Boost Driver Challenge - Technical Discussion Thread – Experts PLEASE step inside.

Just a note before you post:
The primary intent of this thread is to assemble a focus group for hardware design and manufacturing and is specifically directed towards members interested in and capable of working together to solve this challenge. This is not meant to offer ideas of UI features, such as mode selections or voltage cut-off, etc. Please allow our pool of BLF extraordinary talent to colaberate. I ask that you withhold any unrelated comments to maintain focus… if that is even possible around here! :smiley: :smiley:

The Challenge:
Designing a 17mm 3-6V +3A boost driver.

As most of us have been following along through several discussion threads during these past couple of years, the budget compact +3A 4.2V boost driver seems to have eluded us since the advent of XML. My thoughts are: that for such an affordable product to be successful and available to the masses, it must also be profitable for the vendor to produce in mass quantities. The idea of challenging a supplier to produce this specialty driver (with the promise of several more specialty drivers in the future) would guarantee their survival and provide us with a reliable vendor that is both capable and receptive in providing for our needs. Perhaps this is the wrong approach. It occurred to me that there might be several others that have tried but gave up with less than desirable results. Maybe they simply need a reminder and more encouragement to try again. The collaborative approach of open-source might be the added ingredient necessary to see this through. Or just perhaps the goal is unrealistic and impossible; given the budgetary and size constrains and other technical factors. Taking note of the larger driver sizes in most of todays name brand flashlights, perhaps the 17mm constraint needs to be re-examined. Does there even exist an off-the-shelf XML flashlight that boosts properly and efficiently @3A from a single 18650 cell? If so, maybe someone could examine that driver to provide the basis for a similar design.

Even if such a driver were to be produced (and given the capabilities of todays 18650-32650 cells) do you think there will be distinct advantages over the efficiency of 7135 based drivers or other similar offerings? -more specifically, towards the later half of cell depletion where boost will presumably become most effective. Of other concerns and considerations: efficiency and the amount of time while operated during the boost phase, as compared to the potentially higher efficiency of linear drivers before they drop below the curve. Its all speculative and maybe you can offer your expert opinions. In other words, what are the probable real benefits in run time with a noticeably brighter light via boost circuit throughout the discharge cycle, as opposed to the known high efficiencies of 7135 based drivers before the user (not the light meter) notices a significant drop-off? Im trying to explore the real world benefits through the eyes of the user to find if the boost driver justifies itself. Yeah… more speculation and maybe a waste of our time. Maybe this cant be answered until we have a driver to compare. Still, I had to ask…

Probable necessity:
I understand that no one wants to work for free and put forth a large effort, and then only to see someone else profit from their labors. But at the same time, I don’t think we will see any progress unless someone capable of prototyping such a design steps forward to donate their extraordinary talents… to later be copied and mass produced by our Chinese buddies across the pond. Is it even rational to think that such a device could be kept within the sub $7-10 realm if mass produced anywhere else? I doubt it. If anyone can design, manufacture and market this gadget on their own (below $10 delivered), more power to them! :bigsmile:

Basics or more?
Who doesnt want all the bells and whistles? I think everyone would love to see DrJones MCU of choice with his firmware flash, solderable stars to select output amperage and a user programmable UI. It comes as no surprise that we all have individual preferences and needs. If these can be added as part of the package, then more kudos goes to you! But at this phase, I think we will all be more than elated to simply have a working 3 or 4 mode boost driver.

Who will stand up?
Gentlemen, do we have any takers? Can a few of you collaborate and help support this design undertaking for the BLF group? Does the open-source approach sound feasible? At least then we/you would control the design, mcu, code and other variables. Other drivers could also be spawned if you are successful. Your help, opinions and collaboration are very much needed here.

If anything, perhaps we will all learn something through this exercise that might prove invaluable during a later phase. Im still not against contacting other vendors for the challenge, and everyone is free to do as they wish. But I think our internal opinion is more important at this time before choosing a direction to move forward. I feel that lessons have been learned from the other thread and a new approach is needed. Poor results most often occur where there is a lack of initiative, control, trust, communication and understanding.

