Thermal and Optical Challenges for SSL in Automotive Applications

Automotive lighting experienced a major shift during the last decade due to inherent advantages of Solid State Lighting (SSL) products. Light emitting diodes (LEDs), a frontline runner for SSL technologies offer a wide color gamut, exceptional energy efficiency and affordability. However, their implementation poses some challenges for practical applications. Photometric, electrical and thermal properties of LEDs are highly interrelated. Therefore, while a lighting system is designed, those interrelated parameters of the system should be carefully considered to achieve desired performance metrics. Luminous efficacy of LED systems is highly sensitive to junction temperature of the LEDs because optical output of the LEDs changes with temperature. Therefore, designers should consider the dependency between electrical, photometric and thermal quantities to achieve optimal performance. In this study, dependency between photometric, electrical and thermal parameters of an automotive lighting application is presented. It is found that novel thermal and optical technologies are necessary to reach next generation lighting systems for automotive lighting applications.

Introduction

Today's automotive lighting systems basically include headlamp and rear lamp lighting systems whose development have continued over the last few decades. Headlamps' performance has been progressively improved throughout the series of developments. Acetylene or oil were used to run the earliest headlamps which were developed in the late 1880s. Then, a game changer headlamp technology, the first electric headlamp, was released to the market in 1898 [1]. After technology progressed over more than half a century, the first halogen lamp (H1) for use in vehicles as a headlamp was eventually presented in 1962. After that, headlamps running unique light sources started to appear in Europe [2,3]. One of these lighting sources, light-emitting diodes (LEDs), is effectively used today in automotive headlamp applications as they have undergone rapid development since 2004. In fact, the first full-LED headlamp was utilized on Audi R8 sports car [4]. These days, advances in LED automotive lighting have been progressively changed from high class car models to middle class models [5].

In fact, LED technology offers significant advantages in energy efficiency, lifetime, reliability and design in automotive lighting compared to conventional lighting system [6,7]. The use of LED light sources in automotive lighting is especially important in terms of safety concerns since they react very quickly to electrical warnings. The lit time of an LED lamp is less than a standard bulb's in the order of milliseconds so that it makes a significant difference in braking distance when the vehicle is operated at a high speed [8]. One of the recent projections estimates that 20% of front headlights of light vehicles will be powered by LED products by 2030 compared to 2% in 2015 [9]. In fact, predictions show that LED headlamp sector will get bigger at a Compound Annual Growth Rate (CAGR) of 20% and dominate lighting technology. The expanded use of LEDs in front lighting applications is led by Europe and Japan where the LED penetration is expected to be 36% and 45% respectively by 2030 but it was only about 4% and 6% in these regions in 2015 [10]. In addition to upper segment vehicles, LED systems have also started to appear in lower segments recently. The inclusion of LEDs in automotive lighting is expected to be even more when the emerging trends in autonomous vehicles, internet of things and visible light communication are considered [11,12,13]. Since LED technology brings great advantages due to its semiconductor nature, it will play a significant role in many fields. Cutting-edge technologies including AFS, ADB, glare free high beam, automatic leveling and matrix beam are all based on LED light sources and they are successful in illuminating a certain area effectively. Since recent systems include electronics systems with improved capabilities, technological advancements in automotive lighting is typically classified as 'digital lighting' today.

The trends in automotive exterior lighting can be grouped under front (headlamps) and back (rear lamps) lighting systems respectively [14,15]. While novel matrix systems, laser headlamps, LCDs and digital micro-mirror devices (DMD) are the rising trends in future headlamp systems, integration of OLEDs and Micro Lens Arrays (MLAs) into rear lamp systems is also intended for future applications. Considering the trends in front lighting, today's matrix systems are only utilized as a high beam function and still improved. The matrix systems in future applications, however, will combine the low and high beam functions. Thus, they will be able to adapt to driving conditions using a simpler structure. This will allow switching between high and low beams through sensors according to the incoming traffic data [16]. LCD systems with a number of LED chips and micro-optics are also integrated into the headlights as they can enable high-definition illumination. Although the use of an LCD to obtain an adaptive headlamp is first offered in 1989 [17], the current integration of an LED matrix on LCD systems offer new opportunities to reach a resolution at approximately 50000 pixels. Furthermore, digital micro-mirror devices (DMDs) are systems that can focus light to a certain point by electronically moving hundreds of thousands of micron-level mirrors. Each microscopic mirror over the surface represents the pixel of an image and they can be rotated by ±10-12° to ON or OFF state. In the ON state, the pixel is made bright on the screen as the lens is exposed to the reflected light from the light source and it is made dark in the OFF state when the light is forwarded to another field [18].

