Thermal Management: The Main Challenge of LEDs – featured in EE Publishers’ Vector Magazine

This article is featured in the October 2012 Edition of EE Publishers’ Vector Magazine – http://www.ee.co.za/article/blair-hammond-and-associate-326.html

Introduction

A number of articles about the role of the different critical parts of LEDs have been published in Vector previously and in many other lighting publications. None of these articles have directly addressed the single most important aspect of the LED – thermal management.

I will endeavour to convey the importance of thermal management to the readers and highlight that there is an exceedingly small and elite group of manufacturers who are successful in the area of thermal management.

The Challenge

Thermal management is critical to both the performance and lifespan of the LED system. Thermal management embraces efficient heat sink design, high efficiency circuit boards, high thermal conductivity of enclosures and other advanced thermal design techniques.

When I have been conducting workshops or training, I always tell the audience that “you can have the best chipset in the world, but if the thermal management is inefficient, you will still have a poor quality and performance product which will not last for the advertised lifespan”. I have had numerous practical encounters with LED products which have fallen into this category which in all cases left the clients doubting whether LED products were as good as is claimed. I have also adopted the expression that “not all LEDs are created equal”.

Of course, I am fully aware that there are other parts of the LED which are also critical to the performance and lifespan of the LED.

It is important for the smallest LED lighting products such as the MR11 and MR16 down light types to have efficient thermal management, it becomes even more important for high‐power LEDs to have the best and most efficient thermal management.

Effects of Excess Heat

Excess heat reduces short term LED light output and produces colour shift. Long term effects include the accelerated reduction in light output resulting in a shortened lifespan. LEDs are typically tested at an ambient temperature (Ta) of 25°C whereas the normal operating ambient temperature of the environment in a ceiling where most LED down lights are installed often >35°C and sometimes higher >50°C.

Under these conditions the light output could be >10% below the initial light output and even worse than this for products with inadequate thermal management.

The main heat transfer path in an LED device is conduction to the system enclosure. It is therefore essential that the cooling system be developed around the chipset in order to dissipate the heat generated at the junction point (Tj) away from the p‐n junction efficiently.

It is also known that as an LED device ages, so the heat output generated increases over time thereby accelerating the rate of degradation. A common cause of lumen depreciation in white LEDs is due to the yellowing of the phosphor due to the natural gradual degradation of phosphor over time. However, if the thermal management of the device is poor, the increased heat which results from poor heat dissipation will result in more rapid phosphor degradation. It is not always true in such cases that the chip is working less efficiently or that more heat is being generated.

It is of paramount importance that the junction temperature (Tj) of the LED device does not exceed the maximum junction temperature (Tj) specified by the chip manufacturer. This simply implies that the LED device manufacturer has to produce a device which will work within the chip manufacturer’s specifications.

What is considered to be thermal management? It is simply how effectively the heat is dissipated from the chip outwards and away via the heat sink using conductivity, natural convection and radiation.

I have conducted tests on many LED products over some time. In many cases I have found that there is often very poor thermal connectivity between the chipset and the heat sink. In the worst cases that I have studied, thermography has clearly shown that it would appear that there is a thermal “moat” around the heat sink and that very little heat is conducted from the chipset to the heat sink. This has been the case in a number of LED products that we have tested which have been locally produced in South Africa.

Obviously, customers who do not have or in fact are not really expected to have the level of knowledge about LED technology that we have would, in their patriotic desire to support South African business, be purchasing products which would have high lumen depreciation and short lifespan. I must emphasise that not all locally manufactured LED products fall into this category.

Many companies who market LED products do not understand this complex subject. They do not appreciate the possible consequences of early lumen depreciation and reduced lifespan but continue to make claims about product quality and lifespan, many of which fall into the realm of fantasy!

Before continuing, anyone who is involved in the fascinating and challenging world of LED lighting should know about the 10° Rule. The 10° Rule is based on the Arrhenius Equation. The rule states that for every 10°C rise in the ambient temperature, a commensurate rise of 10° will occur in the Tj. It is similarly known that when that occurs and the device is operated under that condition for a prolonged period of time, the lumen output and the lifespan will be reduced by 50%. It follows then, that if a further Ta of 10°C occurs and the device is operated continuously in those conditions, the lumen output and lifespan will be further reduced by 50%. Simply stated, if the Ta increased by 20%, the lumen output and lifespan would be reduced by 75%!

Here is a graphic courtesy of Luxeon Rebel to clearly illustrate the effect of the 10° Rule.

I have found that there are only a few of my fellow lighting engineers who have been in the lighting industry and in more recent years in the LED lighting industry who understand the importance of thermal management in LEDs. Even fewer of them understand the importance of understanding how important the Delta temperature issue is in LED products.

Delta T or Δ T is the change of temperature in a device. In an LED device this is measured at the chipset Tj, at the inner edge of the heat sink to be able to verify that there is thermal conductivity taking place away from the chipset Tj and then finally at the outer edge of the heat sink. The change is measured by taking the Tj (chipset junction temperature) minus the temperature at the outside edge of the heat sink. The Δ T should not exceed 10°C.

It has been alarming to see how many LED products do not perform well in this one critical area. It is surprising to take hold of an LED lamp in one’s hand which has been running for about 30 minutes and find that it is so hot that one cannot grip the device. Further testing reveals noticeable lumen depreciation almost immediately and extremely poor thermal management.

The majority of products which we have tested to determine the Δ T, have had Δ T of between 19° and an incredible 83°C. The latter product failed after only about 3,000 hours which was hardly surprising despite the fact that the distributor claimed a lifespan of 50,000 hrs.

The Heat Sink

The theory of heat sink design is to create a path which will conduct the heat away from the junction point. It requires a high level of engineering and the use of the best available materials to achieve this.

Heat sinks should be designed using the highest grade aluminium and in such a way as to provide the greatest possible thermal surface area. There has to be excellent thermal connectivity between the chip and the thermal plate below the chip.

One overseas manufacturer is highly experienced in heat sink design having had twenty three years in the large computer file industry. In the server industry, they happen to be No 3 in the world. They have been awarded many international design and quality awards. Every product in their extensive range complies fully with all that I have detailed above.

Conclusion

There is far more than meets the eye about LED lighting technology. What makes it even more complex is the fact that there are still other considerations which will be discussed in a future article.

I trust that readers will be stimulated to take a much closer look at the aspects of heat sinks and thermal management.

Vector Magazine Link – http://www.ee.co.za/article/blair-hammond-and-associate-326.html

PHILIP HAMMOND

BHA SCHOOL OF LIGHTING – 22 NOVEMBER 2017

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