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Thermal characterization of multicolor LED luminaire
MR-12366; No of Pages 10 Microelectronics Reliability xxx (2017) xxx–xxx
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Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Thermal characterization of multicolor LED luminaire A.M. Colaco a,d,⁎, C.P. Kurian a, Savitha G. Kini a, S.G. Colaco b, Cherian Johny c a
Department of Electrical and Electronics Engg., MIT, Manipal University, India Department of Electrical and Electronics Engg., St Joseph Engineering College, India c MIT, Manipal University, India d Department of Electrical and Electronics Engg., NMAM Institute of Technology, Nitte, India b
a r t i c l e
i n f o
Article history: Received 20 February 2017 Received in revised form 15 April 2017 Accepted 17 April 2017 Available online xxxx Keywords: Dynamic lighting scheme Multichip package Multicolor LED Heatsink ANSYS Icepak Thermal management Luminaire
a b s t r a c t The LED based dynamic lighting scheme, require compact and thermally efficient luminaire. This paper presents the thermal investigation on the conceptual design of 36 W multicolor light emitting diode (LED) luminaire. The developed prototype design includes configuration and placement of the multichip LED package, RGBW and single die amber LEDs in a 5 × 3 array on the heat sink. LED configurations with low power input are placed between the LEDs having the high power input. The proposed configuration and placement of LEDs reduces the local heat concentration in the centre region of the heat sink. The temperature of 72 °C at LED chip base plate is reduced to 32.1 °C on the heat sink surface. The numerical results are experimentally validated. The proposed method contributes to a reduction in the size of the luminaire and also enhancement of heat dissipation for improving the longevity of the multicolor based LED luminaire. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, energy saving lighting scheme uses Light Emitting Diodes (LEDs) in the cutting edge technology [1,2]. The temporal variations degrade electro-optical properties of the LEDs which form the bottleneck of the LED lighting system [3–7]. The degradation of primary lens and bonding material reduce heat transfer rate from LED chip to the metal core printed circuit board (MCPCB) [8–10]. The overheating of phosphor layer reduces the lifetime of the LED and hence Fan et al. introduced a method to insert a low thermal conducting material encapsulant between the LED die and the phosphor coated material to increase the LED lifetime [11]. In this context, research is gaining more attention towards LED thermal management. Thermal management redresses reliability and lifetime of LEDs. Several investigations have reported that the thermal dissipation from LED case to ambient occur due to series thermal resistances formed between the LED junction to the ambient [12]. The thermal management of LEDs can be categorized into two levels. Firstly the chip level and secondly as the system level. At Chip level heat produced during recombination at p-n junction poses a major challenge in material selection of LED chip substrate. The high heat concentration changes the material property of chip substrate. Hence, Senawiratne et al. [13] introduced GaN as LED die substrate to advance heat dissipation in the ⁎ Corresponding author. E-mail address: [email protected] (A.M. Colaco).
LED die. The LED die is attached to the metal core printed circuit board (MCPCB) using high thermal conductivity thermal paste. Thermal interface material includes thermal paste, thermal pad, silver paste and Au/Sn bonding among these Au/Sn is proved to give the best result [14] for heat conduction. The chip-level thermal management involves computation of the thermal resistance starting from the LED die up to the case (Rjc), which is usually specified in the LED datasheet. System level thermal management includes computation of thermal resistance of the material viz., thermal interface materials (TIM), MCPCB and heat sink, which forms the thermal path to transfer heat from LED case (Rjc) to ambient. The increase in thermal resistance of the thermal interface material and LED substrate is influenced by the ambient temperature and magnitude of input current [15,16]. To increase lifetime of the LED various modalities is proposed in the literature importantly generic system level approach [17], hierarchical life prediction model [18], a nonlinear filter-based approach [19] and recently Miao et al. proposed the step down accelerated degradation testing to limit optical parameters during initial stage [20]. An efficient integration of chip and system-level thermal management enhances lifetime of the LEDs. Jin Taek Kim et al. [21] worked on single chip LED with an 80 mW power to investigate the heat distribution through a backlight unit. The result showed maximum heat dissipation occurred at the lower part of frame panel for length less than 100 mm and in their study, it was observed that eliminating the air gaps between the LED package and frame panel, improved the heat dissipation. Yunqing Tang et al. investigated using Finite element method on the lifetime and thermal
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Please cite this article as: A.M. Colaco, et al., Thermal characterization of multicolor LED luminaire, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.04.026
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behaviour of the LEDs with GaN chip at different convection coefficient and different input power, using the TIM materials such as conductive silver paste, tin alloy solder and graphene solder. The result showed the graphene had low junction temperature at 30 W power input and low thermal resistance compared to conductive silver paste and tin alloy solder [22]. Muna E. Raypah et al. used T3 tester to investigate the thermal resistance coefficient for an SMD chip mounted on different materials such as FR4, and two types of aluminium substrate packages having different dielectric materials of thermal conductivity 2 W/m·K and 5 W/m·K at two current levels 50 mA and 100 mA. FIoEFD simulation tool was used to observe thermal profile. The authors have reported that the aluminium substrate having dielectric material had higher thermal dissipation [23]. Chun-Jen Weng showed FEM technique for simulating LED package with different chip size, cooling condition, heat slug and PCB [24]. Victor C. Bender et al. [4] used monocolored LEDs to investigate the electrothermal performance of the luminaire. The LED luminaire can be designed using any of these (a) monocolor LEDs, (b) mixing mono color single die LEDs and (c) using multicolor multichip package LEDs. The multicolor multichip package has red, green, blue and white (RGBW) dies on single Al base plate. The dynamic color lighting systems often require a compact size luminaire and this is achieved using multicolor multichip package LEDs. The multichip packages LEDs not only reduce quantity and cost of thermal interface material but also the size of the heat sink. The literature show most of the works are related to heat dissipation techniques at chip and system level using monocolor LEDs. Hsueh-Han Wua et al. [25] in his work showed by maintaining larger gap between the chips, high optical properties can be achieved for a GaN-based multichip model. The work by Jong Hwa Choi [26] made a comparative analysis on eighteen red LEDs mounted on single MCPCB. The comparative analysis between the performance without the thermal design, with the thermal grease and with thermal design showed higher optical performance for the thermally designed LEDs. The calibration factor computed at 47.5 °C, 55.2 °C, and 59 °C showed a difference of (0.03 to 0.13 nm/°C) and achieved 0.046 nm/°C at 47.5 °C junction temperature. The authors used FIOTHERM for simulation analysis. Recent investigation on thermal management show integration of external fan for cooling the LED luminaire to increase efficiency. Hui Huang Cheng et al. [27] used longitudinal multi-fin heat sink integrated with a fan for a multipackage LEDs. The investigation for the range of 0.54 W to 0.75 W and 1 W LEDs showed the heat sink with fan had higher convective with reduced junction temperature of 71.76 °C for 0.54 W, 89.91 °C for 0.75 W and 111.50 °C for 1 W respectively. The results were validated using finite element method using ANSYS11. Adam Christensen et al. [28] used forced and natural convection of cooling techniques to investigate the optical, heat convective coefficient and junction temperature on a 1 W, 3 W and 5 W multichip LED package and reported that the forced convection is more efficient and alternate methods such as heat pipes, fluid flow could be deployed for high performance of LED luminaire. The literature survey showed the analyses mainly focussed on monocolor multichip package. However, the thermal investigations on the multicolor multichip packages are limited. This paper presents thermal analysis on discrete multicolor multichip package mounted on heat sink. The forced conduction mode of cooling is deployed for numerical and experimental analysis.
