Author Topic: High efficiency multi mode analogue LED driver  (Read 523 times)

Online royfellows

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High efficiency multi mode analogue LED driver
« on: November 26, 2021, 03:00:39 pm »
I would like to first explain exactly what is meant by efficiency and how it affects the average user of lighting equipment such as headtorches and caplamps, and also describe a new lamp project intended to replace 2 existing models.

First, a bit of explanation. If you connect an LED directly to a battery you will probably get your light disappear in a puff of smoke just like a pantomime genii. So it’s necessary to employ some means of controlling the current to the LED or LEDs. This can range from a simple resistor, through linear drivers which are somewhat better, up to quite sophisticated switch mode drivers. The latter in very simple speak does what it says on the tin, it switches on and off at a very high rate and then smoothes the output into a continuous current. An oversimplification, but necessary for the un-knowledgeable to understand it.

Most of these drivers encountered will be whats known as buck drivers. This type is used when the battery voltage exceeds the voltage required by the LEDs and controls the current. As such, they are quite efficient, however there is a necessity for lower brightness levels, usually referred to as ‘modes’. This is usually done by a process called ‘pulse width modulation’ or PWM, it does not matter about understanding this for the purpose of this thread, but take my word for it that in a lot off the off the shelf buck drivers I have tested, efficiency goes out of the window on the lower output settings.
The product description will often boast up to 95% efficiency, the bit they don’t tell you is that this is at the maximum output, go down to the lowest mode and it can be less than 70%, no kidding. Of course, the current draw at that level will still mean that burn times will be quite long, but it is something of a thorn in ones side to think that a third of the power being drawn from the battery is just going to waste.

There are other efficiency factors such as use of under rated components to save space, typically the series inductor which can be quite bulky. High value plus high current. Mine only needs 2.2 uH, I have seen them on buck drivers as high as 40 uH and obviously underrated for the current to get it into a torch.

An attendant issue is that of falling output as the battery voltage decreases. Obviously, there will be a point when the battery voltage falls to that below the requirement of the LEDs, but this is from the word go. Many people will be attracted to the idea of a ‘slow death’ rather than an abrupt loss of light, but the slow death really should not be from the very start?

Anyway, moving on, I have been for some time now considering the design of a high efficiency driver using analogue mode switching. Well necessity is the mother of invention it is said, and matters have been precipitated by the cessation of availability of the drivers I have been using in my popular Scorpion X12, and also the X8.

Now Stenlight uses an analogue system based around a DC-DC converter and series of reed switches actuated by a little revolving magnet. There is quite a lot of info on the Sten website which is quite factual rather than sales BS. Stenlights are not the most powerful of lamps, and this is for good reason. Regardless of the efficiency, the small amount of energy that is wasted is obviously converted to heat, and all inside a tiny chip of a few millimetres. So there are practical limitations as to output.
 
The DC-DC converter is very efficient mainly because there is no external fly back diode, a source of energy loss. But it also has its own internal switching MOSFETs, actually 3 according to the block diagram, hence the heat issue. I overcome this by mounting my converter sort of ‘piggy back’ on the other side of the PCB from the other components. This enables me to heat sink it directly into the aluminium case of the lamp. It has already been extensively tested at full power, 3 amps.

All for the Stenlight, but my lamp users have grown to accept and love my little waterproof toggle switches, so that is the line of experimentation I have followed.
After many hours on the web I sourced a converter suitable for my purpose and my first testing confirmed an efficiency of a remarkable 98%. This was 3 series Cree XP-L emitters powered from a 11.2V input voltage with the driver set to a 200 mA output. This represents an output of about 550 lumens from the 3 serial LEDs for a battery hit of 150 mA. The battery capacity of the X12 is 3500 mAh, so do your sums. Its worth mentioning that some other lamp manufacturers use prepacked cells of much lower capacity that those I use, Sanyo NCR 18650 GA.

The switching is about 850kHz, so my design revolves around a low inductance ground plane, the overall prerequisite being SAD – short and direct. Deviation from this resulted in quite a few failures, and somehow the inevitable trashed components.

The switching methodology uses 2 serial flip – flop circuits giving 4 logic states controlling Rsense, on this design Rsense is not serial with output, but output to ground.
 Thus -
State 1 - off state – fixed default output
State 2 - output one – state 1 plus output 2
State 3 – state 1 one plus output 2
State 3 – state 1 plus states 2 and 3
Then looping back to state 1.

At this time I have arbitrarily set the outputs as 140mA, 220 mA, 1000 mA, and 2950 mA with the outputs switchable between the 2 beams plus one flood (as the old X12) and three flood LEDs, (as the old X8 but 50% more power). I can easy customise these outputs for individual users.

