Study-on-High-power-LED-Heat-Dissipation-Based-on-Printed-Circuit-Board-外文文献.doc

Study on High power LED Heat Dissipation Based on Printed Circuit Board

WANG Yiwei , ZHANG Jianxin , NIU Pingjuan , LI Jingyi
( 1. School of Information and Communication Engineering, Tianjin Polytechnic University,
Tianjin 300160, CHN; 2. Tianjin Gongda HiYu Solid State Lighting Co, Ltd. Tianjin 300160, CHN)

Abstract: In order to study the role of printed circuit board(PCB) in high power LED heat dissipation, a simple model of high power LED lamp was designed. According to this lamp model, some thermal performances such as thermal resistances of four types of PCB and the changes of LED junction temperature were tested under three different working currents. The obtained results indicate that LED junction temperature can not be lowered significantly with the decreasing thermal resistance of PCB. However, PCB with low thermal resistance can be matched with smaller volume heat sink, so it is hopeful to reduce the size, weight and cost of LED lamp.
Key words: high power LED; printed circuit board( PCB) ; substrate of heat dissipation; thermal resistance; junction temperature CLC number: TN312 Document code: A Article ID: 1007- 0206( 2010) 02/ 03- 0120- 05

1 .Introduction
Light Emitting Diode( LED) , as one kind of solid light emitting semiconductor devices, has lots of performance advantages, such as low voltage, high luminous efficiency, long life time, etc. It is considered as the most valuable light source in the 21st century, which will replace incandescent lamps, halogen bulbs in the future . High power LED(≥1 W) can only convert about 15% of the input power into light, with the rest being lost as heat. Because the surface area of the chip is small( only 1mm x 1 mm ~ 2. 5mm x 2. 5mm) , the heat flux of the chip will reach up to 100 W/cm2 , and it will rise with the increase of the input power. If the heat can not be immediately transferred to the ambient, the junction temperature increases, which will affect the extraction efficiency, light wavelength, device life time and reliability significantly. Therefore, the junction temperature of the LED is an important index for thermal performance.
The heat of high power LED lamp is primarily dissipated by way of conduction, that is, from PN junction to epitaxial layer, epitaxial layer to the PCB, then PCB to heat sink, and finally to air by convection. Therefore, PCB is not only used for physical supporting and electric connecting in LED, but also an important way of heat dissipation. With the increase of the input power of high power LED, ordinary PCBs are not adequate to remove the generated heat .
In this paper, the thermal analysis of PCB used for high power LED lamp is discussed, a simple model of high power LED lamp is designed, and the thermal characteristics including junction temperature and thermal resistance of PCB under the conditions of different powers, are investigated experimentally.

2 Experimental Descriptions
2.1 Experimental Device

Fig. 1

The experimental device is mainly composed by a LED lamp, a DC power supply , SSP8810 LED photometric electric and thermal tester produced by XingPu Optical Inc. of Hangzhou ( thereinafter tester for short ). The LED lamp including 14 LEDs，PCB, aluminum fin-type heat sink and lamp body is shown in Fig 1. In order
to reduce the contact thermal resistance, the thermal grease was filled
between the PCB and heat sink.

2.2 Experimental Principle
2.2.1 Testing Principles of Thermal Resistance
Thermal resistance is usually used to assess the heat transfer performance of the high power LED, and junction temperature is determined by the thermal resistance and the power dissipation. In the high power LED, thermal resistance is an important aspect for thermal performance, called R. It can be calculated by the following equation.
R = ( tJ - tC ) / P (1)
Where R( ℃/W) is the thermal resistance between PN junction and the reference point , tJ is the PN junction temperature, tC is a reference temperature, and P is the total power dissipation.
This Eq. (1) should be modified when the electrooptical efficiency is only 15% in high power LED, while P is the heat transfer rate of the total heat power dissipation, which can be expressed as,
R =( tJ - tC )/P =VtJ /（0. 85Vh*Ih ） ( 2)
Where IH is LED rated current , VH is the forward voltage under rated current , and VtJ is the rise of junction temperature. There is a linear relationship between junction temperature and forward voltage. That is
VV = K ( t J - tC ) = KVt J ( 3)
Where K is a proportional factor, and its unit is V/ ℃. VV , the LED junction voltage change, is relative to the initial value. Using the K factor to calculate the variation of the junction temperature and then by the formula (2) , we can get the thermal resistance.

