Using a fan to cool LED phosphor gel?  - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: January 03, 2023, 03:04:15 pm
content: I know that LED's last longer when run at lower temperatures which sometimes requires them to be actively (fan) cooled. Some of my high power LED's phosphor coating gets quite hot during use (60C or higher; measured using a thermal camera). Should I use a (filtered) fan to actively blow air over this area to cool it down or will this actually be worse for the LED phosphor coating and "dry" it out?My LED COB is of the design attached (the one where you can physically touch the soft phosphor gel).

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 03, 2023, 04:41:04 pm
content: First thing to do is to put a bit of paste on the backside and slap it on a aluminum heatsink. A passive heatsink does not make noise and is a lot cheaper then a fan.If your LED still gets too hot, use a bigger heatsink.Only when your heatsink threatens to become excessively big it becomes time to think about active cooling.Apart from that, it is usually better to run LED's at 50% or so of their rated capacity. When you push more current to a LED, efficiency goes down, which means that most of the extra energy is just turned into heat. If you use two LED's and both run them at a lower current, you can get the same amount of light, but with a higher efficiency, which also means you need a smaller or even no heatsink which reduces cost. With power LED's the efficiency increase is so big that you will recoup the extra cost of the LED by the energy saved. (Unless you live in a country were electricity is very cheap and even general house heating is done with electricity. In such a case it does not matter much whether your LED, or the electric radiator produces the heat, but you will still shorten the life expectancy of the LED).

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: January 03, 2023, 04:46:59 pm
content: Quote from: Doctorandus_P on January 03, 2023, 04:41:04 pmDefinitely will be doing this, as well as running the LED at lower-than the rated power.I guess I shouldn't worry about actively cooling the LED phosphor gel?

author: tooki
author_rank: Super Contributor
timestamp: January 03, 2023, 07:26:10 pm
content: There’s just no point, since the silicone gel isn’t the point of failure, and isn’t a significant path for heat to escape. (Silicone is a terrible heat conductor.)

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: January 03, 2023, 07:48:32 pm
content: Quote from: tooki on January 03, 2023, 07:26:10 pmI would have thought the phosphor gel would lead to the point of failure, by breaking the gold wire bonds between the LED dies?I've come across MANY LED COB's that stopped working and had a very dried out phosphor gel, leading to cracks in the gel. I'd assume this drying + cracking LED to the LED COB death in many instances?

author: Whales
author_rank: Super Contributor
timestamp: January 03, 2023, 11:52:18 pm
content: Adding airflow to the gel side might encourage dust buildup on the surface of the gel. This will have some impact on optical output and self-heating, I'm not sure if its negligible or not.I've accidentally destroyed some LEDs trying to remove dust from their gel phosphor or gel lenses. It "feels" fine to clean them until they suddenly tear (sometimes taking bond wires out too).EDIT: I agree with the other comments about how to get the best life out of high power LEDS:- Lots of heatsinking and airflow on the back- Buy 2x the leds and run each at half the power or less. (You will get much better efficiency, a "100W" COB led run at "100W" is often garbage efficiency. Two at 50W emit more light).

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: January 04, 2023, 12:28:26 am
content: Awesome, thanks!

author: tooki
author_rank: Super Contributor
timestamp: January 04, 2023, 03:10:49 am
content: If you really want to totally eliminate the phosphor gel, then build a remote phosphor system: blue LEDs and a remote phosphor panel.But the main thing is to cool the LEDs, and the way to do that is via the aluminum backplate.

author: inse
author_rank: Super Contributor
timestamp: January 04, 2023, 07:31:42 am
content: 60 degrees does not sound terrible, did you check the datasheet about operating conditions?

author: ebastler
author_rank: Super Contributor
timestamp: January 04, 2023, 07:50:06 am
content: Just a quick clarification, since I don't think it was said explicitly:Ifyou want to add active fan cooling (to keep the size of the heatsink small), the fan's airflow should be directed at the heatsink fins, not at the LED and its phosphor. The thermal energy flows from the LED substrate to the heatsink, so that's where you want to "remove" it with the fan.

author: Whales
author_rank: Super Contributor
timestamp: January 04, 2023, 09:32:40 pm
content: Quote from: ebastler on January 04, 2023, 07:50:06 amHeat is also generated in the phosphor layer. It then conducts away through both the front (air) and back (aluminium) of the LED + emits IR.Overheating the phosphor is a failure mode.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: January 04, 2023, 11:12:50 pm
content: Quote from: Whales on January 04, 2023, 09:32:40 pmOkay, thank you so much for providing this information. When I setup my nice high-CRI lights, I'm going to have a tiny amount of air blowing over the LED phosphor gel, but primarily on the heatsink.

author: tooki
author_rank: Super Contributor
timestamp: January 04, 2023, 11:33:23 pm
content: I think the effects of collected dust from blowing air over the phosphor would be worse. Cool the back more.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 05, 2023, 12:01:40 am
content: I once made a LED light for in my metal workshop.Light output was so high that it heated the dirt on the front of the silicone that the dirt burned into it. You should keep the front of the sillicone clean with some transparent piece of glass or plastic.You can also test this yourself. Just take some high powered LED (for example 20W) and then hold your finger in the beam at a distance of one or two cm from the LED. This gets hot quite quickly.Cooling is done via the aluminum back side.And cheap crappy LED's die sooner, silicone does not dry out, but you never now what they put on the cheapest LEDs.