Download ABSTRACT TANEJA, ROHIT. Making Energy

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