Most efficient driver at low currents

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sac02
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Most efficient driver at low currents

What is the absolute most efficient driver for low currents?

I want to drive a single N219C with a single 18650. I want about 100 lumens, and the most important thing is runtime. Hence the most efficient driver question.

Lightbringer
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Probably a single 7135.

In theory you can make the most power-stingy converter with a buck converter, but in practice, copper losses, iron losses, circuitry losses, etc., all add up.

Plus, buck converters have to adjust to everything from right out of the charger (4.2V or so) to just about the Vf of the LED, whereas linear regs just drop enough voltage to maintain the specced current through the LED.

A fleapower buck converter which is limited to, say, 300mA-400mA or so, can be designed to use a smaller inductor, lighter-weight switching elements, etc., and “tuned” to low-current apps, increasing efficiency.

But, you gotta find/design such a critter.

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One of the members here, HKJ, has tested a quite a few drivers and posted his results here.

You should be able to learn a quite a lot by looking up his very detailed technical reviews.
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I have wondered this before… I did some research and built some lights that I am quite happy with…

I ended up buying a batch of single mode 1.5a drivers, and resister modding them down to the output I want. It turns out the drivers were being over driven anyway, so the power I wanted from them was probably much closer or even inside their efficiency window.

These were and are some of my favorite lights. My goal was 200 lumens for 8 hours. I dont have an IS, but they compare well to other 200 lumen lights, and they definitely run for 8 hours…

I dont have the exact driver I went with, but it looked similar to this. I remember it didn’t have a chip which looked like an MCU like on the atiny boards…

http://www.kaidomain.com/p/S010093.Cree-XM-L-T6-1-Mode-Circuit-Board-5pcs

http://www.kaidomain.com/p/S008846.A9-Buck-Current-Regulated-3_0V-8_4V-1...

netprince
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This driver in HKJ’s review looks familiar, maybe this is what I used:

http://budgetlightforum.com/node/30901

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219c has a low Vf curve and we’re talking about a point laying low on it. Linear won’t do great…

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Agro wrote:
219c has a low Vf curve and we’re talking about a point laying low on it. Linear won’t do great…

all depends from cell capacity curve

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sac02
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Agro wrote:
219c has a low Vf curve and we’re talking about a point laying low on it. Linear won’t do great…

My understanding of exactly how Vf works is admittedly incomplete. I thought low Vf would be preferable in a situation like this – for example we always say that LEDs with low Vf will stay in regulation longer. Is that not related to efficiency?

I guess, can you explain further?

Would an N219B work better?

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Lightbringer wrote:
Probably a single 7135.

Again, my technical expertise on the Electrical Engineering side is lacking. I thought that linear regulators like 7135s were good to keep the light output Regulated, and compared to just a current limiting resistor they would be considered efficient. But don’t they still produce a fair amount of heat at the chip itself (energy being burnt off as heat = energy lost). Or am I incorrect on that?

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Here what I understand / know, correct me if I’m wrong.

Vf is the LED forward voltage, this relates to the current going through the LED. Higher current, higher Vf. This relation is non-linear though. It rises fast on the very low currents and then levels out a bit until you get to the really high currents (weird things happen).

A linear regulator “burns off” extra voltage as heat. Thus a fully charged cell sitting at 4.2V driving a low Vf LED at 1A having a Vf of 2.9V (219C approx) will burn off 1.3V at 1A. Thus you would waste 1.3W of power in the driver.

The upside is that the battery voltage can drop to something like 3.2V or so before the linear regulator does not have enough overhead voltage to keep regulation. A higher Vf needs a higher Vbatt to maintain that margin.

And here is where a well designed buck regulator will win, it will not “burn off” the excess power, instead it will use less current from the cell when the voltage is higher than Vf. Of course, some power is lost in the buck converter, but it will be no where near the loss of a linear driver. (If well designed.)

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Yeah, for low Vf LED, efficiency is lost with Li-Ion full-charge voltage. Better to use a LiFePO4 or Li-Po cell for those, since those chemistries have lower full-charge voltage. Maybe even better, if designing the entire thing from scratch, to use a high quality AA boost driver with Eneloop and low Vf emitter?

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So I guess we are deep enough in at this point that I should give the purpose for this project…

I’m trying to design an ultralight system for lighting a group (4-6 people) campsite when backpacking for up to 10 days. Every gram counts, and the reason that efficiency counts is because it enables me to carry less battery weight (by extracting more runtime).

The first idea I tried was four 12inch lengths of 12V LED strip lighting (with lengths of hook-up wire between them so they could be spaced around the campsite), powered by 4×18650 and a DC-DC cc/cv buck convertor like such:

The runtime was somewhat disappointing, the overall weight was high, and having all the lights wired to a single battery made light placement restrictive.

My idea for version 2.0 was single LED, with single battery and driver. This would eliminate wires, increase light placement options, and potentially reduce weight too.

