Study-on-High-power-LED-Heat-Dissipation-Based-on-Printed-Circuit-Board-外文文献.doc
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1、Study on High power LED Heat Dissipation Based on Printed Circuit BoardWANG 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 3001
2、60, 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
3、 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
4、 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
5、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 bulb
6、s 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 increas
7、e 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
8、 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 elect
9、ric 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 pow
10、er 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
11、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 res
12、istance, 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 resi
13、stance 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
14、 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 -
15、 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 proport
16、ional 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 th
17、ermal 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 determ
18、ined 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 b
19、y 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 sample
20、s 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
21、, 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.
22、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 pair
23、s 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 thermocoupl
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