Fig. 1. (a) Structural modelling of the LED used in ANSYS simulation; (b) multichip LED and bottom view of the LED [33].
0.05 [24,29]. The major sources of heat in LEDs are the active region and phosphor excitation. The heat from the phosphor can be extracted by the method suggested by Fan et al. [11] and also considering the silicone coated phosphor [30]. These models require bidirectional thermal resistance modelling [31]. Recently, Yupu Ma et al. [32] worked on
2. Theoretical background and thermal model 2.1. Steady state thermal analysis The cross section of the LED thermal stack is depicted in Fig. 1a. The Al base plate in Fig. 1b which is placed at the bottom of the chip, extracts heat from LED chip. Secondly, heat is conducted from Al base plate to MCPCB through the thermal paste. Finally, heat is conducted from MCPCB to heat sink through the TIM materials. The mode of radiation is neglected as the Cu slug and Al heat sink has a low emissivity of
Fig. 2. Thermal resistance network of the proposed model.
Please cite this article as: A.M. Colaco, et al., Thermal characterization of multicolor LED luminaire, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.04.026
A.M. Colaco et al. / Microelectronics Reliability xxx (2017) xxx–xxx Table 1 Power input for different LED strings and forward voltage of individual LEDs. Configuration
BNG RGC GC RC Amber
Power input watts @full rated current 350 mA
4.0950 3.6050 2.7300 2.2400 0.8750
LED type
Blue Green Cool, neutral Red, amber
Forward voltage in volts (V) Typ
Max
3.2 3.4 3.1 2.1
3.9 2.5
3
silicone coated phosphor chip and derived bidirectional thermal modelling which includes thermal resistance structure from phosphor to encapsulant and encapsulant to ambient. Secondly the thermal path resistance from junction to the ambient. The proposed method uses the technology [33] where InGaN (blue, green, white) and AlInGaP (red, amber) LEDs are placed on SiC substrate. The heat from SiC substrate is extracted via the LED leads before the phosphor particle reaches the encapsulant lens [34] as a result optical and thermal performance is improved. Hence heat transfer towards the topside is negligible due to the low thermal conductivities of epoxy, silicone and phosphor [26,27,35]. Therefore the total thermal resistance between the junction and the topside of the chip is neglected with respect to the bottom part of the chip. The heat is convected through the side walls of the heat sink and fins. The forced convection at the air velocity 1.2 m/s is considered in the simulation. Assumptions made in the modelling are: 1. 2. 3. 4.
All surfaces are non-radiant Steady state analysis is considered for heat transfer Air is incompressible Conjugated heat transfer was assumed at the interface of the solid and the gas boundary. 5. Gravitational acceleration was assumed to be − 9.81 m/s2 in the z-direction. In a 3D steady-state heat transfer in the LED active region due to Peltier effect, joule heating and thermal gradient is obtained by solving the Laplace equation ∇2 T þ
q ¼0 k
ð1Þ
The forced convection heat transfer at the rate of 1.2 m/s follows on all external surfaces of the heat sink and luminaire are in contact with the external environment E. The heat transfer through the conduction from chip to system level has to satisfy the Eq. (2) [36,37] ∂ ∂T ∂ ∂T ∂ ∂T kx þ ky þ kz þQ ¼0 ∂x ∂x ∂y ∂y ∂z ∂z
ð2Þ
kx, ky and kz are thermal conductivity in x-,y- and z- direction kx
∂T ∂T ∂T nx þ k y ny þ kz nz ¼ hðTS T ∞ Þ ∂x ∂y ∂z
ð3Þ
where h is heat transfer convection coefficient, TS surface temperature of the luminaire and T∞ is the ambient temperature. 2.2. Thermal resistance of the luminaire The thermal resistance and LED junction temperature influence the performance and reliability of the LED. The Fourier law gives the analogous network [38]. The overall thermal resistance from chip to system level for each LED string forming luminaire follows the Eqs. (4), (5), having Rstring_1 four LED configuration of blue, neutral white (BNG), Rstring_2 one configuration of six amber LEDs in series, Rstring_3 two LED configuration of red green cool white (RGC), Rstring_4 two red cool white (RC) and Rstring_5 one LED configuration of green cool white
Table 2 Details of the thermal stack materials.
Fig. 3. (a) Design 1 - amber LEDs on the periphery; (b) Design 2 - Amber LEDs between high power input LEDs and (c) ANSYS modelling of LED luminaire in a room.
