Buck or boost (bFlex or maxFLex)?

[TheSwede]

Hi,

I posted this question on the TaskLED forum but there is not much action over there so I'll give it a try here.

I'm planning to build two headlights with a modular approach.

The "modules" will consist of:
* A LED-lamp with three Cree XR-E LEDs (for MTB)
* A LED-lamp with two Cree XR-E LEDs (for running)
* A TaskLED driver (with a maxFlex OR a bFlex)
* A battery consisting of one (or two) 7,2V 3Ah RC-packs.

The idea is to be able to swap between the LED-lamps depending if I go biking or running (in the dark forrests here i Sweden). I also want to be able to reduce the size of the battery pack depending on the activity (Two RC-packs for MTB and one for running.)

So the setup for MTB/running would be:
MTB: 3 LEDs and two 7,2V 3Ah RC-packs
running: 2 LEDs and one 7,2V 3Ah RC-pack

My question is whether I should use a maxFlex or bFlex to drive the LEDs.

With a bFlex I will not reach high currents (> ~570 mA?) with the running setup.
With a maxFlex I will not be able to go lower than (~570 mA?) with the running setup.

The MTB setup should be alright in any case, right?
* maxFlex: The RC-packs in parallell, V_in = 7,2V I_in < 1,9A, V_LED < 12V, 350mA < I_LED < 1A
* bFlex: The RC-packs in series, V_in = 14,4V I_in < 0,75A, V_LED < 12V, 350mA < I_LED < 1A


Are there any differences regarding efficiency for the two drivers and the two setups mentioned? Or maybe other issues that I have missed?

/Anders - Greatful for comments

 

[iggs]

My understanding is that using a buck controller is more effiecient than a boost controller. It also has less potential to harm your batteries

as the voltage of the batteries drops to equal the forward voltage of the led's the bflex stops controlling the voltage and just lets it go straight through causing a nice gentle drop in the lights and battery

As the batteries empty a boost controller will just start working harder and harder quickly dragging the last drop of the life from the batteries. This can damage some types of batteries.

I'm using a maxflex for my 3 x cree q5 lights as I already had the 6 volt and 7.2 volt batteries. If I'd been starting from scratch I would have gone for 14.4volt batteries and a bflex

 

[gillestugan]

How did your light turn out? Just curious.
/Anders

 

[ifor powell]

Hi TheSwede,

Unfortunatly neither maxflex or bflex/nflex is suitable for the 7.2v and 2 led setup.... You either need more voltage at the batteries for a bflex/nflex (buck) setup or less for the maxflex (boost) setup. If you always used both batteries for 14.4v then the bflex/nflex would the the right option.

For my light weight 2 led light I use a minumum of 3 14500 cells and an nflex.

Ifor

 

[TheSwede]

Hi gillestugan,

A fellow Swede?

To disappoint you I haven't built anything yet.
But new designs come and go rather frequently... :thinking:

Right now a dual MC-E in a "Amoeba"/square housing is on my mind.
However, with my ability to get things going I'm not sure the light will be finished before the spring is here. :sigh:

ifor_powell > My "design" at the moment of the post was not optimal. I have now abandoned the "battery-voltage-close-to-LED-voltage" approach.

/Anders

 

[gillestugan]

Yes, but I can se you are THE Swede...
Regarding the batteries: You can still use the 7,2V together with a maxflex if you wire the MC-E dies in series. And it you put two MC-Es in parallell you will get max 500mA/led and a good efficiency. (and lot of light in a small package)

ELFA has very nice aluminum boxes you might want to have a look at. The walls are fairly thick, so heat transfer is good. http://www1.elfa.se/elfa~se_sv/go.jsp?s=se-sv&a=48C92A57A2FE453CE10000000A0A036A

Too bad the USD has become so expensive. Everything is 30% more expensive than 2 months ago.
/Anders