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
4
LED Characteristics
• “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
ActualSize
We focus on “High Brightness” LEDs (HB LEDs) for Solid State Lighting
Energy ConsumptionUS 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
24%
13%
11%13%
7%
8%
6%
18%Space Heating
Water Heating
Lighting
Air Conditioning
Refrigeration
TV and PC
Washer/Dryer
Other
10% lighting efficiency improvement can cut annual CO2 emissions by 477 Metric Tons
LED Application
Video Walls
US DOE Energy Savings EstimatesLEDs 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
Multiple LEDs arranged in arrays is one way to achieve a legal beam pattern.
Cadillac DTS
Lincoln
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
1990s 2000 2002 2004 2006 2008 2010
20 lm/W
50 lm/W
70 lm/W
100 lm/W
200 lm/W
Red Tail LightsTraffic Lights
MonochromeLCDs in Handsets
Color LCDs inHandsets
Handset Keypads
CameraFlash
Signage
Notebook LCD
Backlighting
LCD TV Backlighting
Street Lighting
MainstreamGeneral
Illumination
AutomotiveHeadlamps
AutomotiveInterior Lighting
LED Flash Lights
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.
Courtesy: Luminus
Up to 6000 lumens @ ~100 lm/W
Courtesy: Toshiba May 2008
Market Projections
LED Driver IC Market SizeLEDs 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
0
100
200
300
400
500
600
700
800
900
2006 2007 2008 2009 2010 2011
Personal Computing
LCD TV
General I llumination
Traffic Lights
Signage
Automotive
Consumer Portables
Mobile Handsets
Mill
ions
of
Dolla
rsLED Driver IC Market Size
HB LED applications
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
Place Photo Here
Incandescent Halogen LED
Light Source ComparisonLED vs. selected other technologies
Efficacy (lm/W)
7 - 20 15 - 30 50 - 100 20 – 50 (Efficiency)
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
HID
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 PerformanceCourtesy: OSRAM
Driving LEDs
Buck Mode LED DriverMR-16 Lamp
Typical input voltage is 12V rms (AC)Typical output power is 1W to 3W
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
Practical ConsiderationsLED Binning
The human eye can detect a brightness difference when the intensity ratio is > 2:1
Luminous Intensity Binning
Lot2 Lot3Lot1
1090 mcd480 mcd
Intensity range in production at rated current:
Bin2
560 mcd 710 mcd 900 mcd 1120 mcd
Bin3 Bin4Bin1
450 mcd
Conceptual description; Actual binning limits may vary
Intensity BinningExample
LED DriversGeneral 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
1) Series
2) Parallel(1) 3) Parallel(2)
Can be constant current sources
1
11
2 3
1 Voltage variations 2 Vf variations; Current hogging
3 Current stress when one or more strings fail
LED Configurations
1) Series
2) Parallel(1) 3) Parallel(2)
1
1 No current variations due to voltage 2 Vf variations; Current hogging
3 Current stress when one or more strings fail
1
2 3
1
Resistor Drive
0
0.1
0.2
0.3
0.4
0.5
9 12 16
Input Voltage Vi
LE
D C
urr
en
t Io
Vf=3.0V
Vf=3.4V
Vf=4.0VVi
Io
Vf
Power loss
Varying current
Changing brightness
Impossible to automatically adjust
for binning
0
2
4
6
8
9 12 16
Input Voltage Vi
Res
isto
r P
ower
D
issi
patio
n Ploss @ Vf =3.0V
Ploss @Vf =3.4V
Ploss @ Vf =4.0V
LED DriversCommon Topologies
• Choice of topology depends on many factors– Input and output voltage range– Efficiency– Cost– Flexibility of use– Reliability
•Flyback•Forward•Resonant•Buck
*Often need PFC
•Buck•Boost•Buck-Boost•SEPIC•Flyback•Forward
LED DriverLED Driver
Linear Switch Mode
DC/DC AC/DC
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
• Constant LED Current
• Output voltage is lower than input voltage
• Not very efficient– V_OUT must be close to V_IN to
maximize efficiency
• Needs good heat management– Pass transistor dissipates heat
I_s
I_LED
+V_FDBK-
Error Amp+
V_REF
V_IN
V_OUT
PD = I_LED X (V_IN – V_OUT)
DIM
TMP
EN
Constant Current DriveLinear Current Source
Linear Current Source; LED Current Regulation
0
0.1
0.2
0.3
0.4
9 12 16
Input Voltage Vi
LE
D C
urr
ent
Io
Vf = 3.0V
Vi = 3.4V
Vf = 4.0V
Linear Current Source Dissipation
0
2
4
6
9 12 16
Input VoltageVi
Line
ar C
urre
nt
Sou
rce
Dis
sipa
tion
Vf = 3.0V
Vf = 3.4V
