In my previous test, I ran a couple of Vero 18 COBs at their typical rated current to see if I could get either of them to go into thermal runaway. Over the 4 hours I tested them, each of the COBs ended up pulling an additional ~50mA each than what they started at, but stabilized at this level. Curious to see if I could get one of my COBs to go thermal, I decided to take another stab at this and hit a couple different chips with considerably higher currents than they’d normally be run at.
Here are my results:
Test 1 – Vero 18 Starting at 1,800mA
The data sheet for these gen. 7 Vero 18s says their typical current is 1,170mA, and their max is 2,340mA, so I wanted to pick something above typical but not too close to max. 1800mA is close to halfway between these 2 points so I figured that’d be a good place to start. The data sheet showed that I could expect this current somewhere around 36.5V, so I set voltage accordingly.
I was again surprised at how stable this setup was. Even at a relatively high current for this model of Vero, it didn’t run away on me. The active heat sinks are doing their job excellently.
Test 2 – Cree CXB3590 Starting at 2,400mA
Having messed with the Vero 18s a couple times, I figured I’d switch to a CXB3590 and throw some real power at it. This COB is on a passive heat sink, so it lacks the extra cooling that the fans I’m running on the Vero heat sinks provide.
In this case, while the COB did pull quite a bit more current over time (300+mA), it DID stabilize and refused to pull any additional current after that point. Time to blast it with even more.
Test 3 – Cree CXB3590 Starting at 3,000mA
The absolute maximum current rating for this COB is 3,600mA. After seeing how the CXB acted at 2,400mA, I expected a few hundred mA of rise in current for this test and set the voltage on the driver to produce 3,000mA of current to start. This would give me enough room to let it rise and keep it under its max.
Well, this did the trick. Apparently, 3 amps is enough to get one of these guys to go thermal and quite quickly. I’m glad I was able to get it to happen, but it took a lot of current to do, and I doubt anyone in their right mind would be feeding their shiny new COBs this much power.
Test 4 – Cree CXB3590 Starting at 3,000mA with Active Cooling
When I was finishing up the 3rd test, I turned on a fan I have in the room and aimed it at the heat sink of the COB just to see what effect, if any, it’d have on the rapidly rising current. I noticed right away that current actually started to drop, and figured I should reattempt a test with 3,000mA+, but with active cooling on the heat sink.
I rigged up the fan to blow directly over the heat sink. This is probably a lot more air than most people would have for an actively cooled heat sink, but I wanted to see if it would be enough to stop the COBs from running away again.
So, what I’ve gathered from this is that yes, thermal runaway can definitely happen when using a constant voltage driver. However, chances are, if you have proper cooling and run at a relatively low drive current, you shouldn’t have a problem with it. It was not until I boosted current way up near the COB’s max that I ran into problems, and even when I did have a chip run away on me at a really high current, I was able to stop the process just by improving my cooling.
The more I test these constant voltage drivers, the more I like them for this application.