Best way to modulate high-power LED at a few MHz - Page 1
Forum section: electronics / projects

author: mark03
author_rank: Frequent Contributor
timestamp: April 22, 2017, 01:47:17 am
content: I'm looking to  modulate a high-power red LED (Cree XP-E) as part of a fun experiment in free-space optical comms. The datasheet contains no information on temporal characteristics--no junction capacitance, no rise/fall times--but from what I've gleaned elsewhere, I believe the modulation bandwidth is higher than the 1-2 MHz I'm interested in.The question is how to do it, coming from an MCU pin. Some options I can think of are:1) NPN transistor switch2) gate driver IC3) Like #2, but apply DC bias to ~ 10% of full output, then capacitively couple the drive waveformI guess the disadvantage of #1 is you'd really like to get a bolus of charge (higher than the rated LED current) at each transition, to quickly charge/discharge the LED capacitance.Any other suggestions?

author: SeanB
author_rank: Super Contributor
timestamp: April 22, 2017, 12:18:22 pm
content: How about a constant current supply ( probably will be wildly inefficient, as it likely will be running off a 12V rail) with the LED connected between the output and ground, and your high speed MOSFET switch doing a shunt across the die. That way minimal stored charge in the LED die capacitance, with it being actively removed, fast turn off and fast turn on limited by the turn off time of the mosfet, which can be increased with correct gate drive, plus it will only see 4V across it when off which helps, and your current source just has to still be compliant when switching between having those 2 loads on it.Should go up to 2MHz with attention to layout and having short leads for the current loop around the die, and really good decoupling of the supply. Your constant current source and the mosfet would have to be mounted on the back side of the LED heatsink, so the leads are short, and you will need an extra heatsink for that as well, though the shorting switch will dissipate very little heat. Worst case as a test you can just use a 12V supply, with a power resistor as current source, and use a resistor of 12R to 27R 20W as a proof of concept device.

author: Marco
author_rank: Super Contributor
timestamp: April 22, 2017, 05:08:53 pm
content: Quote from: mark03 on April 22, 2017, 01:47:17 amBad idea, pushing them into saturation makes them slow. With 5V logic you can use it as emitter follower instead though, with the LED and a current limiting resistor on the emitter, just need a high current gain transistor which can handle the >0.6W it would be burning at 1A.

author: mark03
author_rank: Frequent Contributor
timestamp: April 23, 2017, 07:03:41 am
content: Thanks everyone. I'll give blueskull's circuit a try with a BSS214N. If driving from the MCU pin is too slow I can run through a few paralleled 74LVC inverters.Is the reverse diode there just to protect the FET?SeanB: I have seen the MOSFET shunt idea before. This project will be battery powered, though, so I'd like to not waste a lot of power. Best channel capacity is usually achieved with high-order pulse-position modulation, where the LED is only turned on for one or two time slots in each frame of N slots, where N might be 16 or even higher. So the average current will be quite low, even with one-amp drive.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 23, 2017, 07:57:54 am
content: Quote from: blueskull on April 23, 2017, 07:49:39 amThis would be highly recommended from the functional point of view, too, since while the energy stored in the inductance is being burned, the LED is still lit during that time."A few" MHz is not huge, but I'd still go for really minimizing the loop inductance, like really integrate the whole circuit within a 2x2 cm area with a solid ground plane underneath. No wires to the LED!

author: tggzzz
author_rank: Super Contributor
timestamp: April 23, 2017, 08:26:10 am
content: Quote from: mark03 on April 22, 2017, 01:47:17 amMake sure the dI/dt doesn't cause so much EMI that you disrupt other people and other services. Get that wrong and some serious people will come knocking on your door

author: Kleinstein
author_rank: Super Contributor
timestamp: April 23, 2017, 08:45:33 am
content: It could be important to look at the color of the LED: White LEDs have a phosphor for the yellow part, and that might be slower than the blue led behind it. Anyway white is not the best solution for electric detectors. White and blue LEDs might also have a problem with efficiency at high pulse currents - in some blue/white LEDs the efficiency can go down quite a lot.Si based detectors are usually more sensitive to IR or red light. The NIR diodes are also quite efficient and usually can cope with pulsed operation much better than white ones. However the speed might be limited with some types (especially 950 nm).Optics can do a lot for good sensitivity, just like a good antenna in radio reception.There is no need to drive the LED very fast - it only makes EMI more of a problem. So even the gate diver chips might be an the fast side, but they are small at least. For EMI don't forget local decoupling and inductive filtering in the supply.

author: Marco
author_rank: Super Contributor
timestamp: April 23, 2017, 01:24:35 pm
content: Noise isn't too bad though it seems, there's papers on doing Gbps communication with a white LED (with around 20 MHz bandwidth). Simply putting a blue filter on the photodiode also helped.