Z-Power LEDNote
X10490
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Thermal Management
Design for Acrich2
Rev. 00
March 2012
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[ Contents ]
1. Introduction
2. Thermal management for Acrich2
2-1. Change of Acrich2 characteristics with temperature
3. Thermal modeling for Acrich2
3-1. Thermal resistance of Acrich package
3-2. Characterization parameter of Acrich IC
3-3. Junction temperature calculation
3-4. Junction temperature of Acrich components
3-5. Maximum Tt of IC and Ts of LED
3-6. Characterization parameter of Acrich IC
4. Recommended design for proper thermal management
4-1. PCB design
4-2. Heat sink design
4-3. Interface material design
4-4. Material property
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Introduction
Acrich2 series designed for AC drive (or operation) doesn’t need the converter
which is essential for conventional lighting. Acrich2 has various applications in
the field of general lighting like MR, incandescent, Down-light and Linear light.
Thermal management of Acrich2 products is critical in the design of lighting
products to ensure the highest performance and reliability of the end product.
In this paper, the method for measuring junction temperature of the LED and
Acrich IC are described. Furthermore, to improve thermal characteristics
recommendations and methods for PCB design, heat-sink design and interface
materials are suggested.
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Thermal management for Acrich2
Change of Acrich2 characteristics with temperature
Temperature is one of the most critical factors that determines the optical, electrical and lumen
maintenance characteristics of an LED design, like Acrich2. Normally, luminous flux decreases
gradually with increasing junction temperature. If the maximum junction temperature of an
LED is it exceeded, it could have a severe impact on the LED reliability. The Acrich Integrated
Circuit(IC) is also sensitive to temperature change. If the maximum temperature of the IC is
exceeded the IC may operate abnormally.
(a)
(b)
<Figure 1> Current wave form (a) normal operation (b) abnormal operation
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Thermal modeling for Acrich2
Thermal resistance of the Acrich package
A mechanical cross section of the Acrich package with the thermocouple is shown in figure 2.
<Figure 2> Cross section of Acrich package
Tj is junction temperature of LED chip.
Ts is surface temperature of lead for the package.
Rθi-s is the thermal resistance from junction to package lead.
Tj = Ts + (Rθj-s * PD)
PD is the power dissipation.
Thermal resistance of Acrich packages are shown in table 1.
Acrich package
Package power
dissipation [W]
AZ4
1.12
5630
0.43
[
℃/W]
Rθ
θj-S
Products
5.7
SMJEA3000120
27
SMJEA3000220
SMJEA3001220
SMJEA3002220
SMJEA3003220
<Table 1> Thermal resistance of the Acrich2 package
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Characterization parameter of Acrich IC
A mechanical cross section of Acrich IC with the thermocouple is shown in figure 3.
<Figure 3> Cross section of Acrich IC
Tj is junction temperature of IC chip.
Tt is top temperature of IC surface.
ψi-t is the characterization parameter from junction to IC top surface.
Tj = Tt + (ψj-t * PD)
PD is the power dissipation.
Characterization parameter for Acrich IC are shown in table 2.
Acrich IC
6x6
8x8
IC power
dissipation [w]
℃/W]
ψj-t
[
100V
0.78
16.46
120V
0.64
16.43
220V
0.41
16.40
100V
1.50
5.35
120V
1.23
5.21
220V
0.79
4.98
Products
SMJEA3000120
SMJEA3000220
SMJEA3001220
SMJEA3002220
SMJEA3003220
<Table 2> Characterization parameter of Acrich IC: The value is measured under metal PCB
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Junction temperature calculation
The junction temperature for the LED and IC can be calculated in the following manner. Figure
4 shows thermocouple placements to Ts (Surface temperature for LED) and Tt (Top
temperature for IC). After measurement of Ts(LED) and Tt(IC), using the given parameters,
Rθ(LED) and ψ(IC) values, each junction temperature can be calculated.
Ts (LED)
Tt (IC)
<Figure 4> Thermocouple placement
<Figure 5> Temperature variation of IC and package for SMJEA3001220
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We can use the following example to show the calculations. Figure 6 shows the temperature
variation for the SMJEA3001220 at 220Vrms with a power dissipation of 8.5W.
℃
(LED) is 27℃/W and ψ
℃
Ts (Surface temperature for LED) is 56.1 . Tt (Top temperature for IC) is 64 .
Refer to table 1 and 2, Rθj-s
i-t
(IC) is 5.0
℃/W.
