a run-to-completion scheduler. The HB-LED operation and its control require the following tasks: 1. Initialization: switching frequency, control loop frequency, dimming parameters, overcurrent constants and over temperature constants. 2. Setting and control of the duty cycle in a control loop. 3. Dimming: achieved through a potentiometer. 4. Over current protection. 5. Over temperature protection. 6. In multiple channel operations, out-of-phase operation of loads is done so as to have uniformity in the total current drawn. Figure 7 shows one of the three channels on the board. Computational requirements: 1. Timer interrupt is used for the control loop implementation which runs after every 1 mili second. 2. PWM for the boost converter is provided through a combination of two timers operating in the master-slave mode. Master timer operates on a switching period. Slave provides the duty cycle timing for the boost converter. 13 3. 3 timer channels are used in the slave mode for three duty cycles corresponding to the switching frequency generated by master timer. 4. 2 ADC channels are used per ‘Boost Channel’ for measuring the inductor current for overcurrent protection and the output current for calculating the error and controlling the boost duty cycle in limits. 14 CHAPTER 3 Discontinuous mode of Operation 3.1 Design to operate in DCM The term “Discontinuous Conduction Mode” corresponds to the discontinuity in the inductor current over a period. In other words, the inductor current falls to zero before the cycle ends and rises again in the next positive cycle, which is in contrast to the continuous conduction mode where the inductor current never reaches a zero value. Figure 8 shows the converter board with the dimming MOSFET removed, working on the DCM principle. Dimming MOSFET removed . Figure 8. Boost converter board for DCM 15 DCM occurs under two conditions: 1. Light load in the circuit which does not require much current for its operation. 2. Deliberately putting the circuit under DCM condition by selecting the parameters (inductor value and switching frequency) accordingly. Section 3.2 elaborates on how changing the parameters transition the boost converter from CCM to DCM. Our research pertains to the 2nd option where we operate the circuit in DCM and the component values used for driving HB-LED are given in Table 3: Table 3. List of components for DCM Component Value/Type Inductor 100uH / Ferrite core Capacitor 47uF / Electrolytic Resistor 1 ohm MCU RL78G13 / 16 bit Transistor ( Boost) NMOS The component values used for DCM are same as in CCM except one fewer MOSFET, which is used for dimming in CCM. Keeping the components same provides a smooth transition from CCM to DCM and vice versa. A high inductor value allows the inductor to handle large current ripple [2] and a low switching frequency decreases the switching losses, which will be explored in Chapter 4. 16 3.2 Circuit behavior in DCM The switching period for DCM can be divided into 3 sub intervals: 1. 0 < t < D1Ts 100uH HB-LED Vout 47uF Vin 1 ohm 2. D1Ts < t < (D1+D2)Ts 100uH HB-LED Vout 47uF Vin 1 ohm 3. (D1+D2)Ts < t < Ts 100uH HB-LED Vout 47uF Vin 1 ohm Figure 9 Circuit behavior in DCM 17 Figure 10 shows the inductor current waveform for a HB-LED driver, where the transistor is switched at a frequency of 5 KHz. D1 D2 Ts Figure 10. DCM Inductor Waveform 3.3 Duty Cycle range Calculation for DCM We will introduce an equation that describes the boundary of DCM and CCM: Vin DTsVin > 2 2L D' R Or For CCM 2L > DD' 2 (1) RTs D’ = 1 – D1 Left hand side of equation (1) is called the K value and its right hand side is the Kcrit. 