Materials
Thermal conductivity (w/mK)
Al base plate Thermal paste MCPCB Thermal pad Heat sink-black anodized aluminium
150 0.765 2 5 205
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Rstring
5
¼ RLED
gc
þ Rmcpcb þ RTIM
The equivalent thermal resistance from chip to heat sink is given by Rc
h
¼
1 1 ¼ req Rstring
1
þ
1 Rstring
2
þ
1 Rstring
3
þ
1 Rstring
4
þ
1 Rstring
5
ð6Þ
The thermal resistance from heat sink/system level to ambient is based on the Eq. (7) Rc
h
¼
T j −T h Ph
ð7Þ
Heat spreading resistance of the heat sink (Rc) is given by Eq. (8) pffiffiffiffiffiffiffiffiffiffiffiffi Ap As λkAp RO þ tanhðλt Þ Rc ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffix k πAp As 1 þ λkAp RO tanhðλt Þ
ð8Þ
π3=2 1 λ ¼ pffiffiffiffiffiffi þ pffiffiffiffiffi As Ap
where Ap-contact area of the heat sink base plate, As–contact area of the heat source, t-thickness of the heat sink base plate, k-thermal conductivity of the heat sink base-plate, Ro-thermal resistance of the heat sink. The electrical input is given by the Eq. (9) 4
2
2
6
n¼1
n¼1
n¼1
n¼1
Pin ¼ ∑ P BNG þ ∑ P RGC þ ∑ P RC þ ∑P GC þ ∑ P Amber ¼ 36W
ð9Þ
The total electrical input the luminaire is the summation of electrical input dissipated as heat (Ph) and electrical input converted to light (Popt) P el ¼ P h þ P opt The heat dissipation (Kh) constant is given by the relation Kh ¼
P h P el −P opt ¼ ¼ ð1 wallplug efficiencyÞ P el P el
ð10Þ
To calculate thermal resisitance of the heat sink it is assumed 75% of the electrical power input is dissipated as heat [39]. Solving Eqs. (4)–(10) gives the thermal resistance of the heat sink from system level to ambient as 1.6 °C/W and thermal spreading resistance of the heat sink as 0.0035 °C/W. The total thermal resistance of heat sink is 1.60335 °C/W with the maximum LED junction temperature of 85 °C. The luminaire is designed for 500 lx on the working table, at 4500 K correlated color temperature (CCT). The Table 1 lists the power input for the different string configuration using details from data sheet [33,38]. 2.3. Structural layout of the luminaire
Fig. 4. Meshing of the geometrical model of the LED luminaire and convergence plot of the simulation.
(GC). The thermal resistance network from LED junction to the heat sink is shown in Fig. 2. Rmcpcb ¼
Thichness of MCPCB Area Thermal conductivity
Rstring
1
¼ Rn
bng
þ Rmcpcb þ RTIM
Rstring
2
¼ Rn
amber
Rstring
3
¼ Rn
rgc
Rstring
4
¼ Rn
rc
þ Rmcpcb þ RTIM
þ Rmcpcb þ RTIM
þ Rmcpcb þ RTIM
ð4Þ ð5Þ
The chip Al base plate size is 2.6 mm × 5.40 mm × 0.05 mm. The thermal paste thickness is 0.05 mm, which bonds chip Al base plate to MCPCB. The MCPCB is a star with 22 mm diameter and 2 mm thick [33]. TIM is 0.2 mm thick which interfaces between the MCPCB and black anodized heat sink of dimension 200 mm × 150 mm × 20 mm. Conceptual design includes mixed placement of InGaN (blue, green, white) and AlInGaP (red, amber) LEDs which is categorized into two designs. The design 1 comprises of placing amber LEDs on the periphery of the heat sink as illustrated in Fig. 3a and design 2 includes placing the amber having the low power input amidst the LEDs having high power input as demonstrated in Fig. 3b. The total of six single chip Amber LEDs and nine multichip RGBW LEDs are mounted on the heat sink. The hardware experimentation is conducted at an ambient temperature of 30 °C for forced conduction and similar settings are made for the ANSYS Icepak Fluent solver. The details of the material forming thermal stack are listed in Table 2. The heat is dissipated through conduction from the LED chip to heat sink and heat is expelled from the heat sink to
Please cite this article as: A.M. Colaco, et al., Thermal characterization of multicolor LED luminaire, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.04.026
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Fig. 5. Thermal profile at different layers on the LED luminaire and thermal gradient on heat sink fin for design1.
ambient by convection. The heat dissipation by the radiation is not considered as the heat sink with white paint has thermal resistance only 3% more than matte black in addition to it painting heat sink is less effective, with finned heat sink coating as heat radiated by the fin falls on the neighbouring fin coating [40]. To reflect the real time analysis in modelling the room in ANSYS is considered as ten times the size of the heat sink (Fig. 3c). 3. Results and discussions In the experimental investigation it is difficult to monitor directly the junction temperature of LED, hence as per literature [41,42] temperatures on MCPCB close to chip thermal pad/paste is measured. Fluke, IR thermometer is manually focussed on all LEDs at locations such as case temperature at T1, temperature on the joints between MCPCB and heat sink at location T2, and heat sink surface close to MCPCB at T3. The location of T1, T2 and T3 are shown in Fig. 1a. The simulation geometrical modelling is constructed, starting from chip Al base plate to the heat sink to form the thermal stack from chip to system level. 3.1. Simulation results The ANSYS IcePak uses CFD fluent solver. In the proposed conceptual model, the Reynolds and Peclet numbers are 15148.57 and 10732.9
hence, air flow is turbulent. The mesh quality and convergence plot of the simulation is illustrated in Fig. 4. The geometrical model constructed in ANSYS had 1752211 elements and 1783825 nodes. The quality of the mesh approaches to 1 indicating the good quality. The K-Omega model used in the simulation predicts the turbulence by solving two partial differential equations for two variables K and Omega. The K is the turbulence kinetic energy and Omega is the specific rate of dissipation. According to real-time operation of the LED luminaire in a room, with a forced air velocity of 1.2 m/s at an ambient temperature of 30 °C is monitored in the ANSYS Fluent input.
3.2. Numerical analysis for design 1: amber LEDs on the periphery of the heat sink The thermal gradient obtained for the design 1 is shown in Fig. 5. The maximum temperature of 72.4 °C is obtained for the BNG configuration due to the high power input with respect to remaining configuration. The lowest temperature of 40 °C is obtained for the amber LEDs. The thermal gradient at the centre region of the heat sink fins has a temperature of 32.3 °C at the fin bottom. The centre region of the heat sink surface has a local heat concentration of temperature 33 °C.
