studyonhighpowerledheatdissipationbasedonprintedcircuitboard-外文文献_本科论文

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: 10070206( 2010) 02/ 03- 0120- 051 .IntroductionLight Emitting Diode(

5、 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 incan desce nt lamps, haloge n bulbs i

6、n 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

7、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 f

8、or 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

9、 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 L

10、ED 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 Descriptions2.1 Experimental DeviceFig. 1The experime ntal device is mainly composed by a LED lam

11、p, a DC power supply , SSP8810 LED photometric electric and thermal tester produced by XingPu Optical Inc. of Hangzhou (there in after tester for short ). The LED lamp in cludi ng 14 LEDs , PCB, alu minum fin-type heat sink and lamp body is show n in Fig 1. In orderto reduce the con tact thermal res

12、ista nee, the thermal grease was filled betwee n the PCB and heat sink.2.2 Experime ntal Prin ciple2.2.1 Test ing Prin ciples of Thermal Resista neeThermal resista nee is usually used to assess the heat tran sfer performa nee of the high power LED, and jun cti on temperature is determ ined by the th

13、ermal resista nee and the power dissipati on. In the high power LED, thermal resista nee is an importa nt aspect for thermal performa nee, called R. It can be calculated by the follow ing equati on.R = ( tJ - tC ) / P(1)Where R( C /W) is the thermal resista nee betwee n PN jun ctio n and the refere

14、nee point, tJ is the PN junction temperature, tC is a reference temperature, and P is the total power dissipati on.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 exp

15、ressed 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 jun ctio n temperature. There is a lin ear relati on ship betwee n jun cti on temperature and forward voltage. That isVV = K ( t J - tC ) = KVt J(

16、3)Where K is a proportional factor, and its unit is V/C . VV , the LED junction voltage change, isrelative to the initial value. Using the K factor to calculate the variation of the junction temperature and the n by the formula (2) , we can get the thermal resista nee.2.2.2 The compositi on of PCB T

17、hermal Resista neeThe thermal resista nce(R total) for high power LED lamps in eludes four parts. The first part is the packagi ng thermal resista nee (R1) of the high power LED, which is related to the chippackag ing tech no logy. The sec ond part is the bonding thermal resista nce(R2) betwee n LED

18、s and PCB, which is mainly determined by the bonding material and thickness. The third part is the thermal resista nee of PCB( R3 ) , which is affected by the type of PCB and the thermal con ductivity of each layer in PCB. The last part is the convection resista nce(R4) betwee n the heat sink and th

19、e environment , which is in flue need by various factors such as fin structure, area and en viro nment wind speed . The equati on isR total = R1 + R2 + R3 + R4( 4)In the four thermal resista nces, the thermal resista nee of PCB(R3) will be discussed in this paper.2.3 Experime ntal Procedure400500600

20、700&00900J/mAFirstly, 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 temperat

21、ure of one single LED, the LED at T2 was disconnected with PCB, and the remaining thirteen LEDs formed a serial circuit .Fin ally, for four kinds of PCB samples, the comparis on tests of three differe nt in put powers were con ducted. In put power was obta ined un der three differe nt work ing curre

22、 nts of 400 mA,700 mA and 900 mA, correspo nding to 1.2 W, 2.2 W, 2.9 W. And the test time was set to be two hours. A thermal resista nee of test Rx (R1 , R2 , R3 , R4 ) was measured using the testi ng software system automaticly with available statistics.In the experiment set , the LED lamp was app

23、lied by a DC power. The temperature of the LED lamp was measured by 4 pairs of T-type thermocouples ( with deviation of 0.5 C at 100 C ) : The thermocouple(TI) 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; t

24、he thermocouple( T4 ) was located at the lamp body. Three pairs of thermocouple( T2 - T4 ) are show n in Fig.1.3 Experime ntal Results and An alysis3.1 The Value of K FactorFig.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

25、 measured for three times, and then the averag evalues are figured out. The K factors for the three times are - 2. 3579 mV/ C , - 2. 838 mV/ C and - 2. 458 3 mV/ C , respectively. The average value is - 2. 551 4 mV/ C . So the K factor of high power LED can be obta in ed.Fig.33.2 The Effect of Worki

26、 ng Current on the Jun cti on TemperatureThe 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 in spected stressfully.Fig. 3 shows the effect of working current

27、I ( mA) on junction temperature Tj (C ) . It issig ni fica nt that in the same PCB, the junction temperature of the high power LED in creases with the in creas ing work ing curre nt . The high junction temperature was caused by the cumulated heat loads. Therefore, the effect of the PCB on the jun ct

28、i on temperature stron gly depe nds on the working current of LED. Similarly, Fig. 3 shows that the LED junction temperature of the custom PCB is sig ni fica ntly better tha n that of commercial alu minum PCB, even ifFig.3 The effect ofworking current on the junction temperaturethe work ing curre nt

29、 cha nges. 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 con ductivit

30、y will make more heat be con ducted, and the n 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 C , and that of the commercial aluminum PCB is 63.1 C , so the temperature differenee between the two values

31、 is 10.2 C For the working current of 900 mA, the minimum LED junction temperature on the custom PCBs is 98 C , and that on the commercial aluminum PCB is 103.3 C , so the temperature differenee between the two is only 5. 3 C . It is considered that the custom PCBs are more significant for decreasin

32、g the LED junction temperature un der lower work ing curre nt .3. 3 The Effect of Worki ng Curre nt on the Thermal Resista nee of PCBThe thermal resistance is an important parameter for the heat dissipation of PCB. The worki ng curre nt in crease will cause some cha nge to the thermal resista nce of

33、 PCB.r- _Fig. 4. T he effect of w orking curre nt o n the t hermal resista nee of PCBThe effect of worki ng curre nt I ( mA ) on the thermal resista nee of PCB, Rjc (C /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 se

34、e n that whe n the high power LED work ing curre nt is kept con sta nt, the lowest thermal resistance of custom PCB is 1.3 C /W, while that of the commercial aluminum PCB can reach 5 C /W. It shows that the custom PCB significantly improves heat dissipation of high power LED lamps, because the therm

35、al conductivity of the custom PCB middle layer is higher tha n that of the commercial alumi num PCB. Therefore, the lower thermal resista nce can allow more heat tran sfer to the air, and improve the reliability of LED.3. 4 . The Effect of Differe nt Curre nts on the Perce ntage of PCB Thermal Resis

36、ta nceJ/mAIt is showed in Fig 5 that the effect of worki ng curre nt on the perce ntage of PCB thermal resistance in the whole LED lamp.From Fig 5, the highest percentage of PCB thermal resista nce, P(%) , in the whole high power LED lamp is only 5% , and it can be seen that LED junction temperature

37、 can not be lowered sig ni fica ntlywithdecreas ing PCB thermalresistance. If the PCB thermal resistance is reduced bli ndly, the heat dissipati on 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,

38、so it is hopeful to reduce the size, weight and cost of LED lamp.4 Con clusi onFour different kinds of PCB in LED lamps are investigated. PCB has the minimum thermal resista nce among all the heat dissipati on paths of LED lamps. The PCBs with differe nt thermal resista nces show differe nt effects

39、on jun ctio n temperature. PCB with lower thermal resista nce 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 depe ndence on heat sink. The match usage of lower thermal resista nce PCB and portable heat sinkcan acquire smaller and lighter LED lamp.References:1 Danie