How to make innovative design of LED lighting system

Because LED lighting is not entirely about the technology itself, and the function of lighting itself is not entirely for the purpose of lighting. Of course, innovation includes breakthroughs in the integration of optical, mechanical, electrical and thermal technologies, and it must also take into account the value of consumers' senses such as vision and touch. The application range of LED is extremely wide, and now we will discuss the relevant issues and development directions of innovative design of lighting application systems one by one.

Different lighting applications have different LED performance requirements

In order to expand the market acceptance of LED light sources , it is necessary to meet the functional requirements. Not every LED lighting source must pursue the highest luminous efficiency , lowest price, highest color rendering , longest life, etc., but it varies according to different applications. Let's take the general office work lighting, reading lighting and future home bulb replacement lighting market in indoor lighting as examples.

The primary pursuit of LED light sources is to achieve the best cost-effectiveness, and to obtain the highest lumen output, the lowest power consumption and the lowest price under the condition of safety assurance. Because for the general public, compared with other traditional light sources, spending the least money to buy the most cost-effective product is one of the main motivations for purchase. As for whether the added value of marketing such as no UV, no mercury, high color rendering, and 50,000 hours of life is achieved, it is not the main reason for expanding market acceptance. Of course, the brand represents a certain degree of quality trust, and the visual sense will also affect the color preference. Therefore, how to innovate and design LED light sources that meet market demand will be a topic to be explored on the technical side.

The luminous efficiency of LED light sources has surpassed most traditional light sources (Figures 1 and 2). Only a few low-voltage or high-voltage discharge light sources can compete with them. However, in terms of power supply, control and drive , volume and temperature each have their own advantages and disadvantages. The following will explain each link of LED lighting system design.

Figure 1 Evolution of LED luminous efficiency from 1995 to 2020

Figure 2 LED lighting niche market

Innovative process/AC LED achieves high-efficiency chips

LED epitaxy chips on the market, whether it is Cree 's vertical components, Philips Lumileds' flip-chip components, or Nichia , Epistar 's and other planar components, in order to optimize the performance of the chip itself, are nothing more than using different substrates (light transmission and thermal conductivity considerations) with the efficiency of the optical reflective layer (reflection of light); upper/lower roughening structure (changing reflection and refraction to increase light output); circuit mask design (increasing the light output area to improve the current distribution density, so as to reduce the driving voltage and increase the light output efficiency); chip size optimization adjustment; coupled with epitaxy process parameter variation adjustment, etc. to achieve the highest chip photoelectric conversion efficiency, and in each process, study the materials and processes to provide methods to increase conversion efficiency.

As for which structure is the best innovative design? In terms of luminous efficiency, vertical components may be better; but in terms of manufacturing costs in general office work lighting, reading lighting and future household bulb replacement lighting markets in indoor lighting, Taiwan's current flat chips are far superior to foreign manufacturers in terms of cost performance. In addition, in the application structure design innovation of the chip end, in addition to the innovation of the overall chip efficiency through process improvement; the birth of alternating current (AC) LEDs has enabled LED lighting applications to not only have the option of DC chip series and parallel connection, but also be directly driven by AC mains, thus reducing the cost of transformers, product internal space, different drive circuit specifications, transformer efficiency loss and quality life, and greatly simplifying the problems of module and product design. However, other problems derived from innovations currently faced by the consumer side of the market also arise. The first is that innovations cause consumers to have doubts about unfamiliarity, and they are hesitant and timid in their consumption behavior. This consumer group may not be the end consumer public, but the mid- and downstream buyers in the LED lighting product supply chain. They are hesitant about the product because they are unfamiliar with it, do not understand the AC properties, or are uncertain about the risks of the product, and dare not innovate in product development; the second is that innovations make the consumer public wait and see, but make the early adopters in the consumer group excited. However, the first two problems can be clarified through innovative design and rigorous testing methods in the mid- and downstream LED supply chain, thereby creating product market segmentation and competitive advantages.