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3.3. Numerical analysis of design 2: amber LEDs amidst the high power input LEDs The Amber LEDs are placed between the LEDs having a high power input. The maximum temperature of 74.1 °C is achieved for the BNG configuration and amber LEDs has 40 °C which is demonstrated in Fig. 6. The thermal gradient in the centre region of the heat sink fin has 32 °C at the fin bottom. The design 2 has a uniform distribution of temperature on the heat sink surface. 3.4. Comparison between the designs The design 1 generated a local heat concentration of 33.1 °C on the heat sink surface as shown in Fig. 7a on the contrary, the design 2 shown in Fig. 7b produces uniform heat distribution on the heat sink surface. The design 2 influences the higher heat dissipation. The uniform distribution of heat by design 2 will generate low heat flux (W/m2) on the heat sink surface than design 1. 3.5. Experimental investigation in a real time room The simulation result for design2 showed uniform heat distribution on the heat sink surface hence, an experimental investigation is
conducted for the design 2. The experimental setup is shown in Fig. 8. In the experimental investigation, it is difficult to measure the temperature exactly on the chip and hence, the temperature is recorded at Al base plate. Using FLUKE IR thermometer temperature measurement are taken on the locations (a) MCPCB surface close to LED die package T1(b) TIM junction T2 and (c) heat sink surface region close to MCPCB T3 (Fig. 1a). The low thermal conductance of MCPCB to air makes temperature to reach steady state quickly usually on the full application of the full load current. Temperature reaches steady state after 2 h. The Fig. 9 shows the MATLAB plot for temperatures recorded by the Fluke IR thermometer.
3.6. Lifetime prediction of the LED luminaire The exponential lifetime model is considered to predict the lifetime of the LEDs which is given by Ae−θTj, where A = 477,377, θ = 0.052 [22]. The proposed conceptual design placement, configuration and selection of the heat sink has significantly drifted LED junction temperature of 72 °C to almost ambient temperature. The analytical computation shows maximum junction temperature of 85 °C can occur for the heat sink of thermal resistance 1.6 °C/W. Simulation result shows maximum temperature of 72 °C on chip Al base plate which is within the
Fig. 6. Thermal profile at different layers on the LED luminaire and thermal gradient on the heat sink fin for the design 2.
Please cite this article as: A.M. Colaco, et al., Thermal characterization of multicolor LED luminaire, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.04.026
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Fig. 7. Thermal gradient on the heat sink surface: (a) design 1; (b) design 2.
analytical computation. The exponential lifetime prediction model depicted in Fig. 10a shows at 72 °C, the lifetime of 11,290 h is obtained. The exponential degradation of LED lifetime due to the increase in junction temperature and high input power is demonstrated
in Fig. 10b and c. The proposed design has maximum temperature of 72 °C which ensures longevity and also delivers consistent lumen as per the graph of the relative flux vs junction temperature provided by the data sheet [33]. 4. Discussions The LEDs are sensitive to temperature variation, every 1 °C rise in temperature lead to shift in peak wavelength [43]. The Eq. (11) shows the dependency of the peak wavelength of LED (fp ) on the LED junction temperature (T j ), ambient temperature (T a), the thermal resistance of the heat sink (Rjc), LED current (I) and voltage (V). The rise in LED junction temperature affects the wavelength [44].
Fig. 8. Experimental set up with all LEDs turned ON for design 2.
C d df p λp nk I 2nk ln − ln T a þ Rjc þ NRhs IV 1 ¼ ¼ h β h dT j dT j
ð11Þ
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Fig. 9. Experimental plot of temperature on MCPCB, TIM and heat sink surface close to MCPCB for different LED configuration mounted on heat sink.
Color perception of LED source is evaluated by CCT which is influenced by the wavelength and spectral profile of the LED. The CCT is related to color coordinates by Macamy's polynomial [45] CCT ¼ 449n3 þ 3525n2 þ 6823:3n þ 5520:33
ð12Þ
where n = (x − xe) / (y − ye), xe = 0.3320, ye = 0.1858. The distance of the color from the Planckian locus in 1960 UV diagram is given by Eq.(13). Duv ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðu uoÞ2 þ ðv voÞ2
ð13Þ
The shift in peak wavelength deviate the correlated color temperature of the LED source from the set value. The color rendition of the light source also diverges from actual quality [46]. Hence, for the development of the luminaire it is necessary to investigate the thermal profile at different positioning of the LEDs on the heat sink. The placement of amber LEDs on the periphery of the centre LEDs was efficient to reduce temperature concentration at the centre region of the heat sink. An efficient thermal performance is obtained by the design 2, which directly improves optical performance of the luminaire. The comparison of experimental and simulated values at the contact regions of the LEDs for the design 2 is detailed in Table 3. The experimental investigation has higher values than the simulation result due
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5. Conclusions The paper elucidated the outcome of simulation and experimental investigation on a conceptual design of 36 W multicolor LED with non-uniform power inputs. The multichip LEDs supported to optimise heat sink surface area and design the LED luminaire with reduced materials and size. An investigation is made to realize the relationship between the junction temperature, power input and lifetime. These investigations focussed on real-time thermal performance of multichip package LED luminaire at an ambient temperature of 30 °C, an outcome of this analysis helps to optimise and design more complex configuration using multicolor high brightness LEDs.
Disclosures The authors declare there is no conflict of interest.
Acknowledgements Authors thank Manipal University and NMAM Institute of Technology, India for supporting their research work. Authors thank the faculties of Mechanical department Mr. Vijay G S, Mr. Manjunath M S, Mr. Madhwesh, Mr. Shivkumar and research scholar Mr. Chandrakant Bekal, Mr. Anand Hegde for their guidance and suggestions to conduct the experiment. This research received no specific grant from any funding agency in the public, commercial, or non-for-profit sectors. References
Fig. 10. Variations of: (a) lifetime with rise injunction temperature; b) lifetime with power input and c) junction temperature at different power input.
to following factors: Heat conduction by radiation is not accounted in simulations. However, in real time the heat sink surface and MCPCB conduct heat by radiation. The LED interconnection wires add to the temperature rise of the heat sink surface due to joule heating which is not accounted in ANSYS Icepak simulation. The real-time experimental result correlates well with the simulation result.
