I'm designing a circuit to power a high-brightness UV LED. I'd like to safely drive it as close as possible to maximum irradiance, and I'm having trouble figuring out whether all the components are properly chosen to avoid damaging/stressing anything (especially the LED, which is relatively expensive).
A Raspberry PI will be used to provide the on/off logic. The PI is powered separately, but shares a common ground with the LED power system.
Ideally I'd like the LED to be driven directly by a N-FET, where the gate is hooked up to a raspberry pi GPIO pin:
simulate this circuit – Schematic created using CircuitLab
The first question I have is: am I reading the UV-LED spec (here) correctly?
- Forward Voltage: 3.45V
- Maximum Current: 1400mA
- Electrical Power (Max): 4830mW
The above three make sense, because 3.45V * 1400mA = 4830 mW. Then I see some peculiar items:
- LED Output Power: 1500mW (min), 1700mW (Typical)
- Test Current for Typical Power: 1400mA
How can the "Typical" LED output power be 1700mW when the test current for "typical" power is 1400mA?
Essentially, I'm not sure whether my resistor value should be calculated based on the 1400mA figure (5V-3.45V / 1400 mA = 1.11 Ohms) or whether I should be deriving a current from the 1700mW figure (1700mW = 3.45V X I; I = 492mA; R = 3.1 Ohms) and then using a larger resistor.
The second question I have is regarding the NFET. The NFET I have is rated for 1.7A and 30V (datasheet here). But the package is scarily tiny (SOT-23) and my gut feeling says I should ask before proceeding. I will be running this at 1.4A which is pretty close to the 1.7 rating. Will it work, and do I actually need to get a tiny heatsink for this NFET? Or is it better to get a beefier NFET?
Other than that, I'm making sure to get a hefty power resistor rated at 1-2 Watts, all wires will be 22 gauge, and all PCB traces will be nice and fat. Any other power-related gotchas I should be aware of? Power source details also listed below.
Thanks so much!
Datasheet for the NFET (1.7A / 30V):
https://www.fairchildsemi.com/datasheets/ND/NDS355AN.pdf
Datasheet for the UV LED:
https://www.thorlabs.com/drawings/ca01256cc12b40f8-35473EF6-5056-0103-7951A15FE4DCB58B/M405D2-SpecSheet.pdf
2A+ Power Board and battery:
https://www.adafruit.com/product/2465
https://www.adafruit.com/products/353
Best Answer
You probably want to consider the recommendation in the datasheet: "We recommend using Thorlabs’ DC2200 or LEDD1B LED current drivers"
If you still want to use voltage mode control, I would ask: do you really need to run it at full power?
VF is a typical number than can fluctuate with temperature and vary part to part. So if your application permits it, I would run at lower currents (maybe 700mA).
The LED output power number quoted is for light output power. Since the test current used to generate this output is stated as 1400mA you can estimate the efficiency of your LED and plan for the generated heat accordingly.
Your NFET is close to max rated spec. I would double the current rating if possible. If you don't then I would definitely use a heat sink with some thermal paste.
One last thing that occurs to me is to add a resistor in series with the gate of the FET. Since you will be running high currents (relative to the drive capability of the GPIO), you don't want any glitches in your LED or power supply to shove a ton of current into the GPIO. A 10K resistor should do.