Building a Buck

[Emyr]

I'm building a bike light using some ex-laptop 18650 li-ion cells with a bath waste trap as an enclosure.

I've just ordered three Seoul P4s (SSC's brochureware here)so I have two spare. Only ordered one optic as I have plans for the other two if I don't kill them!

Most of the homebrew bikelight designs use a Buckpuck (expensive, lazy) or a linear converter eg LM7085 (inefficient) but I want to make the most of the 6 good cells I pulled from the laptop.

I've read the wiki article about buck converters and the diagram looks simple enough but I'm struggling with the formulae.

Input:
Looking at typical Li-ion discharge graphs the input will range from 4.2v down to a self-imposed limit of 3.2v (conveniently about the same as the P4's "test voltage").

Output
350mA max, although I'd accept 320mA to make everything last longer... LEDs require constant current, voltage variable to support this current, up to a maximum of 4v From what I've read about LED longevity I should try to implement soft-start to avoid high inrush current peaks but I can worry about that when I'm running out of spare LEDs...

I haven't got much information beyond this, so I don't know where I'd begin trying to solve this. Theoretically with healthy cells I'd be looking at (6 x 2000mAh) / 350mA = 34 hours, and 350/2000 = 1/6C, so ideal for maximizing capacity! I think I have space for at least 10 in this enclosure...

[IanB]

An LED buck driver is basically a DC-DC step-down converter with the added twist that the output is current regulated rather than voltage regulated.

Speaking as someone with no experience in building one (ha!  ), I'd suggest you start by finding a good DC-DC converter chip as the core and then follow the manufacturer's data sheet and application notes to design your circuit around it. If you are lucky you will find an application example with a current limited or current regulated output that you can adapt to suit.

[IanB]

By the way, be careful of bare lithium ion cells like that--it's easy to get a "vent with flame" incident if things go wrong. You should give some consideration to cell protection to guard against over voltage, under voltage and external short circuits. And of course make sure you charge them with a proper lithium ion charger.

[lowimpedance]

Going along the same line as IanB, I would suggest a look at  the ' ON semiconductor ' web site which has quite a lot of design resources for LED lighting.

[Psi]

If you have a good look you might be able to find an IC that does brightness control as well.

Best case the IC will accepts an analog input and you will just need a pot.
Worst case the IC will require a PWM input, so you'd need to make up a 555 timer to generate the signal from a pot.

[mobbarley]

Unless you plan on getting a pcb fab'd you'd probably be better of buying a good led driver - Taskled make lots of good products that you can integrate - or if you want cheap get one from deal extreme - they are cheaper (and some probably perform better) that you can make at home.

On the other hand if you want to design and build a driver - I'd go straight for national's web bench LED calculator and see what it recommends as a starting point - then I'd check out linear tech's website to see what LED drivers they have available.

[Emyr]

Thanks for the replies guys :-)

There's a fabLab here in manchester so I want to design a circular PCB to maximise the board surface I can fit in the enclosure. I'll use surface copperon one side with linking wires on the back if I need any.

Don't all these online part-calculators only work for ICs and Linear transformers? Part of my reason for wanting a buck was to avoid wasting heat inside the enclosure and because it seemed I should be able to build it using discrete through-hole components.

[IanB]

Normally you will want to build a buck regulator (i.e. a switching regulator) around an IC. Unless you are an expert circuit designer, it is unlikely you can make a discrete circuit that does better than a purpose designed IC. To avoid wasting heat inside the enclosure you want your circuit to be as efficient as possible. If you follow the recommendations on the data sheet, the IC circuit will typically be as efficient as you will ever get.

[mobbarley]

Thanks for the replies guys :-)

There's a fabLab here in manchester so I want to design a circular PCB to maximise the board surface I can fit in the enclosure. I'll use surface copperon one side with linking wires on the back if I need any.

Don't all these online part-calculators only work for ICs and Linear transformers? Part of my reason for wanting a buck was to avoid wasting heat inside the enclosure and because it seemed I should be able to build it using discrete through-hole components.

The LED webbench tool will do buck regulators