Table 3 Design 2 comparison of simulation and experimental. Element
Simulation result at the contact region of the LED on the heat sink
Experiment result at the contact region of LEDs on the heat sink
Deviation (%)
MCPCB BNG TIM BNG MCPCB RC TIM RC MCPCB RGC TIM RGC MCPCB GC TIM GC MCPCB AMBER TIM AMBER Heat sink surface
34.3 33.4 33.8 32.7 34.6 32.9 34.7 33.6 32.7 32.6 33.7
37.6 37.2 37.7 38 36 36.9 35 35.5 36 36.7 36.8
8.77 10.21 10.34 13.94 3.8 10.8 0.85 5.35 9.1 11.17 8.42
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Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Thermal characterization of multicolor LED luminaire A.M. Colaco a,d,⁎, C.P. Kurian a, Savitha G. Kini a, S.G. Colaco b, Cherian Johny c a
Department of Electrical and Electronics Engg., MIT, Manipal University, India Department of Electrical and Electronics Engg., St Joseph Engineering College, India c MIT, Manipal University, India d Department of Electrical and Electronics Engg., NMAM Institute of Technology, Nitte, India b
a r t i c l e
i n f o
Article history: Received 20 February 2017 Received in revised form 15 April 2017 Accepted 17 April 2017 Available online xxxx Keywords: Dynamic lighting scheme Multichip package Multicolor LED Heatsink ANSYS Icepak Thermal management Luminaire
a b s t r a c t The LED based dynamic lighting scheme, require compact and thermally efficient luminaire. This paper presents the thermal investigation on the conceptual design of 36 W multicolor light emitting diode (LED) luminaire. The developed prototype design includes configuration and placement of the multichip LED package, RGBW and single die amber LEDs in a 5 × 3 array on the heat sink. LED configurations with low power input are placed between the LEDs having the high power input. The proposed configuration and placement of LEDs reduces the local heat concentration in the centre region of the heat sink. The temperature of 72 °C at LED chip base plate is reduced to 32.1 °C on the heat sink surface. The numerical results are experimentally validated. The proposed method contributes to a reduction in the size of the luminaire and also enhancement of heat dissipation for improving the longevity of the multicolor based LED luminaire. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, energy saving lighting scheme uses Light Emitting Diodes (LEDs) in the cutting edge technology [1,2]. The temporal variations degrade electro-optical properties of the LEDs which form the bottleneck of the LED lighting system [3–7]. The degradation of primary lens and bonding material reduce heat transfer rate from LED chip to the metal core printed circuit board (MCPCB) [8–10]. The overheating of phosphor layer reduces the lifetime of the LED and hence Fan et al. introduced a method to insert a low thermal conducting material encapsulant between the LED die and the phosphor coated material to increase the LED lifetime [11]. In this context, research is gaining more attention towards LED thermal management. Thermal management redresses reliability and lifetime of LEDs. Several investigations have reported that the thermal dissipation from LED case to ambient occur due to series thermal resistances formed between the LED junction to the ambient [12]. The thermal management of LEDs can be categorized into two levels. Firstly the chip level and secondly as the system level. At Chip level heat produced during recombination at p-n junction poses a major challenge in material selection of LED chip substrate. The high heat concentration changes the material property of chip substrate. Hence, Senawiratne et al. [13] introduced GaN as LED die substrate to advance heat dissipation in the ⁎ Corresponding author. E-mail address: [email protected] (A.M. Colaco).
LED die. The LED die is attached to the metal core printed circuit board (MCPCB) using high thermal conductivity thermal paste. Thermal interface material includes thermal paste, thermal pad, silver paste and Au/Sn bonding among these Au/Sn is proved to give the best result [14] for heat conduction. The chip-level thermal management involves computation of the thermal resistance starting from the LED die up to the case (Rjc), which is usually specified in the LED datasheet. System level thermal management includes computation of thermal resistance of the material viz., thermal interface materials (TIM), MCPCB and heat sink, which forms the thermal path to transfer heat from LED case (Rjc) to ambient. The increase in thermal resistance of the thermal interface material and LED substrate is influenced by the ambient temperature and magnitude of input current [15,16]. To increase lifetime of the LED various modalities is proposed in the literature importantly generic system level approach [17], hierarchical life prediction model [18], a nonlinear filter-based approach [19] and recently Miao et al. proposed the step down accelerated degradation testing to limit optical parameters during initial stage [20]. An efficient integration of chip and system-level thermal management enhances lifetime of the LEDs. Jin Taek Kim et al. [21] worked on single chip LED with an 80 mW power to investigate the heat distribution through a backlight unit. The result showed maximum heat dissipation occurred at the lower part of frame panel for length less than 100 mm and in their study, it was observed that eliminating the air gaps between the LED package and frame panel, improved the heat dissipation. Yunqing Tang et al. investigated using Finite element method on the lifetime and thermal
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behaviour of the LEDs with GaN chip at different convection coefficient and different input power, using the TIM materials such as conductive silver paste, tin alloy solder and graphene solder. The result showed the graphene had low junction temperature at 30 W power input and low thermal resistance compared to conductive silver paste and tin alloy solder [22]. Muna E. Raypah et al. used T3 tester to investigate the thermal resistance coefficient for an SMD chip mounted on different materials such as FR4, and two types of aluminium substrate packages having different dielectric materials of thermal conductivity 2 W/m·K and 5 W/m·K at two current levels 50 mA and 100 mA. FIoEFD simulation tool was used to observe thermal profile. The authors have reported that the aluminium substrate having dielectric material had higher thermal dissipation [23]. Chun-Jen Weng showed FEM technique for simulating LED package with different chip size, cooling condition, heat slug and PCB [24]. Victor C. Bender et al. [4] used monocolored LEDs to investigate the electrothermal performance of the luminaire. The LED luminaire can be designed using any of these (a) monocolor LEDs, (b) mixing mono color single die LEDs and (c) using multicolor multichip package LEDs. The multicolor multichip package has red, green, blue and white (RGBW) dies on single Al base plate. The dynamic color lighting systems often require a compact size luminaire and this is achieved using multicolor multichip package LEDs. The multichip packages LEDs not only reduce quantity and cost of thermal interface material but also the size of the heat sink. The literature show most of the works are related to heat dissipation techniques at chip and system level using monocolor LEDs. Hsueh-Han Wua et al. [25] in his work showed by maintaining larger gap between the chips, high optical properties can be achieved for a GaN-based multichip model. The work by Jong Hwa Choi [26] made a comparative analysis on eighteen red LEDs mounted on single MCPCB. The comparative analysis between the performance without the thermal design, with the thermal grease and with thermal design showed higher optical performance for the thermally designed LEDs. The calibration factor computed at 47.5 °C, 55.2 °C, and 59 °C showed a difference of (0.03 to 0.13 nm/°C) and achieved 0.046 nm/°C at 47.5 °C junction temperature. The authors used FIOTHERM for simulation analysis. Recent investigation on thermal management show integration of external fan for cooling the LED luminaire to increase efficiency. Hui Huang Cheng et al. [27] used longitudinal multi-fin heat sink integrated with a fan for a multipackage LEDs. The investigation for the range of 0.54 W to 0.75 W and 1 W LEDs showed the heat sink with fan had higher convective with reduced junction temperature of 71.76 °C for 0.54 W, 89.91 °C for 0.75 W and 111.50 °C for 1 W respectively. The results were validated using finite element method using ANSYS11. Adam Christensen et al. [28] used forced and natural convection of cooling techniques to investigate the optical, heat convective coefficient and junction temperature on a 1 W, 3 W and 5 W multichip LED package and reported that the forced convection is more efficient and alternate methods such as heat pipes, fluid flow could be deployed for high performance of LED luminaire. The literature survey showed the analyses mainly focussed on monocolor multichip package. However, the thermal investigations on the multicolor multichip packages are limited. This paper presents thermal analysis on discrete multicolor multichip package mounted on heat sink. The forced conduction mode of cooling is deployed for numerical and experimental analysis.
