High Power LED Driver Circuits

Step 5 $witching regulators

Switching regulators, aka "DC-to-DC", "buck" or "boost" converters, are the fancy way to power an LED. they do it all, but they are pricey. what is it they "do" exactly? the switching regulator can either step-down ("buck") or step-up ("boost") the power supply input voltage to the exact voltage needed to power the LED's. unlike a resistor it constantly monitors the LED current and adapts to keep it constant. It does all this with 80-95% power efficiency, no matter how much the step-down or step-up is.

Pros:
- consistent LED performance for a wide range of LED's and power supply
- high efficiency, usually 80-90% for boost converters and 90-95% for buck converters
- can power LED's from both lower or higher voltage supplies (step-up or step-down)
- some units can adjust LED brightness
- packaged units designed for power-LED's are available & easy to use

Cons:
- complex and expensive: typically about $20 for a packaged unit.
- making your own requires several parts and electrical engineering skillz.

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One off-the-shelf device designed specially for power-led's is the Buckpuck from LED Dynamics. I used one of these in my power-led headlamp project and was quite happy with it. these devices are available from most of the LED web stores.

Comments

[andybuda]

i want to make a few leds that get charged up in the day and switch on at night ... i need a solar cell photo cell, relay switch and rechargable battries any ideas how to connect the thing up its to light up a plant at night and charge by day...

[ac-dc]

While it may seem pedantic, this description of buck or boost regulators is wrong. They don't constantly monitor the LED current, they don't monitor current at all. Buck and boost mean voltage change - nothing more. Current regulators use a different kind of feedback than boost or buck, generally they sense a change in voltage through a very low ohm feedback resistor.

As for the cons, today they are much less than $20 for the current regulating type, see websites like http://www.kaidomain.com or http://www.dealextreme.com
Frankly at their low prices (typically between $1.50 to $5 per board delivered) it makes little sense to do anything else but buy the regulator boards from them. Only major issue is the most economical regulator boards are designed for flashlights so if the application needs board isolation you have to devise a method (like epoxy to attach to a mounting frame, and to solder on the supply leads where the battery and flashlight body contacts are). I have used several of these and they work very well for the price. I see no reason to spend $20 on a buckpuck, it's highway robbery these days opposed to the past when shoppers were more limited in what was available.

[doctek]

Looking at the web sites you mention, I don't see any LED regulators except some for LED flashlights. These seem to be only for CREE LEDs and then only 1 to 3 of them. Can you explain 1) Am I just not finding more "general purpose" LED regulators? 2) Is it possible to use the regulators on the sites in a more general fashion - that is to drive (for example) 30+ 100mA leds? 3) If that's possible, perhaps you could explain. Thanks.

[ac-dc]

1) Yes, dirt cheap flashlight regulators. In various voltages to suit the need. Surely if you can implement the rest you can solder a positive and negative lead to the bottom of the circuit board, keeping in mind that based on how a battery is designed, the center is positive. I built my bike headlights using a couple and they work great, you can affix them using epoxy or I just slipped some heatshrink tubing over them.

They aren't just for Cree LEDs, they are current sensing based on their individual specs so they regulate to that spec'd current level. Why would you use 30+ less efficient LEDs? Are you sure they are 100mA? The small 5mm encapsulated type are generally 100mW, which is 20mA conservatively or about 30mA with good heatsinking of the leads to the PCB copper.

You have to do some math to determine the logical arrangement as well as consider the input voltage. You give none of these so it would be excessively long postings to cover every scenario. I will give basics instead.

1) Determine your input voltage, keeping in mind the max the regulator can handle (same with any regulator actually).
2) Determine the voltage drop of the regulator. For example if your input is 8V and the regulator drops 1V, you have 7V out maxium.
3) Determine forward voltage of the LEDs. Suppose it's 3.4V @ 300mA. You can do the math to see 7V=n*3.4V, n=2.06
4) Rounded down 2.06 is two, you could put two of the example LEDs in series. That means you would have 15 parallel series of 2 LED to arrive at 30 total LEDs, except you claim 30 LEDs at 100mA each which is 3 amps. This is one area where I wonder if you have 20-30mA LED, or if buying some new why you would buy less efficient 100mA LED instead of 350, 700mA or more LEDs. Since the regulators linked were about 1A, with 100mA LEDs you'd need three boards and splitting up the LEDs, 15/3 = 5 parallel series of 2 LED per board.
5) LEDs don't have exactly the same forward voltage as spec'd, it varies a little so with your 2 x 3.4V series you have 6.8V forward from 7V and 100mA drive current so (7-6.8)/R = 100mA.
6) R = 0.2V/0.1A, you'd put a 2 Ohm resistor in series with each series of 2 LEDs, so you'd need 15, 2 Ohm resistors to make it work in this example.