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For better efficiency you need multi-led (or 4-die led).
Forget about recharhable li-ion. Use primary cells.

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netprince
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Sounds like a neat project, I look forward to see what you come up with…

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kiriba-ru wrote:
For better efficiency you need multi-led (or 4-die led). Forget about recharhable li-ion. Use primary cells.

How does multi-led increase efficiency?

I was interested in DavidEF’s comment about AA cells and boost drivers, especially from a weight perspective. Lithium primaries would work better in cold also.

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sac02 wrote:
kiriba-ru wrote:
For better efficiency you need multi-led (or 4-die led). Forget about recharhable li-ion. Use primary cells.

How does multi-led increase efficiency?

Check Nichia 219C D260 9050 90+ CRI Emitter output test by Texas_Ace

1.0 A 353 lm 122.0 lm/W 353 lm/A
0.5 A 198 lm 141.8 lm/W 396 lm/A
0.25A 111 lm 163.2 lm/W 444 lm/A
0.15A 72 lm 178.8 lm/W 480 lm/A

If you run multiple leds the current on each one will be smaller, then it will be more efficient.

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Not so much. You can check many led tests made by djozz and Texac_Ace and find how much power (current for 7135) is necessary for your target (100lm). Big multi-die leds can have enormous efficiency at tiny current rates, like 200lm/w. For several leds (4) you can take you target (100lm), divide by led quantity (100/4=25) to get each led target (power for buck or boost and current for 7135) and check how total power looks in comparation with single 100lm led config.
I dont now how to make a build with this parts but probably 2x rcr cells with 30mm L6 driver (was tested by Jensen567) powering xhp70.2 (probably target output can be limited by res mod) will give you best hour/weight ratio from parts that can be easily found.

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sac02 wrote:
Lightbringer wrote:
Probably a single 7135.

Again, my technical expertise on the Electrical Engineering side is lacking. I thought that linear regulators like 7135s were good to keep the light output Regulated, and compared to just a current limiting resistor they would be considered efficient. But don’t they still produce a fair amount of heat at the chip itself (energy being burnt off as heat = energy lost). Or am I incorrect on that?

Exactly right. But there’s a tradeoff.

The 7135 will saturate at 0.1V, so if at the rated current (each chip’s share being 350mA), if the Vf of the LED is 2.9V, then it’ll stay in regulation until the cell drops to 3.0V. But right out of the charger when the cell’s at 4.2V, it’ll be dissipating 350mA•1.3V. So that can be quite a bit.

Thing is, buck drivers have their own losses, especially if designed to deliver multiple amps (lower currents being PWMed high-current spikes). So there are copper losses in the wiring (even in the inductor), iron losses (magnetic hysteresis), silicon losses (all the circuitry involved), current through sense resistors, in the feedback loop, etc.

So for higher currents, always running in “turbo” (100%), the buck driver will be more efficient, as wasting 1W of power when dumping 10W, 15W, or more into the LED isn’t all that much. But at lower currents, it’s like using a 600hp Corvette engine in a riding lawnmower. Yeah, it’ll work, but it’ll hardly be efficient.

Now, if you design a buck regulator to be as power-stingy as possible, knowing you won’t be dumping 15W into the load but only 1W or so, scaling down the inductor, “tuning” everything to be as efficient as possible, no added baggage like a µC to do dozens of modes, etc., you can make the buck reg pretty efficient. That brings down the tradeoff point.

But even so, by the time the cell drops to, say, 3.5V, the linear reg will still be more efficient. By 3.2V or so, no contest, the linear reg will always always always be more efficient.

So you gotta figure efficiency (and the tradeoff point) over the whole battery discharge curve, not just the worse-case condition (biggest voltage difference).

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sac02 wrote:
My idea for version 2.0 was single LED, with single battery and driver. This would eliminate wires, increase light placement options, and potentially reduce weight too.

Why didn’t you say so from the beginning? Big Smile

LFP (LiFePO4) cell, crowbarred right across a higher-Vf LED.

Get a nice NW XP-G/-G2 LED and a bunch of LFP cells. Your “regulator” is the power switch. Maybe a small value resistor to slightly limit the current (“low mode”).

Those solar garden-lights use that. Simple, stoopit, and it just works.

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sac02
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Ah, yes I see now. I did know that LEDs were more efficient at lower currents. So for example (fake numbers) four dies could produce 100lm at 0.04A/ea (0.16A total) and is more efficient than one die at 0.2A making the same 100lm. Makes sense. I had not considered XHP70.2, good idea (although more expensive, but it seems higher performance/lighter weight always is more expensive, lol.)

Is the L6 driver known to be efficient, or is it just the first one to come to mind? Would it even work with two CR123a primaries (6.0V)?

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Lightbringer, won’t the current limiting resistor burn off a ton of energy like that?

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The higher electrical efficiency of using LiFePO4 cells will be entirely offset by their lower energy density, if runtime is what you are after. They have about 1/3 the energy density of good Li ion cells.

