thermal management for acrich2 rev 00 - Симметрон ... led x10490 application note rev. 00...

Z-Power LED X10490Z-Power LED X10490

Application Note

Rev. 00 Rev. 00

March 2012March 2012

www.Acrich.com www.Acrich.com

Thermal Management Design for Acrich2


Application Note

Rev. 00 Rev. 00



[ Contents ]

1. Introduction

2. Thermal management for Acrich22-1. Change of Acrich2 characteristics with temperature

3. Thermal modeling for Acrich2 3-1. Thermal resistance of Acrich package3-2. Characterization parameter of Acrich IC3-3. Junction temperature calculation3-4. Junction temperature of Acrich components3-5. Maximum Tt of IC and Ts of LED3-6. Characterization parameter of Acrich IC

4. Recommended design for proper thermal management4-1. PCB design4-2. Heat sink design4-3. Interface material design4-4. Material property


Application Note

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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.


Application Note

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Thermal management for Acrich2

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.

Change of Acrich2 characteristics with temperature

(a)

(b)

<Figure 1> Current wave form (a) normal operation (b) abnormal operation


Application Note

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Thermal modeling for Acrich2

A mechanical cross section of the Acrich package with the thermocouple is shown in figure 2.

Tj is junction temperature of LED chip.

Ts is surface temperature of lead for the package.

Ri-s is the thermal resistance from junction to package lead.

Tj = Ts + (Rj-s * PD)

PD is the power dissipation.

Thermal resistance of Acrich packages are shown in table 1.

Thermal resistance of the Acrich package

<Figure 2> Cross section of Acrich package

<Table 1> Thermal resistance of the Acrich2 package

SMJEA3000220SMJEA3001220SMJEA3002220SMJEA3003220

5630

SMJEA3000120

Products

270.43

Acrich package Package powerdissipation [W]

Rj-S[℃/W]

AZ4 1.12 5.7


Application Note

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Characterization parameter of Acrich IC

A mechanical cross section of Acrich IC with the thermocouple is shown in figure 3.

Tj is junction temperature of IC chip.

Tt is top temperature of IC surface.

i-t is the characterization parameter from junction to IC top surface.

Tj = Tt + (j-t * PD)

PD is the power dissipation.

Characterization parameter for Acrich IC are shown in table 2.

<Figure 3> Cross section of Acrich IC

<Table 2> Characterization parameter of Acrich IC: The value is measured under metal PCB

SMJEA3001220SMJEA3002220SMJEA3003220

SMJEA3000120SMJEA3000220

Products

4.980.79220V

5.211.23120V

16.400.41220V

16.430.64120V

1.50

0.78

Acrich IC IC powerdissipation [w]

j-t[℃/W]

6 x 6

100V 16.46

8 x 8

100V 5.35


Application Note

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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,

R(LED) and (IC) values, each junction temperature can be calculated.

Junction temperature calculation

<Figure 4> Thermocouple placement

Tt (IC)

Ts (LED)

<Figure 5> Temperature variation of IC and package for SMJEA3001220


Application Note

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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.

Ts (Surface temperature for LED) is 56.1℃. Tt (Top temperature for IC) is 64℃.

Refer to table 1 and 2, Rj-s(LED) is 27℃/W and 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 + (Rj-s * PD)

= 56.1℃ + (27℃/W * 0.434W) = 67.8℃

and the calculation for the IC is:

Tj = Tt + (j-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.

<Figure 6> Basic aluminum heat sink

<Top view>

<Front view> <Side view>


Application Note

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<Figure 7> SMJEA3000120 series temperature variation of IC and LED

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.

Junction temperature of Acrich components

<Figure 8> SMJEA3000220 series temperature variation of IC and LED


Application Note

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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


Application Note

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51.265.9120

50.859.6220

56.462.0100

SMJEA3000220 56.459.0120

55.351.9220

68.871.1100

SMJEA3001220 71.869.4120

67.867.9220

92.291.1100

SMJEA3002220 92.688.0120

85.674.8220

48.652.4100

SMJEA3000120

VF[V] Junction temperature for Acrich package [℃]

Junction temperature for Acrich IC [℃]

<Table 3> Junction temperature Acrich2 on a square aluminum heat sink


Application Note

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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,

the 6 x 6 Acrich IC has a thermal characterization parameter of 16.4℃/W (SMJEA3000220,

20Vrms) and the maximum junction temperature of the IC is 125℃, therefore the allowable

max top temperature (Tt_max) is:

Tt_max = Tj_max - (j-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 - (j-t * PD)

= 125℃ - (4.98℃/W * 0.79W) = 121℃

Table 4 gives a summary of allowable maximum Tt of Acrich2 ICs.

Maximum Tt of IC and Ts of LED

121220

119120

117100

8 x 8 Acrich IC

118220

114120

112100

6 x 6 Acrich IC

VF[V] Allowable maximum Tt_max for IC [℃]

<Table 4> Allowable maximum top temperature of Acrich IC measured on the metal core PCB.


