Practical Thermal Management of LEDs

 

Temperature has tremendous effects on LEDs. This is very evident from the practical thermal behavior seen in the devices. In any case, there’s a need to discuss how to manage  the thermal effects in order to make the most out of the LEDs. This is exactly the subject matter of this discourse. There’s a need to look at the analytical method used in calculating the thermal effects as well the  thermal environment in which the LEDs operate in. we’re also going to discuss the various methods of keeping the LEDs cool.


Thermal Analysis of LEDs
To understand the thermal analysis of LEDs, you can borrow a leaf from what happens when you put hot coffee in a ceramic cup. Usually, the coffee cools after about 10 minutes.  But if you put the same coffee into a thermos, it will still be hot even after 10 hours. This shows there’s a difference between the ceramic cup and thermos. The unit that measures this difference is known as “thermal resistance”. It’s quite analogous to electrical resistance found in LEDs. In most cases, the electrical resistance stops heat from flowing instead of resisting the flow of electricity.  The ceramic cup in the above example has low thermal resistance. That’s why heat flows through it very quickly while the heat in the coffee is gone very fast. On the other hand, the thermos has a very high thermal resistance. That’s why the hotness of the coffee is not lost after several hours.

 

Another example you can consider is the hot water used in making the coffee. If you boil a little quantity of water on the stove or burner, it usually reaches boiling temperate very fast. But if you’re to make coffee for 10 extra people, you have to add enough water and boil it on the same burner with the same capacity. Usually, the water will take extra time to reach the boiling temperature. The unit that measures the difference between the little water and the bigger water is known as “thermal capacitance”. This can equally be analogous to electrical capacitance found in LEDs.  It usually stores up heat and increases temperature instead of storing up charge and increasing voltage. A little amount of water has a higher thermal capacitance hence, its temperature rises slowly. But a larger amount of water has lower thermal capacitance while its temperature rises very quickly.

The above examples on coffee and hot water are perfect analogy to the thermal analysis found in LEDs.  Thermal resistors and thermal capacitors are linear devices just as ordinary resistors and capacities are.


Thermal Resistance Calculation
Thermal resistance in LEDs is easy to calculate with analogous examples since it can be very complex. If for instance, you have a heat source that flows through two different avenues to reach the environment, each of the paths is simple a thermal resistor. Hence, both of them are parallel to each other. The total heat transferred is therefore the parallel combination of the two thermal resistors. This is the same in electricity. The ambient is usually at a fixed temperature and it’s also analogous to a fixed voltage source. It’s going to stay at the same voltage or temperature irrespective of how much current you put into it.

 

If for instance, you have an LED that is giving out 3W; its packaging will have a thermal resistance of 100C/W from its junction to the case. Actually, the case will have a thermal resistance of 120C/W in relation to the ambient temperature which stands at 300C. If you carry the thermal calculation   using the right equation, you’ll find out that the die temperature will stand at 960C.

Meanwhile, the ambient temperature refers to the air in the lab. It can stand at 250C.  If for instance, your resistor temperature rises by 500C, the ambient temperature will climb to 750C. This is because the resistor has 2 wires that go into the power supply and they actually conduct heat and also dissipate heat from their make-ups.  If the resistor is sitting on a lab bench, it usually conducts heat through the bench. However, if the bench is a metal, it may serve as the dominant heat removal mechanism for the resistor.

The ambient air usually has 3 methods of removing heat from the resistor. They include convection, conduction and radiation. Convection and conduction rely on their air nature while radiation doesn’t rely on that. In any case, conduction in air is nothing for LEDs in practical terms. Convection is usually very complex and it normally depends on the shape and size of the object in the air. It also depends on the characteristics of the air, the size, shape and position of the objects that surrounds the main object in question. Radiation on its part is usually governed by the Stefan-Boltzmann Law. This law states that a black-body in free space radiates at a rate set by the 4th power of the temperature. This assertion is quite complex. You have to use a specific equation to understand this better.

 

Practical Temperature Estimation in LEDs
The temperature in LEDs can be estimated through  convection and radiation. If you calculate the temperature based on radiation alone, you’ll discover that the actual temperature is much lower than when convection is added.  Convection can actually be minimized. For instance, the convection in air is usually effective at cooling but it can easily improve when the fan or blower is put on. Blowers and fans usually operate by forcing air to move across hot surface. Another area to consider when looking at reduction of thermal resistance is the thermal radiation. It has a lot to offer in the thermal management of LEDs.

Really, the above discourse on the practical thermal management of LEDs seems too technical. This is actually the case. You have to take some time to assimilate the details discussed. You may need to make further research on the overall performance of LEDs in order to understand the practical thermal management discussed above.  LEDs have continued to evolve as the years roll by. The future of such devices is indeed very bright.