Fig. 1. (a) Structural modelling of the LED used in ANSYS simulation; (b) multichip LED and bottom view of the LED [33].
0.05 [24,29]. The major sources of heat in LEDs are the active region and phosphor excitation. The heat from the phosphor can be extracted by the method suggested by Fan et al. [11] and also considering the silicone coated phosphor [30]. These models require bidirectional thermal resistance modelling [31]. Recently, Yupu Ma et al. [32] worked on
2. Theoretical background and thermal model 2.1. Steady state thermal analysis The cross section of the LED thermal stack is depicted in Fig. 1a. The Al base plate in Fig. 1b which is placed at the bottom of the chip, extracts heat from LED chip. Secondly, heat is conducted from Al base plate to MCPCB through the thermal paste. Finally, heat is conducted from MCPCB to heat sink through the TIM materials. The mode of radiation is neglected as the Cu slug and Al heat sink has a low emissivity of
Fig. 2. Thermal resistance network of the proposed model.
Please cite this article as: A.M. Colaco, et al., Thermal characterization of multicolor LED luminaire, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.04.026
A.M. Colaco et al. / Microelectronics Reliability xxx (2017) xxx–xxx Table 1 Power input for different LED strings and forward voltage of individual LEDs. Configuration
BNG RGC GC RC Amber
Power input watts @full rated current 350 mA
4.0950 3.6050 2.7300 2.2400 0.8750
LED type
Blue Green Cool, neutral Red, amber
Forward voltage in volts (V) Typ
Max
3.2 3.4 3.1 2.1
3.9 2.5
3
silicone coated phosphor chip and derived bidirectional thermal modelling which includes thermal resistance structure from phosphor to encapsulant and encapsulant to ambient. Secondly the thermal path resistance from junction to the ambient. The proposed method uses the technology [33] where InGaN (blue, green, white) and AlInGaP (red, amber) LEDs are placed on SiC substrate. The heat from SiC substrate is extracted via the LED leads before the phosphor particle reaches the encapsulant lens [34] as a result optical and thermal performance is improved. Hence heat transfer towards the topside is negligible due to the low thermal conductivities of epoxy, silicone and phosphor [26,27,35]. Therefore the total thermal resistance between the junction and the topside of the chip is neglected with respect to the bottom part of the chip. The heat is convected through the side walls of the heat sink and fins. The forced convection at the air velocity 1.2 m/s is considered in the simulation. Assumptions made in the modelling are: 1. 2. 3. 4.
All surfaces are non-radiant Steady state analysis is considered for heat transfer Air is incompressible Conjugated heat transfer was assumed at the interface of the solid and the gas boundary. 5. Gravitational acceleration was assumed to be − 9.81 m/s2 in the z-direction. In a 3D steady-state heat transfer in the LED active region due to Peltier effect, joule heating and thermal gradient is obtained by solving the Laplace equation ∇2 T þ
q ¼0 k
ð1Þ
The forced convection heat transfer at the rate of 1.2 m/s follows on all external surfaces of the heat sink and luminaire are in contact with the external environment E. The heat transfer through the conduction from chip to system level has to satisfy the Eq. (2) [36,37] ∂ ∂T ∂ ∂T ∂ ∂T kx þ ky þ kz þQ ¼0 ∂x ∂x ∂y ∂y ∂z ∂z
ð2Þ
kx, ky and kz are thermal conductivity in x-,y- and z- direction kx
∂T ∂T ∂T nx þ k y ny þ kz nz ¼ hðTS T ∞ Þ ∂x ∂y ∂z
ð3Þ
where h is heat transfer convection coefficient, TS surface temperature of the luminaire and T∞ is the ambient temperature. 2.2. Thermal resistance of the luminaire The thermal resistance and LED junction temperature influence the performance and reliability of the LED. The Fourier law gives the analogous network [38]. The overall thermal resistance from chip to system level for each LED string forming luminaire follows the Eqs. (4), (5), having Rstring_1 four LED configuration of blue, neutral white (BNG), Rstring_2 one configuration of six amber LEDs in series, Rstring_3 two LED configuration of red green cool white (RGC), Rstring_4 two red cool white (RC) and Rstring_5 one LED configuration of green cool white
Table 2 Details of the thermal stack materials.
Fig. 3. (a) Design 1 - amber LEDs on the periphery; (b) Design 2 - Amber LEDs between high power input LEDs and (c) ANSYS modelling of LED luminaire in a room.
