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I need to drive 4 high power LEDs. Because of their high power, they cannot be connected series as the voltage required by 4 in series would be too high. Their high current requirement (500mA) also means I would like to avoid using current limiting resistors if possible as finding high power, surface mount resistors is a challenge and expensive. I am proposing the following circuit below.

Suggested circuit

I want to ensure there is no chance one LED will go into thermal runaway and believe the constant current sources will prevent this from happening. Is this correct? Do the current drivers eliminate the need for current limiting resistors? Could they introduce any new issues?

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    \$\begingroup\$ Use a boost (not buck) current regulator that does current limiting and wire them in series. \$\endgroup\$
    – Andy aka
    Nov 4, 2021 at 13:24
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    \$\begingroup\$ @MRB If your "LEDs" require 24V, then they are not "LEDs" but lights containing LEDs as components. They could be strings of LEDs or they could have other components as well. Please provide the part numbers and datasheets so we know what exactly you are working with. Use EDIT button to add this information to your question. \$\endgroup\$ Nov 4, 2021 at 13:36
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    \$\begingroup\$ @MRB A single LED cannot withstand a 24V forward voltage. These are LIGHTS using LEDs as components. \$\endgroup\$ Nov 4, 2021 at 14:02
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    \$\begingroup\$ maybe they are COB LEDs \$\endgroup\$
    – jsotola
    Nov 4, 2021 at 15:16
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    \$\begingroup\$ @MathKeepsMeBusy I can assure you these are LEDs with a single die inside. They are UVC LEDs outputting 265nm and therefore require a lot of input power. \$\endgroup\$
    – MRB
    Nov 4, 2021 at 15:21

3 Answers 3

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I need to drive 4 high power LEDs. Because of their high power, they cannot be connected series as the voltage required by 4 in series would be too high.

Since you clarified that these are only 24 V LEDs, I'm going to suggest that you get a constant current boost converter and run in series. 4 modules in series puts you at less than 100 V. This is a very typical voltage for higher power LED lighting and you can easily buy inexpensive drivers for it. A quick parametric search on Digikey for example reveals pages of suitable drivers.

I've built series strings of UVC LEDs. I would not run them in parallel given the high power output and the relatively large chip to chip variations. The current division will be highly unequal. Either series or buy 4 LED drivers.

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  • \$\begingroup\$ can you suggest one of these drivers as I cannot seem to find a suitable one. I should mention these are surface mount LEDs and I am looking for a surface mount driver. \$\endgroup\$
    – MRB
    Nov 4, 2021 at 15:49
  • \$\begingroup\$ In your edit you say either in series or buy 4 LED drivers. Since I already have 4 constant current LED drivers, do you mean I need 4 separate Buck supplies as well? \$\endgroup\$
    – MRB
    Nov 4, 2021 at 15:58
  • \$\begingroup\$ @MRB The LED driver is a Buck/Boost converter, so if you already have them you do not need a second one per channel. \$\endgroup\$ Nov 4, 2021 at 16:29
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    \$\begingroup\$ @MRB It is unclear to me what your actual requirements are. If you simply need to drive 4 24V 500mA diodes, then a product like the Meanwell PLD-60-500B would be a safe and inexpensive choice. If you have other requirements beyond what you have said, edit your post and describe what they are in detail. \$\endgroup\$ Nov 4, 2021 at 16:39
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Even if currents are high, the balancing resistors need to be tiny. Are you sure you can't afford them? I mean, you probably need some milliohms.

Just a random unchecked idea: put a mosfet on every LED's path. A mosfet that will always conduct. I mean, MOSFETs aren't subject to runaway, they're supposed to distribute the load more evenly then, since they are not subject to thermal runaway.

I'll be honest, I'm not sure about it. If someone could confirm or bust my idea in the comments, it would be nice.

Random idea no. 2: individual current source/sink per LED? Probably too many part, I guess.

Also, please specify in your question how high the current is +-. High current can be 1A for LEDs or it can be 100A. Just so that we are all on the same page.

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It really depends on whether you're optimising for cost or efficiency, and the availability of parts for the current/voltage you need.

  • You could simply get 4 buck LED drivers, one for each string. These can be pretty cheap.
  • You can use buck LED drivers with some LEDs in series. There are plenty of LED power supplies that will eg. take 220V to power a 48V LED string.
  • You could use a boost LED driver and the LED in series as Andy aka suggested, if your supply is not mains voltage.
  • You could use the configuration in your post. The current sinks would set the LED current for each string. The DC/DC buck converter would be used to fine-tune the input voltage so that the current sinks have enough headroom (voltage margin) to function, but not so much that they're having to dissipate a lot of power.

The last solution is a bit more complicated (more parts, need to fine-tune things), so it would only be worth it if it's cheaper or if you have the parts at hand. It could also be a little less efficient, although that depends on the specifics.

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