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High power LED measurement methods
Citation preview
New practical method
for measurement of high-power LEDs
CORM 2008, Rensselaer Polytechnic Institute (RPI), Troy, NY, June 9-12, 2008
Yuqin Zong and Yoshi Ohno
National Institute of Standards and Technology
Gaithersburg, Maryland
Temperature Dependence of a High-power LED
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.05
1.10
5 15 25 35 45 55 65 75 85 95
Heat Sink Temperature (C)
Vo
lta
ge a
nd
Flu
x
no
rm
ali
zed
to
T=
25
C
5000
5500
6000
6500
7000
7500
8000
8500
9000
9500
10000
CC
T (
K)
Voltage (V)
Luminous Flux (lm)
CCT (K)
S1 at 350 mA dc forward current
Temperature issue in High-Power LED Measurements
However, SSL product manufacturers use LEDs typically
at much higher operating temperature.
Gaps to fill
LED manufacturers: Pulse measurement at Ta=Tj=25C
Lighting industry (traditional lamp manufacturers, Luminaire
manufacturers, testing laboratories):
DC measurement at Ta=25C.(LED thermal condition not defined)(Condition in LM-79, CIE 127 does not apply to high-power LEDs)
LEDs in ANY conditions (Tj of 10C ~ 100C) need
to be measured.
(Cold LED condition)
LED devices/packages
Existing Measurement Standards
CIE 127:2007
does not address issues on
high power LEDs
CIE TC2-46, TC2-58
CIE R2-36
We need a new, practical method
for high-power LED measurements
Simple Approach for High-Power LED Measurement
Use a metal-core PC board
Mount to a temperature-controlled heat sink (10 - 100 C).
Measure high power LEDs at a full DC current at thermal equilibrium,
or at short pulse.
- Heat sink temperature
- Pin temperature
- Case temperature
- Board temperature
.
Junction temperature (Tj) is the physical quantity of
an LED that reproduces exactly the LED operating
condition universally, anywhere.
Selection of the Temperature Reference
Not always available,
Not always reproducible,
Not always comparable,
Extra cost for the probe,
.
How to set, measure, and control the Tj?
Rated current
Set the heat sink
temp. to desired Tjand stabilize it.
Apply pulse
current, and
measure VF,j
VF,j
Optical quantities
measurement at Tj(in DC mode)
a few min. a few sec. a few min.
Feedback control
of heat sink
temperature
Tj VF,j
The New Practice Method
VF
VF,j
LED Heating Curves
0.95
0.96
0.97
0.98
0.99
1.00
1.01
1.02
0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080 0.090 0.100
Time (s)
Re
lati
ve
Va
lue
LED I
LED Vf
DET Sig
0.95
0.96
0.97
0.98
0.99
1.00
1.01
1.02
0 10 20 30 40 50 60
Time (s)
Re
lati
ve
Va
lue
LED I
LED Vf
DET Sig
0.95
0.96
0.97
0.98
0.99
1.00
1.01
1.02
0 10 20 30 40 50 60
Time (s)
Re
lati
ve
Va
lue
LED I
LED Vf
DET Sig
Mounted on a temperature-controller heat sink
0.95
0.96
0.97
0.98
0.99
1.00
1.01
1.02
0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080 0.090 0.100
Time (s)
Re
lati
ve
Va
lue
LED I
LED Vf
DET Sig
Un-mounted
Comparison of DC and Pulse Results under the same Vf
0.95
0.96
0.97
0.98
0.99
1.00
1.01
1.02
1.03
1.04
1.05
400 450 500 550 600 650 700
DC
/Pu
lse
Wavelength (nm)
Ratio of DC to pulse
Chromaticity x (DC-Pulse) -0.0001
Chromaticity y (DC-Pulse) 0.0001
Chromaticity u (DC-Pulse) 0.0001
Chromaticity v (DC-Pulse) 0.0002
CCT (K) (DC-Pulse) -3.1
Color Rendering Index (Ra) (DC-Pulse) 0.04
Luminous flux (DC/Pulse) 1.0002
Radiant flux (DC/Pulse) 0.9998
A phosphor-based
white LED:
If = 350 (mA),
Vf = 3.672 (V)
Pulse and DC measurements yield
the same results!!
Pulse: before thermal equilibrium.
DC: after thermal equilibrium.
Application of This Method
- IESNA, to develop a LM-xx Approved Method
for Photometric Measurement of High Power LEDs.
Develop a draft in ANSI SSL WG, then forward to the
SSL subcommittee in IESNA Testing Procedures
Committee.
- CIE
-
The whole measurement process can be fully automated.
Can also be used for measuring LED arrays.
Summary
LED can be measured at any junction temperature.
The reference condition is the LED junction temperature,
which makes mounting method and thermal resistance
between the LED and heat sink to be irrelevant to the result.
Pulses are used ONLY to determine the electrical forward
voltage. Optical quantities are measured (typically slow and
more difficult) in a normal DC operating condition.
We have developed a new, practical method for measurement
of high-power LEDs.
ACKNOWLEDGMENTS
Thank Dr. Cameron Miller, the DOE, Cree, Nichia,
and Seoul Semiconductor,