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The Effect Of Cooling On The Lifespan Of Led And Luminous Flux And How To Judge The Cooling Performa

By Author: Leo
Total Articles: 6

Lamp cooling appears to be a popular topic only with the LED light source. While the LED lamps pay attention to heating and cooling, earlier traditional light sources stay unbothered about it.

So why don’t traditional light sources pay attention to cooling?

The earliest light source, incandescent lamp, produces its glow through heat. The heat is never a problem as long as the temperature does not melt the glass.
The halogen lamp is also hot, but the heat does not affect the efficiency and lifetime of the lamp. So long as the heat does not melt the components, there is no problem.
The fluorescent light itself produces little heat, but it is also fitted with insulation, so under normal circumstances, it will not be affected by heat.
The metal halide lamp has a high temperature. As such, there is no need to worry about heat, unless the power is high.

This is a high-power metal halide lamp. The wrong amount of heat may cause the electrode to melt, so it has a very thick electrode connector. In some cases, water may be drawn inside to cool it.

Why IS LED ‘afraid’ of heat?
No matter the wattage of a LED lamp, it must undergo cooling. This is because it has a ‘fear of heat’. Because LED is a semiconductor device, when it encounters a higher temperature, the lifetime performance is reduced, sometimes even leading to the end of its lifespan.
LED chip works with what is known as junction temperature. The temperature of the LED chip is the internal PN junction temperature.

LED works with a little chip, as you can see from the picture.
What is the relationship between the junction temperature and Luminous flux of LED?

As you can see from the picture, when junction temperature is higher, Luminous flux is lower. When junction temperature is higher than LED working condition, it will go down directly.
What is the relationship between junction temperature and LED lifetime?
When the general luminous flux is down to 70%, the life of the LED is likely to end.

As shown in the picture, with the junction temperature at 105 degrees, the luminous flux drops rapidly to 70%, making the LED lamp lifetime about 10,000 hours. If the junction temperature is controlled to 55 degrees, the lifetime can be increased to 100,000 + hours.
In other words, we must find a way to reduce junction temperature, so that the LED can dissipate heat of itself.
So how can we dissipate the heat?
LED cooling principle
We already learned in middle school physics that there are three ways to dissipate heat: Conduction, Convection and Radiation.
CONDUCTION: the process through which heat travels from the hotter part of the body to the cooler part of the body.

Factors affecting heat conduction:
The thermal conductivity of cooling material
The Thermal resistance of cooling structure
The size and shape of heat conduction material
CONVECTION: the use of gas or liquid flow to take the heat away.
RADIATION: the phenomenon of high-temperature objects directly dissipating the heat outside.

Here, there are two LED heat sinks with similar shapes but one is sealed while the other is slotted. It is clear that the heat sink on the right, working with convection, has better heat dissipation, while the one on the left dissipates heat completely by conduction. 

After understanding the relationship between heat dissipation & luminous efficiency, life expectancy, and cooling methods, how do you determine whether the cooling of a lamp is good or not?
How to judge if a LED light heat is good or bad?
A lamp is designed with reasonable cooling and the cooling capacity has a relative relationship with power. The size and weight of the heat sink determines the heat storage capacity, so the heat sink is generally a big size. Heat sink area determines the final distribution capacity and in order to increase this area, it IS usually made into a variety of columns, nets, and other shapes.
The heat sink accounts for a large piece of the total cost for producing led lamps, so while trying to cut costs, many factories produce with insufficient and low-quality materials. Now, the question is, how can we tell that the cooling system of a light is made well?
Of course, the easy way is to measure the junction temperature of LED chip when the light is working. If the junction temperature is controlled within an acceptable range, then the cooling is well done. If the result falls on the contrary, the cooling is bad.
However, it is not easy to measure the junction temperature. Doing so requires a series of professional equipment and methods to measure the LED chip junction temperature inside the lamp. But sometimes, even with the best equipment, we can only see the appearance from outside, not measure it inside.

