Feeling the Heat: The Crucial Role of Thermal Management in LED Performance

Understanding and Optimizing Heat Management in LED Lighting Systems

When it comes to LED lighting, heat management plays a pivotal role in ensuring the longevity and performance of the product. LED lights, despite their energy efficiency, do generate heat during operation. If not properly managed, this heat could significantly reduce both the LED's efficiency and lifespan. Therefore, understanding the science behind LED heat management is crucial before installing any LED technology.

The Importance of Effective Heat Management in LEDs

LEDs, or Light Emitting Diodes, operate by passing an electric current through a semiconductor material. This process generates light, but it also produces heat as a by-product. Without effective heat management, this thermal energy could accumulate and cause the LED's temperature to rise significantly. This can result in diminished light output, colour shifts, and even premature failure of the LED.

The key to efficient LED operation lies in effective heat management – that is, the dissipation of the heat generated during operation. This is achieved through a thermal circuit, which conducts heat away from the LED and dissipates it into the surrounding environment. However, the design and materials used in this thermal circuit play a crucial role in determining its efficiency.

Understanding the Basics of LED Heat Management

The thermal circuit in an LED lighting system generally starts at the LED die (the active part of the LED that emits light when current is applied), and extends through the LED pad, Printed Circuit Board (PCB), heat sink, and finally into the ambient environment. Each component of this thermal circuit is integral to the overall heat management process and has the potential to act as a bottleneck if not properly designed or implemented.

One of the key concepts in LED heat management is 'thermal resistance' – a measure of how effectively a component conducts heat. Similar to electrical resistance, thermal resistance is a property that depends on the material and design of the component. It is typically expressed in degrees Celsius per watt (°C/W). A lower thermal resistance indicates a more efficient heat conductor.

Case Study: Impact of Thermal Resistance on LED Performance

Consider an array of four LEDs, each consuming 1 Watt of power, mounted on an aluminum board and heat sink with a total thermal resistance of 15°C/W. The LED packages themselves have a thermal resistance of 5°C/W.

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.

In this scenario, the junction temperature (the temperature inside the LED) can be calculated as follows:

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

Tjunction (Tj) = Tambient (Ta) + Ptotal * Rhs + Pled *Rled

Assuming an ambient temperature of 60 °C (which might occur if the array was installed in a closed luminaire in a hot environment), we have:

Tj = 60 °C + 4 W * 15°C/W + 1 W*5°C/W

= 60 °C + 60 °C + 5°C

= 125°C

Thus, each LED in the array would have a junction temperature of 125°C.

Now, if the manufacturer of the LED array claims each LED produces 100 lumens and has an L70 lifetime (the time taken for the LED's light output to drop to 70% of its initial level) of 80,000 hours at an ambient temperature of 25 °C, it becomes evident that the actual performance and lifetime differ significantly from these advertised figures. In fact, based on the calculated junction temperature and typical performance graphs for LEDs, each LED in this scenario would produce only about 90 lumens and have an L70 lifetime of approximately 30,000 hours.

The Real-world Implications of Thermal Management in LED Systems

The impact of thermal management becomes even more apparent when we consider real-world scenarios. For instance, a random supplier on Alibaba demonstrates a 40% LED performance drop at 60 °C. While there was no lifetime data available, it's reasonable to infer that the L70 lifetime would be reduced to a few thousand hours at most.To illustrate this further, a graph compiled from data sheets of several manufacturers, showcasing specific LED models, again underscores the crucial importance of thermal management in maintaining LED performance and longevity.

Strategies for Effective LED Heat Management

So, how can we ensure effective heat management in LED lighting systems? Here are some key strategies:

  1. Select High-Quality LEDs: Higher quality LEDs typically have better heat management capabilities. This is because they are often designed with better materials and structures to dissipate heat more efficiently.

  2. Use Efficient Heat Sinks: Heat sinks absorb the heat generated by the LED and dissipate it into the surrounding environment. Choosing a heat sink with a low thermal resistance can significantly improve the LED's heat management.

  3. Opt for Proper PCB Design: The PCB serves as the physical and electrical interface for the LED. A well-designed PCB can aid in heat dissipation, reducing the overall thermal resistance of the system.

  4. Consider Ambient Temperature: The ambient temperature plays a significant role in the overall heat management of an LED system. In hotter environments, additional measures may be needed to ensure efficient heat dissipation.

  5. Employ Thermal Interface Materials (TIMs): These materials can be placed between the LED and the heat sink to improve heat transfer. They fill in any air gaps and create a more direct path for heat to flow from the LED to the heat sink.

Conclusion

Understanding the importance of heat management in LED lighting systems is paramount for their effective and efficient operation. While low-cost LED products may seem appealing, they often come with inefficient heat management and incomplete or misleading data about their performance and lifespan outside of controlled testing environments.

To get the best out of your LED lighting system, it's crucial to consider factors such as the ambient temperature, the quality of the LED product, and how its actual junction temperature impacts performance and lifespan. Proper heat management not only ensures optimal performance but also extends the lifespan of your LED lighting system, making it a cost-effective choice in the long run.

In conclusion, it's not just about the brightness or color of the light, but also how well the product can manage heat. As they say, it's not just about seeing the light, but also feeling the heat!

With the above in mind, we have selected the high performance LEDs from Nichia, Samsung or Seoul to use in most our Lumistrips.

 
 

 

 

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