LED Lighting: A Case Study

LED Lighting: A Case Study
in Thermal Management
This article examines the thermal management of a light
emitting diode (LED)-based lighting system. First, we discuss
the environment in which the lighting system will be used.
Then, we look at the system’s cooling needs and the various
analyses used to confirm that the LED thermal requirements
are being met. The article concludes with a comparison of
the results.
LED-Based Lighting System Requirements
An LED-based lighting system was to be designed to replace
a halogen-based downlight. A downlight is typically installed
in a hollow opening in a ceiling and provides a concentrated
output in the downward direction. A thermal management
analysis was needed to properly design a cooling method for
the LED system, which had to include a natural convection
heat sink. This environment is shown in Figure 1
Ceiling air
temperature
Product Requirements
The lifetime of an LED relates to its junction temperature and
forward current. The new downlight includes three InGaNbased LUXEON cool white K2 LEDs at a forward current of
1000 mA. The maximum operational junction temperature
for these cool white LEDs is 150ºC [1]. The downlight has
a lifetime requirement of 60,000 hours. Figure 2 shows the
lifetimes of the cool white LED for different forward currents,
junction temperatures, and for the B10, L70 lifetime condition
(which implies that for a specific lifetime, 10% of the LEDs
are expected to fail at the specified junction temperature
and forward current.) The failure criterion is when the light
output of the LED has been reduced to 70% of its original
light out. To achieve the 60,000 hours lifetime with a B10,
L70 condition, the junction temperatures required for specific
forward currents are shown in Table 1. From Table 1, with a
forward current of 1000 mA, the junction temperature needs
to be kept below 124ºC
tolifetimes
achieve
a 60,000
hours lifetime.
(B50,
L70)
lifetimes
InGaN
LUXEONK2
K2
(B50,
L70)
forforInGaN
LUXEON
70,000
60,000
Ceiling
Room ambient temperature
Lifetime (Hours)
Downlight
350mA
50,000
700mA
40,000
1A
30,000
1.5A
20,000
10,000
0
90
Figure 1. A Typical Downlight Environment.
6
100
110
120
130
140
150
160
Junction
Temperature
Junction temperature
(C) (C)
170
180
190
200
Figure for
7. Expected
(B50, L70)
lifetimes forVersions
InGaN LUXEONof
K2 the LUXEON
Figure 2. Lifetimes
Different
InGaN
K2 LED [2].
Table 1. Required Junction Temperatures of LUXEON K2 LEDs for
Specific Forward Currents to Achieve 60,000 Hours Lifetime Under
the B10, L70 Lifetime Condition [2].
Forward Current
[mA]
Max Junction
Temperature [ºC]
350
154
700
134
1000
124
1500
112
For this study, in order to achieve a 60,000 hours lifetime the
LED junction temperature must be kept under 124ºC, with an
average year-round temperature of 20ºC. Under maximum
temperature conditions, the junction temperature must be
less than 150ºC at an ambient temperature of 40ºC.
Thermal Management Analysis
The lifetime and maximum temperature conditions were
determined previously; now, a thermal management analysis
is applied to each condition. This is a confidence level analysis
performed to build in safety margins for all unknowns in all
engineering phases. The analysis comprises three sections:
analytical, numerical (CFD) and experimental.
1. Analytical analysis
a. Based on the unknowns in the analysis and
shortcomings of empirical and experimental
correlations, assumptions made in order to
do the analysis
2. Numerical or CFD analysis
a. Unknowns and assumptions made in order
to do the analysis
b. Shortcomings in the numerical code
3. Experimental
a. Incorrect thermocouple placement
b. Variations in thermocouple response
c. Errors in velocity probe calibration
d. Power input measurement
Equation 1 is used for the confidence level analysis, where
Tj is the required junction temperature and CFL is the
confidence level being applied. Additionally, Tj,condiction is the
specified junction temperature and Treference is the reference
or ambient temperature. The temperature difference
between the required junction temperature and the reference
temperature, ΔTcondition, is used when comparing different
conditions.
