Optimization of Helium Inflating on Heat Dissipation and Luminescence Properties of the A60 LED Filament Lamps

LED filament lamp has the characteristics of nearly 360° lighting angle, high brightness, and low energy consumption, turning it gradually into the best substitute for traditional incandescent lamps. At present, due to the limitations of heat dissipation, the development of high-power LED filament lamp is restricted. Helium is a rare gas with small density and high heat transfer coefficient. It can be used as a cooling and protective gas for LED filament lamp. In this paper, we investigated the effects of helium on the heat dissipation and luminescence performance of the A60 LED filament lamps by detecting the changes of junction temperature, color temperature, and luminous flux of different ratios helium inflating in the different power A60 LED filament lamps. Through the experiment, we found the most cost-effective ratio of helium gas in the A60 LED filament lamps without improving the lamp size and the filament diameter.


Introduction
The LED filament lamp has both the appearance of the traditional incandescent lamp and the higher photoelectric conversion of LED lamp.In 2008, Ushio Lighting introduced the first LED filament lamp [1].Panasonic described a flat arrangement with modules similar to filaments in 2014 [2].Unfortunately, the products did not gain widespread market acceptance due to the poor thermal dissipation and luminous flux.This paper investigates the heat dissipation to increase the luminescence of LED filament lamp.The LED filament lamp was usually made up of chips, transparent substrate, and phosphors [3].According to the development of substrate materials, LED filament lamps have undergone three generations from transparent ceramics, quartz glass to sapphire [4,5].
Although the LED filament lamp has the basic structure similar to the traditional incandescent lamp, its luminous structure is essentially different from the incandescent lamp.The incandescent lamp relied on the tungsten filament to emit visible light at high temperature, while the LED filament lamp relied on filaments of the LED.However, because the phosphor completely wrapped around the filament, the heat dissipation effect of the filament was limited [6], which was also the main reason to restrict the high power of LED filament lamp [7].
Besides the lamp size, the filament phosphor diameter, and ionic wind [8,9], this paper mainly explored the application of helium inflating to enhance the heat dissipation effect of LED filament lamp.As the helium is one of the inert gases, with its low density and high thermal conductivity, it was very suitable to be used to extract heat from the LED filaments [10,11].In this paper, the influence of helium on the heat dissipation and luminous performance of LED filament lamps was explored by detecting the change of the temperature and luminous flux of different helium ratios at the A60 LED filament lamps with different power.

Experimental and Theoretical Details
2.1.Thermal Conductivity of Helium.This paper explored the heat dissipation and luminescent properties of LED filament lamps filled with helium.Helium is a colorless and tasteless inert gas.Under normal pressure and temperature, the density of helium is 0.178 g/L.The helium molecule is very small and is just placed behind the hydrogen with the smallest density.As shown in Table 1, the thermal conductivity of helium is 0.144 W/(m• °C) which is a little smaller than the thermal conductivity of hydrogen 0.163 W/(m• °C).As an inert protective gas, helium does not react with the material in the LED filament lamp.In Table 1, the thermal conductivity of helium is higher than argon, neon, oxygen, nitrogen, and air.This was the reason why helium was chosen for heat dissipation and gas protection.

Convective Heat Dissipation Mechanism of the A60 LED
Filament Lamp.The A60 LED filament lamp was the spherical shell with a maximum diameter of 60 mm, as shown in Figure 1.In our experiment, the power values of the A60 LED filament lamps were 4 W, 7 W, and 8 W. The chips were spot welded in series on the holder of filament to form the LED filament.The holders of the filament and LED filament are shown in Figure 2.These four LED filaments were in series to form the A60 LED filament lamp.A linear drive was provided in the lamp holder to support a stable current input for the LED lamp.The driving currents of 4 W, 7 W, and 8 W in the A60 LED filament lamps were 13.5 mA, 22 mA, and 25 mA, respectively.
In the whole lamp, the heat is transferred from the LED filament to the helium when the helium is passing through the LED lamp in the high-temperature work.The density of the heating helium is smaller than the helium at the low temperature, so the expansion rises.And the heat is transferred to the glass shell from the filament, and then the next cycle starts.In the whole process, gas heat conduction followed Fourier law [12].This phenomenon is affected by the natural factors of the fluid itself.When applied to fluid cooling, convection calculation is shown as follows: Among them, Q is the convection heat dissipation (the unit is W), and h is the thermal conductivity (the unit is W/m• °C).A is an effective heat transfer area.The temperature difference between the heat transfer surface and the fluid is ΔT.The Fourier law can be applied to the convection heat transfer process.In the process of heat convection in the lamp, according to equation ( 1), the thermal conductivity is the key factor affecting convective heat transfer under the condition of the same temperature difference and the same area.

