Feeler: universal buck/boost converter/charger

[bikeNomad]

I've been working on a microcontroller-based converter/charger design recently and I
wanted to see if anyone might like to buy such a thing (this will heavily
influence the number I plan to construct).

What it does:

<ul type="square">[*] constant-current Luxeon driver
[*] (or) constant-voltage output for other loads (including incandescent lights)
[*] Li-ion or Ni charger (constant current or constant voltage) with 1% accuracy on voltage sensing
[*] undervoltage protection
[*] overcharge protection, including battery thermal sensing (required)
[*] overtemp sensing on LED (optional)
[*] step-up or step-down from battery to load (including the situation where the battery voltage crosses the load voltage)
[*] good to very good efficiency in either boost or buck modes (haven't finished simulating or testing this yet)
[*] step-up or step-down from charger input to battery (including charge inputs near to the battery voltage)
[*] small (but probably not tiny) size (I'm aiming at a 5x5cm board or smaller if I can)
[*] multiple brightness levels; either pushbutton or analog (i.e. trimpot) input (haven't decided yet)
[*] auto-power-off mode (timer or voltage)
[/list]

The first unit will have the following limits (unless I hear that there's a big interest in some other version):

<ul type="square">[*] 2.7V to 5.5V limits on battery voltage; that is, one of:
[*] 1 Li-ion cell (3.6 or 3.7V nominal; 4.1 or 4.2V charge)
[*] 3 or 4 NiMH (or NiCd) cells
[*] 3 alkaline cells (obviously, no recharging of these)
[*] output voltage limited to 5.5V max (i.e. no 5W Luxeons)
[*] charger input voltage limited to 6V max (i.e. you might blow a fuse beyond that)
[*] output and charge current up to 1.2A or so
[*] 1W (350mA) or 3W (1000mA) Luxeon output ranges
[/list]

Later units will eliminate the 5.5V upper limit on battery and output voltages, probably going to 10 or 12V max at first (it'll take more components to go to 20 or 30V).

Needless to say, I'm a bit wary of producing a Li-ion charger for the hobbyist market, since I can't control how these boards are used, and Li cells can experience, um, rapid disassembly including very hot bits of metal flying around the room.

So I might choose to limit the rechargeable battery choices to Ni types for the time being.

Anyway, which of these features are you most (or least) interested in? Any must-haves?

I've attached a poll. Please vote for the features you must have in this kind of a unit.

Thanks,
Ned Konz

 

[kakster]

This sounds very much like the "Magic Pill" ive been looking for. A single board to handle a wide range of LED/battery configurations. Good effiency would be nice too /ubbthreads/images/graemlins/smile.gif

 

[RussH]

Bikenomad, my incandescent uses are mostly 6v or 12v. On the other hand, I usually use NiMH or gell cells and the self regulation with those (and proper bulb choice) is usually good enough for me.
I'd really like a multi purpose, adaptable charging control, especially if it ends up too big to go in a flashlight. I really like the idea of auto power off. I don't really care for multiple brightness levels much, but I would like to see multiple outputs for charging purposes (same thing?). -RussH

 

[bikeNomad]

There are some problems with a universal external charger:

<ul type="square">[*] for Ni cells, it is very helpful to have a gas gauge to keep from under- or over-charging the cell (in addition to temperature and voltage sensing, of course).
[*] for batteries, you really want to have voltage and temperature sensing for each cell
[*] you need to have thermal sensing, and this needs to be coupled as closely as possible to each cell in the battery being charged. This is difficult if your cells are of different sizes and shapes, or if you have multiple-cell batteries.
[*] for Ni cells, you have to ensure that you have enough charge current to actually detect the end of charge. This is why fast solar charging of Ni cells is not done.
[*] you need a cutoff timer for Li charging, and you have no control over this if the user can change cells at will (this is one of several reasons why there are no bare Li cells sold to consumers)
[/list]

So separating the charger from the cell is not a great idea, especially for Li cells. Li batteries are much worse, since you have the problem of cell mismatch and potential explosion.