I have come across some very high powered LEDs.
Datasheet for LEDs:
http://www.cree.com/led-components-and-modules/products/xlamp/arrays-nondirectional/xlamp-cxa2011
For these high powered LEDs, how can I make a driver circuit that will be energy efficient, or is there a driver for high powered LEDs that I'm just not finding?
The LEDs will eventually be used in a grow light for an aquaponics system, so efficiency is essential.
For best overall LED efficiency (which is your stated aim) you want high efficiency LEDs plus high efficiency drivers. There are many high power LED modules available, but they in many cases use LEDs that are less efficient than the best available. No amount of efficient driving of a very low efficiency LED will make up for its low efficiency.
You mention 0.8A x 40V (which will use multiple LED dies or separate LEDs) = 32 Watts. This input power could be obtained with about 5 to 8 of the top available LEDs and it will often be better to use the best available if top efficiency is wanted.
It will probably be most cost-effective to buy a commercially available LED driver rather than making your own, looking for good efficiency. ebay lists a vast quantity of possible candidates
You also need to specify your energy source. Is this 110 VAC mains, or a 12 Volt battery or?
The standard way of driving LEDs correctly and at maximum efficiency is straightforward (fortunately).
Determine required LED operating current.
From data sheets, determine maximum voltage drop that LED will have at desired current.
Provide a switched mode constant current source that will produce the required current at at least the maximum possible voltage.
Extra points:
Minimize LED temperature with cooling
Operating an LED at below its maximum current ratings will result in somewhat increased efficiency - an increase of perhaps 10% to 20% more light out per Watt-input, at 20% of full power-ratings compared to full power.
LED lifetimes increase with decreasing temperature and with decreasing current. While more current will usually result in more heat produced, these effects are independent of each other.
To get an LED driver of essentially any desired rated current, use of an external MOSFET of suitable rating, driven by a controller that fits, will allow whatever current the MOSFET can handle, which can be very large if desired.
For very best efficiencies, use "synchronous rectification" - this replaces rectifier diodes with switched MOSFETs and gives lower losses.
Efficiencies around 85% from a low voltage DC source to an LED string will be achievable -- over 90% in selected cases and with care.
Comment on the above and we can provide better information.
The cost of consumer commodity PSU's for PC's range from cheap and dirty $50/500W to 1200W @ $300 Corsair Pro, so ranging from $100/kW to $250/kW. But LED approved drivers & PSU's tend to cost much more than this.
If you find a cheaper complete power solution for LED Engines, let me know.
I mount these under aluminum 0.5"x1" extruded end strips uses for tiles for elimination of LED glare and additional heat-spreading and the mount under exterior hand or deck rails.
Moisture protection is added and essential for external use.
To use this for powering LED's depends on your expertise in matching an array of LEDs to the V-I operating points of the ULC. If you can buy from LEDs from the same batch, they V-I curve will be matched, but Disti's refuse to do this, so it takes purchasing skill, otherwise you will have issues.
Basically, it's all about having good low thermal resistance and raising circuit ESR slightly above nominal ESR characteristics with same batch sourcing on LEDs to achieve low cost reliable operation. You need to understand open circuit voltage a rise is expected with no load by the load regulation or ESR of the charger. 2% typ. or 100mΩ for 100W PS on long cables.
Now let's examine @Mike's desire to use Cree's 45W Xlamp cXa2011 LED
All PN diodes have a negative temperature coefficient, k (Shockley Effect) or voltage drop due to temperature rise. The fear with designers is with CV, is "thermal runaway" so they only consider constant current drivers whereas if you have tight control of a batch and sufficient ESR with low Tja, it is stable. When ambient & junction gets hotter, the LED's internal junction voltage drops demanding more current from the voltage difference across the ESR inside.
I'll show how to overcome this effect on the XLamp and in another article show how to measure junction temperature using the LED itself as a thermometer with the V-T curve which is standard in hospitals now.
Let's choose 2 °C/W as heatsink + interface starting point, so Rja = 0.4+2 = 2.4°C/W
- Note: that a decent CPU heatsink is ~ 0.2°C/W for a forced air CPU heatsink ) and an excellent one is <<0.1 °C/W.
V forward = 40V typ, 48V max @ 270mA this is a wide range from batch to batch - So you need a good source LED engines from same batch or ask me.
Thermal equilibrium exists where there is an intersect between the linear temperature with thermal resistance and the quadratic effects of If^2*Rs) If ambient is too high with high thermal resistance, runaway can occur. It can be prevented with low Thermal resistance and adding ESR in the cable.
Answering any or all of the following questions would allow us to better answer your question:
First thing -- Are you sure that these are the proper LEDs for growing your plants?
Most of the LEDs listed are binned in 2 and 4 McAdams steps. The 3000K parts have a CRI of 90. These are meant for indoor lighting where color matching is important -- display case, art gallery, merchandise display, etc. You are probably paying for photometric performance that will not matter to a plant.
Most plant lights that I have seen use monochromatic LEDs -- primarily blue and red. Some will add in a white. The systems vary the amount of red/blue throughout the plants daily growth cycle to improve production. All the requests I have had are for red/blue.
If you really do need white, I would use discrete LEDs. You could use OSRAM Oslon squares, Nichia 219B or Cree XML (XPG or XBD). If you can increase your CCT to 4000K, 5000K, 6000K and reduce your CRI to 60 or 70, you can improve your efficacy.
If the driver is going to be in a damp or wet environment I would use an IP66 (or better) rated driver to ensure reliability. If you are only going to build a couple of systems, I would spend the money and purchase drivers. The driver cost is low compared to these LEDs :)
