LED lamp - connecting leds in parallel

Thread Starter

plumbum_by

Joined Jul 23, 2019
3
I have a question about LED lamp:

QB288V2-1.jpg

Quantum Board

It has 16 strings of LEDs connected in parallel. Each string consist of 18 LEDs connected in serial. It's supplied with constant current source (with max current 2800mA, what is much more than safe current for single LED/led string). I know that connecting LEDs in parallel is not the best solution because of imperfection of LEDs while manufacturing and as a result inequality of currents through every string. But I see that is quite common solution for such lamps.
I have several assumption how it is possible:

- using LEDs with very similar parameters (Vf), using LEDs with same binning. But actually, according to datasheet Samsung LM301B leds have 1V of Vf variation even inside every binning group. Possible in practice this variation is much lower.
- very good thermal dissipation due to metal-core PCB

Could anyone explain how does it work? Are my suggestions the keys for the answer? Is putting serial current limiting resistor for every string and using CV source is more robust solution and why not to do it in this way?
 

schmitt trigger

Joined Jul 12, 2010
101
I also have started seeing this trend. I have a couple of theories:

- They are paying the LED manufacturer to bin the LEDs according to Vf. Unlikely, but not impossible
- They realize that nowadays things are essentially disposable, and therefore don't give a s**t about product reliability. That would be my preferred explanation.

There may be other explanations. I am curious.

EDIT: In the photo, I can see some brownish rectangles which appear to be resistors, but the photo is not clear enough. Could these be ballasting resistors?
 

Sensacell

Joined Jun 19, 2012
2,544
It's all about the isothermal mounting - prevents the LEDS from going into thermal runaway.
That and the dies are from the same wafer.
 

MrSoftware

Joined Oct 29, 2013
1,655
I can't tell from the picture, but I bet each string of LED's has a series resistor for limiting the current through that string. This way it does not matter if only one string is powered, or all strings are powered, the current through each string will be limited.
 

Janis59

Joined Aug 21, 2017
968
Putting many LEDs brute parallel some of em will have probably 10x current and some only 0,1 of normal current. Just them will explode. But when there are larger number of them in seies, then voltage are rather well averaged in boarder of one string, thus if any the neighborous string differs, it is n-fold smaller difference and thus the lamps may suffer remaining alive
 

peterdeco

Joined Oct 8, 2019
72
We parallel LED's at work all the time. As long as we parallel the exact LED's from the same manufacturer and part number, we never had a problem. If one LED is from a different manufacturer with a slightly lower Vf, it dims the whole string.
 

Audioguru again

Joined Oct 21, 2019
674
Over there, they probably pay a little kid a bowl of rice to test the forward voltage of thousands of LEDs and group them.

I have a cheap Chinese flashlight that has 24 white LEDs all in parallel with no series resistor. The current is limited by the three series antique weak "Super Heavy Duty" carbon-zinc AAA battery cells. The brightness of all the LEDs are exactly the same.
The batteries needed replacement soon and I used modern Western alkaline cells and nearly burned out the LEDs. I added a series current-limiting resistor.
 

Janis59

Joined Aug 21, 2017
968
Most strange this occured be problem in my son`s Mercedes-SL. For that damn price I waited be lit a bit smarter inside. Later he found the new rear plafon costs sth about 600 Eur, thus I de-glued, bought a 5-cents worth chinese "frogs" - those large sized LEDs on the metal podest and until today they serves well, but with a resistor.
 

dl324

Joined Mar 30, 2015
9,576
That and the dies are from the same wafer.
Parameters will vary, even on the same wafer. When I was working on microprocessor designs, designers jumped through a lot of hoops to ensure matching of transistors that were placed next to each other (in-die variation, but large die). The transistors were placed as close as design rules allowed (fractions of a micron away).
 

Thread Starter

plumbum_by

Joined Jul 23, 2019
3
But when there are larger number of them in seies, then voltage are rather well averaged in boarder of one string, thus if any the neighborous string differs, it is n-fold smaller difference and thus the lamps may suffer remaining alive
Sounds very reasonable. Samsung LM301B already has 3 binning groups by forward voltage 2.6-2.7V, 2.7-2.8V and 2.8-2.9V, using LEDs with the same binning group and averaging Vf by using a lot of them is the only explanation I see now.
PS. Sorry for my mistake in stating post, surely Vf variation is not 1V, but 0.1V for leds with the same binning index
 
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