Three major rumors of crushing COB light sources

To find out whether COB is suitable or not, you must first look at what COB is a thing - COB is Chip-On-Board, which originally refers to the packaging technology that integrates multiple semiconductor chips into one circuit board. In the LED industry, a plurality of LED chips are specifically packaged on a substrate to form a form of an illuminating light source (Fig. a).

Figure (a) COB actual product renderings

It is this product form that determines the technical characteristics of COB. The so-called "structural determination characteristics" in materials science also apply to products. Only by knowing this, can we carry out the following analysis.

difference:

1. Broadly speaking, there are many types of COB products. As long as the chip is directly fixed on a circuit board or other type of substrate, it can be defined as COB.

2. What is the COB that is not suitable for spotlights? The discussion needs to define the scope first.

We are all called "COB"

COB rumors - small angle of illumination

We first to get rid of a rumor that the COB is widely spread - the angle of illumination is small. The illumination angle of the LED lamp bead is determined by the chip package. From the perspective of product structure, the COB package is nothing more than integrating multiple chips onto a single substrate, which is not fundamentally different from the package of a single chip, so the general COB and single package LED lights are the same - all are 120° light. It is considered that the COB illumination angle is small and it is inferred that the COB is more suitable for the spotlight source. For COB spotlights, the beam angle that defines the beam angle is the deep, light-influenced headlight cup.

difference:

The angle of light output depends on the packaging process. Generally, when a single product wants to be a small-angle package device, it will choose to use the lens plus mode, and adding a lens on the plane can reduce the illumination angle. The light-emitting angle of the horizontally-structured LED chip is theoretically 180°, and can be appropriately enlarged and reduced by the change of the package structure. For example, packaging on a lens substrate can increase the bottom light output to a 270° illumination angle (the filament is a typical). The vertical structure of the chip will have a smaller angle of light. Specifically, which chip and package type are used.

1. Not all COBs are 120°. COB currently has a flat dispensing (the picture above is in the form of a flat dispensing) and a lens package. The COB angle is less than 120° as shown below. If you want to be smaller, you can design different lens shapes according to the light emission simulation.

2. At present, the company's propaganda COB angle may be small, and it is possible to express a product with a small light-emitting angle on a COB product with a smaller luminous surface. The business side also needs to review and not let customers be ambiguous.

COB rumors - better color temperature control and color rendering

The second rumor is that COB's color temperature and color rendering are better than the average single-chip package LED lamp beads. Don't forget, there is no essential difference between the COB and the package material used for a single LED bead. If it is necessary to say that it is also determined by the quality of the materials and production processes, it has nothing to do with the product form. So there is no basis for this argument.

Color temperature control and color rendering are unfounded, agree with your opinion.

Personal point of view: The company may want to advertise that COB light source uses only one COB for each luminaire, and there is no problem that the same luminaire uses many light sources to produce a color temperature difference between the light sources. The COB product shipments are also screened by color separation blocks, and the batch color temperature consistency is good. Due to the large number of productions of low-power products, there are errors between multiple optical test equipments in the production process, which is one of the reasons for the chromatic aberration caused by the use of multiple low-power products on the same luminaire.

COB's big adventure - heat dissipation, luminous efficiency and glare

Speaking of the shortcomings of COB, there are three words that must not be passed. That is, heat dissipation, luminous efficiency and glare. All issues are determined by the COB's own product structure.

For COB, heat dissipation is the first "Achilles' heel." Generally, the size of the 9W COB is a circle with a diameter of about 10 mm, which determines that it can only directly act on the heat source in this area, and the range outside the area is only used as an auxiliary for heat dissipation. For the same 9W SMD, the substrate diameter is generally about 100mm. For heat dissipation, the situation of low heat generation and large area heat dissipation is much better than that of high heat generation and small area heat dissipation.

As for the low thermal resistance that COB manufacturers have been advertised, it is actually said that the thermal conductivity is good.

However, please note that the so-called "good thermal conductivity" is only able to derive the heat generated by the COB, and the subsequent treatment of the COB is completely passive. Just like a highway, the car is running very fast on the road, but it can be blocked at the toll station. This is similar to the situation of COB.

difference:

1. Do not agree to heat dissipation on the light source device. The function of the heat dissipation is the task that the heat sink should perform, and should not be imposed on the light source. The evaluation of the light source should mainly focus on the thermal conductivity, ie the thermal resistance. Or evaluate the temperature difference from the junction temperature of the chip to the solder joint of the PCB. The good thermal conductivity is that the temperature of the chip can be transferred to the heat sink in a shorter time.

