Creating a custom watercooled led light ... struggling

[thefreeman]

@frippe75

Not really an expert (obviously ) but thought paralleling LEDs had the potential of thermal runaways. Not sure how that would work on a single channel and one “LED strip” broke.

It is indeed often recommended to not parallel LEDs, with two LEDs with a Vf difference the one with lower Vf will draw more current than the other, which means it will heats up more which in turn decreases its Vf, drawing even more current…etc. In practice it’s not really an issue with high power LEDs, there isn’t much Vf variation in the first place and unless overdriven near their limit there is a lot of room for some current imbalance, the Vf also increases substantially as current increases (with XHPs at least), counteracting heat Vf decrease. Many high power multi LED flashlights use parallel LEDs, an Acebeam X50 for example has 12XHP70.2 in parallel (6V). Quite a few LED bulbs I disassembled had a combination of series and parallel configurations, an XHP LED actually has 4 LEDs in a single package, wired in 4S (12V) or 2S2P (6V), though they might be binned to have less Vf variation and it’s probably better to series 2 6V XHP than parallel 2 12V ones. If it can simplify your circuit then IMO it’s a viable option.

@frippe75

There is an app-note that has a nice table of alternate configs…

Design2 is what I’m thinking of but a much higher I_LED … 5A range.

But the note … “The RMS current rating of L1 should be at least 50% higher
than the specified ILED.” … So 8A maybe…

Ohhh… And the price of that inductor jumps…

I wonder why it needs such high inductance, 68uH is quite high and indeed it’s a problem since it needs to be huge for the current/saturation rating needed.

@frippe75

@thefreeman, what you wrote about not getting the Vin too close to the Vout got me thinking. The above alternative of going Vin ~36V and Vout 24V (i.e 3 drivers per Light of a total of 6x 12V XHP70.3HI’s) will get me into trouble on the inductor side. Both expensive and redesign of my existing LM3409 driver PCB.

Thinking about voltage drop. Voltage drop in the circuit, duty cycle limitation, battery->cable->lamp … and so on.

My preferred option of stacking 2x 12V LiPOFE4 battery for a 24V configuration…
Will eventually put me at that 12V draining the batteries closer to zero…
So until I get to 14% battery capacity left I will be at 25 - 1 = 24V at the “lamp level”.
Maybe not a good way of doing it??

Doing 36V instead would at least give me 36.5V by stacking three batteries and reducing one driver per lamp…
A buck/boost driver would have been better but more complicated,

Any input here would be really appreciated.

Your chart shows the rest voltage, the voltage of the battery under load will be lower, how much depends on the battery internal resistance, which comprise of the cell’s DCIR and battery management system (BMS : overvoltage/undervoltage/overcurrent protection) circuitry resistance if there is one.

For example a LiFePo4 cell like this one, has a voltage of ~3.3V with a 0.2A draw at 50% SOC, but it drops to 3V at 20A draw, which would be 12V in a 12V(4S) battery, and this is a high power density cell (very low DCIR).

Then there is the Vf at the desired current, which is going to be higher than the ”6V” or ”12V” nominal Vf, see this test of the XHP70.2, for example at 6V10A the actual Vf is 6.63V (13.26V at 12V5A).
So even with the 100% duty cycle capable LM3409 you wont be able to drive your LEDs at your needed current with a x nominal battery voltage to x nominal Vf. With 24Vin, 1x12V or 3x6V LED would be the maximum, with 36Vin, 2x12V or 5x6V.