PD = 21.7V * 0.02A = 0.434W
The junction temperature for the LED is calculated using the following formula:
Tj = Ts + (Rθj-s * PD)
℃ + (27℃/W * 0.434W) = 67.8℃
= 56.1
and the calculation for the IC is:
Tj = Tt + (ψj-t * PD)
= 64
℃ + (4.98℃/W * 0.79W) = 68℃
Figures 7 - 10 show the saturation curve over time of Ts for the LED and Tt for the IC. We have
used a basic aluminum heatsink for reference. Refer to figure 5.
<Top view>
<Front view>
<Side view>
<Figure 6> Basic aluminum heat sink
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Junction temperature of Acrich components
Graphs of Tt of the IC and Ts of the LED are measured below in figures 7 - 10. A basic square
aluminum heat sink is used as shown in figure 6. A 1.2W/mK thermal adhesive tape is used to
attach the PCB to the Heat-sink.
<Figure 7> SMJEA3000120 series temperature variation of IC and LED
<Figure 8> SMJEA3000220 series temperature variation of IC and LED
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<Figure 9> SMJEA3001220 series junction temperature variation of IC and LED
<Figure 10> SMJEA3002220 series junction temperature variation of IC and LED
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SMJEA3000120
SMJEA3000220
SMJEA3001220
SMJEA3002220
VF[V]
Junction temperature
for Acrich package [ ]
Junction temperature
for Acrich IC [ ]
100
52.4
48.6
120
65.9
51.2
220
59.6
50.8
100
62.0
56.4
120
59.0
56.4
220
51.9
55.3
100
71.1
68.8
120
69.4
71.8
220
67.9
67.8
100
91.1
92.2
120
88.0
92.6
220
74.8
85.6
℃
℃
<Table 3> Junction temperature Acrich2 on a square aluminum heat sink
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Maximum Tt of IC and Ts of LED
In order to operate the Acrich2 normally, the junction temperature of the components (IC and
LED) must operate lower than the maximum junction temperature. We can calculate the
maximum junction temperature under different operating conditions by using the previous
formulas and examples.
Acrich IC
There are two different Acrich ICs, one is a 6mm x 6mm and the other is an 8mm x 8mm.
The 6 x 6 Acrich IC is used on the SMJEA3000120 and SMJEA3000220 and the 8 x 8 Acrich IC is
used on the SMJEA3001220, SMJEA3002220 and SMJEA3003220. These two devices have
different thermal characterization parameters, therefore different Tt maximums. For example,
℃/W (SMJEA3000220,
the 6 x 6 Acrich IC has a thermal characterization parameter of 16.4
℃
20Vrms) and the maximum junction temperature of the IC is 125 , therefore the allowable
max top temperature (Tt_max) is:
Tt_max = Tj_max - (ψj-t * PD)
= 125
℃ - (16.4℃/W * 0.41W) = 118℃
If we look at the 8 x 8 Acrich IC, it has a thermal characterization parameter of
℃
5.0 /W(@SMJEA3001220, 20V) and the maximum top temperature of the IC is:
Tt_max = Tj_max - (ψj-t * PD)
= 125
℃ - (4.98℃/W * 0.79W) = 121℃
Table 4 gives a summary of allowable maximum Tt of Acrich2 ICs.
VF[V]
6 x 6 Acrich IC
8 x 8 Acrich IC
Allowable maximum Tt_max for IC [
100
112
120
114
220
118
100
117
120
119
220
121
℃]
<Table 4> Allowable maximum top temperature of Acrich IC measured on the metal core PCB.
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Acrich package
℃
The 5630(5.6mm x 3.0mm) Acrich package has a thermal resistance of 27 /W which used on
the SMJEA3000220, SMJEA3001220, SMJEA3002220 and SMJEA3003220.
℃
The maximum junction temperature of the 5630 Acrich package is 125 , therefore the
maximum permissible surface of lead temperature Ts_max is:
Ts_max = Tj_max - (Rθj-s * PD)
= 125
℃ - (27℃/W * 0.434W) = 113℃
The AZ4 Acrich package which is used on the SMJEA3000120 has a thermal resistance of
℃
5.7 /W . The maximum permissible surface of lead temperature is:
Ts_max = Tj_max - (Rθj-s * PD)
= 125
℃ - (5.7℃/W * 1.12W) = 118℃
Table 5 shows a summary of the allowable maximum Ts of Acrich2 packages.
VF[V]
Allowable maximum Ts_max for LED [
5630
All
113
AZ4
All
118
℃]
<Table 5> Allowable maximum surface of lead temperature of Acrich package
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The characterization parameters of the Acrich ICs change with power consumption as shown
below in figure 11.
