I have 2 COB chips rated upto 400ma at 36v connected in parallel with a 600ma constant current driver (54v max) On testing both led worked fine but had minor difference in lumens at some levels as I tried dimming them till 10%
I am assuming that in parallel both will have an equal amount of current draw
So, are my assumption correct or are there any irregularities in parallel connection ? Also is there anything that can go wrong due to inrush current?
Please help me with some suggestions
There is no guarantee that without some form of ballast resistor or other mechanism to help compensate for differences in characteristics and temperature that they will conduct equal current- if you want guarantees, connect them in series. But you can't do that with your existing supply.
You could try putting individual 50Ω 10W resistors (actual dissipation about 4.5W each), one in series with each string. That will drop an additional 15V. Even 5Ω~6.8Ω 1W resistors may make a noticeable difference if current imbalance is the issue.
Keeping them the same temperature as each other (even if that temperature varies) will also help. The hotter one will tend to hog more current when they are connected in parallel, which will make it even hotter.
There will also be differences in light output between two samples of the same LED string at the same current due to manufacturing tolerances and perhaps temperature.
Obviously, each COB chip contains at least 12 small LED chips, and they are connected in series. I think the first thing to check is whether the internal resistances of the two 36V COB chips are the same. And in the parallel circuit, even a small difference in line resistance may cause inconsistent current.
