The Tech Blog

When it comes to LEDs, it’s not just about the light they emit; it’s about how they handle the heat! LEDs are sensitive beings; they don’t bode well under excessive heat. Heat adversely affects their efficiency, light output, and overall longevity. If you’ve ever seen a decline in LED brightness over time, heat is the problem. Excessive temperatures can lead to a phenomenon known as “thermal runaway,” where the LED’s performance spirals out of control, shortening its lifespan and compromising its luminosity.

LEDs generate heat primarily through the forward voltage across the semiconductor junction. Higher the current, more the heat produced. Heat output can be assumed to be around 80% of the input power given to the LEDs(rough estimate). Do note that forward voltage decreases as the junction temperature of an LED increases. Temperature increase can cause colour shifts in emitted light too. There will be a graph in the LED datasheet called the temperature derating graph, which plots LED current vs LED temperature. It gives a safe operating region for an LED for a given temperature. Always keep within its limits.

So how do you keep your high-power LEDs cool? Heatsinks are the most common solution. They come in all shapes and sizes. Thermal heatsink design is simple but too long to explain here. Dave from EEVblog has a few videos on how you can do that, with thermal resistances from the heatsink’s datasheet, to keep your LED junction temperature in check. A commonly overlooked aspect by newbies is LED thermal vias and connecting them to the large ground planes. Surprisingly, you can utilize PCBs, both Aluminium and FR4, as effective heatsinks, provided they are sufficiently large. For critical applications, consider integrating a PTC thermistor. Typically positioned near the LED at a test point, it actively measures real-time heat, enabling the regulation of driver current via feedback. This, in turn, acts as a safeguard against thermal runaway, ensuring the stability and long life of your LED system.

Not all LEDs are created equal! When selecting LEDs, one crucial aspect to consider is their beam pattern. The datasheet section to look into this would be the radiation or luminous intensity chart. It will be a circular polar chart or XY straight line plot (which is just the polar chart stretched out linearly, check images). The graph shows the intensity on one axis and the angle on the other. All that graph shows is how the light is spread out in space and a cross-sectional view of the 3D beam pattern with the LED at the center when you cut it along a particular axis. With this graph, you can understand how the illumination pattern of the LED will be.

Some datasheets opt for simplicity, providing the LED’s beam angle rather than a chart. The beam angle, defined by FHWM (full width at half maximum) is just a fancy way of signifying the angle at which the intensity drops to 50%. This angle influences light spread—narrow beams cover longer distances with heightened intensity, ideal for applications like LED spotlights. Understanding the radiation pattern proves critical when strategically arranging multiple LEDs to achieve desired lighting outcomes.

In the realm of SMD LEDs, the physical embodiment of the radiation pattern is done by the tiny transparent lens on top of the LED die. Manufacturers manipulate lens properties to yield diverse radiation patterns. Yet, not all patterns are achievable in-built, prompting the use of mountable lenses. These lenses, often made of moulded PMMA, come with holders and can be fixed on top of LEDs for unique lighting patterns. They’re extremely cheap and are worth considering if you want some unique lighting patterns from your LEDs.