LED heat management explained

LED heat management explained
Before installing LED technology, we should consider how operating temperature can result in a substantial difference between the advertised and actual performance of a LED lighting product.

Determining factors are the LED quality, product design and its heat management. 

It is paramount that much of the heat generated while in operation of the LED is channeled away. If it occurs, a LED based lighting product will have performance as advertised, long life and will be energy efficient. 

The basics of LED heat management

LEDs use electricity and this process generates heat. This heat needs to be channeled away from the LED in the ambient as efficient as possible. Designing the LED itself and the luminaire for this purpose is called heat management. The heat that needs to be channeled away is directly proportional with the luminous flux and power consumption of the LED.

Heat is channeled away via a thermal circuit.

A simple example of a thermal circuit starts with the LED die, continues via the LED pad to the PCB, the heat sink and finally the ambient.

Each element of this chain plays a key role in the overall heat with which the data is channeled away 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 to pass through it, a phenomenon called thermal resistance. 

Thermal resistance is the equivalent of a bottleneck in a data network. To have minimum thermal resistance, proper materials and smart design are required.

Thermal resistance is measured in  °C/W. A value  1°C/W means that for every Watt of LED power consumption the thermal resistance  (heat not transferred) is 1°C.

Let's say we have an array with 4 LEDs (as in image below) each with the power consumption of 1 Watt (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.

We install the array in a closed luminaire that functions in a hot environment. This can result in

an ambient temperature of 60 °C (Ta). We will then calculate the junction temperature (Tj ) to use with the graphs above.

Tj= Ta + Ptotal * Rhs + Pled *Rled

Tj = 60 °C + 4 W * 15°C/W + 1 W*5°C

Tj = 60 °C + 60 °C + 5°C =125°C

Each LED from the array will have a junction temperature of 125°C.

This information will show us the gap between actual and advertised performance and lifetime.

If the manufacturer of the LED array claims each LED has 100 lumen and 80.000 hours L70 lifetime at an ambient temperature of 25 °C we can easily notice the gap exists. 

With the above calculation and graphs, the resulted values for each LED are 90 lumen and 30.000 hours L70 lifetime.

There are many cases in which the gap between real and advertised is much more significant.

Below are examples.

Figure 1: A graph from a random supplier on Alibaba. At 60 °C we are looking at a 40% LED performance drop. While there was no lifetime data available, we could safely assume the L70 lifetime is a few thousand hours at best.

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 a cooled testing chamber.

With the above in mind we have selected the Nichia 757G LEDs as the LED of choice for most our Lumistrips. 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.

LumiFlex Nichia 757G Flexible LED Strips

High Power MaxLine Nichia Modules

Backlight Nichia LED modules

LinearZ Plug & Play Nichia LED modules

 

 

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