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I'm Working on LED load design circuit which is up-to 64V@1.2A. I thought of giving LED's in series and parallel combination with switches to control the voltage but while searching I found one circuit. enter image description here

From theoretical point of view both are same but what's the main difference in both the circuits and which one performs better.IS there anything related to thermal protection?

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    \$\begingroup\$ Generic schematics showing LEDs directly in parallel or with parallel strings of series LEDs are highly suspicious and should not be trusted because it depends entirely on the non-generic details of the specific LED as to whether either or both are suitable. \$\endgroup\$ – Andy aka Jun 1 at 11:25
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Both are bad, with the right potentially being worse, and harder to implement. You need a current regulation per string, not for three strings.

These are diodes: if one of them is conducting a little better (e.g. due to normal production variations, ie. there will be one that conducts a little better), then that diode will carry more current than its "neighbors" in your right scheme, and that means it gets hotter. Hotter means for diodes that they conduct better. So, that's a recipe for thermal runaway, where the diode that is already hotter gets hotter and hotter by transporting a larger and larger amount of the total current.

In the left case, there's three strings of diodes that are in parallel, which at least statistically reduces the variance between these, but it's not sufficient for reliable operation.

You should use the left design in that you use strings of LEDs. But, you need to give all these strings their own current regulation – either by having one "Current regulator" for every string, or at least by placing a resistor in series with every string, so that if a string pulls more current than the neighbor, the resistor drops more voltage, counteracting that runaway effect.

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