The Effect of Chip Junction
Temperature on the Performance of LED.The relationship between the main wavelength of LED filament lamp and the junction temperature of the chip is shown as follows:   In equation ( 2), λ 0 T and λ 0 T 0 are the main wavelengths of the emitted light at T and T 0 , respectively.ΔT is the temperature difference, that is to say the main wavelength of the chip changed correspondingly with the temperature difference, thus changing the spectral composition of the chip [13].The increase of the junction temperature also affected the color temperature of the LED filament lamp, making the color temperature drift.The lifetime of LED chip increased exponentially with the decrease of the junction temperature.When the temperature of the chip rises from 40 °C to 50 °C, the life span of the chip could be shortened from 42,000 hours to 18,000 hours [14].In this paper, through the influence of helium on the heat dissipation, luminescence performance and color temperature of the A60 LED filament lamps were explored by detecting the change of the junction temperature and luminous flux in the A60 LED filament lamps with different powers.
2.4.Testing Measurements.The instrument LEDT300/H was specially designed and developed for measuring the junction temperature of LED filament lamps.The main test method was to put the test lamp in the chamber of the LEDT300B/H and then connected the thermocouple with the current input line of the lamp to test the junction temperature.That was because the junction and the current input line connected directly, the temperatures between them were the same.When the lamp was working in the normal working voltage range, the exact value of the working voltage was obtained when it reached the stable state, and the junction temperature of the lamp in the stable operation was obtained by combining the value of the voltage temperature coefficient.
The spectrum integration sphere PMS-80 was mainly used to test the photoelectric parameters of the LED filament lamps.The working way of the integral ball was to place the test light source or lamp in the center of the integral ball.After applying the working voltage, the light was reflected multiple times on the inner wall of the integrating sphere and finally concentrated on the window hole.The external connecting device of the integrating sphere concentrated the light at the window hole and finally calculated the photoelectric parameters of the test light source or the lamp, which included the parameters of luminous flux and color temperature.

Results and Discussion
The positive chip mode was used in the A60 LED filament lamps to meet the process stability in this experiment.The transparent ceramic substrate was used to improve their performance price ratio in this lamp.The A60 LED filament met the European state standard.The A60 LED filament with a power of 4 W had a size of 38 mm × 1 5 mm, with 24 LED chips in series.The LED filaments with the powers of 7 W and 8 W both had the sizes of 50 mm × 1 8 mm, containing 25 LED chips in series inside.The difference between them was the power of the internal LED chips.The following figures are about the junction temperature and luminous properties of the A60 LED filament lamps (powers of 4 W, 7 W, and 8 W) under different helium inflating quantities (50%, 60%, 70%, 80%, and 90%) with the different pressures ranged from 0.5 Pa to 0.9 Pa.The junction temperature, luminous flux, and color temperature data diagram were tested at room temperature of 25 °C.