2.2.2 The composition of PCB Thermal Resistance
The thermal resistance(R total) for high power LED lamps includes four parts. The first part is the packaging thermal resistance (R1) of the high power LED, which is related to the chip packaging technology. The second part is the bonding thermal resistance(R2) between LEDs and PCB, which is mainly determined by the bonding material and thickness. The third part is the thermal resistance of PCB( R3 ) , which is affected by the type of PCB and the thermal conductivity of each layer in PCB. The last part is the convection resistance(R4) between the heat sink and the environment , which is influenced by various factors such as fin structure, area and environment wind speed . The equation is
R total = R1 + R2 + R3 + R4 ( 4)
In the four thermal resistances, the thermal resistance of PCB(R3) will be discussed in this paper.

2.3 Experimental Procedure

Firstly, in order to identify the four PCB samples conveniently, three custom PCBs were labeled as P1, P2, and P3, and the last commercial aluminum PCB was labeled as D1. Secondly , fourteen LWW5JM high power LEDs were welded on each PCB sample. In order to measure the junction temperature of one single LED, the LED at T2 was disconnected with PCB, and the remaining thirteen LEDs formed a serial circuit . Finally, for four kinds of PCB samples, the comparison tests of three different input powers were conducted. Input power was obtained under three different working currents of 400 mA,700 mA and 900 mA, corresponding to 1.2 W, 2.2 W, 2.9 W. And the test time was set to be two hours. A thermal resistance of test Rx (R1 , R2 , R3 , R4 ) was measured using the testing software system automaticly with available statistics.
In the experiment set , the LED lamp was applied by a DC power. The temperature of the LED lamp was measured by 4 pairs of T-type thermocouples ( with deviation of ±0.5℃ at 100 ℃ ) : The thermocouple(T1) was located at the ambient; the thermocouple(T2) was located at the pin of LED; the thermocouple(T3) was located at the heat sink of lamp; the thermocouple( T4 ) was located at the lamp body. Three pairs of thermocouple( T2 - T4 ) are shown in Fig.1.

3 Experimental Results and Analysis
3.1 The Value of K Factor
Fig.2 shows the K factor of the high power LED. In order to keep the value of K factor more accurately , the K factor should be measured for three times, and then the averag evalues are figured out. The K factors for the three times are - 2. 3579 mV/℃ , - 2. 838 mV/℃ and - 2. 458 3 mV/℃, respectively. The average value is - 2. 551 4 mV/℃. So the K factor of high power LED can be obtained.

Fig.3

3.2 The Effect of Working Current on the Junction Temperature
The junction temperature is very critical in evaluating the design and reliability of the PCB for the thermal control of the high power LED lamp. So the junction temperature of LED needs to be inspected stressfully.
Fig.. 3 shows the effect of working current I ( mA) on junction temperature Tj (℃) . It is significant that in the same PCB, the junction temperature of the high power LED increases
with the increasing working current . The high junction temperature was caused by the cumulated heat loads. Therefore, the effect of the PCB on the junction temperature strongly depends on the working current of LED. Similarly, Fig.. 3 shows that the LED junction temperature of the custom PCB is significantly better than that of commercial aluminum PCB, even if Fig.3 The effect of working current on the junction temperature
the working current changes. It is
mainly because the thermal conductivity of custom PCB is higher than that of commercial aluminum PCB, the heat can be quickly transmitted to the heat sink. Therefore, the LED junction temperature is directly related to the thermal conductivity of PCB. The higher thermal conductivity will make more heat be conducted, and then the junction temperature is lowered.
In Fig. 3, at the working current of 400 mA, the minimum LED junction temperature on the custom PCBs is 52.9 ℃, and that of the commercial aluminum PCB is 63.1 ℃, so the temperature difference between the two values is 10.2℃ For the working current of 900 mA, the minimum LED junction temperature on the custom PCBs is 98 ℃, and that on the commercial aluminum PCB is 103.3 ℃ , so the temperature difference between the two is only 5. 3 ℃. It is considered that the custom PCBs are more significant for decreasing the LED junction temperature under lower working current .

