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LED Lighting
Muzahid Huda
Bay Chips, Inc.
October 17, 2009
LED Lighting
• LEDs; Some basics
• Recent developments – more watts, more
lumens
• Market projections
• Driving LEDs
• Example LED Driver circuits
• Summary
What is an LED?
• Light Emitting Diodes or LEDs can
come in many different shapes
and packages. But one thing
each of these has in common is
that it is a Semiconductor based
material that Emits Light
Courtesy: OSRAM
LED Characteristics
4
• “Pure” light source
– LEDs emit a narrow spectrum light when forward biased
• Why all the excitement?
–
–
–
–
–
–
Resistant to shock and vibration
Saturated colors – don’t need filters
Small size enables new applications and designs
Fully dimmable without color variations (PWM)
Cold start capable (-40°C) – For outdoor, industrial, automotive
Fast response (< 40ns)
– Long Life
Wide Range of Applications
We focus on “High Brightness” LEDs (HB LEDs) for Solid State Lighting
A
c
t
u
Energy Consumption
US Residential Lighting
• Lighting accounts for 3rd largest (11%) electrical
energy consumption
• Annual US consumption for illumination was 138
Terawatt hours in 2007
US Home Electricity Usage
Space Heating
18%
10% lighting efficiency
improvement can cut
annual CO2 emissions by
477 Metric Tons
24%
Water Heating
Lighting
6%
Air Conditioning
Refrigeration
8%
13%
7%
13%
11%
TV and PC
Washer/Dryer
Other
LED Application
Video Walls
US DOE Energy Savings Estimates
LEDs Magazine Oct ‘08
• Switch to LEDs could save $22B in electric costs
– Colored-light applications:
• Traffic signals and pedestrian crossings; decorative lights; exit
signs; signage
– Indoor applications:
• Recessed downlights; refrigerated display cases; retail displays;
task lights; office undershelf lights; under-cabinet lights
– Outdoor white-light applications:
• Street and area lights; step, path, and porch lights
Styling advantage - Arrays of LEDs
Cadillac DTS
Lincoln
Multiple LEDs arranged in arrays is one way to achieve a
legal beam pattern.
Courtesy: OSRAM
Recent Developments
Recent Developments
• Higher power levels
– Useful for most lighting applications
– 6000 lumens @ 60W available
• Improved luminous efficacy (“Efficacy”)
– >>100 lumens/Watt exist
– Commercial efficacies routinely exceed 80 lm/W
Luminous Efficacy
vs. LED Application
200 lm/W
Mainstream
General
Illumination
Automotive
Headlamps
LED Flash
Lights
Street
Lighting
LCD TV
Backlighting
100 lm/W
Automotive
Interior
Lighting
70 lm/W
Signage
50 lm/W
Color LCDs in
Handsets
Notebook
LCD
Backlighting
Camera
Flash
Handset
Keypads
Monochrome
LCDs in Handsets
20 lm/W
Red Tail Lights
Traffic Lights
1990s
2000
2002
2004
2006
2008
2010
LED Development Trends
• Japanese Philosophy:
– Not the increase in power handling capacity per die per se, but
the efficacy that is more important to improve
• US Philosophy:
– Focused more on increasing power handling capacity
– Highest wattage LEDs available from Luminus, Inc.
