Red, Green and Blue LED-based White Light Sources

Jim GainesPrincipal 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.

LED Spectra

LED spectra vs. temperatureTypical LED spectra

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.

Red, Amber, Green and Blue LED-based White Light Source

forColor Rendering Index > 90

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.

Multi-chip Packages

forcompact, color-variable light

sources

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)

Advance LED Drivers3 years of market experience

12W17W

25W40W

60W80W

LEDIllumination

SystemsSemiconductors LEDs

Control Electronics

Thermal Management

Thank you.

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