It will be well into the New Year before this lamp becomes available due to the need to continued testing.

The efficiency does vary as I have had results as low as 92%, but the current output does not. Its steady as a rock on the lower settings down to about 9V, switch off is at 8.1V. The intention is to fit in a flashing red that will cut in at about 9.5V.

The current production of Scorpion X12 batteries is a very compact aluminium cased unit that weighs in at 280 grams, so no great weight to carry on your helmet. Capacity is 3500 mAh at nominal 11.1V. Expect long burn times, yes.

I welcome comment from the knowledgeable on here, also questions, although obviously I don’t want to give away all of my secrets, and don’t intend to. Don’t even ask for the schematic.
I also have to say that there could still be unforeseen issues that could turn this whole project belly up.
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UK Caving

High efficiency multi mode analogue LED driver
« on: November 26, 2021, 03:00:39 pm »
Warmbac

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Re: High efficiency multi mode analogue LED driver
« Reply #1 on: November 26, 2021, 03:41:36 pm »
So is this magical IC specifically designed for driving LEDs in a highly efficient manner directly from a suitable battery pack?

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Online Cantclimbtom

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Re: High efficiency multi mode analogue LED driver
« Reply #2 on: November 26, 2021, 04:11:59 pm »
Thank you, I understand now why some torches cause flickering/strobing when recorded on some GoPro settings, but wondering why a small capacitor isn't in series with the emitters to avoid that

Maybe this is only a feature for simpletons like me, but after recently trying your 3 position toggle

* Walk Mode
* Low
* Turbo (photo or looking up/down stuff)

IMHO that's unbeatable so I'm puzzled why all these logic states etc it seems a step backwards, each to their own.
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Online ChrisJC

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Re: High efficiency multi mode analogue LED driver
« Reply #3 on: November 26, 2021, 04:18:03 pm »
Thank you, I understand now why some torches cause flickering/strobing when recorded on some GoPro settings, but wondering why a small capacitor isn't in series with the emitters to avoid that

The circuit I designed (for which I have published the circuit diagram!) specifically avoids that by having a true constant current supply, which is adjusted for the various brightness levels. No strobing.

An awful lot of circuits just flash faster than the eye can see (just). I notice that most car rear lights are actually flashing on and off because when they pass me I see a series of lights rather than a continuous smear.

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Re: High efficiency multi mode analogue LED driver
« Reply #4 on: November 26, 2021, 04:47:23 pm »
So is this magical IC specifically designed for driving LEDs in a highly efficient manner directly from a suitable battery pack?

Chris.

I would say yes after the many hours spent studying data sheets etc, but probably not by the manufacturers intent.
Figures are figures and numbers are numbers.
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Re: High efficiency multi mode analogue LED driver
« Reply #5 on: November 26, 2021, 04:55:26 pm »
Thank you, I understand now why some torches cause flickering/strobing when recorded on some GoPro settings, but wondering why a small capacitor isn't in series with the emitters to avoid that

Maybe this is only a feature for simpletons like me, but after recently trying your 3 position toggle

* Walk Mode
* Low
* Turbo (photo or looking up/down stuff)

IMHO that's unbeatable so I'm puzzled why all these logic states etc it seems a step backwards, each to their own.

Its all in what I have posted, the run of X12 and X8 lamps as they were has ended in an abruptness only exceeded by the CAL access agreement.

However the new lamp, if it goes to plan, should incorporate all the features of the previous two plus longer burn times and more consistent output.
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Re: High efficiency multi mode analogue LED driver
« Reply #6 on: November 27, 2021, 06:14:40 pm »
Most *caving* lamps, from what I have seen use linear regulators, the small ubiquitous 7135's. I can only assume due mostly to robustness, and ease of design, cheap to produce.. Of course, these supply a fixed current each of 350mA but are usually connected in parallel to increase available current. They are controlled by a micro which itself will output either a DC voltage for maximum brightness, or PWM signals with varying ratios to control perceived brightness.

What's interesting about a linear regular though, is they can potentially be more efficient as the battery voltage nears the forward voltage of the LED, and for obvious reasons, but until that lower voltage is reached, the excess is wasted as heat as you say.

Interesting to note that very few designs these days use buck or boost technology. As you suggest above the Stenlight does, and they decided to avoid microcontrollers for reasons only known to themselves. Personally, I cannot see any reason why the output of their lamps hasn't increased other than the fact they are just happy with their design, brightness proven reliability and longevity in use.

I am interested to know how you are getting such a high efficiency. Typically you'd expect a figure of about 92% again depending on operating conditions and battery levels etc...
But 98% seems quite remarkable. How about other operating currents?

