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I have a 2 strips of 3 high powered LEDs as the loads. The load is hooked up as :

Config 1
Strip 1 : 3 Green LEDs (3.3Vf, 4A) in series => 4A and 13.2V load

Strip 2 : 3 Red LEDs (2.8Vf, 4A) in s in series => 4A, 11.2V load

Config 2
Strip 1 : 3 Green LEDs in parallel => @ 3.7 max battery, a single LED sources close to 8As, so for parallel it would 24As

Strip 2 : 3 Red LEDs in parallel => @ 3.7 max battery, a single LED sources close to 7As, so for parallel it would 21As

Config 3
Strip 1 : 3 Green LEDs (3.3Vf, 4A) in parallel => 12A, 3.3Vf load

Strip 2 : 3 Red LEDs (2.8Vf, 4A) in s in series => 12A, 2.8Vf load

I can use either 3.7/7.4 50C (1S/2S) battery, with whatever capacity required. So the three driving approaches would be:

  1. As per my previous question, it was concluded that a Boost with CC is more applicable than buck with CV. So, 2S wired to current mode Boost converter outputting 4As to Config 1 strips. The output connected to two FETs to switch between the strips.
  2. Using one approach flashlights seem to use - a direct drive FET driver. Directly connecting the battery to the LEDs. 1S battery connected to two FETs to Config 2. It sounds like a dangerous approach but is by far the least complex and most compact approach. There's a ton of FETs out there that source massive currents, and the heat losses would be minimal as vbatt and vf are close. Why wouldn't this approach work?
  3. Using another approach that flashlights use - LDOs with current sink like the popular 7135. S1 connected to a Stack 32x 380mA AMC7135 to source 12As and then two FETs to switch between the strips (Config 3). This would run until battery drops below a functioning Vf.

Main question
I understand that 2 & 3 are not approaches that are usually chosen for high power LEDs but with the target goals being - High lumen output and a 60sec duration, these seem like the most compact approaches. Only unsure, what else to consider or if these are even feasible.

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2 Answers 2

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Multiple amps still use the symbol A, do not pluralize it; As is either arsenic or Amp-seconds, which are coulombs.

1 is the only workable option here. Connecting LEDs directly to the battery without any kind of current limiting will either trip your battery's overcurrent protection, burn out your LEDs, fry your pass element, or some combination of the above. 3.7V lithium ion batteries start out above 4V, quickly drop below, and hang out around 3.7V for a while until they drop further when the battery is depleted. 3.7V is only a nominal rating; your 3.3V and 2.8V LEDs won't stand a chance.

The 7135 is inefficient, which is not what you want on a battery powered system. And whenever you find yourself installing 32 of the same component, you should be asking if there's a simpler option. This is also never going to be as compact as option 1.

Now is also the time to be thinking about how you're going to cool these LEDs.

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If you're limited to a 2s pack (7.4V), seems like approach 1, using separate current-limiting boost converters for each color is a reasonable approach.

By wiring in series with a current control you will achieve even illumination for each strip, while reducing currents to more manageable levels.

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  • \$\begingroup\$ What is even illumination is not a concern? \$\endgroup\$
    – roaibrain
    Commented Apr 1 at 4:37
  • \$\begingroup\$ Also is it not possible to use one current limiting boost converter for both colors? And switch between each color using a N-FET \$\endgroup\$
    – roaibrain
    Commented Apr 1 at 4:38
  • \$\begingroup\$ The larger the current, the bigger (and more expensive) the FET. Yes, you could switch them, but if you use series connection with a boost for each it will be more cost-effective and reliable. Boost converters are surprisingly economical because they’re made in high volumes. \$\endgroup\$ Commented Apr 1 at 14:43

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