18 State of the system can be described by looking at (2) and (3) below: K > Kcrit K < Kcrit for CCM for DCM (2) (3) For our Design, L = 100uH, Ts = 200us & R (load) = 25 ohms; K = 0.1, where Kcrit(max value) = 0.141 (4/27). Figure 11 shows the range of Duty Cycle achieved in our design where it operates in DCM. 0.16 0.14 X: 0.3 Y: 0.147 0.12 59% 14% 0.1 0.08 0.06 <-------------DCM-------------> 0.04 0.02 <-CCM 0 0 0.1 CCM-------------> 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Figure 11. Range calculation depending upon K 1 19 3.4 Duty Cycle equations For a DCM operation, the variation of the output voltage (Vo) with respect to the input voltage (Vi) can be represented as: Vo 1 + 1 + 4 D1 2 / K =M = Vi 2 where, D1 is the time for which the boost switch stays ON, and K = 2L/RTs (0.1 in our system) This equation is only valid for K < Kcrit (D), i.e. DCM operation (Kcrit = .141) Thus, our system operates in DCM for K = 0.1. Equations below [6] are useful for providing the duty cycle values and describing the state of the system. 1 1− D For K > Kcrit (CCM) 1 + 1 + 4D 2 / K 2 For K < Kcrit (DCM) Vo = Vi 20 Vo-to-Vi ratio for Duty Cycle variation 5 4.5 4 3.5 Vo/Vi 3 2.5 2 1.5 1 0.5 0 0 0.1 0.2 0.3 0.4 0.5 0.6 Duty Cycle 0.7 0.8 0.9 1 Figure 12. M (Vo/Vi) with respect to Duty Cycle for DCM, K=0.1 Linearity in voltage regulation is achieved for a K value of 0.1. We lose on the linearity of the control, as well as the efficiency, on decreasing the K value. This design also serves as a reference to designers for selecting the parameters according to the output efficiency requirements and the range of DCM operation. 21 3.5 Dimming in DCM mode 100uH Diode Vout Boost MOS Vin HB-LED 47uF PWM 1 ohm1 ADC_IN (Over Current Protection) 1 ohm ADC_IN (Output Current) Figure 13. Circuit in DCM with one MOSFET Controlling the HB-LED dimming in DCM operation relies on the property of the inductor instead of using a dimming MOSFET in the continuous conduction mode. Varying the duration of the third subinterval (Section 3.2), (by changing the duty cycle using the potentiometer knob) has an effect on the output current, resulting in dimming of the LED. The system was tested in the range of duty cycles over which the boost converter operates in DCM, i.e. range for which the K value is 0.1. Figure 14 and Figure 15 shows the variation of output current as the duty cycle is increased from 10% to 40%. 22 Figure 14. DCM input and output current LHS in figure 14 shows Input (Inductor) Current with respect to Switching Frequency. RHS shows Output Current with respect to Switching Frequency. Duty Cycle is 10 percent. Figure 15. DCM input and output current LHS in figure 15 shows the Input (Inductor) Current with respect to the Switching Frequency. RHS shows the Output Current with respect to the Switching Frequency. Duty Cycle is 40 percent. 23 CHAPTER 4 Analysis and results 4.1 Electrical efficiency For our design with the K value at 0.1, the duty cycle range is between 14% and 58%. This leads to an operating output voltage between 5.8 volts and 12.1 volts. The method to compute and compare the power efficiency of DCM with CCM relies on keeping the output power as a constant parameter and observing the input power consumption. The following input and output current waveforms were observed while maintaining 1786 mili watts as the output power of the converter. Output power is fixed for comparison of CCM and DCM operations. Figure 16. CCM Output voltage 24 Figure 17. CCM Input current Figure 18. CCM Output current 25 Figure 19. DCM Output voltage Figure 20. DCM Input Current 26 Figure 21. DCM Output Current Applying standard formula for electrical efficiency, η = Vout * I out Vin * I in Plugging the observed r.m.s values of the parameters in the above equation: η CCM = 10.2 * 176 * 10 −3 = 72.5 % 5 * 495 * 10 −3 η DCM = 9.4 * 195 * 10 −3 5 * 440 * 10 −3 = 83.3 % 27 4.2 Inference from results DCM gives a higher efficiency than CCM for the HB-LED string, as a load. The duty cycle for DCM in this experiment is 37.5%, whereas for CCM it is 58%. In other words, switch is conducting (ON) for a shorter duration in DCM than in CCM. Secondly, lower switching frequency in case of DCM results in smaller switching losses with respect to CCM. Thirdly, the DCM driver has lower DC losses [3], whereas CCM suffers from both AC and DC losses. Losses in the switch, the inductor, the diode and the capacitor are not modeled in this study. 