LED quality and heat management: why operating temperature is so important

Getting the most from a product based on LED technology can be somewhat similar with aiming to obtain the maximum advertised speed of a internet connection subscription.  For example, a 8000 Mbs internet subscription will seldom provide actual 8000 Mbs download speeds.  The best network equipment, a high end computer  and speed guarantee by the internet provider are usually required.

Similarly there can be substantial differences between the advertised and actual performance of a LED based product. The principal factors are the LED design and heat management, how much of the heat generated while in operation is channeled away.

In brief if we manage the heat well, a LED based lighting product will have long life and be energy efficient. If it does not, we should not choose LED over other lighting technologies.

Exactly what poor heat management can do i have presented in a previous post.

Moreover, being a more complex product, a LED lamp or fixture is almost certainly more  expensive to produce, with a bigger effect on the environment as well. Only long lifetime and lower energy consumption will offset this.

The basics of LED heat management

To create light a LED uses 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.

We can make a parallel with data transfer from the internet via a network to a home computer. 

Each element of the network (Internet connection, Broadband Modem, Wireless Rooter and the Wireless or PCI Adapter) plays a key role in the overall speed with which the data reaches the home computer. Each has the potential to be a bottle neck, to slow things down.  This must be prevented by choosing the right equipment. 

In the case of a thermal circuit, each element prevents some of the heat to pass through it, a phenomenon called thermal resistance. 

A LED luminaire should be designed to channel away heat as a network is build for speed. 

 Thermal resistance is the equivalent of a bottle neck 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.

Lets 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.

Lets say we install the array in closed luminaire that functions in a hot environment. This can mean 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 L70 lifetime at an ambient temperature of 25 °C we can easily notice a gap exists.  With the above calculation and graphs the realistic values for each LED are 90 lumen and 30.000 hours L70 lifetime.

The picture is not always so good. There are many cases in which the gap between real and advertised is very significant.

Below are two 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 datasheets 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.