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

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

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:

@Crux
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:

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.