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

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:

DSPics? I just threw up a little when I read that :Sp
Never was a Microchip fan, I have used them enough (16 and 18 series) but never really appealed to me.
These days I’m a little biased though, current employment and all :wink:

Sorry that I disturbed your stomach.
I’m not a Microchip fan either. But some of the DSPics have dedicated hardware for smps. The only others I know with those features are within the Ti c2000 family. That’s even more exotic, at least to me. :smiley: Synchronous rectification is so nice I would even touch a Microchip part for that. :stuck_out_tongue:

Are there constant current boost drivers that can deliver 3A from 2xAA?

Last time I looked (several years ago) you had to use a high current fixed voltage boost regulator, sense resistor, and an op amp to make it work as a constant current source. You multiplied the voltage from the sense resistor with the op amp and then used that as the feedback voltage for the regulator.

Maybe I’m missing the target here? 3-6V 3A to power multiple LEDs in series or what? 3-6V to power a single LED with 3A needs a buck/boost.

If possible, include compatibility for 2 & 3 NiMH/alkaline and you'll really have something special here. 3 NiMH shouldn't be a problem, and even if it has reduced output with only 2 NiMH, that'll still be far ahead of what's available now.

I’m not aware of any. Most capable of that current have an undervoltage lockout of at least 2.8…3V.
But one could convert a voltage regulator in the way you described. Would probably fit on an 16mm veroboard cutout if you have a calm soldering hand. :wink:

Yes, perfectly right, the driver needs buck/boost capabilities. (but hasn’t to be of the buck-boost topology)

This one from Fasttech has the specs to do 3A, but it’s been out of stock since January. I’d like to get one, just to see if it can do it. I suspect that it can only do 3A when running on a full charged Li-Ion. Since it is output voltage adjustable, there has the possibility to run two emitters in series.
Too bad it looks like they will probably not get any more.

Relic, I just submitted a request to FT for the driver you mentioned.

Its been awfully quiet over here. Does anyone have any more ideas or strategies? Please dont give up trying.

What about that 1A boost driver that was looking to scale to 3A in a future iteration?

Hey, who’s talking about giving up? :wink:
I jotted down a few ideas I want to try out on my ‘when I finally got some freetime’ list, but unfortunately I will not be able to build anything within the next 6…8 weeks.
If this is not done by others in the meantime, I will happily contribute what I can after that time. :wink:

So, here is the circuit I'm using in a custom light. This isn't intended to be the solution to this thread - I was just asked to upload it...

Yes it's complicated, and may not PWM well, but it does work for my intended application. Which as I stated earlier is 2amps maximum.

Feel free to comment and question!

A HUGE thanks goes out to all the intellects for your energetic participation in trying to come up with a solution. Your efforts are imensly appreciated. I think the need and desire for such a driver has been well established. Regardless of who or where the driver is manufactured and sold, I believe it will sell in high quantity if it can remain affordable. While we have an unreliable claim of a possible driver larger than 17mm being able to provide good boost results, (for obvious reasons) the 17mm target size still remains the goal until all other options have been exhausted.

Aside from the huge physical & intellectual challenges, I also realize that this isnt something that can quickly or inexpensively be thrown together and tested. The hurdles are enormous, but yet we have members that still arent willing to give up trying. THANK YOU!!

Please, if there are others that have more ideas to add to the discussion, please feel free to join in. If you wish to keep your ideas private for a possible joint venture, please do PM others with your plans and ideas if you think it will help. My initial thoughts about surrendering a design to a chinese manufacturer for production was simply in the interest of keeping it affordable to the masses. Looking ahead: if anyone can have these made by a preferred local vendor at good rates, then more power to them… and for the rest of us who want to buy.

Please help keep this thread alive.

Great inputs PPtk!
I will point out that we are not talking about a high ratio boost driver though. This is one cell driving one emitter, with boost kicking in once the cell drops below Vf. Still not easy, but peak currents will not be as extreme as a high ratio boost.
In reality, I do believe the only way to do this will be a two-board solution (messy, costly), since the parts are probably not going to fit on one board. Even if some passives are moved to the battery side, the emitter side will be very crowded. This is the primary issue that Bill from Lightmalls had with the design. By reducing the drive to around 3.5A, it got a little better.