| Light-Emitting Diodes(LEDs)have attracted the attention of researchers and the public as a new generation of lighting application in the context of the increasing energy crisis,and they are popularized all over the world.However,with LEDs gradually shifting towards developing high power applicaitons,the thermal management becomes increasingly prominent.Although many new technologies have been developed for LED cooling,commercial LED products consume 70%of their electrical input power in the form of heat.The influence of self heating effect on LED luminous efficiency stands very prominent,and the heat consumption of high-power LED is more serious.This paper first studies existing measuring methods of LED junction temperature,and proposes a feasible one.Second,we study the influence of LED temperature rise on the quantum efficiency.Third,we study the influent of voltage on the energy conversion efficiency.And finally optimize the heat sink.Follows are main works and innovation points:Measuring method of the junction temperature of LED.The temperature is closely related to the carrier concentration,which determines many electrical properties of the device in semiconductors.Therefore,accurate measurement of junction temperature plays an important role in researches of LED.At present,various methods to measure the junction temperatures enjoy their own advantages and disadvantages.Users can choose one of them according to practical occasions.Based on the Shockley equation,we propose an experimental method that determines junction temperatures in light-emitting diodes by measuring currents while holding the low forward voltages constant.In this procedure,we first calibrate current-temperature-relationship parameters under the condition of negligible thermal generation.Results show a nearly linear relationship between the algorithmic currents and temperature reciprocals under the condition of constant voltage.Thus the junction temperature can be obtained by collecting the current of LED with the help of the temperature sensitivity parameter.We test the blue and white LED's current at different temperatures to verify the correctness of the theoretical model.The comparison of junction temperatures measured by the forward voltage method and the proposed method has been conducted,and<2K deviation has been observed.We have found that the current is more sensitive with the temperature when the test voltage ranges between 2 V to 2.5 V,and reaches peak at 2.4V yielding most sensitive measurements of the junction temperature.The proposed method has the following advantages:1.It reduces the system error caused by the LED's self heat effect and improves the accuracy of the measurement.2.the sensitivity is two orders of magnitude higher than that of the forward voltage method,which is more advantageous in the case of slight temperature change.3.This method avoids the dark room environment and the expensive spectrometer equipment to measure and analyze the spectrum,reducing the cost of the enterprise and the laboratory.The effect of the self heat on the quantum efficiency.With the increase of the driving current,carrier concentration in the quantum well will increase,leading to enhancement of Auger recombination or carrier overflow.Meanwhile,the thermal power becomes larger and the junction temperature rises,in return decreasing the quantum efficiency.The design of the heat dissipation package aims at transferring this part of heat to the outside air to weaken its negative effect on LED.To quantify the influence of this heat on the quantum efficiency of LED,we qualify the influence of current and junction temperature on the quantum efficiency by mathematical deduction.We present the thermally-induced penalty(TIP),and drive the LED quantum efficiency with the self-heating effect only for a given bare LED.In our model,we found that excluding TIP,the quantum efficiency of blue or green LED decreases with the current increases,while it does not appear for red LED.From the experimental results,temperature rise will cause the decrease of quantum efficiency caused seriously in all LED.Highlights in this section lie:(1)using TIP to quantify the self heating-effect on LED quantum efficiency and performance evaluation of LED package,which can determine the heat dissipation capability of excellent package intuitively;(2)eliminate the influence of self thermal effect in the determination of quantum efficiency.The effect of voltage on the energy conversion efficiency of LED.Energy conversion efficiency is the most direct reflection of LEDs' energy saving function.When the forward voltage Vf<Vg,the energy of the injected electron is lower than the average photon energy emitted,resulting the radiation recombination needs to absorb external energy.Therefore,in the small voltage region,the increase of quantum efficiency can also be promoted by raising the external voltage or increasing temperature.When Vf= Vg,the quantum efficiency of the LED reaches peak,and the energy of the electron matches the average energy of the radiation photon.When Vf>Vg,we believe that,if the energy of the carrier of LED is greater than that of its radiant photon,the remaining energy will be released in the form of heat once radiation.According to this hypothesis,we theoretically analyze the relationship between the energy conversion efficiency and the quantum efficiency with the external voltage,and proposed a simple mathematical model.We build the experiment to verify the reliability of the method using red,green,blue,and UV LEDs as experimental samples.In comparison with traditional methods,our approach maintains a decent accuracy of no more than 1.4%error compared with the result by spectrometer.As the spectrum changes with current,red shift or blue shift will happen,which will cause some error directly to the final measurement results.However,the error of spectral shift is only 1.2%under general working conditions,which can be neglected.There are two reasons for the decrease of energy conversion efficiency under the condition of large current.One is the decrease of quantum efficiency,the other is the excessive carrier voltage due to the high external voltage,and the latter is the main reason for the decrease of energy conversion efficiency.Optimization of the heat sink.The LED chip is constructed with a small a small area and needs the auxiliary radiator to bring the heat generated into the air.Cooling the LED chip can improve the efficiency and plays an important role in energy-conservation advantage of LED.The heat dissipation effect of heat sink can be improved by using fins that are extensions on exterior surfaces of the peripheral arrangement.We choose a general multiple ray-fin plate as the research subject.Then we build a physical model of the sample and analysis it by COMSOL numerical simulation.Compared with the results measured by thermocouple,the maximum average temperature error of the heat source is less than 3%under the same heat power.When the number of grids is changed,the average temperature of the heat source varies insignificantly,indicating grid independence of simulations.Changing the length and width and the quantity of the fin,we study the cooling ability of the fin with different length,width,and the quantity of the fin.The results show that(1)as the fin length increases,the cooling capability of the heat sink will increase first and then decrease due to the small fin spacing preventing air from external circulation;(2)as the fin number increases,the result are the same as fin length;(3)fins whose cross sectional shapes are trapezoidal can provide greater cooling effects than their rectangular counterparts.We summarize several attentions in fin design.(1)the temperature of the circular plate center is the highest because it is near the heat source.The cold air outside the circular plate can cool down the fin directly.Since the temperature difference around the circlar plate is large and the flow rate there is also fast,leading to high convective heat transfer efficiency,therefore,we should increase the area or quantity of fins around edg properly.(2)the center temperature of the center of the fin group is the highest,but the peripheral velocity is almost zero,resulting in the retention of hot air and unfavorable heat dissipation.Cold air should be imported from the outside.(3)the length of fins should be desined appropriately to avoid obstructing air flow from outside,leading to the overall cooling effect worse.(4)improving surface radiation coefficient can effectively improve heat sink heat dissipation. |
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