Tj − Treference
∆Tcondition
=
≤ CFL (1)
Tj,condition − Treference Tj,condition − Treference
Table 2. Confidence Factor Level, CFL, for Different Types of
Analyses.
Type of Analysis
CFL
Analytical
80%
Numerical
80% to 85%
Experimental
90%
A confidence level of 90% is used in this study. Re-arranging
Equation 1 yields Equation 2. Applying the lifetime conditions
to Equation 2 determines the temperature difference for the
lifetime condition.
(
)
(2)
∆Tlifetime = 0.9 × (124 − 20 ) = 93.6 K
(3)
∆Tcondition =
CFL × Tj,condition − Treference
The maximum temperature difference
determined, as shown in Equation 4.
can
also
∆Tmaximum = 0.9 × (150 − 40 ) = 99 K
be
(4)
From Equations 3 and 4, the lifetime condition is the most
severe condition. Re-arranging Equation 1 yields Equation
5, which, applied to the lifetime condition, gives us Equation
6.
(
)
Tj =×
CFL Tj,condition − Treference + Treference
September 2009 |Qpedia
(5)
7
Tj = 0.9 × (124 − 20 ) + 20 = 113.3 ºC
(6)
(10)

Pe= Pl + Q
j
Re-arranging Equation 10 yields Equation 11:
Therefore, the junction temperatures to be determined by
different analyses must be less than 113.3ºC at an ambient
of 20ºC.
Analytical Analysis
As a starting point, an LED junction temperature of 108ºC
is assumed, with a required forward current of 1000 mA.
The usable light tool [3] gives a light efficiency of 9.4% and
electrical power dissipation, Pe, of 3.53 W. The light efficiency
is the ratio of the light power, Pl that the LED emits to the
electrical power input, Pe. This is also given by Equation 7,
which can be re-arranged in the form shown in Equation 8.
ηl =Pl Pe
(7)
Pl = ηlPe
(8)
(11)
Q=
Pe + Pl
j
Substituting Equation 8 into 11 yields Equation 12. Rearranging Equation 12 gives Equation 13:
= P − η P
Q
j
e
l e

=
Q
Pe (1 − ηl )
j
(13)
Because all other values of Equation 13 are known, the heat
dissipated by the LED can be calculated.
(12)
 = 3.53 × (1 − 0.094=
Q
) 3.2 W
j
(14)
Pl
Standard FR-4 boards can be used for LEDs with up to 0.5
W of dissipation, but metallic substrates are required for
higher levels [4]. Because the LED heat dissipation is 3.2 W,
a metal core board type PCB was used. Figure 4 is a sketch
of the LED junction to heat sink. It shows each material
that the heat from the LED must transfer through before
it reaches the heat sink. Figure 4 also provides a thermal
resistance diagram based on the sketch. The resistances
are considered to be in series.
Pe
.
Qj
Figure 3. Control Volume Around an LED.
Consider the control volume around the LED in Figure 3.
The electrical power input, Pe enters the control volume while
 and the light power, P leave the
the heat dissipated, Q
j
l
control volume. Applying an energy balance to the control
volume yields Equation 10.
∑ Ein = ∑ Eout
The metal core board’s spreading resistance, Rmetalcore can
be determined using the spreading resistance calculation
method explained in [5]. The effective in-plane thermal
conductivity can be calculated using Equation 15, as
described in [6]:
where t is the total thickness of the PCB, tc,i and tg,i are the
Ng
Nc
(9)
∑ k c t c,i + ∑ k g t g,i
=i 1 =i 1
k p,e =
(15)
t
September 2009 |Qpedia
9
FUTURE COOLING
P re p re g
1 5 0 µm
R j-s
M e ta l
co re
b o a rd
R s-solder
R m etalcore
A lu m in iu m 1 .6 m m
In te rfa ce m a te ria l
C o pp e r
1 7 .5 -70 µm
S o ld e rp aste
1 5 0 µm
R interface
R hs
base,
spreasding
H e a tslu g
H e a tsink b ase
Figure 4. Heat Sink-to-LED Junction and Corresponding Thermal Resistance Diagram.
thicknesses of the copper and glass-epoxy or prepreg/
dielectric layers, and kc and kg are the thermal conductivities
of the copper and glass-epoxy, respectively.