2. For heat dissipation, the situation of low heat generation and large area heat dissipation is much better than that of high heat generation and small area heat dissipation. The assumption of this heat dissipation mode is based on the conduction path. If the heat is radiated from the way of radiation, the product with high temperature has a higher emissivity. Therefore, it is recommended to consider three modes of heat dissipation: conduction, convection and radiation system cooling. At the same time, some COBs have large luminous areas and low power. Such COBs are generally not used on spotlights and do not meet the requirements of spotlights.

3. As mentioned in the article: The size of the general 9W COB is a circle with a diameter of about 10mm, which determines that it can only directly act on the heat source in this area, and the range outside the area is only used as an auxiliary for heat dissipation. For the same 9W SMD, the substrate diameter is generally about 100mm. A: The area of ​​10mm is the light-emitting surface, the fixed position of the chip and the package area, and the remaining area is not used for heat dissipation, but for heat conduction. B: The diameter of the SMD substrate is about 10mm. It is not used for spotlights. We can calculate the maximum size of the 9W common spotlight specification: MR16\PAR20. The light source substrate area is larger than the lamp, which is obviously impossible.

4. Just like a highway, the car is running very fast on the road, but it can be blocked at the toll station. This is similar to the situation of COB. This example is very good, just to say that the COB has a strong thermal conductivity. The bottleneck is the matching problem of the heat sink. The faster the heat dissipation rate of the heat sink, the lower the junction temperature of the COB product. Products with poor thermal conductivity will get stuck in the bottleneck of heat conduction, so they will only get stuck at the toll booth.

The second drawback of COB is light efficiency. Since a plurality of LED chips are closely arranged in a small area, the light emitted by the single chip in the horizontal direction encounters the adjacent chips and continuously forms a total reflection, and is finally absorbed by the packaging material and cannot be emitted. For SMD, as long as the spacing is reasonable, there is no such problem (see Figure b). It is this total reflection that makes the COB's luminous efficiency 10% lower than the surface mounting of the LED lamp bead from the beginning. At the same time, the encapsulation material absorbs the heat caused by the horizontal light and the heat generated by the dense arrangement of the chip itself, resulting in a high COB operating temperature, which again affects the chip light effect. Even with the same chip, the COB is about 20 lm/W less than the surface mount.

Figure (b) Schematic diagram of the COB package, the circle in the figure represents the substrate used for the package, and the box represents the LED chip.

difference:

1. There is no evidence that the light efficiency of the same chip package COB is 20% lower than that of a packaged single device. The COB light source product generally has high power and the surface temperature of the package glue is high. Considering the temperature resistance of the package glue, a glue with a refractive index of n=1.41 is used; while the power of the single device is small, the surface temperature of the glue is low, and the refractive index is generally used. N=1.5 or more glue. The higher the refractive index, the more favorable the light output of the LED chip (n=2), and the poor temperature resistance of the glue due to the characteristic of the glue is poor (the molecular structure of the glue causes characteristics).

2. The densely packed chip in the above example is a COB product for spotlights. There are also a lot of COB products with large gaps in the chip, and COB products have lenses on each chip, which also avoids direct light interference from the chip.

3. Please pay attention to the attributive: The article uses “small” and “closely arranged” when referring to COB products. The SMD products are used with the words “as long as the spacing is reasonable”. It can be seen that the comparison method of the old military doctors is: the comparison of the light effect of the narrow, closely arranged COB products and the SMD products with reasonable spacing. This is not comparable,

4. Let's talk about the spotlights again. Spotlight products will be more important to light effects when considering the central light intensity. Spotlights are area illumination of the entire lamp, unlike bulbs or streetlights that are widely illuminated. For example, the bulb can achieve a total lighting efficiency of 130 lm / W; the street light also has 150 lm / W, but the spotlight rarely has a product of 100 lm / W.

The third drawback is glare. Also based on the small area and high power of COB, there is inevitably a problem of glare. Basically, the spotlights using COB must be equipped with a very deep lamp cup, in addition to the need to distribute light to prevent excessive glare.