In summary, yes there are many ways to configure LED arrays you just have to do the math to see how you need to go to get from where you are starting to where you need to be. With some combinations and input voltage options the flashlight regulators may not be ideal, if your Resistor value goes too high it becomes more lossy, but on the other hand a cheap way to lose some current through a handful of resistors may not be a problem if the power source is not a low capacity battery.

Doctek I think where's your stuck is on the basics of combining LEDs in series and parallel combinations and calculating out the parameters. Beyond examples you need to be more clear about defining the project requirements whether they be max brightness, minimal size, max efficiency, minimal cost, max resistance to extremes of temperature, max lifespan, max battery life or some combination of these. Once you have clearly defined your goal then choosing parts and adding complimentary circuits to reach the goal will become a more clear process.

I suggest looking at some of the LED calculators found via Google search and doing some more basic circuits before trying to tackle something more advanced, costly and time consuming.

[doctek]

Thanks for the thorough, thoughtful, and helpful response, ac-dc. It is greatly appreciated. Here's some more info on what I'm doing and the LEDs I'm working with. What I'm trying to make is an "IR Spotlight" for night time videography. As suggested in this forum, I found BG Micro to be a good source and bought a kit having 36 LEDs and a round circuit board. The LEDs are BG Micro part number LED 1101. They're rated 1.5V, 100mA, 850nm, 20 degree. I've built the little circuit and it works pretty well - maybe not quite as powerful as I'd hoped, but I'm still experimenting. So far I've powered it with a nominal 16V, 700mA wall wart. The LEDs are wired in 4 rows of 9 LEDs plus a 100 ohm resistor in each row. My goal is to have this totally battery powered, with eventual solar charging of the batteries. The camera I'm using has an RF link. I'll use a motion detector and a AT-Tiny to only turn on the camera when there's motion, and only turn on the IR LEDs if it's dark. So having an efficient power system is important. Keeping it cheap just adds to the challenge. After doing a bit of Googling, I'm checking into the LM3410; but if there's something already out there that gets the job done, I'm open. Thus my interest in the chargers you mentioned. I just want to know there limits. The two web sites you mention are a bit thin on specs. Thanks again for your help.

[ac-dc]

Presuming you want to use that circuit board the flashlight requlators aren't suitable since a series of nine of those (assuming BGMicro spec is correct) is minimum 13.5V out. The LM3410 does look like an option, though with 1500mA out limit you'd use more than one, the math works out nicely that you can use one LM3410 driver circuit per each series/row of 9, and not need that 100 Ohm resistor since you can set the LM3410 to the output current you want (ie 100mA). Since LM3410 has max input of 5.5V, getting good runtime from batteries would require using fairly high capacity cells, it's unfortunate it wasn't a higher input so a series of 2 Li-Ion could be used. Therefore I would think about using 4 x C cell (roughly 4000mAH) NiMH cells as a pack, and it's more forgiving of unregulated charging than Li-Ion would be w/o adding separate charging circuit. You can do the math on that to determine what runtime you will have, or if less runtime is needed then perhaps AA NiMH to save size and weight but think about how much power the circuit uses, it's going to require a fairly large, expensive solar panel and a bright day to charge a pack in a reasonable amount of time. I always find solar panel specs are crap, you never get even close to the peak rating in normal use so to avoid frustration (or maybe I just don't know enough about solar panel calculations) it's better to assume you won't get even 1/3rd rated power and overbuild the panel array. Frankly I think you should avoid solar charging unless you really, really need it, and grab some random wall wart limited to a low current (C/15 or so) trickle charge if that is fast enough for your needs, or of course you can built yet another more elaborate charging circuit for that too, but the time, cost and size keeps going up.

[dan]

A buck/boost LED driver regulates current. we are not talking about voltage-regulating buck/boost device since that requires using additional circuitry such as one of my circuits shown in this project. In the year since I wrote this article buck/boost LED drivers have come down in price.