Equation 15 can be modified to accommodate the PCB’s
aluminum layer, as shown in Equation 16. Additionally, the
coverage percentage of each layer can be taken into account
by the factor βi,
Nc
Ng
∑ βik c t c,i + ∑ βik g t g,i
=i 1 =i 1
k p,e =
+ β ALk AL t AL
Material
Coverage
[%]
Conductivity
[W/m∙K]
Thickness
[µm]
Copper
50
385
70
Dielectric
100
3
150
Aluminum
100
180
1600
(16)
t
The other thermal resistance needs are as follows:
where tAL is the aluminum thickness and kAL is the thermal
conductivity of the aluminum.
The PCB’s material properties are shown in Table 3. Using
the spreading resistance calculation and effective in-plane
thermal conductivity methods previously mentioned, along
with the PCB’s material properties, the spreading resistance
in the metal core board was calculated as Rmetalcore = 1 K/W.
Table 3. PCB Material Properties.
10
1. Rs-solder is the thermal resistance in the solder under
FUTURE COOLING
the LED slug. It is 146 µm thick with a thermal
conductivity of 50 W/m∙K and an area of 22.5 mm².
This results in a thermal resistance of 0.13 K/W.
2. The interface resistance is assumed to be 0.2 K/W.
This is comparable to the resistance of Chomerics
T405-R thermal interface material.
3. The spreading in the heat sink base, Rhs base,spreading is
assumed to be zero.
4. The junction-to-heat slug thermal resistance of the
LED is 9 K/W [1].
Consider the thermal resistance in the heat transfer path
from the junction to the heat sink base shown in Figure 4.
These resistances are considered to be in series, and the
junction-to-heat sink resistance is the sum of the individual
resistances. Using Fourier’s law of heat conduction in a onedimensional differential form, the heat transfer rate between
the junction and the heat sink can be expressed by Equation
17. Because the required lifetime junction temperature, the
heat dissipated by the LED, and the thermal resistance from
the junction to heat sink are known, Equation 17 can be rearranged to calculate the heat sink temperature, Equation
18.
 = Ths − Tamb
Q
hs
Rhs−amb
Rhs−amb =
(17)
 R
Ths= Tj − Q
j j−hs
(18)
Ths − Tamb

Q
(21)
hs
=
Rhs amb
80.34 20
= 6.28 K/W
9.6
(22)
Figure 5. Thermal Resistance Diagram of LED Junction to
Tj = 113.3 ºC
.
Qj
.
Qj
.
Qj
Rj-hs
Rj-hs
Rj-hs
Rhs-amb
 = Tj − Ths
Q
j
R j−hs
(20)
.
.
Qhs = 3xQj =
9.6 W
Ths = 80.34 ºC
Tamb = 20 ºC
Ambient.
Ths= 113.3 − 3.2 × 10.3= 80.34 ºC
(19)
Because there are three LEDs on the heat sink, the sink
must be able to transfer 3 x 3.2 W = 9.6 W from a heat sink
temperature of 80.34ºC to an ambient of 20ºC. Using the
thermal resistance diagram shown in Figure 5, the thermal
resistance from the heat sink to ambient can be calculated
using Equation 21. From Equation 22, the heat sink thermal
resistance must be less then 6.28 K/W or the heat sink must
be able to dissipated 9.6 W at a temperature difference of
60.34 K.
12
For the analytical simulation, two methods available to
determine the heat sink thermal resistance. The first is
to refer to the heat sink’s data sheet, which, in this study,
shows that 9.6 W can be dissipated at a 56.3K temperature
difference (see Figure 6.) This is less than the required 63.4
K temperature difference.
