Red, Green and Blue LED- based White Light Sources Jim Gaines Principal Engineer
Historical Perspective
From Tubes to Transistors (glass-electrode to semiconductor)
Now: From Lamps to LEDs (SSL)
Contents
White light generation and associated challengesHigh color rendering with RAGB LEDsMulti-chip packagesConclusions
Light Emitting Diodes (LEDs)
Used for signals for 30+ yearsHB LEDs are established in several
areas:• Handheld phone backlights• Traffic lights• Car brake lights• Large, stadium-size displays
(NASDAQ building)LEDs are moving towards use in illumination
•Car headlights•Niche applications (step lights, flashlights,architectural lighting)•Mix red, green and blue LEDs to make•white and variable-colored light
White light from RGB-LEDs
Theoretically the most efficient solutionUnique ability to vary lamp colorBy adding amber (RAGB), high color rendering index can be obtained.
y
x0
0
Triangle vertices shift with:CurrentTemperatureTimeLEDs
1
1
The Need for Color The Need for Color CControlontrol
A Complete RAGB LED Illumination System
EMI filter Rectifier
Vin SwitchingModeDC-DCPowerConverter
Current source
Current source
Current source
LED(R)
LED(G)
LED(B)
MixingOptics
Microprocessor ControlOptical feedback
Electrical feedback
Parameter/Coordinates Identification, Calibration,Color mixing algorithms, Compensation functions,Dimming algorithms, Protections, …...
Thermal feedback
Userinterface
Current source LED(R)
LED Drive and Control
Color Mixing & SensingControl and Digital Design
LED(A)LED(A)
Issues for Practical Implementation of RGB LED
Lighting
• Spatial color mixing
• Control system design
• Photosensor placement and light sampling
• Effect of current waveform on sensing and perception
• Stability of system during dimming and color changes
• Thermal management
Multidisciplinary problem, involving optics, power electronics drivers, feedback control systems, thermal management and system architecture.
Challenges for RAGB Four-Color Mixing
With RGB, there is a single set of LED currents that produce a given set of color coordinates.With RAGB, different combinations of R, A, Gand B can yield the same color.We can use this extra degree of freedom to maximize the Color Rendering Index.
Dynamic Color Control for LED Light Sources
64 LEDsTemperature feedbackLuminous intensity feedbackIndividual color sensing
Feedback Control
Color Rendering IndexRA > 90
00.00020.00040.00060.00080.001
0.00120.00140.00160.0018
25 30 35 40 45 50 55 60
Heatsink temperature (Degree C)
Del
ta u
v 2800K3200K4600K
8586878889909192939495
25 30 35 40 45 50 55 60
Heatsink temperature (Degree C)
Col
or re
nder
ing
inde
x (C
RI)
2800K3200K4600K
8586878889909192939495
25 30 35 40 45 50 55 60
Heatsink temperature (Degree C)
Col
or re
nder
ing
inde
x (C
RI)
2800K3200K4600K
8586878889909192939495
25 30 35 40 45 50 55 60
Heatsink temperature (Degree C)
Col
or re
nder
ing
inde
x (C
RI)
2800K3200K4600K
8586878889909192939495
25 30 35 40 45 50 55 60
Heatsink temperature (Degree C)
Col
or re
nder
ing
inde
x (C
RI)
2800K3200K4600K
Color variation ∆uv < 0.002
For a heatsink temperature change of 25C, at 3 different color temperatures.
Metal-core PCB
Heatsink
Schematic LED-System-in-Module
User inputs:DC power,Intensity&Colorsignals
Si submount w/electronics(including sensors)
Heatsink
First-stage optics
LEDs
Discretes
16 1W LEDs @ 75lm/W ≈ 90W Halogen reflector lamp
RGB without Feedback
Color change during warm-up
Color change when Color change when a single LED dimsa single LED dims
0.245
0.25
0.255
0.26
0.265
0.27
0.235 0.24 0.245 0.25 0.255 0.26 0.265 0.27
x
y
Lamp 2Lamp 3Lamp 4Lamp 5Lamp 7Lamp 8Lamp 10
Low TempHigh Temp
0.235
0.245
0.255
0.265
0.275
0.285
0.295
0.23 0.235 0.24 0.245 0.25 0.255 0.26 0.265x
y
1G2R3B4G5B6G7G8R9R10G11G12B13G14B15R16G
Conclusion
There are plenty of challenges to arrive at multi-colored LED general illuminationTwo examples of approaches to address these challenges:
- RAGB lamp (electronic controls)- Multi-chip RGB lamp (color mixing optics)