Increasing demand for innovative packaging design

The packaging design ranges from the early light source type to the plastic pinless chip carrier package ( PLC C) side view or top view type, all of which are single crystal or multi-chip packaging types, with a driving current of less than 120 mA. In the early days, it was mainly used in 3C consumer electronic products such as mobile phone screen backlight, digital photo frame backlight, notebook computer screen backlight, etc. However, after entering general lighting, many manufacturers still use low-wattage PLCC packaging components as light sources, and then combine multiple light sources to realize the light source module of lighting products (Figure 3). No innovative design has been applied to LED light source packaging.

Figure 3 Lighting module with multiple LED light sources

Innovative projection/floodlight packaging must be comprehensive

For floodlight bulbs or spotlight sources, such as E27 bulbs, PAR lamps, AR111, MR lamps, embedded downlights, track headlights and other light source products, the LED package light source tends to have single-point high lumen output (projection type) (Figure 4)/multi-point combination output (floodlight type) (Figure 5), high color rendering index (CRI>80), low thermal resistance coefficient (<3k/W), uniformity of light color temperature , color temperature deviation under temperature increase, long life/high reliability, etc. How to innovate from the perspective of packaging (Figure 6) to meet market demand can be explained as follows:

Figure 4 Single point high lumen output

Figure 5 Multi-point combination output

Figure 6 Wafer Level Silicon Chip Packaging

Before implementing innovation, we must first understand the ultimate goal of the product and meet the expected market trends in LED packaging light source specifications.

First, we must select the most appropriate chip, understand the chip structure, chip optoelectronic characteristics and chip cost performance, and then design the best packaging structure. Currently, high- power chip packaging above 45 mil can achieve 150 to 160 lumens per watt when driven by 350 mA current. Today, Taiwan chips can produce 160 lumens per dollar, which is absolutely better than foreign manufacturers under the competition of other major manufacturers in various countries.

Secondly, the best die attach materials and process technology must be used for die bonding, using high thermal conductivity silicone, silver glue (>15W/mk) or even solder flux (Cu-Sn-Au) to solder the chip to the metal substrate, or using the Eutectic Bonding process to fuse the chip to the silicon chip. The selection of these die bonding processes is to reduce the chip node temperature, improve the electro-optical conversion efficiency and increase the light efficiency under the same current, reduce the proportion of future light decay caused by material aging, and improve the reliability of die bonding. In addition, the design of the substrate for packaging monomers used to be mainly low-power leadframes, and gradually moved to high-power metal slugs, then to low-temperature co-fired (LTCC) ceramics, high-temperature co-fired (HTCC) ceramics, chips directly fixed on various metal substrates (Chip on Metal Board), and then chips fixed on 8-inch silicon chips. With the continuous increase in power, light type requirements, and the increase in thermal density per unit area, the selection of packaging chip substrate design must innovate and develop materials that can accelerate heat conduction, extract light output, and reduce aging and attenuation caused by material variation. The structural design must be changed due to the different chips selected, so as to achieve the optimization of the overall performance of the package. It is not possible to package the best light efficiency component by purchasing a general leadframe or public board ceramic and placing a chip with the maximum conversion power (Power Flux).

In addition, in today's LED lighting applications, although there are various colors of decorative applications, the main focus is still on white light. However, the way to produce white light is nothing more than red, green, blue light , mixed light, ultraviolet light to excite red, green and blue phosphors, or the most common blue light plus yellow phosphors. But when it comes to innovation, three directions are specifically mentioned here. The first is high color rendering index (CRI). CRI is greater than 80 or even greater than 90, and innovative development must be made in phosphor materials. For packaging research and development, it is necessary to conduct experimental comparisons of phosphors of various bands with red phosphors close to 610-630 nanometers, and even 655 nanometers red phosphors under different packaging structures and blue light band combinations to find the best combination of light output efficiency, color rendering index or color temperature. In addition, the different packaging glue materials, dispensing methods and whether the phosphors are precipitated or not will more or less increase the variables that are difficult to control in packaging. Secondly, innovations in the process of sealing and phosphor coating, from syringe dispensing to mold filling, printing, high-precision spray filling (Conformal Coating and Inject Printing) and other innovative processes, improve production capacity, yield, light output efficiency, and improve the uniformity of light output. In addition, the so-called Remote Phosphor process is more innovative and commercialized, which extracts the phosphor from the chip surface and attaches the phosphor to the light-transmitting structure in different ways at a certain distance. Philips has also applied this innovative technology to light bulb products, and its light efficiency results are not far from the academic experimental results or theories published by research institutions and academic institutions in the past, and can indeed reach more than 100 lumens per watt and CRI>80. Finally, in the innovation process of lighting packaging components, attention should be paid to color rendering and light output efficiency (Figure 7).