Characterization parameter [ /W]
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Characterization parameter of Acrich IC
℃
<Figure 11> Characterization parameter of Acrich IC
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Recommended design for proper thermal management
PCB design
The PCB is the most critical factor determining the thermal characteristics of Acrich2. FR4 is the
most commonly used material for PCBs, however FR4 has a very low thermal conductivity due
to the FR4 dielectric material. The following method is used to improve the thermal
characteristics for an FR4 board by adding thermal vias between the top copper layer and
the bottom copper layer. Better thermal performance can be achieved by using a metal core
PCB which has a much better thermal conductivity and can improve the thermal dissipation.
<Figure 12> Cross section of PCB: Metal core PCB, FR4 PCB and FR4 with thermal via PCB
Metal core PCB
Table 6 below shows typical thermal conductivity according to thickness for metal core PCBs.
Layer
Thermal conductivity [W/mK]
Thickness [µ
µm]
Aluminum
150
1600
Dielectric layer
2.3
100
Copper (Top)
398
50
<Table 6> Thermal conductivity of metal core PCB
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The thermal resistance for a metal core PCB(MCPCB) can be calculated by using the following
equations:
Rθ = t / (k * A)
t is layer thickness
k is thermal conductivity
A is area
For a 1661mm2 area(such as the SMJEA3001220 PCB):
Rθ = Rθaluminum + RθDielectric + RθCopper
= (t / (k * A))aluminum + (t / (k * A))Dielectic + (t / (k * A))Copper
℃/W
= 0.03
℃/W.
However, the actual thermal resistance for an MCPCB is much larger than 0.03
This is
because the effective (heat) area is smaller than the whole PCB area. The LED is not spread
across the whole MCPCB.
FR4 PCB
Table 7 below shows typical thermal conductivity according to the thickness of FR4.
For 1661mm2 area,
Rθ = RθCopper + RθFR4 + RθCopper = 4.8
℃/W
Layer
Thermal conductivity [W/mK]
Thickness [µ
µm]
Copper (Bottom)
398
50
FR4
0.2
1600
Copper (Top)
398
50
<Table 7> Thermal conductivity of FR4 PCB
℃
However, the actual thermal resistance for FR4 is much larger than 4.8 /W, because the
effective (heat) area is smaller than the FR4 material. The LED is not spread across the whole
PCB.
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FR4 with thermal vias
Thermal vias in FR4 are filled solder material like SnAgCu compound. Table 8 below shows
typical thermal conductivity according to the thickness of the FR4 with via.
The heat from the LED is able to pass more easily through FR4 with a thermal via from the top
layer to the bottom layer because of the lower thermal resistance of the via. The equations to
calculate thermal resistance for an FR4 board with thermal vias is below:
Rθ = RθCopper + (RθFR4 // RθThermal via) + RθCopper
= (t / (k * A))copper + {(t / (k * A))FR4 // (t / (k * A))Thermal via} + (t / (k * A))Copper
= 3.7
℃/W
In case of FR4 with six vias and a diameter of 0.3mm per via and 1661mm2 area of PCB, the
thermal resistance is 3.7
℃/W.
This is a 23% improvement over the initial 4.8
℃/W
derived
from Table 8.
If the effective thermal area (small heat source) is considered, the improvement gap increase
around 50% over.
Layer
Thermal conductivity W/mK]
Thickness [µ
µm]
Copper (Bottom)
398
50
FR4
0.2
1600
Thermal via (Solder)
58
1600
Copper (Top)
398
50
<Table 8> Thermal conductivity of FR4 with thermal via PCB
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Temperature simulation parameters for the IC and LED
• Product: SMJEA3002220
• Voltage: 220Vrms
• Thermal pad: 100mm, 1.2W/mK
• Heat sink: Refer to figure 14
<Figure 13> Temperature comparison as kinds of PCB
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Heat sink design
One of the most effective and simplest cooling methods is to use a heat sink. In order to
achieve good heat transfer between the components (IC and LED) and ambient temperature,
the heat sink must have an optimal structure.
Normally, the heat sink material that is used is aluminum due to its high thermal conductivity,
low weight and low cost.
For bulb applications, the heat transfer is done using free convection, but the structure of the
heat sink must have an optimal size, a number of fins and gaps between each fin to allow
for good air flow. The gap and quantity of fins is very important. The more fins, the more
surface area, but a gap is needed to allow the air to pass.
The following section describes example simulations using Flowtherm and provides the results of
different bulb heat sinks for the SMJEA3001220 and SMJEA3002220. The examples will show
different heat sink sizes and fin quantities.
At simulation, the following are fixed: an aluminum metal PCB and 1.2W/mK thermal tape is
used to adhere the PCB to the heatsink.
