When it comes to replacing or redesigning a light fixture, many people turn to LED lights for their efficiency benefits over traditional sources. However, even though LED lights are significantly more efficient than a filament bulb, they still generate heat that must be considered in the overall design. In this article, I will explain what heat does to your LED lights and what you can do about it.
Heat Output of LED Lights
A 100W tungsten filament bulb is about 10% efficient, meaning it only generates 10W of light energy for every 90W of heat energy. The “light energy” is what designers need to match with other sources to get an equivalent amount of light, which is why high-efficiency bulbs can call themselves equivalent to a 100W bulb while only actually consuming a fraction of that energy.
While LEDs themselves can be extremely efficient, other components of the system, such as the driver or lens, tend to reduce the efficiency of an LED light bulb to about 70%. That is an incredibly energy-saving over the 10% efficient filament bulb, but still means that about 30% of the power consumed by the bulb gets emitted as heat rather than light.
| Bulb Type | Input Power | Light Output | Heat Output |
|---|---|---|---|
| Tungsten | 100W | 10W | 90W |
| LED Bulb | 100W | 70W | 30W |
| LED Bulb | 15W | 10.5W | 4.5W |
How Do LEDs Work?
To understand how LEDs work, it’s important to note that they operate quite differently from traditional filament bulbs. While heat aids the efficiency of filament bulbs by lowering the electrical power required to heat the filament to the point of emitting light, LEDs emit light (and heat) based on the PN junction of the diode.
The PN junction creates an energy gap that an electron must achieve before it can be absorbed into the other side of the diode junction and emit light. This energy gap is indicated by the forward voltage of the LED. When heat is introduced, the voltage of the LED decreases, causing the electrons to start at a higher energy level. As a result, the efficiency of the LED decreases, and it emits less light.
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LED Constant Current vs. Constant Voltage
Most LED drivers are constant current and limit the LEDs as described above. While it seems like a good solution would be to move to constant voltage control, which would allow the LEDs to draw more current as the voltage decreases, it actually creates a positive feedback loop in the diode junction that causes the LED to draw more and more current, generating more and more heat until it burns up. Even if the power supply has a current limit, constant voltage supplies are not designed to regulate at that limit and will likely stop functioning once their limit is exceeded. One way or another, something in your system will fail.
Thermal LED Runaway and Dimming Lights
As an engineer who works with LED systems, I understand the importance of avoiding both thermal runaway situations and dimming lights. One solution is to use temperature-sensitive driver chips, which are often placed in series with each string of LEDs in a fixture. This balances parallel strings and compensates the allowed current based on fixture temperature.
Here are some key points to consider when dealing with thermal LED runaway and dimming lights:
- Use temperature-sensitive driver chips:
These chips will limit the current to a range that is safe for the LEDs and prevent runaway while also increasing the current to compensate for lower voltages. - Compensating for temperature with power supplies is not enough:
While some power supplies claim to compensate for temperature, this is only effective when ambient temperature fluctuates. Temperature of concern is usually localized to the LEDs and can only be properly regulated by a chip that is on the same board and in the same situation as the LEDs themselves. - Proper heat sinking is crucial:
The easiest solution is to allow proper heat sinking for the LED board to prevent junction temperature from reaching the point where LED dimming is noticeable. Using thermal vias, generous copper pours, metal heat sinks, and fans where appropriate will help keep the LED system cool. - Dimming is only noticeable at 50% brightness:
Without a comparison point, our eyes typically do not detect dimming until the light source is at 50% of the original brightness, so things have to get pretty hot for the average application to be affected by the dimming effect. - Let fixtures cool down before investigating dimming:
If you do notice dimming in a fixture, it’s important to let it cool down before investigating as it will be hot.
Tips for Managing Heat
In summary, while LED lights are significantly more efficient than traditional sources, they still generate heat that can affect their performance. To prevent thermal runaway or dimming of LED lights, it is important to properly manage heat through the use of temperature-sensitive driver chips, thermal vias, copper pours, heat sinks, and fans where necessary. By taking these steps, you can ensure that your LED lighting system runs efficiently and effectively for years to come.
