How to lit 3W LEDs from Li-Ion battery??

[xbrite]

Dear Gents,

Do you guys have any idea how to lit a 3W AND dual series connected 3W LEDs from the same 2 cell series connected Li-Ion batteries of which the battery voltage varies from 8.4V (at full charge) to 6.6V?

For a single 3W LED, a step down converter can be used but this cannot be used for dual 3W as the Vf falls in the middle of the varying battery voltage. I thought about SEPIC but the efficiency seems too low. I prefer the efficiency higher than 90%..

Any idea for a single 3W LED (Vf is approx 3.7V at 1A)from a single Li-Ion battery (Voltage varies from 4.2V to 3.3V)??

Cheers..

 

[Kill-O-Zap]

Is parallel LED's, rather than series, an option?

 

[Doug Owen]

Seems to me like you're up against it. The numbers are all against you.

You have a couple of ways to go. If you're hung up on switching converters (which I suspect), the best option might be a larger battery so Vb is always bigger than Vf. FWIW, I wouldn't depend on efficiencies greater than 90% unless you've got a lot of experience....

Or you could realize that the voltage says above 3.6 or so Volts for almost all the useful capacity, just don't count on the last 10% of the charge. Then a conventional LDO regulator can get you that elusive 90%.

Doug Owen

 

[xbrite]

Kill-O-Zap, Parallel is not a good idea as most of the case, the Vf of the two LED's are not exactly the same and so the current thru one LED is different from the other LED by large. This means a noticeable brightness difference between LED's.

Cheers..

 

[Kill-O-Zap]

xbrite, if you're making just one of these lights and have the luxury of testing and fine tuning, a small amount of resistance on the lower Vf LED might be a pretty reasonable solution, I would think more efficient than going to a buck/boost converter.