Who? Not me! :smiley:
Hey, if Im not the right guy as the proverbial “Project Manager” (now or later) I will gladly pass the torch along to someone else. I hold no title… Im just trying to help get this started and see it through. Like most of us, I simply want a good budget boost driver to become available to the masses. Speaking for the rest of us mortals, please help us make this become a reality. Its so far overdue!!

Thank you for reading and please post your ideas!

Most Sincerely,


I nominate Keith as Project Manager, no hesitation there. He did a good job with the other one, all things considered. I have to bow out of managing this time, due to extreme time constraints. I will definitely participate in a technical way as well as any other way that I can.

Thanks for doing this: at one time I did have the expertise, before a brain injury, but may be able to help in other ways. I do know an EE, and will ask for help from him.

Lately I have been getting some practice in with eagle, so if no one else can do that I probably could manage.

I haven’t tried it at 3A, but the graph in the data sheet suggests that the Texas Instruments TPS63020 will do it.
I’m using this at 2A in a light I’m currently making. Dimming may need to be done by some method other than PWM, I wasn’t able the get the Enable input to work properly at a high enough frequency to avoid flicker. The dimming scheme I used switches the value of the sense resistor, this hardware approach precludes programmable brightness (without a soldering iron).
This probably isn’t the best approach - it’s just what came to mind…


Crux, as you seem to be knowledgeable about this would you mind posting your current circuit diagram?

It’s not a big surprise that something like this is not readily available as cheapo chinese driver board.
If you look around, there is barely any dedicated LED driver chip that can handle this kind ouf output current in this input voltage range.
With 2A, you could probably even get a nice synchronous and monolithic one, but 3A is tough. So the chinese manufacturer (or anyone else) cannot just simply slap the typical application schematics from the datasheet onto a pcb as they usually do. Someone actually has to invest some thought into it. (like adding external switches to an existing driver, or converting a buck/boost converter IC to constant current mode with a shunt resistor and an external error amp or something like that.)
17mm also is a quite small form factor. Might get tough with leaded components, might be fine with qfn packages etc.

If it is worth efficiency-wise depends on the converter topology (respectively its overall efficiency). Efficiency of the linear regulator rises while battery voltage approaches the LEDs forward voltage + the regulator drop voltage. Below that, output current will decrease. So it also depends strongly on the battery’s discharge curve.
If one could get a high efficiency synchronous converter, it would be quite some gain over an linear regulator. How much cannot be estimated without battery discharge curve.
But what you get is a more or less constant output over the whole battery charge. To me that would be enough to want one, even if overall efficiency is on par or less than linear driven. :slight_smile:

tps63020 is a really nice synchronous converter. Sadly it cannot deliver 3A output over the complete input voltage range. (look at the graphs on page 6 datasheet.)

Let’s list the main components we will need for this driver, the pieces that will take up space on our precious 17mm of 2-sided real estate:

  • MCU for control, modes, memory, low battery, etc.
  • Inductor (aka coil) for the boost converter.
  • N-channel MOSFET for the boost converter.
  • Blocking diode for the boost converter.
  • Boost controller (is this optional, see below)
  • Various passives (resistors, capacitors) and a diode or two.
  • Negative contact ring
  • Positive contact pad
  • LED+ and LED- pads

One way to save space would be to eliminate the boost controller. It is theoretically possible to directly control the MOSFET switch with an MCU.

  • Use a high frequency PWM output to adjust the drive.
  • Use an ADC input to monitor the drive current for the feedback loop.

I’ve implemented a boost converter using an MCU in the past. It’s really not that hard to do. The difficult task is keeping the output constant when the input power changes quickly. With a Li-Ion battery as the supply, this usually isn’t a big challenge. it will not be a $0.30 MCU anymore, more like $0.50 or $0.60 but it saves on the boost controller and precious board space.

I could draft up a rough schematic, and one of our resident PCB specialists can see if the parts will fit on the board.
Any takers? :wink:

The current in the inductor for a boost converter is greater than the emitter current. Finding a small enough inductor that won’t saturate at those currents is very doubtful. To reduce the size of the inductor you would need a VERY high switching freq… like over 1 MHz. Driving a FET gate at those freqs is inefficient and requires a few amps of instantaneous current.