4.3 Inductor ripple in DCM Inductor current (peak-to-peak) directly depends on the inductor value as well as the switching frequency. In our design, the inductor value is kept at 100uH so that the current saturation limit for this inductor can sustain high ripples that are observed in DCM. Another reason for choosing an inductor value of 100uH, over 10uH or 20uH, is the availability of 100uH value with a high saturation limit at low cost. This is because the cost of an inductor goes up with an increasing current saturation limit. Figure 22 depicts the change in inductor ripple as the switching frequency is increased. 28 Indcutor current vs Switching Frequency Inductor Current( mili Amperes) 4500 4000 3500 3000 2500 2000 1500 1000 500 0 5 12.5 40 Switching frequency (KHz) 62.5 Figure 22. Indcutor current variation with respect to Switching Frequency At higher frequencies, i.e. 40 KHz and 62.5 KHz, the system transitions into CCM and the current no longer falls to zero. In order to operate the system at high switching frequencies and to maintain discontinuous mode of conduction i.e. K < Kcrit, designers have to opt for a lower inductor value as shown in Figure 23. Low value inductors with higher saturation limit are hard to find, and may be costly. 29 Inductor value (uH) Inductor value vs Switching frequency ( Constant Ripple) 300 250 200 150 100 50 0 5 12.5 40 Switching frequency (KHz) 62.5 Figure 23. Inductor value for different Switching frequencies The effect on inductor ripple is not only observed with changing switching frequency, but with changing duty cycle as well. Figure 24 displays how the ripple increases with increasing duty cycle. 2500 15 2000 10 1500 1000 5 500 0 0 20 37.5 50 59 Duty Cycle (%) with fsw = 12.5 KHz Indcutor ripple (mA) Output Voltage (Volts) Vout and Inductor ripple vs Duty Cycle (DCM) Figure 24. Vout and Inductor ripple vs Duty Cycle Output Voltage Inductor ripple 30 4.4 Trade-offs in DCM operation Another design consideration for the LED drivers or in fact for any load is to consider the impact of switching frequency on the efficiency of the system. Once a system is designed and taken in for production, the only variable parameters are the switching frequency and the duty cycle, owing to their software control. It is observed that as the switching frequency is decreased, in other words if the K value decreases, efficiency decreases. Designers have to study the trade-offs between the duty cycle range and the efficiency requirements for their load. This is because a lower K value provides a broader range of DCM operation but with lower efficiency. Table 4 provides the efficiency values associated with different K values. In our design, we cannot go below a K value of 0.04 due to high current ripples, at very low duty cycles, through the inductor, which has a saturation point at 1.2 amperes. Table 4. Efficiency for different K values K (2L/RTs ) Switching Output Frequenc Voltag e y (KHz) (V) 0.04 5 Output Source Curren Curren t t (mA) (mA) Input Voltag e (V) 9.8 170 725 5.1 9.71 164 500 5.1 0.1 12.5 9.57 141 320 5.1 0.14 17.2 8.9 120 245 5.1 0.06 7.4 Output Power (mW) Sourc e Power (mW) 3697. 1666 5 1592.4 4 2550 1349.3 7 1632 1249. 1068 5 Efficienc y (%) 45.057% 62.449% 82.682% 85.474% 31 Efficiency Efficiency vs K 90.000% 80.000% 70.000% 60.000% 50.000% 40.000% 30.000% 20.000% 10.000% 0.000% fsw = 17.2 kHz fsw = 12.5 kHz fsw = 7.4 kHz fsw = 5 kHz 0.04 0.06 0.1 0.14 K Figure 25. Efficiency for different K values Inducotr cost ($) Inductor cost vs Value ( same saturation current) 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 20uH 30uH Inductor value 100uH Figure 26. For same saturation limit, cost of inductor is higher for lower inductor value. 