I’ve been working on the current drivers for the LEDs and have arrived at a common (but still interesting) dilemma; is it best to choose efficiency or flexibility?
The old design was not really optimal and neither was the PCB layout. There were a number of issues with ground bounce and stray capacitance that needed sorting. So while struggling with those issues I, true to form, started messing about with additional complexity 🙂
Just for the heck of it I tried a different LED array configuration with 28 rows of 4 LEDs. The idea was to see if we could get a better performance out of the drivers, and with very little tweaking I got 96% efficiency (blue and green) and 94% efficiency for red. That is certainly awesome, (although it is for optimal conditions). It means that very little battery power is lost in the drivers. Great for battery life!
The trouble is that this configuration sets the max input voltage for an external power to around 9.5 Volt. The lower level is defined by the internal battery charger at about 8.5 Volt. So, for practical purposes we would be limited to 9 Volt external power . That is not so good. People got all sorts of obscure contraptions external battery packs and power supplies they want to use to power their gear.
Another issue with the short LED strings is that although the emitters are well matched, the variation becomes noticeable as light and dark patches on the panel at low power. This does not actually affect use that much, but it means that some emitters will burn brighter that others, and potentially burn out before their time. Not so good. So we would need current mirrors to compensate for the LED differences, but for strings of only 4 LEDs, the high loss in the current mirrors would eat up of the efficiency gain.
So, I’m back to using a 8 by 14 array (8s14p). The driver efficiency is about 85%. Still good, but certainly not in the same class as 96/94%. The advantage is that it looks like we don’t need current mirrors, small differences in LED forward voltage drop evens out nicely. And we are much more flexible on the kind of external power we can accept. Upper limit is 18 Volt. Lower limit is still about 8.5 Volt which is the cut-off voltage for the charger.
Actually, in theory it should be possible to run the light on a voltage as low as 4 Volt. The internal batteries would not charge, but you should get normal operation from the rest of the unit. I’m going to have to test that.