I have done this too, but it comes with a high price. The control loop will be really slow. Might not be a problem for a LED lighting application tho.
The problem is that you lose the in-cycle current limiting an analog controller usually offers. This might end really really bad for the switching transistor unless you take one with very high current ratings, combined with an inductor so large the current cannot rise to a critical level at maximum duty cycle. (or you add external circuitry for peak current limiting).
This might be a little to much for a 17mm board.

Flashlight applications have a very well defined and very slowly varying load. That suits MCU based controllers very well. You can characterize the required PWM setting for a given current (and battery voltage) and use that as the initial condition for the control loop.

Texaspyro, dave_, all valid points. The coil would need to be 5A for a 3A driver, at least. Driving at 1MHz is not feasible for an MCU, so the coil size would have to increase. Direct driving a FET above 50kHz is not efficient at high currents.

I’m not too concerned about the speed of the control loop. For this application, running as fast as the ADC can go should be fine. The supply is nice and stable, and with hard limits on the PWM duty, it can avoid catastrophic failures in loop response.

Now, if we can find a way to get both the MCU and the boost controller on there, I’m all for it. I was just suggesting a possible alternative that might allow us to cram 9lbs into the 5lb bag instead of cramming all 10lbs. :slight_smile:

Thanks to everyone for your efforts. PLEASE dont give up in these discussions. You can make it happen! Also, feel free to invite others to this thread if you feel they might be able to contribute. You’re all off to a great start…

I somewhat doubt a converter of that power level will fit on a single sided board in that tiny area togehter with the µC. I probalby would build it on something like 0.5mm RF4 and give the controller it’s own board. But from what I read many people here dislike multi board drivers. :smiley:
I’m not so much worried about control loop instability but more about the current peaking within a cycle. A inductor that would physically fit into the little space at a current rating of ~5A would be in the range of maybe 5…10µH. Just quickly estimate the time constant of that inductor in the head. On time would have to be really short to avoid to high current. With adequate switching frequency, high resolution pwm and extensive testing/data collection it might work out tho. I really like digital control, but ultra fast analog control has it’s charme too. :wink:
I whish I had time to play around with that, but that sadly has to wait a few months before I get some more freetime. :frowning:

Current spikes are an issue, but remember, this is a boost converter. As long as we limit the maximum on time of the FET to no more than the theoretical saturation time of the coil at VinMax (say 4.3V), Anything else is just Direct driver through the coil, blocking diode and into the LED. Until Vin-losses drops enough to prevent direct drive (no FET switching) from delivering the current we want, the FET doesn’t turn on at all. Once we see current drop below the desired level, we start switching to boost things up where we want them again. This goes on until the battery reaches the low threshold….

OK, I just realized that I missed something when removing the boost controller. There’s no way to create modes without adding another MOSFET into the path. because a boost driver has the battery connected to the LED via the coil and diode, there needs to be a way to switch this with PWM in order to create modes. Even with the boost controller, the extra FET may still be necessary. Hmm…

Any chance a dual board setup can be used?

One would be a single sided board with the contacts and some components, and the second could be a double sided one.

I think there has been some significant development over at LPF, as most diodes require higher forward voltages that what a single li-ion can supply.

Here’s one for example, but it’s limited to about 2A:

Edit: The main reason why LPF doesn’t need 3A+ drivers (yet) is because laser diodes with the current technology can take only 1.8-2.5A at a Vf of 4.5V before dying. However, I’m sure we can get this working, and that would be awesome.

Sounds good. Don’t want it to let it burst into flames when someone tries out his new ultra low resistance cells. :smiley:

Why not use buck-boost/sepic/cuk topology and then implement modes through set point of the control loop?

A buck-boost would do it, but that also requires two FETs. No free lunch today. :frowning:
I’m not exactly a hardware designer expert, more of a firmware guy with peripheral knowledge of how hardware works (get the hardware working, then fix it in software :wink: ).

There is a single switch variant of the buck-boost topology. It inverts the output voltage, but that should not be a problem.
Hmm, firmware guy . . . you are not by any chance into DSpics and desperately want to implement synchronous rectification in SW? :stuck_out_tongue: