# What is thermal runaway and why does it happen?



## Murphy625 (Jan 29, 2014)

What is thermal runaway and why does it happen?

My first thoughts is that the LED chip gets too hot which increases its resistance and then the increased resistance causes more heat, then more resistance, bla bla bla.. until poof.. 

Do I have this right?

So, thermal runaway is easy to prevent by over sizing the heat sink yes?

thanks.


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## jtr1962 (Jan 29, 2014)

Thermal runaway happens when you drive an LED with a constant voltage source, as opposed to a constant current source. The key reason thermal runaway occurs is because an LED's forward voltage decreases as temperature rises. Or put in layman's terms, it's resistance decreases with temperature. With a constant voltage source the decreased resistance results in the current increasing. This is turn increases the amount of power going to the LED and makes it hotter, decreasing the resistance further, and increasing the current, until either the power source or the LED is destroyed. Thermal runaway is best prevented by using a constant current source. A constant current source will adjust the voltage going to the LED so the current remains constant. The LED can still overheat if the heat sink is inadequate, but you won't have a runaway increase in current and heat as you would using a constant voltage source.


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## DIWdiver (Jan 29, 2014)

Another way thermal runaway can happen is if you have several LEDs or strings of LEDs in parallel. If there is something different about one LED, like maybe a poor connection to the heatsink, or even just the worst spot on the heatsink, and it gets hotter than the others, its forward voltage drops and thus it draws current from the other LEDs.

Because a small change in voltage produces a large change in current, this can get away from you in a hurry.

There are numerous ways to prevent it. Right-sizing the heatsink is one, and using current limits is another. You can also avoid paralleling, or add a small resistor in with each string. You can also make sure you have LEDs from the same batch. I built a driver years ago for a 3S10P array. Actually it's a dual driver, running a 3S4P and a 3S6P array. After some years and 1000+ units, they decided it was so reliable that they wanted to raise the current limits.


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## Murphy625 (Jan 30, 2014)

Thanks!

Great info...


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## Mr. Tone (Jan 31, 2014)

Wow, some great replies here. I did not know the true details of this scenario so I am glad the OP made the thread and got 2 great replies! Once again, I have been _enlightened_ by CPF. :candle:


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