Vf = 4.0V
Vi
Io
Vf
Power loss
Constant current
Constant brightness
Can integrate adjustments for
binning
Linear Control
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
= Duty cycle
Boost
Is2 = ILED
Is1
VLED/Vi = 1/(1-)Vi
Buck
Is = ILED
VLED/Vi = Vi
Buck BoostIs1
Is2 = ILED
VLED/Vi = /(1-)Vi
• 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; Vi can vary• No output cap needed in CCM operation• Simple, low cost• Synchronous for high efficiency
Constant Current DriveSwitch-mode Current Source
Vi
Io
Vf
Power loss (Small)
Constant current
Constant brightness
Can integrate adjustments for
binning
Switch-mode Control
Switch-mode Source Power Dissipation
0
0.05
0.1
0.15
9 12 16
Input Voltage Vi
Curr
ent S
ourc
e Di
ssip
atio
n Vf = 3.0V
Vf = 3.4V
Vf = 4.0V
Topologies vs. ApplicationsGuidelines
DESIGN BASE BY POWER RANGE
AC or DC IN Architecture Power Range Applications
DC IN
Buck
0.5W - 2W
Luminaires, LED Bulbs, Down Light, Linear Lamps, Bar Lamps, Decorative
3W - 6W
8W - 12W
15W - 20W
25W - 40W
50W - 65W
Boost
8W - 12W
LED Bulbs, FL Replacement, Down Light, Linear Lamps, Bar Lamps
15W - 20W
25W - 40W
50W - 65W
Buck-Boost
8W - 12W
Garage lighting, Emergency, Automotive
15W - 20W
25W - 40W
Cuk
8W - 12W
Garage lighting, Emergency, Automotive
15W - 20W
25W - 40W
Forward, Flyback 25W - 40W
Panels, Down Light, Linear, Bar50W - 65W
AC or DC IN Resonant
50W -100W
Street lamp, Flood Light, Search Light, Marine Lihts
100W – 250W
250W – 400W
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 widelyabove and below LED string voltage, theDriver automatically adjusts duty ratio tomaintain constant output voltage.
Useful for automotive lighting:
9V battery voltage can vary from over 14Vdown to 6V.
Buck-Boost LED Driver (DC/DC)Analysis
1. Continuous Mode operation
2. Q1 is ON:
Vi = Ldi/dt = L*IP/Ton
3. Q1 is OFF:Vi – Vo = Vi – (VLED +Vi) = -VLED = -Ldi/dt = -L*Ip/Toff
4. Combining:VLED / Vi = Ton / Toff = Ton / (T – Ton) Note: T = Ton + Toff = 1/f
= (Ton/T) / ((T – Ton)/T) = d / (1 – d) d = duty ratio
5. When:d < 0.5: VLED / Vi > 1 Voltage buck moded = 0.5: VLED / Vi = 1 Pass through mode d > 0.5: VLED / Vi > 1 Voltage boost mode
6. Operating mode depends on: Sum of all the LED forward voltages (no. of LEDs)
Ton Toff
Cu
rre
nt
Ion Ioff
IDC t
VD
VD = VLED + Vi
IP
Vi
Vo
Ion
Q1: ON/OFF
VD
VLED
+
-
Pi = Po/ where = efficiency
Po = VLED * {IDC * (1-d) + IP * (1-d)/2}Set Ip using sense resistor R4
R4
Resistor vs. Linear vs. Switch-modeComparing LED Driver Schemes
Linear Current Source Efficiency
0
0.2
0.4
0.6
9 12 16
Input Voltage Vi
Ele
ctri
cal
Eff
icie
ncy Vf = 3.0V
Vf = 3.4V
Vf = 4.0V
Switch Mode Current Source Efficiency
0.90.920.940.96
9 12 16
Input Voltage Vi
Ele
ctri
cal
Eff
icie
ncy
Vf = 3.0V
Vf = 3.4V
Vf = 4.0V
RESISTOR LINEAR SWITCH MODE
PROS Simple Simple; No inductor
Very efficient
Lowest parts cost
Low cost No. of LEDs not limited by Vi
No EMI No EMI
Constant Io Constant Io
Other functions are discrete
Integrated (Dim, programming)
Integrated functions (Dim, programming)
CONS 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
Need to suppress EMI
Resistor Drive Eficiency
0
0.2
0.4
0.6
9 12 16
Input Voltage Vi
Ele
ctri
cal
Eff
icie
ncy 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
• 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 = PFC X CONV
– Example: Efficiency = 0.93 X 0.92 = 85.6%
Courtesy: Power Integrations
PWM
RampSignal
t
LED Dimming
• PWM Dimming– LED current is pulsed– Peak value of current is
constant– No color shift
• Analog Dimming– LED current is varied– May cause color shift
• Some drivers have an on-board ramp generator
– An external analog voltage compared to the ramp provides PWM signal to dim the LED
– This is called Analog Controlled PWM Dimming
TON1
T
TON2
T
PWMDimming
AnalogDimming
Lower brightness
Higher brightness
ILED
ILED
Higher brightness
Analog voltage
Analog Controlled DimmingAnalog Controlled Dimming
14W LED DriverWide Input/Output voltage Range
3W to 24W LED DriverOperates from 6V to 30V
Driver IC
AC/DC in,DIM Input
Inductor
Power Switch
LED +LED –GND
DIM and Fault Protection 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
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