Up to 6000 lumens @ ~100 lm/W
Courtesy: Luminus
Courtesy: Toshiba May 2008
Market Projections
LED Driver IC Market Size
LEDs Magazine Oct ‘08
• HB LED driver IC market to exceed $1.9 billion in
2011
– Growth driven by general illumination, signs and displays, and
automotive applications
– 2007 -2011 Compound Annual Growth Rate (CAGR) of 38%
• The $865 million market for driver ICs in 2006 was
dominated by mobile phone applications
– Mobile phone share will drop from 77% in 2006 to 50% in 2011
– LED driver ICs for laptop display backlight applications will see
revenue increase
LED Driver IC Market Size
HB LED applications
900
Personal C omputing
LC D TV
General Illumination
Traffic Lights
Signage
Automotive
C onsumer Portables
Mobile Handsets
800
Millions of Dollars
700
600
500
400
300
200
100
0
2006
2007
2008
2009
2010
2011
Applications
Issues and Challenges - LEDs
• Energy Efficiency
– Needed to “measure up” to current fluorescent technology
• Photometry & Color
– Different applications need different light distributions & color
temperatures
• Thermal Management
– Light output & lumen degradation
– Reliability issues due to heat
• Lumen Maintenance & Life
– Lumens depreciate over time
– Life test parameters to specify lifetime
Applications
Issues and Challenges - Electronics
• Energy Efficiency
– Drivers that operate at optimum efficiency
– Efficiency measurement techniques
• Performance and Functionality
–
–
–
–
Size and cost
Conducted and radiated EMI
Harsh environments eg. Surge, lightning, load dump, cold start
String compatibility
• Thermal Management
– Wide temperature range and high humidity
• Reliability & Life
– Lifetime of passive components: capacitors, magnetics
Light Source Comparison
LED vs. selected other technologies
Incandescent
Halogen
HID
LED
20 – 50
(Efficiency)
Efficacy
(lm/W)
7 - 20
15 - 30
50 - 100
Light
Output
(Lumens)
500
1900
3200
25 - 600
Life time
(hrs)
400 – 10000
200-1500
1000 - 2500
>50,000
CCT (K)
2500 - 3000
3000-3300
4150
2700 - 10000
Place Photo Here
Forward Lighting
– 2007 saw the launch of the first LED
headlamps.
– LEDs match the color temperature of
HID as well as create stylistic brand
recognition.
Courtesy: OSRAM
Life of Vehicle Performance
Courtesy: OSRAM
Driving LEDs
Buck Mode LED Driver
MR-16 Lamp
Typical input voltage is 12V rms (AC)
Typical output power is 1W to 3W
Practical Considerations
LED - Binning
•
•
LED Binning
What is Binning? / Why is it required?
•
Upon completion of assembly, LED’s are measured for
brightness, color, and forward voltage
•
They are placed into “bins” according to each
characteristic
Types of Binning
•
Intensity Binning
•
Color Binning
•
Forward Voltage Binning
The human eye can
detect a brightness
difference when the
intensity ratio is > 2:1
Intensity Binning
Luminous Intensity Binning
Example
Intensity range in production at rated current:
Lot1
Lot2
Lot3
480 mcd
1090 mcd
Bin1
450 mcd
Bin2
560 mcd
Bin3
710 mcd
Bin4
900 mcd
Conceptual description; Actual binning limits may vary
1120 mcd
LED Drivers
General Considerations
• LEDs are current driven devices
• LED Drivers must deliver a constant current to the
LED or LED string(s)
• Power supply voltage may fluctuate
– The driver must regulate its output current even if the input
fluctuates
LED Configurations
Can be constant current sources
1
3
2
1
1
2) Parallel(1)
1) Series
3) Parallel(2)
1
Voltage variations
3
Current stress when one or more strings fail
2
Vf variations; Current hogging
LED Configurations
1
2
3
1
1
2) Parallel(1)
1) Series
3) Parallel(2)
1
No current variations due to voltage
3
Current stress when one or more strings fail
2
Vf variations; Current hogging
Resistor Drive
Power loss
Io
Changing brightness
Vi
LED Current Io
Varying current
0.5
0.4
Vf=3.0V
0.3
Vf=3.4V
0.2
Vf=4.0V
0.1
Vf
0
9
Impossible to
automatically adjust
for binning