In addition to front lighting, there are also various ideas that make scientists and engineers eager to improve back lighting in the automotive industry. First, OLEDs are utilized only in signaling functions today in automotive lighting since they have much lower luminance than LEDs.  Therefore, researchers are paying attention to the increase of lumen output of OLEDs for future applications. Furthermore, although micro lens arrays (MLAs) have been currently used in various entertainment units of micro-projectors, they can also be utilized in projecting light in the shape of certain signs or warnings on the road or pedestrian walk surface and this could bring a lot of benefits in traffic.

Although these technologies are under development for future applications, optothermal problems and reliability issues associated with them arises because of high power conditions over a limited space. Table 1 shows the consumed electrical power and generated thermal power values of a typical LED over a PCB of a Daytime Running Lamp (DRL) measured at various LED solder fitemperatures in a range of 30° to 120°C. The optical conversion efficiency was determined as 27.45% [8].

Table 1: Power consumption of a typical LED over a PCB of a Daytime Running Lamp (DRL)

The resulting elevated temperatures of LEDs do not only cause reduced lifetime but also lead to a significant drop in the amount of generated light and optical efficiency. In fact, the dependency of optical, electrical and thermal characteristics of LEDs needs to be addressed for the integration of these novel technologies to the current applications. Thus, this study focuses on determining the relationship between optical, electrical and thermal traits of LEDs for automotive applications.

In this study, a test board with multiple amber and red LEDs is designed and manufactured to mimic an automotive LED lighting PCB. This LED light engine with a conventional flex FR4 based PCB is used to understand the relationship between optical, electrical and thermal characteristics of red and amber LEDs. Usually an FR4 LED light engine is used in an exterior rear lighting system of automobile. It has a double-sided structure having electronics at the front and LEDs at the back side (Figure 2). Identical FR4 flex PCBs with a thickness of 200 μm are attached to FR4 substrate with the dimensions of 66x80x2.75 mm. There are 10 red LEDs and 6 amber LEDs at the front side and driver electronics exist at the back side of the engine.

Figure 2: FR4 based LED light engine (a) LEDs' side (b) electronics' side

FR4 based LED light engines are driven at 6 different power levels. First, only red LEDs are driven at six different input electrical powers between 0.5 and 3W. Then, only amber LEDs are separately driven at the same input powers. In order to determine the input power of the LEDs, electrical measurements are conducted with a digital oscilloscope. There are two branches in parallel for the electrical connection of red LEDs and serially connected five red LED chips are placed at each branch. Six amber LED chips, however, are connected in series in the SIGNAL branch.

FR4 based LED light engine is connected to a power supply with position, stop and ground cables. Both differential and current probes are connected to an oscilloscope for this measurement. Thus, voltage difference across one of the branches and electrical current passing through that branch are determined. The setup for electrical measurements of LEDs is given in Figure 3.

Figure 3: Experimental setup for electrical measurements

The measurements were conducted for various power levels in the range of 0.5W and 3W. As it is noticed in figure 4, the growth of electrical input power of both LEDs decreases and its ratio to total input power decreases as the LEDs are driven at higher electrical power levels. At the lowest total input power of the circuit, 80% of this power (494.5 mW) is supplied to red LEDs while only 33.5% of total input power is given to the same LED at the highest input power. A similar case is also observed for the amber LEDs. As 91.2% of total input energy is provided to the amber LED at the minimum power level, it reduces to 64.5% when the input power is elevated to the highest level.

Figure 4: Change in electrical input power of red and amber LEDs with respect to total electrical input power to PCB

In addition, thermal experiments were conducted for each LED color and power condition to observe the change in thermal characteristics of the LED light engine. Red LEDs and amber LEDs were separately operated in two cases for six different power conditions. Total input powers supplied to the LED light engine in these experiments were 500 mW, 1000 mW, 1500 mW, 2000 mW, 2500 mW and 3000 mW. While input power provided to only red LEDs were 396 mW, 737 mW, 879 mW, 931 mW, 968 mW and 997 mW, they were 433 mW, 907 mW, 1284 mW, 1606 mW, 1835 mW and 1929 mW for amber LEDs.