32 4.5 Lumens Output for a Design A human perceivable metric for a light source is lumens rather than electrical power [4]. One of the primary reasons for industries in adopting LEDs as a light source is their high luminous intensity or the luminous flux output with lower power consumption as compared to other sources of light. Output electrical wattage can be related to luminous flux as: Luminous flux = Output Wattage * Luminous Efficacy In our design, the load is a HB-LED strip which has a luminous efficacy of 31.25 lumens per watt [8]. Thus, the reason for keeping the output power as a constant parameter in Section 4.1 is to have the same luminous flux. In other words, accurately comparison, of the efficiencies of the two systems, requires keeping the same amount of lumens perceived by a human eye (luminous flux) and observing the input power requirement i.e. input voltage and input current. 4.5.1 Lumens Output range of Design The range of load handling capability of our design is a useful parameter, because in a system the hardware has to be changed when operating beyond a certain load value. Reasons for changing the hardware are primarily the current saturation limit of the inductors. Secondly, beyond this range, the system either transitions to CCM or gives very low efficiency. Table 5 gives the maximum and the minimum power output for our design operating in DCM, with only software configurable parameters (the switching frequency and the duty cycle). 33 Table 5. Lumens Range for Our Design Load Range Minimum load Peak load Output Voltage V Output Current mA Output Power mW Inductor ripple(rms) mA K Duty Cycle (%) 8.6 56 480 113 0.04 8% 10.6 400 4240 766 0.1 55% Luminous Flux (lm) 15 132.5 Figure 27. Development Board with HB-LED as load 34 4.6 Cost-effectiveness of DCM driver It has been shown that DCM results in a higher efficiency than CCM for driving high brightness LEDs. DCM is also a more cost-effective solution in driving HB-LED loads due to the reduction of one transistor, which takes a major portion of the cost of switching DCDC regulators. Compared to CCM, DCM is a better choice for controlling HB-LEDs as it Cost ( $) leads to higher efficiency and a 16% cost reduction. 3.25 3 2.75 2.5 2.25 2 1.75 1.5 1.25 1 0.75 0.5 0.25 0 RL78 MOS zener diode diode resistance 10k ohm resistance 1 ohm Cap inductor DCM CCM Mode of operation Figure 28. Cost Analysis 35 CHAPTER 5 DCM for Multiple Channels 5.1 Multiphase Operation 100uH Diode Vout HB-LED Boost MOS 47uF PWM 1 ohm1 1 ohm 100uH1 Diode1 Vout1 HB-LED1 Boost MOS1 47uF1 PWM1 1 ohm3 1 ohm2 100uH2 Diode2 Vout2 Boost MOS2 Vin2 HB-LED2 47uF2 PWM2 1 ohm5 1 ohm4 Figure 29. Multiphase Circuit in DCM 36 5.1.1 Operating 3 channels in multiphase Efficiency of the system can be increased in a three channel multiphase operation through to the following factors: 1. Using the same microprocessor for the computation of PWM signal and sensing. 2. Most of the modern day processors have high number of ADC and timer channels, 3 channels can be easily utilized. 3. Duty cycle factors for all the three channels are out of phase with each other in one cycle, thus avoiding overshoot in input current requirements. Figure 30. Overlapping of PWM signals resulting in shoot-up in overall load current Figure 31. Interleaved PWM signals resulting in uniform Overall Load Current 37 5.1.2 Computational Requirements and Software Changes 1. 2 timers per channel (master-slave combination) are used for PWM generation. 2. 2 ADC ports per channel are used for inductor current and output current sensing. 3. As this is an interrupt driven system, interrupt for 1st channel PWM negative edge triggers the 2nd channel PWM, and the 2nd channel PWM negative edge triggers the PWM signal for the 3rd channel. 4. Timer interrupts for the PWM are disabled once all the 3 channels are triggered. Interrupts are enabled after every 1 second and step 3 is repeated again. This is done to ensure minimum overlapping of PWM signals of the 3 channels, which in turn reduces overlapping of the input current requirements. Channel 1 Channel 3 Channel 2 Figure 32. 