Wiring LED's - Parallel or Series/Parallel

[bigk1]

Hello all! I need to modify an LED strip for higher brightness. Currently has 16 Cree CP41B-RD (2.5-3v Forward Voltage at 70mA each) powered from 12v.

These are currently arranged in a strange 4 x 4 series / parallel parallel with resistors at every step, i.e from the + rail, it goes through a resistor, then feed 4 LED's in parallel which then is wired in series to another set of 4 LED's in parallel through resistors and so on. (Series-Parallel)


The issue I have is to do it as efficiently as possible (save every mW!).

So I have a few choices to make, I can keep using resistors and calculate the replacement values or move to a current/voltage controlled converter.

1. Which is more efficient in this scenario, resistors or a buck converter? (Research indicates converter)

2. Do I keep it at Series/Parallel or wire all LED's as Parallel only? Is one more efficient than other? If kept as series-parallel, would I still require resistors in-between each group of LED's?

[ebastler]

You did not explicitly say so, but I assume you want to keep using the Cree CP41B-RD LEDs, right?

The 70 mA you mention are the specified absolute maximum rating for these LEDs (for steady current), https://cree-led.com/media/documents/CP41B1143.pdf. Are they already being driven at that current in your LED strip? If so, better leave the thing alone. Otherwise, your LEDs might get brighter, but not for long...

If you have not measured the actual current yet, you can do so without unsoldering anything by measuring the voltage drop U across the series resistor, as well as the resistor value R, and calculate the current via I = U/R. (Apologies if this was already obvious to you!)

[tunk]

What's the voltage drop across the LEDs? If it's 2.5V, then the efficency is 83%, for 2.75V it's 92%.
I guess both are in the same ballpark as a buck converter - if so, it's much simpler to replace the
resistors.

Edit: Is it 12.0V, or is it e.g. from a "12V" battery?

[bigk1]

Thanks for replying, I should have made it clear!

I replaced all the LED's with the Cree's... as these are the "brightest" I could get and now want to drive them as bright as possible with consuming the least power. The old (unknown) LED's were around 20-30mw each.

Kind regards,

[ebastler]

These are currently arranged in a strange 4 x 4 series / parallel parallel with resistors at every step, i.e from the + rail, it goes through a resistor, then feed 4 LED's in parallel which then is wired in series to another set of 4 LED's in parallel through resistors and so on. (Series-Parallel)

I forgot to ask you about this part. Are you sure you saw that correctly? One resistor plus four LEDs in series would make much more sense to me, and then another group of (resistor plus four LEDs in series) in parallel with that, etc.

Anyway, that's how you should wire the Cree LEDs, I'd suggest.

You will be increasing the power consumption of the LEDs alone by nearly a factor of 10 (from 20 mW to 200 mW per LED) if you drive the Crees at 70 mA. Do you have adequate heatsinking/cooling for that LED strip?

[bigk1]

Had a closer look again, it's even stranger than what I said!

So from + it goes to a resistor, 4 LED's in parallel, resistor in series to 4 LED's in parallel, resistor in series to 4 LED's in parallel and then finally, in series, there are 4 parallel LED's.

Original LED strip was made by Valeo, so assuming their engineers worked out how to make it with as little cost as possible.

I've attached the diagram below!



Edit - In regards to heat output, I won't be able to use heatsinks, however my plan is to reduce temps if required by reducing voltage / current until temps are acceptable. As lights won't be used for longer than 2-3 minutes at a time, heat isn't a huge concern

[sleemanj]

70mA/LED gives 280mA through the string max. 

In the resistors that is 1.3 watts wasted total.

I would either short out all the resistors and use a 210-280mA led driver, or I would short out 2 of the resistors and replace the third with a low (not critical) value 5w resistor and use an adjustable voltage buck (or boost, depending on initial source) dc-dc converter to tweak the voltage until suitable brightness, or 210-280mA was achieved whichever came first.

Using 210mA will allow you to have one led in a parallel set die and the other 3 not be over-driven.

[ebastler]

I've attached the diagram below!