IR thermal images of the FR4 based LED light engine at different input power levels are presented for red and amber LEDs in figures 5 and 6 respectively. While electronics operated to drive red LEDs are given in figures 5c, 5d, 5e, and 5f, electronics in charge of driving amber LEDs are located on the right-hand side of the electronics side of the engine which corresponds to the left-hand side of the LEDs' side of the engine. As it is indicated in figure 5, hot spots over the red LEDs become more critical as input power increases. Temperature of the right-hand side of the LED light engine where LED1, LED2, LED3, LED4 and LED5 are placed is higher and several local hot spots are observed over these LEDs. Thus, nonuniform temperature distribution is experienced over the LED light engine.  On the other hand, local hot spot firstly forms around amber LEDs as seen in figure 6. It can be inferred from the figure that hot spots expand to surrounding of these LEDs as input power starts to increase. In the last two experiments, temperature of the left-hand side became higher and local hot spots appeared more explicitly over LEDs 12, 15 and 16.

Figure 5: IR thermal images of the LED light engines when electrical input power of red LEDs is (a) 396 mW, (b) 737 mW, (c) 879 mW, (d) 931 mW, (e) 968 mW and (f) 997 mW

Figure 6: IR thermal images of the LED light engines when electrical input power of amber LEDs (a) 433 mW, (b) 907 mW, (c) 1284 mW, (d) 1606 mW, (e) 1835 mW and (f) 1929 mW

Maximum temperatures of red and amber LEDs at six different power conditions are presented in figures 7 and 8 respectively. A linear increase is observed in maximum temperature of the red LED 6, 7, 8, 9 and 10 and all amber LEDs. However, maximum temperature of certain red LEDs (LED 1, 2, 3, 4 and 5) and amber LEDs (LED 15 and 16) exhibits a sharp increase after a specific input power value. This happens after the second lowest input power level around 737 mW for red LEDs and the second highest input level around 1835 mW for amber LEDs due to the heat dissipation from the electronics at the back side. It is also noticed that the slopes of the first 5 red LEDs are approximately twice of the slopes of the other 5 LEDs.

Figure 7: Maximum temperatures of red LEDs at different electrical input powers

Figure 8: Maximum temperatures of amber LEDs at different electrical input powers

Figure 9 shows the maximum temperature of red and amber LEDs. While x-axis shows total heat generation of red and amber LEDs, which is calculated as the difference of total input power of the LEDs and total radiant flux of the LEDs, y-axis shows the change in temperature of LED 2 and LED 12, which reach the highest surface temperature among ten red and six amber LEDs respectively. In addition to the supplied electrical power on LEDs, heat conduction from operated electronics also led to a temperature rise of LED 2 and LED 12. Therefore, the increasing rate of the maximum LED temperature is more than total heat generation's.

Figure 9: Maximum LED temperature with respect to total heat generation of red and amber LEDs

Optical Measurements

After electrical and thermal experiments, optical experiments were conducted on the same LED light engine to determine photometric and radiometric characteristics of the light engine. Optical experiments were conducted on FR4 based LED light engine with FR4 flex PCB at various power conditions. Optical spectrum of red and amber LEDs at the given power conditions are presented in figures 10 and 11 respectively. It is seen that peak wavelength shifts towards the right due to an increase in the input power and bandgap energy shrinkage. In fact, bandgap energy is temperature dependent because of electron-phonon interactions and lattice vibrations [19].

During red LED tests, as total input power supplied to LED light engine is elevated by 2.5 W, peak wavelength shifts by 4 nm. On the other hand, as the input power of amber LEDs is raised from 0.5 W to 2 W, peak wavelength shifts right by 6 nm and radiant flux at peak wavelength rises by 1.7 mW. When the input power of amber LEDs is altered from 2 W to 4 W, peak wavelength shifts right by 3 nm and radiant flux at peak wavelength drops by 0.8 mW. Therefore, the efficiency decrease can be inferred from the drop in relative radiant flux.

Figure 10: Flux spectrum of red LEDs at various electrical power conditions

Figure 11: Flux spectrum of amber LEDs at various electrical power conditions

Figure 12 demonstrates the peak wavelength increment with respect to input electrical power of the LEDs. Peak wavelength rise of red LEDs accelerates after the third lowest power condition due to junction temperature elevation. In amber LEDs, the increase in peak wavelength accelerates at the highest power condition.

