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ABSTRACT
Cooling is a critical process in the thermal designs of high-voltage light-emitting diodes (HV-LEDs), whose junction temperatures are in turn an essential criterion for evaluations of LED aging, light-color quality, and luminous efficacies. In the present work, both numerically and experimentally, we have identified two cooling-related parameters, namely thermal crowdedness and nonuniformity, that are related to the lowering of junction temperatures of 17 microchips. Based on these two parameters, we have further proposed three regimes that facilitate the optimization. In addition, great care must be taken to incorporate the difficulty of tackling the high voltage into electrical-resistance selections. The 3D transient conjugate heat transfer is simulated using the finite element package named COMSOL. Finally, we hope that the present study can provide the photonic and cooling industries with the guidance for optimizing cooling characteristics in HV-LEDs based on these two parameters and three regimes.
Nomenclature
| A1, A2 | = | size of the microchip, |
| cp | = | specific heat with p held constant, |
| cv | = | specific heat with v held constant, |
| kair | = | thermal conductivity of air, |
| ks, kf | = | thermal conductivity of solid and fluid, |
| K | = | coefficient in the linear relationship between the forward voltage and the junction temperature, K/V |
| g | = | gravity acceleration, m/s2 |
| mil | = | conventional length unit used in LED industry |
| n | = | normal direction to the interface between solid and fluid |
| p | = | pressure of the fluid flow surrounding the assembly, Pa |
| Q | = | thermal power of the heat source, W |
| R | = | gas constant, |
| R1, R2, R3 | = | electrical resistance used in the test circuit, Ω |
| Ths | = | temperature of the heat sink, K |
| Ti | = | the junction temperature of the individual microchip in the chip array, K |
| Ts, Tf | = | temperature of solid and fluid, K |
| u, v, w | = | flow velocities in x, y, and z directions, mil/s |
| |v| | = | absolute velocity of the air, mil/s |
| = | voltage of sample assembly, | |
| VS | = | voltage provided by the external current source, V |
| = | voltage variation of sample assembly, R2 and R3, V | |
| ΔVS | = | voltage variation of the external current source, V |
| ΔT | = | difference of the average junction temperature in the simulation, K |
| ΔTexp | = | difference of the average junction temperature in the experiment, K |
| α | = | thermal crowdedness, defined in Eq. (13) |
| β | = | nonuniformity, defined in Eq. (15) |
| ϵ | = | gap between a pair of microchips located in the central column, mil |
| μ | = | dynamic viscosity, N · S/m2 |
| ρ | = | density of the fluid flow surrounding the assembly, Kg/m3 |
| σ | = | standard deviation of 17 microchips’ junction temperature, K |
| Subscripts | = | |
| f | = | fluid |
| i | = | node of the microchip |
| exp | = | experiment |
| s | = | solid |
| S | = | source |
Nomenclature
| A1, A2 | = | size of the microchip, |
| cp | = | specific heat with p held constant, |
| cv | = | specific heat with v held constant, |
| kair | = | thermal conductivity of air, |
| ks, kf | = | thermal conductivity of solid and fluid, |
| K | = | coefficient in the linear relationship between the forward voltage and the junction temperature, K/V |
| g | = | gravity acceleration, m/s2 |
| mil | = | conventional length unit used in LED industry |
| n | = | normal direction to the interface between solid and fluid |
| p | = | pressure of the fluid flow surrounding the assembly, Pa |
| Q | = | thermal power of the heat source, W |
| R | = | gas constant, |
| R1, R2, R3 | = | electrical resistance used in the test circuit, Ω |
| Ths | = | temperature of the heat sink, K |
| Ti | = | the junction temperature of the individual microchip in the chip array, K |
| Ts, Tf | = | temperature of solid and fluid, K |
| u, v, w | = | flow velocities in x, y, and z directions, mil/s |
| |v| | = | absolute velocity of the air, mil/s |
| = | voltage of sample assembly, | |
| VS | = | voltage provided by the external current source, V |
| = | voltage variation of sample assembly, R2 and R3, V | |
| ΔVS | = | voltage variation of the external current source, V |
| ΔT | = | difference of the average junction temperature in the simulation, K |
| ΔTexp | = | difference of the average junction temperature in the experiment, K |
| α | = | thermal crowdedness, defined in Eq. (13) |
| β | = | nonuniformity, defined in Eq. (15) |
| ϵ | = | gap between a pair of microchips located in the central column, mil |
| μ | = | dynamic viscosity, N · S/m2 |
| ρ | = | density of the fluid flow surrounding the assembly, Kg/m3 |
| σ | = | standard deviation of 17 microchips’ junction temperature, K |
| Subscripts | = | |
| f | = | fluid |
| i | = | node of the microchip |
| exp | = | experiment |
| s | = | solid |
| S | = | source |