Also, you mention the lack of flyback diode, which I cannot believe a BUCK design can work without the function such a diode provides... I assume this function is internal to the driver IC? I wonder if they have used a very fast-acting FET to provide a lower forward voltage?
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Re: High efficiency multi mode analogue LED driver
« Reply #7 on: November 27, 2021, 07:30:03 pm »
No its all in what I post, I suppose the devil is in the detail same as the data sheets.
My circuit is not a conventional buck circuit, its a DC-DC converter same as what Stensat use. I'm afraid here you got the story wrong.  These are very efficient, but usually the limitation is the heat produced inside the chip, which is what I expect limits the Stenlight. As I explain, I have used a novel way of heatsinking.

As I say, the DC-DC converter is very efficient. My designs match the LED arrays in series to the input voltage, so voltage overhead is low. Say typically the X12 which is 12.5 ish volts off charge, shutting down at 8.1V to protect the cells. 3 series Cree XP-L on full chat are about 10v forward voltage, so overhead is slim, although they will still glow fairly bright at the shutdown voltage. This probably goes to explain the high efficiency. Off course, neither a converter or a buck circuit can is capable of driving the LEDs at anything greater than the input voltage, which is the start of the death cycle. I will be as I said, putting in a low battery warning on the new model.

My impression is that a lot of electronics engineers are very afraid of heat. But my background, well part of it, was in IT. I was a PC and local network system engineer, Access developer, and Visual Basic programmer. What a mouthful never mind the willy wave!
Anyway, I well remember the early days of the fast AMD processors and the copper core heatsinks made for them. They came with a blue cellophane protective strip over the copper, and woe betide anyone assembling a unit who forgot to remove this before mounting on the CPU. First boot would be King Tut.
One trashed processor.

The heat produced at 3 amps in the little converter is diddly compared to that.

Off thread, but there are two things that adverts get right relative to two different products.
One is "Intel Inside", yes they are far superior to anything else and computers built with them last for years. I remember in an office an old machine that I fixed that kept freezing up. Opening the back I found the CPU heatsink and fan assy hanging off. Refixed it worked fine, guess what processor. Answer on the postcard.

Oh yes, the other product is Calgon, saves a fortune in washer repairs and replacements, especially around here where its hard water.

Caving lamps, I dont know so much about 7135s. They have a maximum voltage of 6 volts which puts them outside the frame for more powerful lighting. However, I rather expect that they are tops with the DIY builders and Oldham converters.

Of course the builders of high quality lamps like Nora and Scurion produce their own PCBs, but I cannot comment on these as I have not had chance to look at one. I do get the impression that a lot of manufacturers of specialist caving lamps are using boost circuits and low voltage batteries., either single or parallel cells.
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Re: High efficiency multi mode analogue LED driver
« Reply #8 on: November 27, 2021, 08:03:01 pm »
My circuit is not a conventional buck circuit, its a DC-DC converter.

This might be a naive question, but is a buck converter not a DC-DC converter?

Offline Tangent_tracker

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Re: High efficiency multi mode analogue LED driver
« Reply #9 on: November 27, 2021, 08:37:11 pm »
Yes, it is. electrical energy (charge) is stored in an inductor. To step down you effectively monitor the current as the inductor stores said energy (they do not charge instantaneously hence why voltage level can be monitored as stored charge increases) and switch off when it reaches the desired amount. The stored of the energy is then dissipated by the load until the current reaches a preset amount before the cycle is started over again. Most Chinese modules use this principle, however, they are cheaply manufactured and offer efficiency at the lower end of the spectrum.
Not sure where the fear of heat dissipation is with these designs though.
What makes these circuits efficient is that the transistor driving the inductor is either on or off, so minimal power is dissipated across said device as heat, very much like a switch. The only alternative to this method that I am aware of is the use of linear regulators that act more like a variable resistor, one that varies to give a predefined output. You can still use pulsed signals to drive these, but efficiency will always be an issue due to the overhead dissipated as heat.

Just looking now and I have found an automotive driver that offers both buck and boost within a single package, and seemingly without a traditional flyback which will improve total overall efficiency. This device is said to offer up to 98% efficiency which is impressive! I will have a further read... (https://www.analog.com/media/en/technical-documentation/data-sheets/lt8391.pdf)


Regarding low voltage for Li-ion cells, yes this is something that folks who play with li-ion batteries and their own designs rarely understand. The fact is, it is undesirable to let your cells drop below approximately 3v each. Relying on the built-in protection boards is not a good idea as many do not cut off at the advertised 2.7v and by then you are still damaging the cell. Most commercial torches turn the lamp off or to a very low current at about 3v per cell.

