XLamps - High V & High Amp

So I have an opportunity to venture into XLAMPs made by Cree. Search Mouser for this part number: 941-CXA20110P00J00E3 (in case the link doesn't work.)

I'm looking at 40VDC and 270mA. Ultimately I want to control these with an AVR, so there will be some mosfets in play here to flip the thing on and off. But here's the thing, how the hell do I get a steady 40VDC? Just about everything I find provides 48V. So I'm looking for suggestions from this group. I'm open to any of the following:

a) build an AC-DC converter/transformer to go from standard (US) mains AC to 40VDC b) purchase a ready made AC-DC converter/transformer that outputs 48VDC and build a circuit to drop that to 40VDC c) find and purchase a ready made AC-DC converter/transformer that outputs 40VDC

With each lamp consuming 270mA, I'm thinking some high Amps are in order here so I can string a few together. So something in the range of at least 4Amps. Higher ones allows me to put a couple of strings on the same line as opposed to having to buy more power supplies.

Let the creative juices flow.

Instead of driving the LED with a constant 40V you should focus on driving it at a constant 250ma (with a voltage higher than its forward voltage of 40V).

A google search for “LED driver” or “constant current” will lead you to lots of inexpensive options.

Duh! I keep forgetting that. You're right, it's constant current that I need, not the voltage (as long as it's over 40V, it'll be fine.) However, I don't want a driver that can drive a single one. I want to be able to string a bunch of them together and light them up. So does it make sense to get a high current driver and arrange the lamps in parallel to distribute the current, or get a driver for the 250 (or 270mA) and drive the LEDs in series?

Is there a pro/con to either of the setups? With series/parallel setups, what happens when one of them fails? With parallel, the rest remain lit, but does the current now change because one of them has failed? With series, like old xmas lights, I assume everything dies.

Or should I drive each one with its own driver?

The easy but inefficient way is to get a 48v regulated SMPS (there are lots to choose from, unlike 40v power supplies) and use a resistor in series with each chain. 30 ohms 3W should do.

Another possibility is to make each mosfet into a constant current circuit using a series resistor between source and ground and an NPN transistor (assuming an N-channel mosfet) connected with base and emitter across the resistor, and collector to mosfet gate to rob it of gate drive. In this case, the excess power of about 2.2W per LED chain will be dissipated in the mosfets, so they will require modest heatsinks.

The efficient way is to use PWM (which you may be doing anyway), max PWM ratio of 40/48 assuming a 48v supply, and use an inductor and diode to smooth the current. Increase the PWM frequency to allow you to use a reasonably small inductor.

dc42: The efficient way is to use PWM (which you may be doing anyway), max PWM ratio of 40/48 assuming a 48v supply, and use an inductor and diode to smooth the current. Increase the PWM frequency to allow you to use a reasonably small inductor.

This is still on a 48V constant current supply?

All my suggestions were ways to use a standard 48v regulated (i.e. constant voltage) supply, which is a readily-available component. The term "48v constant current supply" doesn't make sense. However, the description "270mA constant current supply, max output voltage 48V" makes sense in the same way that "48v constant voltage supply, maximum output current 3A" does.

Yeah, I used the wrong term there. I meant a 48V supply that caps out at a certain current. Pretty much what you said. Technically, it shouldn’t matter what that max current is because I can regulate that with resistors. What I have to figure out is how to wire these suckers up with a mosfet so I can control them with an Arduino (which also means having to add a voltage regulator to drop that 48V down to <12V for the Arduino itself.) I’m thinking of just getting the enclosures that go with them and forego soldering the pads. The enclosures can then also hold a lens cap over it.

Ok, I must not be looking for the right thing here. Looking for AC-DC PSU's with >48VDC with a constant current I noticed two thing: a lot of them only output 700mA, and a few go as high as 1.3Amps. That's it. And all of them start at about $35 and up, some as high as $190 ...

So yeah, I must not be searching for the right thing here. "Led drivers" or "constant current", neither got me very far.

I think the only really low-cost power supplies rated 100W+ are the sort that PCs use. What current rating do you need? I did a search for 48V @ 2A or more from one of my suppliers, and the cheapest I found was http://uk.farnell.com/xp-power/vcs100us48/psu-low-cost-cased-100w-48v-2-08a/dp/1821472. If you want a constant current supply, you'll pay a lot more, so I suggest you use a 48v constant voltage supply and one of the techniques I suggested earlier.