12
16
Resistor Power
Dissipation
Input Voltage Vi
8
6
4
Ploss @ Vf =
3.0V
2
Ploss @Vf =
3.4V
0
9
12
Input Voltage Vi
16
Ploss @ Vf =
4.0V
LED Drivers
Common Topologies
• Choice of topology depends on many factors
–
–
–
–
–
Input and output voltage range
Efficiency
LED Driver
Cost
Flexibility of use
Reliability
Switch Mode
Linear
DC/DC
•Buck
•Boost
•Buck-Boost
•SEPIC
•Flyback
•Forward
AC/DC
•Flyback
•Forward
•Resonant
•Buck
*Often need PFC
DC/DC LED Drivers
• Drive LEDs from DC power source
– AC/DC conversion usually occurs independently upstream
• “Simple” to implement
– LEDs are DC devices (Unidirectional current)
• Two Types
– Linear Drivers
– Switch Mode DC/DC Drivers
Linear LED Driver
• Simple
– Low parts count
PD = I_LED X (V_IN – V_OUT)
• Constant LED Current
• Output voltage is lower than
input voltage
V_OUT
V_IN
• Not very efficient
– V_OUT must be close to V_IN to
maximize efficiency
• Needs good heat management
– Pass transistor dissipates heat
I_LED
DIM
TMP Error Amp
+
EN
V_REF
I_s
+
V_FDBK
-
Constant Current Drive
Linear Current Source
Power loss
Linear Current Source; LED Current
Regulation
Io
Constant brightness
Vi
Vf
LED Current Io
Constant current
0.4
0.3
0.2
0.1
0
9
12
16
Input Voltage Vi
Can integrate
adjustments for
binning
Linear Current Source Dissipation
Linear Current
Source
Dissipation
Linear Control
Vf = 3.0V
Vi = 3.4V
Vf = 4.0V
6
4
Vf = 3.0V
2
0
Vf = 3.4V
9
12
Input VoltageVi
16
Vf = 4.0V
Switch Mode DC/DC Driver
• Most use inductors for energy storage and delivery
– Switch Capacitor types use capacitors for energy storage
• Energy (Current) ramps up in inductor during switch
“ON” period
– Energy (Current) delivered to LED string(s) during switch “OFF”
period
• Switching frequency is usually >20kHz
– Avoids audible noise
– Commonly in the 100’s of kHz range
Switch-mode Driver Topologies
Vi
VLED/Vi = 1/(1-d)
VLED/Vi = d
Vi
Is = ILED
Is2 = ILED
Is1
Buck
Boost
• VLED > Vi
• No output short protection
• Non-isolated
• Two current sense
resistors (two control loops)
• VLED > Vi, = Vi, or < Vi
• No output short protection
• Non-isolated
• Two current sense
resistors (two control loops)
VLED/Vi = d/(1-d)
Vi
• VLED < Vi; Vi can vary
• No output cap needed in
CCM operation
• Simple, low cost
• Synchronous for high
efficiency
Is2 = ILED
Is1
Buck Boost
d = Duty cycle
Constant Current Drive
Switch-mode Current Source
Power loss (Small)
Switch-mode Source Power Dissipation
Io
Constant brightness
Vi
Vf
Can integrate
adjustments for
binning
Switch-mode
Control
Current Source
Dissipation
Constant current
0.15
Vf = 3.0V
0.1
Vf = 3.4V
0.05
Vf = 4.0V
0
9
12
Input Voltage Vi
16
Topologies vs. Applications
Guidelines
DESIGN BASE BY POWER RANGE
AC or DC IN
Architecture
Power Range
Applications
0.5W - 2W
3W - 6W
8W - 12W
15W - 20W
25W - 40W
Buck
50W - 65W
Luminaires, LED Bulbs, Down
Light, Linear Lamps, Bar
Lamps, Decorative
8W - 12W
15W - 20W
25W - 40W
Boost
50W - 65W
LED Bulbs, FL Replacement,
Down Light, Linear Lamps,
Bar Lamps
8W - 12W
15W - 20W
Buck-Boost
25W - 40W
Garage lighting, Emergency,
Automotive
8W - 12W
15W - 20W
DC IN
Cuk
25W - 40W
Garage lighting, Emergency,
Automotive
25W - 40W
Forward, Flyback
50W - 65W
Panels, Down Light, Linear, Bar
50W -100W
100W – 250W
AC or DC IN
Resonant
250W – 400W
Street lamp, Flood Light, Search
Light, Marine Lihts
Example LED Driver Circuits
Buck-Boost LED Driver (DC/DC)
Example
Versatile Topology:
For fixed input voltage, automatically
adjusts output voltage to accommodate
long or short string of LEDs.
Conversely, if input voltage varies widely
above and below LED string voltage, the
Driver automatically adjusts duty ratio to
maintain constant output voltage.
Useful for automotive lighting:
9V battery voltage can vary from over 14V
down to 6V.
Buck-Boost LED Driver (DC/DC)
Analysis
-
Vi
1.
Continuous Mode operation
2.