Figure 7 LED color rendering and luminous efficiency changes

Lowering junction temperature improves color rendering

In actual lighting use, no one looks at the instantaneous lumen efficiency at Tj=25℃. In the process of package R&D and innovation, more attention should be paid to the variation of temperature and light output, which will also significantly affect the color temperature difference and slight color rendering variation in actual use (Figure 7). For example, in bulbs and embedded downlights, the ambient temperature of LEDs can easily reach 60-80℃. In other words, if the lighting system design is not optimized, it is very likely that the temperature of the LED junction will reach 120-150℃, and the actual thermal effect will cause the reduction of light output (Hot-cold Factor) by more than 15%. To improve this problem, the thermal resistance of each interface of the entire lighting system must be reduced to achieve the purpose of reducing the junction temperature and reducing the color temperature deviation (Figure 8).

Figure 8 LED ambient temperature and relative beam characteristics change

Three major issues need to be considered in innovative module design

Completing the best chip packaging component has already determined more than half of the optical characteristics. The important issues that need to be considered in module design are thermal management, power control and circuit design, and secondary optical mechanism design. If these three items are not well designed, it is easy to lose more than 50% of the light output efficiency in the module system. Even if it reaches 150 lumens per watt, after the system is assembled, it will be found that the terminal product is only 60 to 70 lumens.

The above three problems will be solved in different ways: First, use high thermal conductivity materials, such as module circuit boards, increase the air contact area on low thermal conductivity boards, disperse the LED components (heat sources) or separate or keep the heat-generating electronic components away from the LED light source, and avoid using any low thermal conductivity insulation layer to increase the resistance (thermal resistance) in the LED heat conduction process. Secondly, use high reflectivity insulating paint on the circuit board surface to increase the reflection of downward light, use high-efficiency constant current control components to reduce power consumption, and use temperature control mechanism to adjust the constant current output as a protection mechanism, and use high-voltage AC-DC conversion circuit to reduce transformer loss. In addition, use durable high light transmittance or high reflectivity materials to make secondary optical lenses or reflective optical components, and use professional optical simulation software to calculate and design the best light output efficiency.

Innovative lighting system design must take into account innovation/irreplaceability/marketability

In this system that combines light, machinery, electricity and heat, in addition to selecting or designing the best LED components, matching the best module thermal management and circuit design, and using secondary optics, the product design closest to consumers is completed. Of course, product design does not start at this stage, but is based on the market demand at the earliest stage of this article. All aspects of system design are for the purpose of the final product. Under the framework of innovative LED lighting, the end product itself must be groundbreaking, irreplaceable and marketable.

图9由圆形单片组成光源体组合的镂空立体灯具，虽然是 欧司朗 光电 半导体 (OSRAM Opto S emi conductors)以有机发光二极管( OLED )设计而成的概念，但此概念在目前是可以商业化且充分兼顾LED照明功能的优越性，以及对热对流和热传导的设计考虑。至于日本Pan ason ic电工所设计的薄型下照灯(图10)，亦具备一般传统光源所无法的取代性，超薄的尺寸透露轻巧的视觉感受，若采用AC LED更可将厚度再薄型化，当然热处理上必须考虑散热面积、表面材质的应用处理，加上空气对流设计，藉以降低空气接触面的热阻，加速散热效果。

Figure 9 OSRAM OLED concept lighting

Data source: Panasonic Electric
Figure 10 Thin downlight developed by Panasonic Electric

As for outdoor lighting, architectural lighting, special lighting such as aquaculture, medical and other LED lighting application innovation designs, the research and development innovations in the light source packaging are basically not much different, but in the module design, power circuit control and optical structure design, each has its own use environment restrictions, menu appearance and wavelength uniqueness, which still requires LED R&D personnel to work hard to create.