I am stumped why they arranged it that way, frankly. With the four LEDs in parallel, what would balance the current distribution through the LEDs? Is there some intrinsic balancing behavior -- if an LED's power consumption goes up, its forward voltage drop increases so the other LEDs see more current, or such?

The three different values for the series resistors are even stranger. Obviously the same current flows through all of them -- why on earth would one not use the same resistor value for all three?

[Zero999]

LEDs shouldn't be driven at the full power rating, continiously, because it shortens their life.

Temperature is an issue, even if it's only on for 3 miniutes, because it's significantly longer than the thermal time constant of the tiny LEDs.

If you want efficiency, ditch the resistors and use a switched mode, constant current power supply to drive the LEDs. Ideally they should also be put in series, but that's probably impractical because it will require a fairly high voltage, which will pose a safety hazard.

How many LEDs are there in total? EDIT: I should have read the orignal post, stating 16.

Are you powering it from the mains, or a battery?

[gcewing]

Maybe the different resistor values are to even out the temperature rise across the board? Perhaps the ones at the ends can afford to be bigger because heat can be dissipated faster there.

The Vf of a diode does increase slightly with current, so parallel diodes will share current provided they're well matched. I'm not sure I'd like to rely on that if I were designing one, though.

[Siwastaja]

As efficient as possible? Simple: there is no place for resistors. Resistors are crappy, inaccurate and lossy way of regulating current using regulated voltage supply. Simple, yes, if you already have a constant voltage supply and must use it. But a much better (and the only, if efficiency is paramount) option is a switch-mode converter that runs in constant current mode i.e., feedbacks based on current, not voltage. LEDs are directly connected to the output. LEDs are in series. If multiple strings are used, each string needs its own switch-mode converter. As these converters cost money, longest possible strings are preferred but eventually the voltage gets too high for comfort.

Typical non-isolated topologies are buck and boost. Both work equally well driving LEDs in constant current mode. The choice depends on the available supply voltage and number of LEDs required. I'd hazard a guess that a boost topology is more common.

If power is coming from mains, then the mains supply itself can be directly designed to be a current supply instead of voltage supply. Then the name becomes "LED driver". This is the correct and modern way of driving LEDs. The supply drives the specified (fixed or adjustable) current, voltage will be whatever it is. They have some maximum voltage though and protection against overvoltage event.

[bigk1]

LED strip is used in a car (14.4v DC constant). There are 16 LED's in total.

Why the resistors are arranged as they are have stumped me too. Whilst I simplified the diagram, first resistor is two 16ohms in parallel. Second resistor is 4 x 15ohm in parallel, third resistor is 5 x 15ohm resistors. Assume it's for cost cutting rather than heat dissipation, as first two resistors are much warmer than rest. Credit where credit is due, all 16 LED's light up evenly and it has been working fine for the last 10 years!

I already have a Current & Voltage regulator based on a LM2596 waiting to go in, which seems a much simpler solution as it allows to reduce LED loads with a quick adjustment to ensure it doesn't overheat.

It's just deciding whether to stick with the current series / parallel layout or change to all 16 being in parallel and whether resistors are still required.

Changing to all 16 LED's in series is not possible due to the board needing to be redesigned and space constraints...

[Siwastaja]

Resistors are never required per se. It's just that LEDs need current limitation circuit, especially because of negative Vf temperature coeff; the hotter they get, the lower forward voltage they have.

Due to that funkiness, voltage supply won't work at all, even if carefully tuned. So now you can add a series resistor, and the LED current will be less dependent on the Vf variations, but it still is. And here's the optimization; the more voltage you drop over the resistor, the better the current regulation gets, but efficiency is wasted. Or you can drop only a tiny part of total voltage over the resistor but then it's close to not having a resistor at all, i.e., LED variations start significantly affecting current again.

If you just connect LEDs parallel, one with the lowest Vf hogs the most current, gets hotter, starts having even lower Vf and hogs even more current. Direct parallel connections are undesirable but acceptable when LEDs are matched (same batch, similar Vf), thermally coupled together and bond wires add a bit of resistance.

