基于纹波特性的白光LED器件动态及稳态光通量模型

Dynamic and Steady Luminous Flux Model for White LED Devices Based on Ripple Properties

在明确发光二极管(LED)器件输出动态光通量、负载电功率、发光效率、热功率系数之间关系的基础上,将理论模型拓展至包含LED驱动控制器的参数(频率、增益系数、电压幅值)以及散热器温度,从而通过调节LED器件参数以及驱动特性,控制白光LED照明系统最佳工作点的范围,进而分析照明系统闪烁指数、闪烁百分比、电压幅值、频率和LED光源参数之间的内在规律。提出一种基于纹波特性的白光LED器件动态光通量特性理论模型,该理论模型可准确预估白光LED照明系统在不同纹波特性下的动态照度变化、闪烁指数、闪烁百分比。通过测试不同工作条件下LED照明器件的照度、闪烁指数以及闪烁百分比,并与计算值进行比较,二者的平均误差为7.1%,计算结果和测量结果具有良好的一致性,证明了所提出的基于纹波特性动态及稳态光通量模型的有效性。由测试结果可知,LED光源在不同工作条件下闪烁指数以及闪烁百分比发生明显变化,这意味着需要通过合理地控制LED光源和驱动系统的参数,使照明系统的闪烁指数和闪烁百分比变化最小,从而满足IEEE国际标准对闪烁效应的要求。

Objective There are increasing concerns about flicker as lightemitting diode(LED)source products enter the market,which is linked to the driver.In a passive circuit,the light output exhibits a sinusoidal function of rippled current′s sinusoidal waveform.Color shifts and variations in luminous flux during dimming are generally unacceptable in general lighting.There are complex interconnections among white LED lighting system parameters,dynamic optical properties,ripple currents,LED light source device parameters,and LED driver parameters.However,the above issue lacks the necessary theory for clarifying the superiority.Therefore,an investigation of the illuminance and flicker variation of white LEDs driven by sinusoidal waveforms is presented in this paper.It incorporates factors such as illuminance,flicker index,percent flicker,voltage amplitude,frequency,amplification factor,and heatsink temperature into a relatively realistic model over dimming.This paper aims to present a method for designing LED systems with sinusoidal driving schemes that minimize flicker indexes and percent flicker variations systematically.In order to meet the flicker requirements set forth in IEEE Standard,the proposed model assists power supply engineers in controlling LED source and driver parameters.Methods Based on the interaction of photometric,electrical,and thermal factors of semiconductors,the maximum luminous flux,flicker index,and percent flicker of LED sources are modeled.Many parameters can affect luminous flux,flicker index,and percent flicker,including the heatsink temperature,the thermal resistance of the LED source,the heat dissipation coefficient,luminous efficacy,and driver parameters.A white LED lighting system has been used to demonstrate the proposed flicker modeling process.Light flicker analyzer(LFA3000)shows the waveform of an LED system′s light output with a sinusoidal wave of specified parameters.White LED system with different heatsink temperatures is electrically driven.A wideband amplifier(Texas Instruments ATA122D)in the direct current(DC)component from the DC power supply adds the signal function(Gigol DG500).The light output of the LED is captured from the detector(LFA3000)using the highspeed signal amplifying function.The luminance of LED samples is measured after thermal stability with constant heatsink temperature from 25℃to 85℃.The voltage amplitude ranges from 3 V to 5 V.The amplification factors vary from 2 dB to 6 dB.The frequency ranges from 100 Hz to 2000 Hz.LED source and photodetector are connected by the dark tube.There is a spacing of 20 cm between the source and the photodetector.Therefore,the ambient light does not influence the measurement results.Results and Discussions According to Eq.(8),heatsink temperature,and maximum electrical power,it is possible to predict the dynamic illumination of the LED source.A plot of the predicted and measured illumination variation is shown in Fig.5.The results appear to be fairly in agreement.When the heatsink temperature is 30℃,the variation illumination of the LED source ranges from 2688 lx to 4512 lx,and the variation range is about 59.8%.Increasing the electrical power to 2.1 W results in a variation illumination range of 11252-21033 lx,with a variation range of around 53.2%.At a heatsink temperature of 85℃,the variation illumination is 2532-4399 lx at a maximum electrical power of 0.35 W,while the variation range is about 57.5%.As electrical power and heatsink temperature increase,there is a decrease in the variation range.This can be attributed to several reasons.First,with an increase in current density injected into the quantum well and junction temperature,the reduction of band gap and electron mobility will decrease.It means that the radiative recombination of electrons and holes in the potential well will decrease with increasing nonradiative recombination.The reduction of the internal quantum efficiency is caused by an increase in the number of electrons overflowing the potential well.As shown in Figs.8 and 9,the average and maximum deviations between the calculations and measurements are about 7.1%and 12.8%,respectively.The illumination of white LED devices increases with increasing voltage amplitudes and amplification factors.It decreases with increasing frequency.When the voltage amplitude varies from 3 V to 3.5 V(amplification factors of 1 dB,heat sink temperature of 25℃,and frequency of 100 Hz),the illuminance increases from 3966 lx to 5889 lx,with a variation range of 32.6%.When the frequency changes from 100 Hz to 2000 Hz(amplification factors of 1 dB,heat sink temperature of 25℃,and voltage amplitude of 3 V),the illuminance decreases from 3966 lx to 2059 lx,with a variation range of 48.1%.When the heat sink temperature ranges from 25℃to 65℃(frequency of 100 Hz,amplification factors of 2 dB,and voltage amplitude of 4.5 V),the illuminance decreases from 13206 lx to 12904 lx,with a variation range of 2.3%.According to the proposed model,the deviations between theoretical and experimental results may be caused by the following factors:1)the proposed model does not include the droop effect of multiple quantum wells and the nonlinear relationship between amplification factors of current ripple and carrier concentration;2)the proposed model does not contain the relationship between Fermi energy level and voltage amplitude and cannot accurately predict threshold of carrier overflow potential well;3)the proposed model does not contain the threedimensional heat flow conduction and fails to accurately establish junction temperature of the device under different operating conditions.The ripple frequency of the LED device is 100-2000 Hz.Therefore,the allowable percent flicker is 0.3-66 according to IEEE standard 1789—2015(Fig.6).With a frequency of 100 Hz and maximum electrical power of 0.35 W,percent flicker is 0.276 and 0.289 under heatsink temperatures of 30℃and 85℃,respectively.It is noted that the values of percent flicker are lower than the requirements of IEEE standard.When the maximum electrical power is increased to 2.1 W,the percent flicker increases to 0.341 and 0.356 under heatsink temperatures of 30℃and 85℃,respectively.It should be pointed out that the values of percent flicker are higher than the requirements of IEEE standard.Conclusions A realtime LED measurement method is demonstrated in this paper,so as to analyze and develop dynamic light outputs in real time.The dynamic illuminance,percent flicker,and flicker index of LED sources can be calculated independently as a function of heatsink temperature,frequency,voltage amplitude,amplification factors,and electrical power.By using the proposed model,it is possible to convert the dynamic light output from LED sources into flicker indexes and percent flickers under different conditions.There is good agreement between measured and calculated optical and flicker results,even when measured at different heatsink temperatures and driver parameters.According to dynamic optical and flicker performance,the tool allows designers to optimize LED system designs.Therefore,researchers and engineers can determine dynamic illuminance and flicker index using the LED and driver datasheets instead of optical instruments.