Not sure if its in your price range but an ebay search for led driver 48v bring up a nice adjustable driver for 27$ or the cheaper lm317hv and A mosfet and make your own driver

There are chips that are constant-current switch-mode converters for driving strings of LEDs, some boost, some buck:

LT3591 is rated at 40V, 0.3A and requires inductor and caps, for instance, but only in DFN (surface mount) package MAX16822AASA+ is available in SOIC package (possible to hand-solder) but is only a buck (step-down) siwtching regulator (can take inputs up to 63V).

@dc42: Yeah, that's pretty much what I'm seeing on my vendors as well. The 700mA ones tend to be in the low 30s to 90s depending on brand and whether it is dimmable or not, with the higher ones being quite a bit higher.

@winner10920: Personally, I don't know how well I would trust something like that from eBay, especially if it's something that's coming from say China. :)

@MarkT: you missed the part about it needing to be an AC-DC converter ...

I've used some 5$ 1w dimmable led drivers from ebay with no faults on the 3 im using id hope the 30$ ones would be as nice Honestly 99% of the time what you get from ebay works fine, once I got a bad module worth 3$ and once they sent me the wrong color led, other than that id say it may be worth a shot, maybe atleast 1 to try

Often, the eBay sellers just sell from the same source as sells to the higher-priced brand name integrators.
Unfortunately, sometimes they sell batches that quality control has rejected.
Even more unfortunately, it’s often the case that even the brand name integrators will just buy from the cheapest bidder, and even if you pay more, you end up with cheap stuff that doesn’t meet spec, and burns out ahead of time.
Because everyone buys on price, it’s almost impossible to buy on quality. Race to the bottom!

When it comes to AC/DC power supplies, you may need to roll your own using a transformer and a regulator of some sort. You can buy board-mount power supplies, but when you want uncommon voltages or high power, it will start to cost you. The 48V “board mount” ones at Digi-Key are all < 700 mA at reasonable prices, or a 1.25A supply for $100.
This guy has a “programmable current limit” if that’s what you want:
http://search.digikey.com/us/en/products/LT1074HVCT%23PBF/LT1074HVCT%23PBF-ND/889162
http://cds.linear.com/docs/Datasheet/1074fds.pdf
Available in through-hole versions. And does up to 5A! (Also, cheaper version does 2A)

Add a transformer, a rectifier bridge, an inductor, some capacitors, resistors, and schottky diodes, and lay it out very carefully on your circuit board, and you’re good to go…

Another option is to use two 24V regulators, one in “negative” mode, and creating a 48V differential that way. If you set it up to be current limiting, though, those two regulators may end up oscillating against each other if you’re not careful.

There’s a reason these things cost money :slight_smile:

Yeah, I’m starting to realize no matter what, ultimately I’m looking at spending some serious dough. Ok, slightly off track … I’m putting these LEDs on the back burner for the time being. The next set of testers I have are closer to voltages we’re more used to fiddling with, 3V forward voltage, 350mA.

If I get a driver capable of 9V/3.3A (constant voltage), I can then power an Arduino, while also providing power to the LEDs themselves (so they don’t pull current from the Arduino pin). I’ll use a resistor to control the current to the LED, and a mosfet to turn it on and off with the Arduino. Yes?

Two questions:

  • If I want to drive the LEDs at 300mA, and I want to have a pair per “string”, would the attached schematic work, both as a single pair, as well as an expanded set of pairs?

  • Can I use a PWM signal to the mosfet to fade the LEDs? Any particular detail I need to be aware of when choosing a mosfet (other than voltage and amount of current)?

(edited to add the part number for the mosfet)

Yes, on the general set-up. Yes, on the using PWM with a MOSFET. (There's some gate switch time in the MOSFET, but it's measured in something like picoseconds to unlikely to matter in this application :-) Yes, the diagram looks fine. Those resistors may get hot :-) You want to find a MOSFET with a low Rds/on when driven by 5V into the gate -- 50 milliOhms or better is quite achievable.

Thanks jwatte. Resistors can be changed to higher wattage of course. The simple online calculator I used said they would be dissipating 900mW so that's why I picked a 1W resistor. But I can certainly go higher to a 1.5W perhaps?

This little guy (NTD5867NLT4G) is a 60V/20A, 39 mOhms N-Ch. About 20 cents more than the one I currently have in the circuit, which is a 64 mOhms one.

So, what does the RDS value mean or what does it do?

RDS is the resistance between drain and source -- i e, how much of a "resistor" does the transistor look like. This affects how much of full power you get out, which is important for amplifiers and motors and stuff. But more importantly it affects how much heat is dissipated by the transistor, and thus how hard you can push it before you need heat sinks. Half the RDS means half the heat developed at same current.

Given that you burn off one watt per channel in resistors already, waste in the transistor might not be your biggest concern, though :-)

I don't think I have much of a choice there. :) Thanks for the explanation though.