The glare is not caused by the use of COB, but there is no glare without COB. And in the design of the luminaire, it is necessary to consider the treatment of glare, not only by adding a deep cup.

difference:

1. Definition of glare: Dazzle refers to the visual condition in the field of vision due to unsuitable brightness distribution, or extreme brightness contrast in space or time, resulting in visual discomfort and reduced object visibility. A bright sensation in the field of vision that the human eye cannot adapt to may cause disgust, discomfort, or even loss of visibility. In the field of view, a certain local area appears too high brightness or excessive brightness changes occur before and after. Therefore, the original sin of glare cannot be said to be caused by COB.

2. Very deep reflectors are generally used to make lamps with smaller angles. As for the efficiency of the reflector cup, the COB's reflector cup efficiency is close to that of a single device (the same angle).

3. According to the text: In addition to the light distribution needs to be more to prevent too strong glare. Can it be understood that there is no strong glare after COB is equipped with a deep cup? From a rigorous point of view, is there still or not?

Since COB has so many problems, it has become the main source of spotlights. Very simple, because the COB product form is closest to the traditional light source, so the original lamp cup, lamp, design can be copied (notice, this is similar to the logic of the filament lamp).

difference:

1. The COB is not the closest to the traditional light source, but the way it is used is close to the traditional light source. The more convenient the user is, the more widely the product can be promoted. The COB does not require a patch and does not require the design of a circuit board, which is a convenience for manufacturers of luminaire manufacturers.

2. The article mentions that “so the original lamp cups, lamps, and design methods can be copied.” The focal length of the traditional reflector is different from the COB-matched reflector. It cannot be copied. Each COB product needs a specific angle of spotlights, which needs to be simulated by optical design, reflective cup mold experiment, perfected mold, and finally become a general product.

Discussion on the application characteristics of spotlights:

Spotlights mainly consider local lighting and are mainly used in commercial lighting. Commercial lighting has higher and higher power requirements for lamps, and the angle requirements are getting smaller and smaller, and the size of the lamps is small. With a small surface, the high-power COB is produced to meet the needs of customers. If a single device is used for mounting, the area of ​​the overall light source is large, resulting in the need for a larger reflector or lens (the area of ​​the light source and the light-emitting surface of the reflector or lens are magnified), which is difficult to match the needs of the end user.

The emergence of COB light sources was around 2008. At that time, it was to solve the problem of "ghosting" of LED lamps. Under the illumination of LED lamps, the object will produce several shadows that do not completely overlap, which makes the eyes feel dizzy. There were two solutions at the time:

1) Add a film (light-emitting plate) that is not completely transparent before the LED light source. This solution has a big problem. At that time, the luminous efficiency of the LED is not high, and the light-emitting panel makes the overall light effect lower. Based on this concern, the second solution came into being...

2) Start with the improvement of the light source and eliminate the “ghost” on the light source. This is the original motivation for COB development, but it was quickly abandoned. The reason is very simple. The COB belongs to the secondary package. The technology and process are relatively complicated. With the technology of a single piece of COB at that time, as long as it exceeds 35W, there is no way to maintain the quality stability in mass production, and its light efficiency and heat dissipation are not comparable to surface mounting. The first show of COB in the market quietly ended.

In the current environment, the development of COB has its rationality, which is essentially different from the subsequent redevelopment of COB. The second spring of COB was re-invented in 2012 as a new light source. The motivation is the market's disappointment over the long-term stagnation of LED products. However, the technical problems that lie ahead of the COB have not improved over time, and are still hindered by the encapsulation and high-power quality stability.

In 2015, the COB was "fired" again. If the first two developments of COB promoted the LED industry, then this time is purely to "connect with the old tradition", which is essentially a retrogression. It is true that COB solves the problem of “ghosting”, but previous developments have proved that COB is more harmful than good in this respect. Although the significant improvement in lamp performance has created a good technical foundation for COB packaging, it has finally met the application needs of the market. All of this looks good, but the underlying logic of the COB product form makes it impossible for the best technology to make up for its own flaws. And it is precisely based on the objective temptation of good technology that designers who are unfamiliar with LED characteristics are rolling further and further on the wrong road.

How the product is understood, how it is designed;

How products are designed and treated.

So the industry is not down for no reason!

The minority has said more than once that a good product is a humanity appeal, while a bad product attempts to impose some inappropriate features on the user to make a profit. These two points are reflected in the COB spotlights.

The technological development of LED chips has promoted the development of COB products, and the trend of product development will eventually lead to convenience, which is the humanity appeal of COB becoming the mainstream of spotlights. COB can overcome the problem of ghosting. Some spotlights don't want to have a side spot, let alone a ghost. If you use a single device arrangement, there will be a problem of ghosting.

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