3 channels in DCM 38 Figure 33. PWM signals with different duty cycles Input Current Wave with 3 peaks corresponding to 3 PWM signals 39 CHAPTER 6 Software 6.1 Software Control An interrupt driven system for the controlling of HB-LEDs was designed by Tharunachalam Pindicura, to work in CCM. Software has been modified to work for the DCM operation. StartUp Entry HW Init Switching and Control loop Frequency init Timer Interrupt (1ms) Display Output Power Lumens Output PWM Interrupt (80us) Idle Waiting for Interrupt If PWM Interrupts Enabled Control loop ISR ISR for fsw Duty Cycle Control Sensing Inductor & Output Current Over Current Protection Enables 2nd/3rd Channel Disable PWM Interrupt PWM Interrupts enabled after 1 second to avoid Current Shoot due to Duty Cycle Overlap Figure 34. Software state machine 40 6.1.1 Control loop and PWM signaling The following actions are performed under a control loop ISR, occurring at 1 mili second: 1. Check for the over current through the inductor. 2. Sense the output current through the LED. 3. Potentiometer is used to control the duty cycle. This differs from the earlier CCM control technique, where the potentiometer was used for controlling the dimming transistor’s ON time. 4. Calculate the power output and the lumens output. 5. Perform a close loop control through PID correction and adjustment of the switching frequency based on the error. Same PID control is used as developed by Tharunachalam Pindicura for the CCM control. 6. Enable the PWM timer interrupts for all three channels after 1 second. This is done to autocorrect the overlapping of duty cycles of 3 PWM signals. This step is performed if the sum of ON periods of all three channels is smaller than the switching period. 7. PWM signaling is provided through the use of 3 Master-Slave Channels, i.e. total of 6 timer channels. ON periods for the three PWM signals are interleaved in one switching period to avoid large source current requirements. Interrupts from 3 the slave channels are used to achieve interleaving. 41 6.1.2 ADC and sensing ADC channels [7] on the RL-78 are used for: 1. Output current sensing required for closed loop control. Output current is also used for output power calculations. 2. Over current protection: Current going through the inductor is sensed in the control loop to check for over current. Polling mode is employed for ADC sensing rather than an interrupt mode. It is done to avoid nesting of interrupts, as the control loop runs as a timer interrupt service routine. For operating 3 channels, we have used 6 ADC channels in total. 6.2 Software Performance and Profiling Major changes in the code for DCM are attributed to the operation of 3 channels using interleaved PWM signals. Memory footprint of the code and the speed of control loop execution have been analyzed. 6.2.1 Memory footprint Code is profiled using a code developed by Dr. Alexander Dean. Regions along with their sizes are identified. Code is profiled for low and high optimization levels. Table 6 shows the result of profiling for both CCM and DCM code. 42 Table 6. Memory footprint Code Optimization level CODE DATA CONST Memory(bytes) Memory(bytes) memory(bytes) DCM Low 16,579 1,127 7,793 DCM High 14,350 1,127 7,775 CCM Low 15,908 2,632 8,167 CCM High 13,871 2,632 8,149 Table 7 lists the regions of interest in our software. 20% less memory footprint is observed for the control loop with high optimization level. Number of regions for CCM code is fewer than that for DCM. This is mainly because of extra PWM channels required in the DCM code. Table 7. Memory footprint of Regions of interest in DCM DCM Code with High Optimization level Size Region (bytes) 28 Init_control_loop 17 Init_dimming 234 Main 31 R_TAU0_Channel0_Start 20 R_TAU0_Channel2_Start 20 R_TAU0_Channel4_Start 12 R_TAU0_Channel7_Start 38 CB_Channel7_Interrupt 423 Control_loop DCM Code with Low Optimization level Size Region (bytes) 43 Init_control_loop 18 Init_dimming 276 Main 25 R_TAU0_Channel0_Start 24 R_TAU0_Channel2_Start 24 R_TAU0_Channel4_Start 12 R_TAU0_Channel7_Start 42 CB_Channel7_Interrupt 671 Control_loop 43 6.2.2 Execution time for