Figure 12: Change in peak wavelength with respect to electrical input power of red and amber LEDs

Radiant power change with respect to input power of the red and amber LEDs are presented in figure 13. Although there is a linear relationship between radiant flux and input power of red LEDs, radiant flux is raised by 2.5% as input power increases by 7% at the last three power conditions. However, after some point, radiant flux starts to decrease although input power increases. While input power of amber LEDs is elevated, junction temperature of the LEDs also increases due to the ineffective thermal management of FR4 based LED light engine. Because of the rise in junction temperature, optical output of the LED drops in this case.

Figure 13: Variation of radiant power with electrical input power of red and amber LEDs

As input power of red LEDs increases from 395.5 mW to 996.5 mW, luminous efficacy decreases by 13.6 lm/W as seen in figure 14. A sharp fall in luminous efficacy is experienced in the last case compared to the first five cases since junction temperature rises more at higher driving currents. On the other hand, as input power of amber LEDs increases from 432.8 mW to 1928.5 mW, luminous efficacy decreases by 24.6 lm/W. Until the fourth experimental case, luminous efficacy increases slowly, after that it starts to decrease with a positive acceleration. The fall in the luminous flux causes a decrease in luminous efficacy. As a result, the power level of 1600 mW is critical for amber LEDs.

Figure 14: Change in luminous efficacy with respect to electrical input power of red and amber LEDs

Results and Discussions

Thermo-electro-photometric relationship is developed for red and amber LEDs for an automotive exterior lighting system. Discussion for thermo-electro-photometric relationship begins with red LEDs and continues with the amber LEDs. As it is suggested in figure 15, heat generation of both red and amber LEDs increases linearly as input LED power is elevated. However, due to the additional effect of heat generation of the electronics, maximum LED temperatures of red and amber LEDs rise growingly.

Figure 15: Change in total heat generation and maximum LED temperature with respect to input electrical power

Figure 16 shows the change in luminous flux with respect to input power of red and amber LEDs. It is noticed that luminous flux of red LEDs does not change significantly when input LED power exceeds 879 mW while luminous flux of the amber LEDs starts to decrease after the input LED power of 1606.1 mW. Thus, driving LEDs with the input powers after certain point is ineffective for automotive lighting systems. A similar trend is observed for both LEDs in the change of luminous flux and maximum LED temperature with respect to input LED power. An inverse relationship between junction temperature and luminous flux can be inferred from the graph. As it is also shown in the graph, the increasing rate of the maximum temperature slightly rises at the last power condition.

Figure 16: Change in luminous flux of LEDs and maximum LED temperature with respect to electrical input power

As it is experimentally observed, input electrical power is converted to radiant energy and heat generation. The amount of heat generation varies depending on the performance of cooling technologies in lighting systems. Figure 17 indicates the change in radiant flux and heat generation of red and amber LEDs with respect to LED electrical power. When input power of red LEDs is 395.5 mW, 43% of the electrical power turns into radiant flux. However, if it is raised to 996.5 mW, 37% of the electrical power is converted into radiant flux. In the case of amber LEDs, when the input power of the amber LEDs is 432.8 mW, 12% of the electrical power turns into radiant flux. On the other hand, if it is elevated to 1928.5 mW, only 5% of the electrical power results in radiant flux. Consequently, the conversion rate of the red and amber LEDs decreases by 6% and 7% respectively because of the increase in junction temperature.

Figure 17: Amount of heat generation and radiant flux of LEDs at various driving conditions

The conversion rate is presented with respect to maximum LED temperature in figure 18. A relationship between maximum surface temperature of an LED and the conversion rate is developed. While maximum temperature increases, the conversion rate of the LED decreases. This correlation is generated for a multi-chip LED board and it is assumed that all LEDs have identical radiant power and LED temperature. However, due to nonuniform heat distribution, LEDs may have different radiant flux and temperatures.

Figure 18: Relation between maximum LED temperature and conversion rate for amber LEDs

Summary and Conclusions

LEDs are now widely accepted for a large number of applications from general lighting to displays, automotive lighting to horticultural use. In this study, we have presented a number of thermal, electrical and optical challenges for automotive lighting applications. It is found that LEDs with over 5x efficiency and 8x lifetime expectancy compared to conventional lighting products, will quickly replace conventional automotive lighting products. However, cost is still a burden and slows the penetration. While high end vehicles started quickly adapting these technologies, there is still some room for improvement of those products. High junction temperature, optical distribution problems, glare over other drivers and pedestrians, lifetime and electronics packaging at compact volumes still pose challenges for the industry.

Acknowledgement:
Authors thank to EVATEG Center at Ozyegin University for enabling the use of computational and experimental facilities as well as FARBA Corporation for providing test samples for the experimental studies.

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