Influence of Different Helium Inflating Ratios on the
Junction Temperature of the A60 LED Filament Lamps.It can be seen from Figure 3 that with the increase of helium gas inflating, the junction temperature of all the A60 LED filament lamps decreased.For the 8 W power LED filament lamp, the lowest helium inflating ratio of 50% got the highest junction temperature of 113.5 °C.In the same inflated helium proportion, the 4 W filament lamp had the lowest junction temperature.This was because the power value of 4 W was lower, the working current was smaller, and the heat also reduced accordingly.Compared with the A60 LED filament lamp with different power charge, the higher inflating ratio was conducive to the heat dissipation of the filament and the junction temperature.
In Table 2, the A60 LED filament lamps were driven under the power 4 W and the voltage 270 V. Compared with the LED filament lamps of 4 W power, the A60 LED filament powers and junction temperatures of 7 W and 8 W obviously reduced.Because the substrate size of the 7 W and 8 W power filaments was much larger than the 4 W power filament, the heat dissipation was faster.The 7 W and 8 W power filaments had the length of 50 mm, while the 4 W power filament had  3 International Journal of Photoenergy only 38 mm.Apart from that, the heat exchange area of 4 W power filament was smaller.Therefore, both the helium gas inflating and the size of filament influenced the heat dissipation.

Influence of Different Helium Inflating Ratios on
Luminous Flux of the A60 LED Filament Lamps. Figure 4 is the luminous flux data of the three different power A60 LED filament lamps under different ratios of helium inflating.For the same power level, the luminous flux increases as the ratio of helium inflating increases.It can be explained by the fact that the junction temperature decreases with the helium inflating ratio.
In this experiment, we found that with 80% helium inflating, the luminous flux of the A60 LED filament lamps were 480 lm, 815 lm, and 1070 lm for the power values of 4 W, 7 W, and 8 W, respectively.These values were higher than the nominal luminous flux of the commercial A60 LED filament lamps of 470 lm, 806 lm, and 1055 lm also for the power values of 4 W, 7 W, and 8 W, respectively.At the value of 80% helium gas inflating, these series of the A60 LED filament lamps can maintain the nominal luminous flux value requirement, saving more energy than the lamp with helium ratio of 90% and improving the performance price ratio.

Influence of Different Helium Inflating Ratios on the
Color Temperature of the A60 LED Filament Lamps. Figure 5 shows the color temperature of the three power A60 LED filament lamps at different helium inflating ratios.The inflating ratio affected the junction temperature of the filament, and the junction temperature affected the color temperature change.From the data, it can be seen that the color temperature of the A60 LED filament lamp of every power decreased with the increase of the inflatable ratio, which was in accordance with equation (2) that the temperature of the filament affected the color temperature of the filament.Of course, the main factor determining the color temperature of LED filament was the ratio of blue and yellow phosphor.

Conclusions
From the experimental analysis, it was found that the higher helium inflating ratio had a significant effect on the heat dissipation and luminous properties of the A60 LED filament lamps.In the case of low helium inflating ratios, the increase of helium volume can brought obvious performance improvement.The direction for the improvement of LED filament lamp is to develop high-power LED filament lamps.The greater power value means that the filament needs to carry higher current, leading to higher junction temperature.The high junction temperature had a heavy influence on the luminous flux, color temperature, and life of the A60 LED filament lamp.Therefore, the thermal analysis of LED was very important for judging the feasibility of LED filament lamps.A good balance between luminous flux and helium inflating volume helped enterprises to reduce the cost of LED filament lamps and improve their performance.In our research, we found that with 80% of helium inflating without changing the size of the lamp and filament, the A60 LED filament lamp can maintain the nominal luminous flux value requirement.Consequently, it can reduce more costs than a lamp with helium ratio of 90% and improves the performance and market competitiveness of the A60 LED filament lamp with different power values.

Figure 1 :
Figure 1: (a) The A60 LED filament lamp and (b) its structure parameters.

Figure 2 :
Figure 2: (a) Holder of the filament and (b) LED filament in the A60 lamp.

Figure 3 :
Figure 3: Junction temperatures of the A60 LED filament lamps (4 W, 7 W, and 8 W) with different helium inflating ratios.

Figure 4 :Figure 5 :
Figure 4: Luminous flux changed with different helium inflating ratios in the A60 LED filament lamp.

Table 1 :
Thermal conductivity of several common gases.

Table 2 :
A60 LED filament lamp junction temperatures and filament lamps under 4 W-driven power.