Q1 is ON:
R4
3.
VLED
Ion
Vi = Ldi/dt = L*IP/Ton
Q1 is OFF:
Vi – Vo = Vi – (VLED +Vi) = -VLED = -Ldi/dt = -L*Ip/Toff
+
Combining:
VLED / Vi = Ton / Toff = Ton / (T – Ton)
= (Ton/T) / ((T – Ton)/T)
= d / (1 – d)
Vo
VD
Q1: ON/OFF
When:
d < 0.5: VLED / Vi > 1
d = 0.5: VLED / Vi = 1
d > 0.5: VLED / Vi > 1
6.
d = duty ratio
Voltage buck mode
Pass through mode
Voltage boost mode
Operating mode depends on:
Sum of all the LED forward voltages (no. of LEDs)
Pi = Po/n where n = efficiency
Po = VLED * {IDC * (1-d) + IP * (1-d)/2}
Set Ip using sense resistor R4
Current
5.
Note: T = Ton + Toff = 1/f
IP
Ion
Ioff
t
IDC
VD = VLED + Vi
VD
4.
Ton
Toff
Resistor vs. Linear vs. Switch-mode
Comparing LED Driver Schemes
Simple; No
inductor
Lowest parts
cost
Low cost
No EMI
No EMI
Constant Io
Other
functions are
discrete
CONS
Very efficient
0.6
Vf = 3.0V
0.4
Vf = 3.4V
0.2
Vf = 4.0V
0
No. of LEDs
not limited by
Vi
9
12
16
Input Voltage Vi
Linear Current Source Efficiency
Constant Io
Integrated
(Dim,
programmin
g)
Integrated
functions
(Dim,
programming)
Low efficiency
Low
efficiency
Complex
Io varies with
Vi and LED Vf
Thermals
No. of LEDs
limited by Vi
No. of LEDs
limited by Vi
Adjust R in
production
Resistor Drive Eficiency
Electrical
Efficiency
Simple
SWITCH
MODE
Need to
suppress EMI
0.6
Vf = 3.0V
0.4
Vf = 3.4V
0.2
Vf = 4.0V
0
9
12
16
Input Voltage Vi
Switch Mode Current Source
Efficiency
Electrical
Efficiency
PROS
LINEAR
Electrical
Efficiency
RESISTOR
0.96
0.94
0.92
0.9
9
12
Input Voltage Vi
16
Vf = 3.0V
Vf = 3.4V
Vf = 4.0V
Off-Line LED Drivers
• Drive LEDs directly from AC power
– Replaces standard light bulbs/lamps
• Needs AC to DC conversion
– Bridge rectifier at input
Courtesy: Power Integrations
• Often needs input/output isolation
– Flyback, forward or resonant mode converters
– Resonant converters offer highest efficiency
• Often need Power Factor Correction
– Front end PFC reduces overall efficiency
– Efficiency = nPFC X nCONV
– Example: Efficiency = 0.93 X 0.92 = 85.6%
LED Dimming
•
–
–
–
•
LED current is pulsed
Peak value of current is
constant
No color shift
Analog Dimming
–
–
•
Lower
brightness
PWM Dimming
LED current is varied
May cause color shift
PWM
Dimming
–
–
ILED
Analog
Dimming ILED
Some drivers have an onboard ramp generator
An external analog voltage
compared to the ramp
PWM
provides PWM signal to
dim the LED
Ramp
This is called Analog
Controlled PWM Dimming Signal
Higher
brightness
TON1
TON2
T
T
Higher
brightness
Analog Controlled Dimming
Analog voltage
t
14W LED Driver
Wide Input/Output voltage Range
3W to 24W LED Driver
Operates from 6V to 30V
Driver IC
AC/DC in,
DIM Input
LED +
LED –
GND
Inductor
DIM and Fault
Protection Switch
Power Switch
Summary
•Solid-State Lighting continues to evolve with certain
expectations:
•Greatly increased lifetime = Lower Maintenance Costs $$$
•Uses a fraction of the power compared to traditional light– Greater
Energy-Efficiency = Lower Energy Costs $$$
•LED lighting market is growing at 38%
•Many technical challenges need to be overcome –
These are many areas of opportunity to work on