3. 3 The Effect of Working Current on the Thermal Resistance of PCB
The thermal resistance is an important parameter for the heat dissipation of PCB. The working current increase will cause some change to the thermal resistance of PCB.

Fig.. 4.. T he effect of w orking current o n the t hermal resistance of PCB

The effect of working current I ( mA ) on the thermal resistance of PCB, Rjc (℃/W) , is shown in Fig 4. Three custom PCBs was labeled as P1, P2,and P3, and the commercial aluminum PCB labeled as D1. It can be seen that when the high power LED working current is kept constant, the lowest thermal resistance of custom PCB is 1. 3 ℃/W, while that of the commercial aluminum PCB can reach 5 ℃/W. It shows that the custom PCB significantly improves heat dissipation of high power LED lamps, because the thermal conductivity of the custom PCB middle layer is higher than that of the commercial aluminum PCB. Therefore, the lower thermal resistance can allow more heat transfer to the air, and improve the reliability of LED.

3. 4 .. The Effect of Different Currents on the Percentage of PCB Thermal Resistance
It is showed in Fig 5 that the effect of working current on the percentage of PCB thermal resistance in the whole LED lamp.
From Fig 5, the highest percentage of PCB thermal resistance, P(%) , in the whole high power LED lamp is only 5% , and it can be seen that LED junction temperature can not be lowered significantly with decreasing PCB thermal resistance. If the PCB thermal resistance is reduced blindly, the heat dissipation of the whole high power LED lamp would not be improved greatly. However, the lower thermal resistance PCB can be matched with smaller volume heat sink, so it is hopeful to reduce the size, weight and cost of LED lamp.

4 Conclusion
Four different kinds of PCB in LED lamps are investigated. PCB has the minimum thermal resistance among all the heat dissipation paths of LED lamps. The PCBs with different thermal resistances show different effects on junction temperature. PCB with lower thermal resistance has smaller advantage in lowering the junction temperature. However, the PCB with lower thermal resistance can be used with smaller and lighter heat sink. Therefore, the application value of custom PCB does not effect the degree of reducing junction temperature, but lowering the dependence on heat sink. The match usage of lower thermal resistance PCB and portable heat sink can acquire smaller and lighter LED lamp.

References:
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IEEE Journal on Selected Topics in Quantum Electronics, 2002, 8( 2) : 310..321.
[2] Lin Yuan chang, NguyenTran, Zhou Yan, et al. Materials challenges and solutions for the packaging of high power LEDs[ C] / / 2006 International Micro systems, Packing , Assembly Conference Taiwan, 2006:1-4.
[3] Arpad Bergh, George Craford, Anil Dugg al, et al. The Promise and Challenge of Solid State Lighting [ J] . Physics Today , 2001, 54( 12) : 42-47.
[4] HsuJT, Han W K, Chen C, et al. Design of multichips LED module for lighting application[ J] . Pro c. SPIE, 2002,
4776: 26-33.
[5] WANG Duo xiao, LI Jia. The research of high power LED packaging technology[ C] / / 2008 fifteent hsession of China Institute of Electronics, elect ronic components, Annual Conference, 2008: 141-150( in Chinese) .
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