As others have said using multiple LEDs will be more efficient. I think a quality buck driver will be more efficient than a linear driver in this case. For example the XHP70.2 produces 418 lumens at .3A and 5.35V. If we say
the battery voltage is 8V then the efficiency is only about 67% with a linear driver, while a quality buck driver can be greater than 90% efficient.

But as lightbringer explained as the battery voltage drops the linear driver efficiency increases, so overall the efficiency difference between linear and buck won’t be that high.

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I know it doesn’t answer your question directly but here’s my answer. I was in USMC infantry for years multiple deployments. I know weight counts trust me getting shot at in mountains with 100+ lbs on your back is not fun. But in all reality. You already probably have a small 18650 light that has a low mode that will run 15-20 hours check the specs and do a runtime test yourself. If you can get close to 17-20 hours per battery you’d only need the light and a 4 pack case of batteries. I’d choose 3400/3500 variety.one 100 gram flashlight and 250 grams in batteries. Is under a pound and won’t make much difference in your load in all seriousness.
But I don’t know your physical conditioning or what not. But I know what it’s like to hike in your own food and water for a week or so and ammunition, body armor with 110+ degree temps. Under a pound for your lightening needs is great and easily doable with what you have now. Even you carried our or two more batteries would still be under or around a pound. But my thinking if you can get close to 20 hours. A couple hours before the sun comes up you have light coming in you can start to see inbetween night and morning like 5 am it’s dark but not nighttime dark. So even if a battery lasted 17 18 hours it would still work 2 days per battery. One in the flashlight w 4 case or 2 2cases with ncr GA or ncr and your covered. Even a extra battery for peace of mind is under a pound still. 10 days would be covered. Just my 2 cents on it alot of lights have a low mode around 100ish lumens. Your original plan was to carry your device and 4 18650s anyway. Same weight as a flashlight and 4 batteries.

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Yeah, Li-Ion seems to be the most advanced (highest energy density) for sure.

The potential efficiency of a buck driver would have to be weighed against the impact of needing a second cell – more weight. Could use smaller cells like 2×18350,but Wh/gram is lower than 18650 cells.

But now maybe I see where Lithium Primary cr123a might work – at low currents (0.3A) the xhp70.2 Vf is 5.35v per TA’s test, so with 2xcr123a the battery voltage and Vf would be very closely matched. Is that thinking correct?

sac02
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Speed4goal, I’m drunk on the UltraLight backpacking kool-aid – I used to brag about how much my pack weighs, now I “brag” about how little it weighs (“brag” in quotes because it’s not that light compared to many.) (and I understand that in the military you don’t get a lot of choice to carry a lighter pack)

My current base weight is about 11.5lbs, and I’m attempting to get closer to actual “ultralight” at 10lbs, so 1lb is a lot. I’m in good shape and could lug around a 50lb pack, but the UL mindset and reduced pack weight allows me to cover more ground when hiking, and more importantly I ENJOY the hike, rather than suffer through it as a necessary evil to get to the campsite.

My lighterpack:
https://lighterpack.com/r/3kkgbi

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sac02 wrote:

But now maybe I see where Lithium Primary cr123a might work – at low currents (0.3A) the xhp70.2 Vf is 5.35v per TA’s test, so with 2xcr123a the battery voltage and Vf would be very closely matched. Is that thinking correct?

No.
You can take big capacity from primary cells if youll discharge them much below 3v. So for 2 cells and single xhp70 you need boost driver.

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Ah, OK.

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sac02 wrote:
Lightbringer, won’t the current limiting resistor burn off a ton of energy like that?

It should be quite small. Need to test first how much current without any resistor, then try small values if you want to limit current further.

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Oh ok I got you, I was thinking it was around the same weight as your first prototype device. Nothing wrong with going ultralight everyone has their thing. I’ll probably always be stuck in the mindset to bring more supplies then needed just incase. Probably around 25 30lbs

But hey it gives you a reason to build and play with a new toy and show everyone look what I built.

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sac02 wrote:
kiriba-ru wrote:
For better efficiency you need multi-led (or 4-die led). Forget about recharhable li-ion. Use primary cells.
How does multi-led increase efficiency? I was interested in DavidEF's comment about AA cells and boost drivers, especially from a weight perspective. Lithium primaries would work better in cold also.
Well, with Energizer Ultimate Lithium AA primaries, you could use 2x and not need a boost driver. These have about the same mAh capacity as a good Li-Ion 18650 and even 2x will weigh less than 1x Li-Ion 18650. You will not have the ability to recharge them, and they are pricey for single use. At the low current draw, voltage will be better matched than Li-Ion. Use multiple LED's _in parallel_ to increase the efficiency. Putting them in parallel keeps the Vf the same (or actually _lowers_ it due to lower current draw per LED). "Here's HKJ's review/test":http://lygte-info.dk/review/batteries2012/Energizer%20Ultimate%20Lithium... of the Energizer Ultimate Lithium AA Primary.

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