The second method is to use an analytical model of the
heat sink (whose part number is ATSEU-077B-C4-R0.) The
results of the analytical analyses are shown in Table 4.
and tested. This was done in order to verify the results of
the analytical and numerical analyses. The LEDs were
calibrated using the forward voltage method, also referred
to as the electrical method. In the forward voltage method,
the LED is calibrated at a sense current. Thereafter, the LED
is tested at the required forward current of 1000 mA. When
steady-state has been reached, the junction voltage at the
sense current is measured and the junction temperature can
be calculated from the calibration curve. A detailed example
of the forward voltage/electrical test method is given in [8].
ce G raph
9.00
10.00
11.00
12.00
ation [W ]
ed
DThs-am bient [K ]
T herm al P erform ance G raph
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
11.00
12.00
P ow er D issipation [W ]
Vertic al m ounted
Figure 6. Experimental Results using the ATSEU-077B-C4-R0
Heat Sink.
Numerical Results
Based on the analytical results, a model of the downlight
was created. It was simulated in a free air environment. The
boundary conditions for a free air environment are discussed
in [7]. The results of the numerical analysis are shown in
Table 4.
A
B
Qj
Tam b 2 0 °C
Tj, m ax = 1 1 3 °C
He a t sin k
Ba se
In te rfa ce
material
MCPCB
S id e o f h o u sin g
not shown
C o m pa ct
model
LED
Figure 7. Numerical Results of the Downlight Analysis.
Experimental Results
An experimental model of the downlight was manufactured
C
(c)
Figure 8. Images of the Experimental Analysis showing the LEDs
(a), the Experimental Set Up (b) and an Infrared Picture of the
Lighting System (c).
Comparing the Analytical, Numerical and Experimental
Results
(c)
Table 4 summarizes the analytical, numerical and
experimental results for the LED lighting system. The table
shows that the results obtained using the different methods
are within 10% of each other and have a high confidence
level. The maximum temperature difference calculated for
the CFD results is 93 K. Further, the experimental results
September 2009 |Qpedia
13
FUTURE COOLING
Table 4. Comparison of the Analytical, Numerical and Experimental Data, Normalized to an Ambient Temperature of 20ºC.
Units
Analytical,
with
experimental
hs-data
Analytical,
only
CFD
Experimental
Tambient
°C
20
20
20
20
Iforward
mA
1000
1000
1000
1000
Light efficiency
%
9%
9%
9%
9%
Pdissipated,total
W
9.6
9.6
9.6
9.6
Theatsink base
°C
68
76
75
71
Tboard
°C
73
81
84
78
Tj, led
°C
102
110
113
107
Comparison of methods
%
95%
103%
105%
100%
ΔTj-amb
K
82
90
93
87
TRUE
TRUE
TRUE
TRUE
Parameter
Less than the required temperature
difference of 93.6K
have a temperature of 87 K. Both of these results are below
the required 93.6 K for the lifetime condition. Therefore, the
analyses have shown that the LED-based downlight system
satisfies the lifetime temperature condition. The LED-based
downlight lighting system end product is shown in Figure 9.
Figure 9. LED-based Downlight Whose Cooling Solution was
Developed by ATS Europe B.V.
Summary
This article explains the development of an LED-based
downlight system. The LED lighting system uses 3 LUXEON
K2 LEDs at a forward current of 1000 mA. The article
discusses analytical, numerical and experimental analysis
methods A comparison of the different analysis results are
14
given. For reliability, it is recommended that at least two
independent results be obtained, and that these not differ by
more than 20%.
References:
1. Luxeon K2 Technical data sheet, DS51, http://www.
philipslumileds.com/pdfs/DS51.pdf, September 2009.
2. Luxeon K2 Reliability Datasheet RD06, http://www.
philipslumileds.com/pdfs/RD06.pdf, September 2009.
3. Future Electronics, Usable Light Tool, www.
futurelightingsolutions.com.
4. Petroski, J., Thermal Challenges in LED Cooling,
Electron­ics Cooling Magazine, November 2006.
5. Spreading Thermal Resistance: Its Definition and
Control, Qpedia eMagazine, September 2007.
6. Shabany, Y, Component Size and Effective Thermal
Conductivity of Printed Circuit Boards, ITHERM, 2002.
7. Boundary Conditions for Natural Convection CFD
Simulations, Qpedia eMagazine, June 2009.
8. Hulett, J. and Kelly, C., Measuring LED Junction
Temperature,
http://www.photonics.com/Content/
ReadArticle.aspx?ArticleID=34316, September 2009/