COB LEDs are very popular nowadays in LED lighting business. We talk and write about COBs, and our customers use COBs in their luminaires, but what exactly is COB? First of all, the abbreviation COB comes from words Chip-on-Board.
Citizen Electronics’ COB frontside, The yellow substance is phosphor, which turns the blue light of the chip white
In COB packages many LED chips are usually attached to substrate with non-conductive adhesive. LED chips are wire bonded together to make different LED setups. The amount of single LED chips, inside a one COB LED package, can vary from few pieces up to several hundred pieces. Substrate is located on base material. Base material of COB LED is usually MCPCB or ceramic PCB. COBs often have blue diodes and use yellow phosphor layer to convert light to desired color temperature.
General drawing of a COB LED.
In early years of 21st century there were few SMD LED packages, which could be considered almost as COB packages due to their construction. Generally COB LEDs became available and popular in LED lighting market around year 2007. At first there were first quite a lot doubts towards COB LEDs in the market . Mainly because this package construction enabled LED manufacturers to put high powers in small package. Over then, this “high power” meant over 10W,
Also there was very little experience of COB LEDs, so these LEDs had a lot to prove. Although now several lifetime tests have shown that COB packages are very reliable, if heat management is done properly.
When thinking about heat management, one important feature is thermal resistance. But it is worth noticing, that you can’t define which COB LED is better to conduct heat just from thermal resistance value. You should actually test LEDs in your own application.
Today COB LEDs are available from few hundred lumens up to 30,000 lumens. This means that
Citizen Electronics’ COB backside, aluminium PCB, which will conduct the heat to the heat sink effectively. However it won’t be enough to cool the LED
almost every light source can be replaced with COB LED. So available powers go from few watts up to almost 200 watts. The most powerful COBs require exessive heat sinks because they generate a lot of heat.
COBs offer great variety due to possibility to have many different LED setups even inside one COB package. Usually LED manufacturers have different lumens packages available in same size COB, so lighting manufacturers is able to use e.g. same connectors and optics in different solutions. Also it is good to remember that usually you can underdrive or overdrive COB LEDs and those might have quite wide driving range. This allows you to drive LEDs with very high efficacy, you can make balanced solution or you can make economical lumens.
COB LEDs are generally used in luminaires where lamp or single spot light sources have been used. So basically COB LEDs are used in almost every kind of luminaire. Although COB LEDs are used less often in linear lights or panel lights, some solutions of that kind have been made with COBs. Luminaires which have really high luminous flux are usually made with more than one COB LED to distribute heat flux and to ease design of heat sink.
Sochi Olympic cauldron, Lights made using Citizen Electronics’ COB (Copyright: Zers Pride LLC)
COBs offer very high luminous fluxes from small size packages and thus allow flexible design of luminaire. This gives you more freedom, when designing a luminaire. COB packages usually have excellent uniformity of light from light emitting surface, this is important e.g. if you want to avoid multiple shadow effect, which might occur with SMD based solution. It is also very easy to test COB LEDs. You only need COB LED, constant current driver and e.g. piece of aluminium, which can be used as heat sink.
LED Shadows: On the left, a shadow from a COB. On the right, a shadow from a LED with multiple light sources
We can see that COB LEDs have been very important development step in LED lighting business. This great package design has allowed to increase powers used inside one LED package tremendously. Also one good indicator of success of COBs is that today every major LED manufacturer and package maker has COB LEDs in their selection.
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