First, for verification purposes between real tests and simulations, we will measure Tt and
Ts for the SMJEA3001220 with the bulb heat sink. The bulb heat sink used is shown in Figure
15. Table 9 shows the results between measured and simulation for verification purposes.
℃
℃
Tt [ ]
Ts [ ]
Experiment
70.5
70.2
Simulation
70.6
70.4
<Table 9> Comparison data between experiment and simulation for SMJEA3001220 with bulb heat sink
7.0mm
7.0mm
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<Figure 14> Basic bulb heat sink structure
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Figure 15 shows the temperature variation of IC and LED with modification to the fin quantity of
the heat sink.
< Simulation parameters >
• Product: SMJEA3001220
• Voltage: 220Vrms
• Thermal pad: 100µm thickness, 1.2W/mK thermal conductivity
• Heat sink: Refer to figure 14
<Figure 15> Temperature variation with change in number of fins
℃ and 70.4℃
are increased to 76.2℃ and 76.1℃. The IC
As the simulation shows, a heat sink with 20 fins has a Tt and Ts of 70.6
Respectively, but with a 0 fin heat sink, Tt and Ts
and LED junction temperature are calculated to be:
Tj_IC = Tt + (ψj-t * PD)
= 76.2
℃ + (4.98℃/W * 0.792W) = 80℃
Tj_LED = Ts + (Rθj-s * PD)
= 76.1
℃ + (27℃/W * 0.434W) = 88℃
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The bulb heat sink shown in figure 14 is not an optimal structure for the SMJEA3001220. It is
just one example, therefore more optimization may be done changing the size, fin gap, fin
quantity and shape to even further reduce the junction temperature.
The next simulation is for SMJEA3002220 which has a 12W power dissipation. Figure 17 is the
simulation result by changing the heat sink size. In simulation, an aluminum heat sink , metal
core PCB and 1.2W/mK thermal tape are used for the input parameters, however these heat
sink conditions shown in Table 10, are not the most optimal structure either for the
SMJEA3002220. More optimization of the heat sink structure and use of high quality thermal
material can improve the thermal characteristics.
Fin
Base
Free space
Length
Heat sink
Length
[mm]
Free space
depth [mm]
Base
Thickness
[mm]
Case I
Case II
Case III
Fin
Diameter
[mm]
Quantity
[ea]
area
[mm2]
64
50
39
11
80
Gap
[mm]
12320
20
100
18312
3.6
25914
<Table 10> Simulation parameters for SMJEA3002220 heat sink
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< Simulation parameters >
• Product: SMJEA3002220
• Voltage: 220V,RMS
• Thermal pad: 100µm, 1.2W/mK
<Figure 16> Simulation results for the SMJEA3002220
As mentioned earlier, for a complete understanding of whether a certain heat sink will dissipate
the appropriate heat for Acrich2 products, Tt and Ts must be checked and these values must be
no more than Tt_max and Ts_max as shown in table 4 and 5.
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Interface material design
Thermal interface material can help control junction temperature of the Acrich2 as well. It is
used to fill the air gap between the Acrich2 PCB and the heat sink. Thermal interface materials
are thermally conductive and electrically isolating. They come in pad (tape) or liquid
dispensable types.
Figure 17 shows simulation results using different thermal interface materials. Thermal
resistances of interface materials can go from 0.52
℃/W to 2.25 ℃/W.
Thermal pad material performance (thermal resistance) depends on the pressure used in the
assembly process. Actual product performance is directly related to the surface roughness,
flatness and pressure applied.
< Simulation parameters >
• Product: SMJEA3001220
• Voltage: 220V,RMS
• Thermal pad thickness: 100mm
• Thermal pad area: 1661mm2 (SMJEA3001220 PCB size)
• Heat sink diameter: Refer to figure 14
<Figure 17> Temperature variation of IC and LED as value of thermal resistance of interface material
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Material property
Material
Thermal conductivity [W/mK]
Aluminum_Pure
237
Aluminum_4.5% Cu, 1.5% Mg, 0.6% Mn
177
Aluminum_4.5% Cu
168
Copper_Pure
401
Copper_90% Cu, 10% Al
52
Copper_89% Cu, 11% Sn
54
Copper_70% Cu, 30% Zn
110
Copper_55% Cu, 45% Ni
23
Gold
317
Iron_Pure
80.2
Iron_99.75% pure
72.7
Nikel_Pure
90.7
Nikel_80% NI, 20% Cr
12
Nikel_73% Ni, 15% Cr, 6.7% Fe
11.7
Silicon
148
Silver
429
Tin
66.6
Tungsten
174
Aluminum oxide, sapphire
46
Silicon carbide
490
Silicon dioxide
1.38
Silicon nitride
16.0
Glass
1.4
<Table 11> Thermal conductivity
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