But as you clearly are after a really good and efficient circuit, don't parallel any LEDs, resistors or not. If the string cannot be 16 LEDs, then just have multiple strings but each with their own driver, i.e., parallel the drivers not LEDs.

Now if you want good efficiency and sane integration level / size, the driver should not be an ancient switcher IC like LM2596. Sure it was great in 1990's. Look in the LED driver IC category at distributors, there are plenty. If you take the efficiency seriously and the string length is less than some 8 LEDs, synchronous rectification may be a requirement, getting rid of some 0.5V of extra drop.

In any case, if efficiency is truly important, the choice of LEDs dominates here. It doesn't make a lot of sense to optimize the driver from 80% to 98% efficiency if the LEDs themselves are some cheap 80 lm/W crap while over 200 lm/W is easily available.

[BeBuLamar]

I would series 4 of them with a resistor to get the right 70mA. I think you have to experiment. Have 2 of these circuit in parallel. I would use 2 separate resistors for the 2 parallel branches.

[Siwastaja]

Finally remember that calculating the LED current using voltage source and series resistor involves two calculations,

Imin = (Vin_min - Vf_max) / Rmax
Imax = (Vin_max - Vf_min) / Rmin

Do it this way unless you are sure you are fine without. The range is staggering with low values of R, or small difference between Vin and Vf.

[StillTrying]

I am stumped why they arranged it that way, frankly. With the four LEDs in parallel, what would balance the current distribution through the LEDs?

I can't think why they're wired the bad way either, it's the same number of components when wired the right way.

I don't think you have to worry too much about the overall power efficiency when there's 4 LEDs in series, most of the time it's going to be over 80% anyway because there's only the one dropper resistor in the string.

[Zero999]

Four LEDs in series on 12V is marginal, but for 14.4V automotive use it's more reasonable, just as long as you're happy with it dimming when the engine is turned off.

I'd just use a linear constant current regulator. It will have over 80% efficiency and I don't see why that's the most important thing, as it's an automotive situation, where they shouldn't be the prime concern.

You could run them as 8 strings of 2 in series, at 450mA (around 56mA per LED, as it's always good to under-drive them, which minimally reduces the brightness),  with a buck switching regulator IC, to ensure it gives the same light output, over a wide range of voltages, without excessive losses.
LM3405A
https://www.ti.com/lit/ds/snvs508d/snvs508d.pdf

TPS92511
https://www.ti.com/lit/ds/symlink/tps92511.pdf

[bigk1]

Ready to be perplexed? Decided to do a quick bit of testing, removed the resistors and replaced with shunt wires. Using a current limited power supply at 10v, ~0.5a, all LED's light up & heat up evenly, from 5v to 12v, 0.1a to 1a!  Guess those valeo engineers did know what they were doing after all!

Thanks for your input!

[mariush]

You could just use a proper LED DRIVER chip to regulate current and skip resistors completely.

There's hundreds of such chips on Digikey alone : https://www.digikey.com/en/products/filter/pmic-led-drivers/745

Considering the use, I'd probably modify the strip so there's 6-8 leds in series to allow the use of a step-up (boost) driver, which boosts 10-14v from car battery to 2.5-3v x 6-8 while monitoring the current.

But you could go the basic route and use a simple linear driver for each group of 3-4 leds ... some are as basic as the chip and a current sense resistor to set the maximum current.. here's one such example: https://www.digikey.com/en/products/detail/nexperia-usa-inc/NCR420ZX/9998337
There is a voltage overhead of approx. 1.4v with these linear drivers but can't beat simplicity

[Zero999]

Ready to be perplexed? Decided to do a quick bit of testing, removed the resistors and replaced with shunt wires. Using a current limited power supply at 10v, ~0.5a, all LED's light up & heat up evenly, from 5v to 12v, 0.1a to 1a!  Guess those valeo engineers did know what they were doing after all!

Thanks for your input!

1A sounds like quite a high current. Did the LEDs get hot? Are you sure you correctly reverse engineered it?

[StillTrying]

all LED's light up & heat up evenly, from 5v to 12v, 0.1a to 1a!

How can four 2.5-3V LEDs in series light up at 5V.