Looking for appropriate LED driver for this setup

Electromechanical Optoelectronics
1

Hello,
I wish to hook up these Bridgelux LED strips, and find a correct driver.
Configuration:

  1. three BXEB-L1120Z-50E4000-C-B3 connected in parallel;
  2. each strip is Vf: 39VDC, If: 700mA;
  3. so connected in parallel the driver must supply 2100mA at 39VDC
  4. I am in North America with supply voltage at 120 VAC.
  5. So far I am considering a ‘constant current’ LED driver by Mean Well: ELG-150-C2100B
  6. It is a constant current driver set at 2100mA, capable of delivering 36VDC to 72VDC to the strips.

My questions:

  1. is my approach viable? ie 3 strips in parallel with this driver?
  2. should I consider a different LED driver for this setup? A constant voltage driver instead?
  3. any other comments recommendations will be greatly appreciated!

Regards

Ron, Montreal, Canada

2

Hello,

Welcome to the DigiKey tech forum community.

The Mean Well ELG-150-C2100B should work fine with that set up as it will provide the constant current of 2100mA, which the 3 of those strips would require. However, ELG-150-C2100B is not in stock currently and there would be a 14 weeks lead time approximately if you were to place it on order.

There are similar ones to it that we have in stock in link below:
LED Drivers | Electronic Components Distributor DigiKey

Hope that helps.

3

I find no problem with your plan so far.

4

Hello presse55,

Welcome to the DigiKey TechForum.

The short answer to your question is maybe. We need to be very careful with parallel connection as described in this application note:

Variation in the forward voltage of the individual module can result in current hogging, where a lower total Vf module may see a higher forward current compared to a higher Vf module connected in parallel. This may produce non-uniform flux and color, and may affect the reliability of the lighting system.

That same application note goes on to describe a dual channel “parallel” option using their Vests Flex dual channel driver as shown below.

Dual channel Bridgelux Vesta Flex LED drive.
Dual channel Bridgelux Vesta Flex LED drive.1003×613 98.1 KB

Please give me a few days and I’ll work on a proper technical answer along with a few rules of thumb for this “current hogging” issue.

Best Wishes,

APDahlen

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5

Hi presse55,

APDahlen is correct. Connecting LED strips in parallel is often problematic, even when using all of the same part number and binning. There are several reasons for this, but it primarily comes down to the fact that LEDs are current controlled devices rather than voltage controlled. Their forward voltage will vary as current passes through them in a non-linear manner.

Manufacturing variability from one part to the next can cause variations in the current/foward voltage relationship, and when one adds up several LEDs in a strip, these variations can compound and cause an imbalance in forward voltage for a given current, or stated differently, an imbalance in forward current for a given forward voltage. At a minimum, this will result in uneven brightness between strips. An additional effect is that the higher current experienced by some of the parallel strips relative to others will cause an uneven aging rate between the strips.

Additionally, LEDs have a negative temperature coefficient with respect to temperature versus forward voltage. This means that as an LED heats up, its forward voltage will drop. If one LED strip is slightly warmer than another, then its forward voltage will drop for a given current. This means that it will draw more of the current when strips are connected in parallel, and as it draws more current, it will heat up further, possibly causing a thermal runaway condition and resultant failure of the strip.

Furthermore, if one of the parallel strips fails open, then all of the current that the failed strip had been drawing will then be redirected through the remaining parallel strips, which can potentially overdrive them and cause them to fail as well.

The point is, even though it is sometimes possible to run like strips in parallel, one must make sure they are carefully matched and ensure that they are kept at the same temperature, which is not easy to reliably accomplish.

Because of the issues described above, it is generally recommended that one drive LED strips in series. This eliminates the issue of variable forward voltages and also prevents the failure of a single strip from damaging the remaining strips. If one strip fails open, the series string will quit illuminating, but once the failed strip is replaced, the string will be functional again.

With all this in mind, here are some options to drive three BXEB-L1120Z-50E4000-C-B3 strips in series at 700mA.

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6

Thank-you APDahlen and David_1528 for your detailed responses.
I will be changing my topology to series connected LED strips.
I am ordering a replacement driver this very day.
Many thanks!

Ron,
Montreal, Canada

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