the Control loop The control loop is implemented in a timer’s ISR and the timing information (below) compares execution time of the control loop in DCM and CCM (under high optimization level). DCM takes 41% less time to execute than CCM, due to fewer computations for dimming. Figure 35. Execution time DCM - 580 microseconds Figure 36. Execution time CCM - 994 microseconds 44 CHAPTER 7 Conclusions and Future Work 7.1 Conclusion 1. Electrical efficiency for the same lumen output is proved to be higher for DCM than CCM. This is attributed to the low source current required by inductor in the discontinuous conduction mode. Also, DCM operates at low switching frequency as compared to CCM which further contributes to the overall efficiency. 2. Cost effective solution is developed by using DCM instead of CCM because of reduction of 1 MOSFET in the circuit, which contributes significantly to the cost. Getting higher efficiency at low cost makes this solution more suitable for lighter loads. While operating the circuit in DCM, extra caution has to be taken to keep the duty cycle within range depending on the K value for the circuit. 3. Keeping switching frequency at 12.5 KHz rather than 5 KHz further decreases the ripple peak thus, keeping the inductor cost down. This is done keeping in mind that switching frequency satisfies the DCM condition i.e. K < Kcrit. 4. Although profiling the software proves that there is no significant change in code size while operating the circuit under CCM or DCM, DCM does provide a speedup in terms of the control loop execution time. This implies additional scope for multitasking within this design. Optimizations for code size and speed results in smaller memory footprint and faster processing of control loop. Reducing 45 memory footprint brings down the cost of the whole system as memories account for a big proportion of cost in a microcontroller. 7.2 Future Work The boost converter daughter board comprised of a ferrite core inductor and an electrolytic capacitor for our analysis. Circuit needs to be analyzed for efficiency with other options like air core inductors and thin film or ceramic capacitors [5]. Currently, the software works as a standalone application for providing closed loop control for a boost converter. Considering the small memory footprint and scope of multitasking in DCM, the behavior of this code in multitasking application needs to be studied. Also, as LEDs are increasingly being used in the screens these days, it will be interesting to study the difference in requirements of LED drivers required for screens, and development of DCM based drivers for the same. 46 REFERENCES [1] Pindicura, Tharunachalam, Analysis of microcontroller based High Brightness LED drivers. [2] Brigitte Hauke, Basic Calculation of a Boost Converter's Power Stage, Texas Instruments [3] Travis Eichhorn, Boost Converter Efficiency through Accurate Calculations, National Semiconductors, Powerelectronics.com [4] Evaluating Light Output, Philips; http://www.colorkinetics.com/support/whitepapers/Evaluating_Light_Output.pdf [5] J. M. Alonso, D. Gacio, J. García, M. Rico-Secades Universidad de Oviedo, “Analysis and Design of the Integrated Double Buck-Boost Converter Operating in Full DCM for LED Lighting Applications”. [6] W. Erickson Robert, Dragan Maksimovic, “Fundamentals of Power Electronics” [7] Rl-78 Hardware User Manual, Renesas [8] LED strip specifications; http://www.ledii.com/pdf/5050smd%2030led%20ribbon%20series.pdf 47 APPENDIX 48 Appendix A A.1 Bill of materials Table 8. Bill of materials for three channel HB-LED Driver (Quantity = 1000) Component Inductor (100uH) Capacitor (47uF) Resistor ( 1 ohm) Vendor Digikey Digikey Digikey Resistor (10k ohm) Diode Digikey Digikey Zener Diode (3.3 V) Transistor MCU (RL78G13) Digikey Digikey Digikey Part Number 595-1352-2-ND 565-2568-2-ND 541-10.0AFDKR-ND RMCF0805FT10K0CTND 568-6530-2-ND MMSZ4684-TPMSCTND 2SK2315TYTR-ND R5F101JEAFA#V0-ND unit Price( $) 0.2635 0.2904 0.075 0.00378 0.155 0.03 0.465 1.3