Application Note

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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 - (Rj-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 - (Rj-s * PD)

= 125℃ - (5.7℃/W * 1.12W) = 118℃

Table 5 shows a summary of the allowable maximum Ts of Acrich2 packages.

118AllAZ4

113All5630

VF[V] Allowable maximum Ts_max for LED [℃]

<Table 5> Allowable maximum surface of lead temperature of Acrich package


Application Note

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The characterization parameters of the Acrich ICs change with power consumption as shown

below in figure 11.

Characterization parameter of Acrich IC

<Figure 11> Characterization parameter of Acrich IC

Ch

arac

teri

zati

on p

aram

eter

[℃

/W]


Application Note

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Recommended design for proper thermal management

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.

Metal core PCB

Table 6 below shows typical thermal conductivity according to thickness for metal core PCBs.

<Figure 12> Cross section of PCB: Metal core PCB, FR4 PCB and FR4 with thermal via PCB

Layer Thermal conductivity [W/mK] Thickness [m]

Aluminum 150 1600

Dielectric layer 2.3 100

Copper (Top) 398 50

<Table 6> Thermal conductivity of metal core PCB

PCB design


Application Note

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The thermal resistance for a metal core PCB(MCPCB) can be calculated by using the following

equations:

R= t / (k * A)

t is layer thickness

k is thermal conductivity

A is area

For a 1661mm2 area(such as the SMJEA3001220 PCB):

R= Raluminum + RDielectric + RCopper

= (t / (k * A))aluminum + (t / (k * A))Dielectic + (t / (k * A))Copper

= 0.03℃/W

However, the actual thermal resistance for an MCPCB is much larger than 0.03℃/W. 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,

R= RCopper + RFR4 + RCopper = 4.8℃/W

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.

Layer Thermal conductivity [W/mK] Thickness [m]

Copper (Bottom) 398 50

FR4 0.2 1600

Copper (Top) 398 50

<Table 7> Thermal conductivity of FR4 PCB


Application Note

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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:

R= RCopper + (RFR4 // RThermal via) + RCopper

= (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 [m]

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


Application Note

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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


Application Note

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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.

Heat sink design

7.0mm

7.0m

m

<Figure 14> Basic bulb heat sink structure

70.6

70.5

Tt [℃] Ts [℃]

Experiment 70.2

Simulation 70.4

<Table 9> Comparison data between experiment and simulation for SMJEA3001220 with bulb heat sink


Application Note

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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 >


• Voltage: 220Vrms

• Thermal pad: 100m thickness, 1.2W/mK thermal conductivity

• Heat sink: Refer to figure 14

As the simulation shows, a heat sink with 20 fins has a Tt and Ts of 70.6℃ and 70.4℃

Respectively, but with a 0 fin heat sink, Tt and Ts are increased to 76.2℃ and 76.1℃. The IC

and LED junction temperature are calculated to be:

Tj_IC = Tt + (j-t * PD)

= 76.2℃ + (4.98℃/W * 0.792W) = 80℃

Tj_LED = Ts + (Rj-s * PD)

= 76.1℃ + (27℃/W * 0.434W) = 88℃

<Figure 15> Temperature variation with change in number of fins


Application Note

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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.

Heat sink

Gap[mm]

area[mm2]

Quantity[ea]

Diameter[mm]

Thickness[mm]

Free space depth [mm]

Length[mm]

20

Fin

100

80

64

3950 18312Case II 11

Base

25914

12320Case I

3.6

Case III

Length

Base

Fin

Free space

<Table 10> Simulation parameters for SMJEA3002220 heat sink


Application Note

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• Voltage: 220V,RMS

• Thermal pad: 100m, 1.2W/mK

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.

<Figure 16> Simulation results for the SMJEA3002220


Application Note

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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.



• Voltage: 220V,RMS

• Thermal pad thickness: 100mm

• Thermal pad area: 1661mm2 (SMJEA3001220 PCB size)

• Heat sink diameter: Refer to figure 14

Interface material design

<Figure 17> Temperature variation of IC and LED as value of thermal resistance of interface material


Application Note

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Material property

16.0Silicon nitride

490Silicon carbide

174Tungsten

429Silver

11.7Nikel_73% Ni, 15% Cr, 6.7% Fe

90.7Nikel_Pure

80.2Iron_Pure

23Copper_55% Cu, 45% Ni

54Copper_89% Cu, 11% Sn

401Copper_Pure

177Aluminum_4.5% Cu, 1.5% Mg, 0.6% Mn

52Copper_90% Cu, 10% Al

168Aluminum_4.5% Cu

237Aluminum_Pure

72.7Iron_99.75% pure

317Gold

110Copper_70% Cu, 30% Zn

66.6Tin

148Silicon

12Nikel_80% NI, 20% Cr

1.4Glass

1.38Silicon dioxide

46Aluminum oxide, sapphire

Material Thermal conductivity [W/mK]

<Table 11> Thermal conductivity

thermal management for acrich2 rev 00 - Симметрон ... led x10490 application note rev. 00...

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