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Thermal Management Design for Acrich2

Rev. 00 March 2012 www.Acrich.com

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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) 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.

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

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.

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

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)

Thermocouple placement

Temperature variation of IC and package for SMJEA3001220 Rev. 00 March 2012 www.Acrich.com

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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.







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.

SMJEA3000120 series temperature variation of IC and LED

SMJEA3000220 series temperature variation of IC and LED

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Z-Power LEDNote X10490 Application SMJEA3001220 series junction temperature variation of IC and LED

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

℃

℃

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

℃]

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

℃]

Allowable maximum surface of lead temperature of Acrich package

Rev. 00 March 2012 www.Acrich.com

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

℃

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.

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

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

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

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

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

Comparison data between experiment and simulation for SMJEA3001220 with bulb heat sink

7.0mm

7.0mm

Rev. 00 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

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℃ Rev. 00 March 2012 www.Acrich.com

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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

Simulation parameters for SMJEA3002220 heat sink

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< Simulation parameters > • Product: SMJEA3002220 • Voltage: 220V,RMS • Thermal pad: 100µm, 1.2W/mK

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

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 Thermal conductivity

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