Heat management of LEDs
- By Ledrise Led Professional
- Dec 15, 2020
The optimal use of a product based on LED technology can be made difficult, as high operating temperature can lead to a significant decrease in performance and lifetime.
Important factors for this effect are LED quality, product design and heat management: how much of the heat generated during operation is dissipated.
If the heat is well managed, an LED-based lighting product will have the announced performance, long life and energy efficiency.
The basics of LED heat management
LEDs use electricity and this process generates heat. This heat must be conducted away as efficiently as possible from the LED in the environment. The design of the LED itself and the luminaire for this purpose is referred to as thermal management. The heat to be dissipated is directly proportional to the luminous flux and power consumption of the LED.
The heat is dissipated via a heat circuit.
A simple example of a thermal circuit starts with the LED cube, goes via the LED pad to the circuit board, the heat sink and finally to the environment.
Each element of this chain plays a key role in the total heat with which the data is diverted and has the potential to be a bottleneck to slow things down. This must be prevented by choosing the right equipment.
In the case of a thermal circuit, each element prevents some heat from passing through it, a phenomenon known as thermal resistance.
Thermal resistance is the equivalent of a bottleneck in a data network. Suitable materials and an intelligent design are required to achieve minimum thermal resistance.
The thermal resistance is measured in °C/W. A value of 1°C/W means that for each watt of LED power consumption, the thermal resistance (heat not transferred) is 1°C.
Suppose we have an array of 4 LEDs (as in the picture below) each with 1 Watt power consumption (Pled = 1 Watt, Ptotal=4 Watt at 350mA).
Each of the LED packages has a thermal resistance of 5°C/W (Rled). The LEDs are placed on aluminum board and heat sink with a total thermal resistance of 15°C/W (Rhs)
At specific points in the thermal circuit (equivalent to the data network) temperature values are defined. For the array in our example the below are of interest:
- Tjunction (Tj)
- Tambient (Ta)
With these temperature values we can discover how a LED will perform (Relative Luminous Flux) and what lifetime will it have (L70 Lifetime). Both depend on the temperature inside the LED, called junction temperature, a relationship usually represented in graphs as the ones below.
Let's say we install the array in a closed luminaire that works in a hot environment. This can mean an ambient temperature of 60 °C (Ta). We then calculate the connection temperature (Tj) to be used with the diagrams above.
Tj= Ta + Ptotal * Rhs + Pled *Rled
Tj = 60 °C + 4 W * 15°C/W + 1 W*5°C
Tj = 60 °C + 60 °C + 60 °C + 5°C = 125°C = 125°C = 125°C
Each LED in the array has a connection temperature of 125°C.
This information shows us the gap between actual and advertised performance and longevity.
If the LED array manufacturer claims that each LED has 100 lumens and 80,000 L70 lifetime at an ambient temperature of 25°C, we can easily determine that there is a gap. With the above calculations and diagrams, the realistic values for each LED are 90 lumens and 30,000 hours L70 lifetime.
The picture's not always so good. There are many cases where the gap between real and advertised is huge.
Below you will find two examples.
Figure 1: A diagram of a random supplier on Alibaba. At 60 °C we see a 40% drop in LED power. Although no lifetime data was available, we could safely assume that the life of the L70 would not exceed a few thousand hours.
Figure 2: A graph compiled from data sheets of several manufacturers, with the specific LED models mentioned at the bottom.
In conclusion, we should always consider the ambient temperature relevant to our project, how good is the LED product at heat management and how the actual junction temperature impacts LED performance and lifetime.
There should also be an understating that a low price usually means poor LED performance at high temperature, inefficient heat management and absent, incomplete or misleading data about the performance and lifetime of the product outside of a cooled testing chamber.
With the above in mind we have selected the Nichia 757G LEDs as the LED of choice for most our LED strips. The proprietary Nichia design of the 757G has the best resistance to high temperature on the market and thus ensures the best performance with the easiest thermal management.