Materials
Thermal conductivity (w/mK)
Al base plate Thermal paste MCPCB Thermal pad Heat sink-black anodized aluminium
150 0.765 2 5 205
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Rstring
5
¼ RLED
gc
þ Rmcpcb þ RTIM
The equivalent thermal resistance from chip to heat sink is given by Rc
h
¼
1 1 ¼ req Rstring
1
þ
1 Rstring
2
þ
1 Rstring
3
þ
1 Rstring
4
þ
1 Rstring
5
ð6Þ
The thermal resistance from heat sink/system level to ambient is based on the Eq. (7) Rc
h
¼
T j −T h Ph
ð7Þ
Heat spreading resistance of the heat sink (Rc) is given by Eq. (8) pffiffiffiffiffiffiffiffiffiffiffiffi Ap As λkAp RO þ tanhðλt Þ Rc ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffix k πAp As 1 þ λkAp RO tanhðλt Þ
ð8Þ
π3=2 1 λ ¼ pffiffiffiffiffiffi þ pffiffiffiffiffi As Ap
where Ap-contact area of the heat sink base plate, As–contact area of the heat source, t-thickness of the heat sink base plate, k-thermal conductivity of the heat sink base-plate, Ro-thermal resistance of the heat sink. The electrical input is given by the Eq. (9) 4
2
2
6
n¼1
n¼1
n¼1
n¼1
Pin ¼ ∑ P BNG þ ∑ P RGC þ ∑ P RC þ ∑P GC þ ∑ P Amber ¼ 36W
ð9Þ
The total electrical input the luminaire is the summation of electrical input dissipated as heat (Ph) and electrical input converted to light (Popt) P el ¼ P h þ P opt The heat dissipation (Kh) constant is given by the relation Kh ¼
P h P el −P opt ¼ ¼ ð1 wallplug efficiencyÞ P el P el
ð10Þ
To calculate thermal resisitance of the heat sink it is assumed 75% of the electrical power input is dissipated as heat [39]. Solving Eqs. (4)–(10) gives the thermal resistance of the heat sink from system level to ambient as 1.6 °C/W and thermal spreading resistance of the heat sink as 0.0035 °C/W. The total thermal resistance of heat sink is 1.60335 °C/W with the maximum LED junction temperature of 85 °C. The luminaire is designed for 500 lx on the working table, at 4500 K correlated color temperature (CCT). The Table 1 lists the power input for the different string configuration using details from data sheet [33,38]. 2.3. Structural layout of the luminaire
Fig. 4. Meshing of the geometrical model of the LED luminaire and convergence plot of the simulation.
(GC). The thermal resistance network from LED junction to the heat sink is shown in Fig. 2. Rmcpcb ¼
Thichness of MCPCB Area Thermal conductivity
Rstring
1
¼ Rn
bng
þ Rmcpcb þ RTIM
Rstring
2
¼ Rn
amber
Rstring
3
¼ Rn
rgc
Rstring
4
¼ Rn
rc
þ Rmcpcb þ RTIM
þ Rmcpcb þ RTIM
þ Rmcpcb þ RTIM
ð4Þ ð5Þ
The chip Al base plate size is 2.6 mm × 5.40 mm × 0.05 mm. The thermal paste thickness is 0.05 mm, which bonds chip Al base plate to MCPCB. The MCPCB is a star with 22 mm diameter and 2 mm thick [33]. TIM is 0.2 mm thick which interfaces between the MCPCB and black anodized heat sink of dimension 200 mm × 150 mm × 20 mm. Conceptual design includes mixed placement of InGaN (blue, green, white) and AlInGaP (red, amber) LEDs which is categorized into two designs. The design 1 comprises of placing amber LEDs on the periphery of the heat sink as illustrated in Fig. 3a and design 2 includes placing the amber having the low power input amidst the LEDs having high power input as demonstrated in Fig. 3b. The total of six single chip Amber LEDs and nine multichip RGBW LEDs are mounted on the heat sink. The hardware experimentation is conducted at an ambient temperature of 30 °C for forced conduction and similar settings are made for the ANSYS Icepak Fluent solver. The details of the material forming thermal stack are listed in Table 2. The heat is dissipated through conduction from the LED chip to heat sink and heat is expelled from the heat sink to
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A.M. Colaco et al. / Microelectronics Reliability xxx (2017) xxx–xxx
5
Fig. 5. Thermal profile at different layers on the LED luminaire and thermal gradient on heat sink fin for design1.
ambient by convection. The heat dissipation by the radiation is not considered as the heat sink with white paint has thermal resistance only 3% more than matte black in addition to it painting heat sink is less effective, with finned heat sink coating as heat radiated by the fin falls on the neighbouring fin coating [40]. To reflect the real time analysis in modelling the room in ANSYS is considered as ten times the size of the heat sink (Fig. 3c). 3. Results and discussions In the experimental investigation it is difficult to monitor directly the junction temperature of LED, hence as per literature [41,42] temperatures on MCPCB close to chip thermal pad/paste is measured. Fluke, IR thermometer is manually focussed on all LEDs at locations such as case temperature at T1, temperature on the joints between MCPCB and heat sink at location T2, and heat sink surface close to MCPCB at T3. The location of T1, T2 and T3 are shown in Fig. 1a. The simulation geometrical modelling is constructed, starting from chip Al base plate to the heat sink to form the thermal stack from chip to system level. 3.1. Simulation results The ANSYS IcePak uses CFD fluent solver. In the proposed conceptual model, the Reynolds and Peclet numbers are 15148.57 and 10732.9
hence, air flow is turbulent. The mesh quality and convergence plot of the simulation is illustrated in Fig. 4. The geometrical model constructed in ANSYS had 1752211 elements and 1783825 nodes. The quality of the mesh approaches to 1 indicating the good quality. The K-Omega model used in the simulation predicts the turbulence by solving two partial differential equations for two variables K and Omega. The K is the turbulence kinetic energy and Omega is the specific rate of dissipation. According to real-time operation of the LED luminaire in a room, with a forced air velocity of 1.2 m/s at an ambient temperature of 30 °C is monitored in the ANSYS Fluent input.
3.2. Numerical analysis for design 1: amber LEDs on the periphery of the heat sink The thermal gradient obtained for the design 1 is shown in Fig. 5. The maximum temperature of 72.4 °C is obtained for the BNG configuration due to the high power input with respect to remaining configuration. The lowest temperature of 40 °C is obtained for the amber LEDs. The thermal gradient at the centre region of the heat sink fins has a temperature of 32.3 °C at the fin bottom. The centre region of the heat sink surface has a local heat concentration of temperature 33 °C.
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3.3. Numerical analysis of design 2: amber LEDs amidst the high power input LEDs The Amber LEDs are placed between the LEDs having a high power input. The maximum temperature of 74.1 °C is achieved for the BNG configuration and amber LEDs has 40 °C which is demonstrated in Fig. 6. The thermal gradient in the centre region of the heat sink fin has 32 °C at the fin bottom. The design 2 has a uniform distribution of temperature on the heat sink surface. 3.4. Comparison between the designs The design 1 generated a local heat concentration of 33.1 °C on the heat sink surface as shown in Fig. 7a on the contrary, the design 2 shown in Fig. 7b produces uniform heat distribution on the heat sink surface. The design 2 influences the higher heat dissipation. The uniform distribution of heat by design 2 will generate low heat flux (W/m2) on the heat sink surface than design 1. 3.5. Experimental investigation in a real time room The simulation result for design2 showed uniform heat distribution on the heat sink surface hence, an experimental investigation is
conducted for the design 2. The experimental setup is shown in Fig. 8. In the experimental investigation, it is difficult to measure the temperature exactly on the chip and hence, the temperature is recorded at Al base plate. Using FLUKE IR thermometer temperature measurement are taken on the locations (a) MCPCB surface close to LED die package T1(b) TIM junction T2 and (c) heat sink surface region close to MCPCB T3 (Fig. 1a). The low thermal conductance of MCPCB to air makes temperature to reach steady state quickly usually on the full application of the full load current. Temperature reaches steady state after 2 h. The Fig. 9 shows the MATLAB plot for temperatures recorded by the Fluke IR thermometer.