Even with a professional imager, you can only see the outside temperature and not an accurate measurement of the LED clip junction temperature inside.
A designer or contractor cannot measure the junction temperature when they choose the lights. The easiest way to know is by touch: Is it best when it’s hot? Or is it best when it’s not hot?
First of all, it is not scientific to measure the temperature of led lights simply by touch, since different people have different levels of sensitivity towards temperature. But in the absence of any test equipment, you can get a rough estimate of the lamp temperature by touching it with your hand. The lamp temperature should be lower than what will scald the hand.
When a LED lamp is working properly, one of the signs is a lower temperature cooling, but a lower temperature is not necessarily always a good sign.
So why is a lower temperature heat sink not necessarily a good thing? The major problem is the conduction of heat. When the heat cannot be smoothly transferred to the chip, it accumulates around the heat source. The high temperature causes the heat to transfer to the heat sink, making one feel the temperature is not high.
If the heat sink is very hot, either the cooling system is bad or the cooling capacity is insufficient. In such a case, the cooling area is inadequate, which in turn slows down the rate at which heat is exchanged with air. This results in the lamps dissipating the heat with the air at a different temperature, causing it to feel very hot.

Some heat sinks appear very thick, almost like “effective cooling area” is not enough to describe them. To be clear, an “effective cooling area” is a cooling system with parts that allow free passage of air and through which air can easily leave the heat sink area. Other materials cannot allow free access to air for all parts; only to the heat capacity materials or heat radiation areas.
Based on physical testing by touch, the following are tips on how to systematically identify the advantages and disadvantages of led lamps cooling:
"Half-hour illumination method" test for junction temperature

Since we cannot directly measure the junction temperature, is there an indirect way to get the junction temperature? Fortunately, there is. As the general LED junction temperature increases, the luminous flux declines. Then, as long as we measure illumination changes in the same position, we can determine the change of junction temperature.

i.Choose one of your outside lights. It is best to turn off the other lights at night.
ii.Turn on the cold lights and immediately measure the position of illumination. Note the reading as "cold illumination."
iii.To keep the lamp and the illumination at the same position, make sure the lamps continue to work.
iv.Half an hour later, take another measurement. Record that reading as "hot illumination."
v.If the two readings are similar (10 ~ 15%), the lamp cooling system is basically good.
vi.If the two readings are far apart (more than 20%), then the lamp cooling system is questionable.

The suitable range of “Half-hour illumination method”
Using the "Luminous Flux vs Junction Temperature" calculation, you can see from this graph, that when we know how many Lumens the luminous flux decreased, we can indirectly arrive at how many degrees Celsius the junction temperature rose to.

Figure: Osram S5 (3030) chip. The luminous flux dropped 20% than the 25℃ and the junction temperature rose to 120℃.

Figure: Sensus series chip, the luminous flux dropped 15% than the 25℃, and the junction temperature rose above 105℃.
As we can see from the figures above, if half an hour later, the range of “hot illumination” to “cold illumination” is down by 20%, then the junction temperature has exceeded the chip tolerance range. Basically, we can conclude that the cooling system is unqualified.
This is what happens in most cases. But are there any special cases? Of course:

Figure: OSLON Square 1-5W chip. Junction temperature rises from 25 ℃ to 40 ℃ . The flux rises first and then declines. When it goes up to 120 ℃ the luminous flux decreases by about 10% only. Therefore, for the chip, we use the "half hour illumination method" to measure heat, but can only get 5-8% change.

MingfaTech Manufacturing Limited, a LED thermal solution leading provider, headquartered in Dongguan City of China, who is a certified LED heat sink supplier of many branded LED modules. MingfaTech’s design and supply on star heat sinks and pin fin heat sinks, especially the breaking-through LED cooling series of eLED , xLED , tLED , BuLED , EtraLED, SimpoLED, GooLED, HibayLED coolers have been helping the clients improve their designs with the heat dissipation in better efficiency and more aesthetic appearances, substantially making LED in a real sense as well. With the professional know-how focused on LED cooling, MingfaTech has been bringing high value to all its customers’ esteemed brands, like Philips, Orsam, Xicato ,Lumileds ect. MingfaTech has gained very good reputation from hundreds of valued customers in America, Europe, Middle East, Asia, Australia and rest of the world. With more than seven years’ experiences in LED coolers developing, molding, manufacturing as a trustworthy turnkey solution partner for all clients in LED industry, the company is gradually growing up bigger and bigger, as well as more professional in LED thermal management. 

For more information about MingfaTech Manufacturing Limited,
please check www.mingfatech.com or mail to sales@mingfatech.com

Sales & Marketing Center:
Suite 1107, Star City Building No. 52, XueQian Street, DongCheng District, DongGuan City, GuangDong Province, ChinaTelephone:

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