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So the design is still under discussion but EE has agreed on strings-in-parallel circuit and wants to regulate source to each series.

See previous post What circuit for LED grow light? for more info and details. Essentially, it was about which circuit to use for a grow light with diverse diodes and whether each series string needs to be regulated.

The EE referred me to this article in EDN Overcome the challenges of driving parallel LED strings Article states, "The simplest way to drive parallel strings is with a fixed voltage source and a series current-setting resistor. When higher performance and protection features are required, individual string regulation offers the highest level of regulation accuracy and flexibility." Starting with resistors in the series, the author then presents circuit solutions of increasing performance and complexity.

mapplejacks suggested "If you want to avoid that, you need a current source on each string. Assuming your total voltage on each string is about the same (i.e. the same number of each type of diode), you can use a resistor for this"

  1. So, to implement individual string regulation to each series, how complicated should we get for an under 100W grow light?
  2. Do we need integrated circuits before each series string or will resistors be adequate?

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    \$\begingroup\$ If all in series is not an option, you need per string either straight up regulation or at least a resistor to increase the slack between lowest and highest Vf difference to acceptable levels. \$\endgroup\$
    – winny
    Dec 3, 2023 at 17:08
  • \$\begingroup\$ A "current" generator with multiple outputs could be used. Controlling the generator ... control the current of every string. \$\endgroup\$
    – Antonio51
    Dec 3, 2023 at 17:47
  • \$\begingroup\$ Thanks winny! I guess my question is what is an "acceptable level" of Vf difference b/t each series-string? In practice, I don't find regulation used in grow lights today. As a result, I'm very skeptical about over-designing a circuit for 2.2v-3v LEDs in a grow light. \$\endgroup\$
    – Effexxess
    Dec 3, 2023 at 18:08
  • \$\begingroup\$ @winny For a grow light you typically do not want to do either of those things since they waste energy and the goal is to be as efficient as possible. Instead, you typically run the diodes well below max (to keep efficiency high) and accept that the current does balance evenly. This is not ideal since the hotter strings are less efficient, but it's less wasteful than the alternatives. \$\endgroup\$ Dec 3, 2023 at 18:23
  • \$\begingroup\$ user1850479, thank you for all the valuable info. When you mentioned "run the diodes well below max" are you talking about current or Vf? \$\endgroup\$
    – Effexxess
    Dec 3, 2023 at 18:29

3 Answers 3

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I have a large panel of white LEDs that is constructed in a manner similar to your diagram. It has a small current sensing resistor in each string, but only one of those has a connection brought out for feedback to an external regulator. The idea is that the strings are similar enough so that if the overall voltage is controlled to regulate the current through one string, the current through all of the strings will be "equal enough".

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See previous post What circuit for LED grow light? for more info and details. Essentially, it was about which circuit to use for a grow light with diverse diodes and whether each series string needs to be regulated.

You already know based on your research and analysis of commercial products that individual strings are not regulated independently:

enter image description here

(Taken from your last question)

There is no shame in learning from existing products especially in a market that is extremely mature and has converged to a more or less optimal design. Most likely if you try something different you'll come to a worse design (higher costs, lower efficiency), in which case you would have been better off buying the (better) commercial product.

The EE referred me to this article in EDN

This is the same engineer who told you to put all your LEDs in parallel instead of in series? And the same engineer who wanted to drive them constant voltage instead of constant current? If someone consistently gives you terrible technical advice, probably it means they don't know what they're doing and you should find someone else to get help from.

I guess my question is what is an "acceptable level" of Vf difference b/t each series-string?

This is typically why you'd hire an engineer, but basically you need to match Vf close enough that you can drive the array of parallel strings at the target current without burning up the string with the lowest Vf. As your current approaches the diode max you will have to match them very closely. If you're going to run them at a more reasonable level you won't have to match them so closely since your operating points will be far from max.

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  • \$\begingroup\$ Absolutely, user1850479!! I'm doing teardown of SP1000 EVO now, since using same LM301H EVO diodes. I'm learning from the general wisdom of the market. \$\endgroup\$
    – Effexxess
    Dec 3, 2023 at 18:12
  • \$\begingroup\$ I have to admit, I think I misunderstood about using strictly parallel circuits. Confused with the hybrid circuit. \$\endgroup\$
    – Effexxess
    Dec 3, 2023 at 18:16
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The determining factor for your design is LED forward voltage (Vf) spread.

If all have identical Vf and you drive your series-parallel string with constant current, you will have equal current in each string.

The world is however not perfect, so there will be natural variations in Vf between LEDs, especially identical LEDs for different batches.

Let's take Seoul S1W0-3030XX8003 for example:

enter image description here

Within the same bin, you need to assume 100 mV difference between individual LEDs. If you are mass producing them and don't buy them binned at all, you need to assume the full 200 mV spread. Within the same reel of LEDs, you should have the same bin, save for some really shady manufacturers. But if your EMS does not guarantee that all LEDs on the same panel is from the same reel, you can't assume it.

So how much does 100 mV matter?

There are redeeming factors. The LEDs are not ideal in the sense that they only have forward voltage with an infinitely sharp I-V knee but rather equivalent resistance, causing a slope. For an infinite parallel array with all but one being 2.8 V and one is 2.7 V, the 2.7 V one will see 2.3 times higher current than the others. You can design for this, but it means significantly underdriving your LEDs, causing bad economy since you need to buy more LEDs.

enter image description here

Your array isn't infinitely parallel, so it won't be that bad. Your 2.7 V one will take more current than the 2.8 V ones, but less than 2.3 times. Please simulate your exact series-parallel situation with one sting all having the lowest Vf rated and all the others the highest. Your model needs to include the equivalent resistance of the LED to give the correct I-V characteristic.

Another factor is that the LEDs with the lowest Vf that will see higher current will also heat up more, and the white LEDs have negative temperature coefficient, worsening the situation. Red LEDs on the other hand have positive tempco.

enter image description here

Your best solution for efficiency is to run them all in series. It is however the least fault tolerant solution and one LED going open will turn off everything. A Zener diode with higher voltage than your max Vf across each LED is one way to mitigate it, but there is a static power loss penalty from it since they will leak a little bit of current.

If you are stuck with a series-parallel string like yours, you need to either straight up accept the result from max spread within your Vf bin(s). Tighter Vf spread = less overcurrent needs to be derated for. If unacceptably high, the simpleast but least efficient remedy is a resistor for each string. If you have small production quantity, you can hand-match just the low Vf strings when they appear and add just enough resistance to balance out and with zero ohm for the high(est) Vf one(s). With high volume production, this won't be an option and you have to design for worst case scenario you can accept and suffer from that even your string with highest Vf has the same resistor in series as your lowest Vf one. An intermediate option is to have an active device such as a linear constant current sink for all strings but one. That will come with its own set of tolerances but will improve your situation.

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  • \$\begingroup\$ Great, thanks winny. Want to see a clever twist on the series-parallel string in a LED grow light? Learned that by merging the traces the current doubles on the red diodes--smart idea. electronics.stackexchange.com/questions/691670/… \$\endgroup\$
    – Effexxess
    Dec 4, 2023 at 7:00
  • \$\begingroup\$ @Effexxess Yep, they stole “my” idea/read the datasheets and realized red LEDs are made for higher current than white to be effective. Same Vf issues as stated above though. \$\endgroup\$
    – winny
    Dec 4, 2023 at 7:55

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