3.6. Lifetime prediction of the LED luminaire The exponential lifetime model is considered to predict the lifetime of the LEDs which is given by Ae−θTj, where A = 477,377, θ = 0.052 [22]. The proposed conceptual design placement, configuration and selection of the heat sink has significantly drifted LED junction temperature of 72 °C to almost ambient temperature. The analytical computation shows maximum junction temperature of 85 °C can occur for the heat sink of thermal resistance 1.6 °C/W. Simulation result shows maximum temperature of 72 °C on chip Al base plate which is within the
Fig. 6. Thermal profile at different layers on the LED luminaire and thermal gradient on the heat sink fin for the design 2.
Please cite this article as: A.M. Colaco, et al., Thermal characterization of multicolor LED luminaire, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.04.026
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Fig. 7. Thermal gradient on the heat sink surface: (a) design 1; (b) design 2.
analytical computation. The exponential lifetime prediction model depicted in Fig. 10a shows at 72 °C, the lifetime of 11,290 h is obtained. The exponential degradation of LED lifetime due to the increase in junction temperature and high input power is demonstrated
in Fig. 10b and c. The proposed design has maximum temperature of 72 °C which ensures longevity and also delivers consistent lumen as per the graph of the relative flux vs junction temperature provided by the data sheet [33]. 4. Discussions The LEDs are sensitive to temperature variation, every 1 °C rise in temperature lead to shift in peak wavelength [43]. The Eq. (11) shows the dependency of the peak wavelength of LED (fp ) on the LED junction temperature (T j ), ambient temperature (T a), the thermal resistance of the heat sink (Rjc), LED current (I) and voltage (V). The rise in LED junction temperature affects the wavelength [44].
Fig. 8. Experimental set up with all LEDs turned ON for design 2.
C d df p λp nk I 2nk ln − ln T a þ Rjc þ NRhs IV 1 ¼ ¼ h β h dT j dT j
ð11Þ
Please cite this article as: A.M. Colaco, et al., Thermal characterization of multicolor LED luminaire, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.04.026
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Fig. 9. Experimental plot of temperature on MCPCB, TIM and heat sink surface close to MCPCB for different LED configuration mounted on heat sink.
Color perception of LED source is evaluated by CCT which is influenced by the wavelength and spectral profile of the LED. The CCT is related to color coordinates by Macamy's polynomial [45] CCT ¼ 449n3 þ 3525n2 þ 6823:3n þ 5520:33
ð12Þ
where n = (x − xe) / (y − ye), xe = 0.3320, ye = 0.1858. The distance of the color from the Planckian locus in 1960 UV diagram is given by Eq.(13). Duv ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðu uoÞ2 þ ðv voÞ2
ð13Þ
The shift in peak wavelength deviate the correlated color temperature of the LED source from the set value. The color rendition of the light source also diverges from actual quality [46]. Hence, for the development of the luminaire it is necessary to investigate the thermal profile at different positioning of the LEDs on the heat sink. The placement of amber LEDs on the periphery of the centre LEDs was efficient to reduce temperature concentration at the centre region of the heat sink. An efficient thermal performance is obtained by the design 2, which directly improves optical performance of the luminaire. The comparison of experimental and simulated values at the contact regions of the LEDs for the design 2 is detailed in Table 3. The experimental investigation has higher values than the simulation result due
Please cite this article as: A.M. Colaco, et al., Thermal characterization of multicolor LED luminaire, Microelectronics Reliability (2017), http:// dx.doi.org/10.1016/j.microrel.2017.04.026
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5. Conclusions The paper elucidated the outcome of simulation and experimental investigation on a conceptual design of 36 W multicolor LED with non-uniform power inputs. The multichip LEDs supported to optimise heat sink surface area and design the LED luminaire with reduced materials and size. An investigation is made to realize the relationship between the junction temperature, power input and lifetime. These investigations focussed on real-time thermal performance of multichip package LED luminaire at an ambient temperature of 30 °C, an outcome of this analysis helps to optimise and design more complex configuration using multicolor high brightness LEDs.
Disclosures The authors declare there is no conflict of interest.
Acknowledgements Authors thank Manipal University and NMAM Institute of Technology, India for supporting their research work. Authors thank the faculties of Mechanical department Mr. Vijay G S, Mr. Manjunath M S, Mr. Madhwesh, Mr. Shivkumar and research scholar Mr. Chandrakant Bekal, Mr. Anand Hegde for their guidance and suggestions to conduct the experiment. This research received no specific grant from any funding agency in the public, commercial, or non-for-profit sectors. References
Fig. 10. Variations of: (a) lifetime with rise injunction temperature; b) lifetime with power input and c) junction temperature at different power input.
to following factors: Heat conduction by radiation is not accounted in simulations. However, in real time the heat sink surface and MCPCB conduct heat by radiation. The LED interconnection wires add to the temperature rise of the heat sink surface due to joule heating which is not accounted in ANSYS Icepak simulation. The real-time experimental result correlates well with the simulation result.
Table 3 Design 2 comparison of simulation and experimental. Element
Simulation result at the contact region of the LED on the heat sink
Experiment result at the contact region of LEDs on the heat sink
Deviation (%)
MCPCB BNG TIM BNG MCPCB RC TIM RC MCPCB RGC TIM RGC MCPCB GC TIM GC MCPCB AMBER TIM AMBER Heat sink surface
34.3 33.4 33.8 32.7 34.6 32.9 34.7 33.6 32.7 32.6 33.7
37.6 37.2 37.7 38 36 36.9 35 35.5 36 36.7 36.8
8.77 10.21 10.34 13.94 3.8 10.8 0.85 5.35 9.1 11.17 8.42
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