I recently programmed a pic micro to act as a battery manager which also reports low voltage by way of pulsing an LED, as I was surprised that nothing is available commercially!

If anyone wants want to play around with just give me a shout!
« Last Edit: November 27, 2021, 08:47:11 pm by Tangent_tracker »
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Re: High efficiency multi mode analogue LED driver
« Reply #10 on: November 27, 2021, 09:08:42 pm »
Here are two typical Chinese modules. The one on the left uses three linear regulators in parallel. The positive input on these requires very little current and can be driven with a PWM signal to give the desired light output. The LED's always run at full current but the disadvantage is they dissipate the overhead as heat, always while driving the LED.

The device on the right is a buck or DC-DC converter. They cleverly use the big thing with windy wires to both limit initial current to the load (and hence allow monitoring current as it increases with time) and then release stored energy when the desired limit is reached. Because very little energy is released as heat, they are far more efficient.




https://photos.app.goo.gl/3BrpwU1ShL9KiegT8
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Re: High efficiency multi mode analogue LED driver
« Reply #11 on: November 27, 2021, 09:16:14 pm »
Yes, note my use of the word conventional, most as you say have flyback diode, as well as external switching MOSFET. Was "conventional" the right word? "Usual" whatever.

Li Ion cells, data sheet for what I se gives 2.5V, the 2.7 is a safeguard, and my management boards monitor individual cells.
I am surprised about the unreliability of the protected cells circuits as some years ago they adopted a dedicated chip produced by Seiko Inductiries.

Interesting discussion, but we are probably over a heads of a lot of people.
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Re: High efficiency multi mode analogue LED driver
« Reply #12 on: November 27, 2021, 09:20:25 pm »
I cant get to see your photos.
I rather expect the 7135 one is 3 or 4P with Atmel microcontroller? Is this a good guess?
Like to see the other, may be something I have tested.
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Re: High efficiency multi mode analogue LED driver
« Reply #13 on: November 28, 2021, 04:57:04 pm »
Well Roy its a good job you didn't go into to much detail with the original post  :lol:
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Re: High efficiency multi mode analogue LED driver
« Reply #14 on: November 28, 2021, 05:34:41 pm »
... ... obviously I don’t want to give away all of my secrets, and don’t intend to.... ...

....
My circuit is not a conventional buck circuit, its a DC-DC converter same as what Stensat use... ...
... My designs match the LED arrays in series to the input voltage, so voltage overhead is low. Say typically... ...
I'm learning a lot from and enjoying your posts, thank you Roy, but I think I'm amused by this in a similar vein to Paul?
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Re: High efficiency multi mode analogue LED driver
« Reply #15 on: November 28, 2021, 06:00:44 pm »
Well Roy its a good job you didn't go into to much detail with the original post  :lol:

Well, I would say from what I posted a really savvy engineer could probably identify the chip!
The 'novel idea' is in the mode switching
I am only being a bit coy on it because I am a commercial producer not just a hobbyist, and there are people out there..........................................

A point, all the 4 mode buck drivers I have seen from HK seem for some reason to have the lowest setting far too low to be of any real use, and the next one up not much more. Then a massive jump to the high settings. I puzzle on this, maybe I am missing something.
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Offline Paul Marvin

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Re: High efficiency multi mode analogue LED driver
« Reply #16 on: November 28, 2021, 06:18:13 pm »
Well Roy its a good job you didn't go into to much detail with the original post  :lol:

Well, I would say from what I posted a really savvy engineer could probably identify the chip!
The 'novel idea' is in the mode switching
I am only being a bit coy on it because I am a commercial producer not just a hobbyist, and there are people out there..........................................

A point, all the 4 mode buck drivers I have seen from HK seem for some reason to have the lowest setting far too low to be of any real use, and the next one up not much more. Then a massive jump to the high settings. I puzzle on this, maybe I am missing something.

Nope I think you have it covered Roy   :lol: :clap2:
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Re: High efficiency multi mode analogue LED driver
« Reply #17 on: November 28, 2021, 06:42:18 pm »
......lowest setting far too low to be of any real use, and the next one up not much more. Then a massive jump to the high settings. I puzzle on this, maybe I am missing something.
Because then you can make and sell a lamp capable of 10,000lm (or other wild claim) and also state the battery life is 20hours
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Re: High efficiency multi mode analogue LED driver
« Reply #18 on: November 28, 2021, 07:19:38 pm »
Well, I would say from what I posted a really savvy engineer could probably identify the chip!

They could. There are a lot to look at though, so whether they could find the time to trawl is a different question...

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