Laser Diode Power Conversion

W

Thread Starter

Wryverine

Hey hey....

I need to drive a fiber coupled 50mW, 520nm diode from a 12V DC source. I'll probably only need 20mW out the end of the fiber but want the capability to go higher depending on further coupling losses.

For the driver, I have to drop voltage to 6.5V-7V and then want to regulate the current from 140mA (minimum threshold) to 350mA to control the power. The specifications sheet calls out a max of 7.5V at 400mA.

I originally made a quick breadboard with two LM317 in series to drop the voltage and then control the current. While the circuit worked well on the DC power supply I was using, both regulators rapidly became quite toasty (duh)!

I'm looking to now redesign it as a pcb with a LT8606 (switching regulator) to drop voltage and put out 350mA max followed by a LM317 to regulate current. I currently have it designed with space for a 5W heat sink attached to the LM317.

Does this sound like a reasonable approach to drive this diode while minimizing heat output? The end result will most likely go into an aluminum/plastic bud box type closure.

Recap:
Power in: 12V DC
Power out: 6.5V @ 150-400mA
Board mounted in relatively small enclosed container.

https://imgur.com/a/L8XflN5 - Schematic
 
Considering your original circuit ie: 12Vdc in to 7.5Vdc out at 0.5A, that's only 2.25Watts dissipation in the LM317.
Fitting a small heat sink to the LM317 should keep the heating of the LM317 within acceptable limits.

Did you confirm that the Vout across the diode emitter was in fact 7.5v when the system was running?
 
Thank you for replying. The voltage was correct as was the current per a handheld meter.

I guess I should clarify something: they (lm317) never powered off due to reaching maximum temperatures, but I plan on using this for hours at a time in a small box so I was concerned about that being a problem.

Also, it looks like this laser diode comes with a photodiode (manufacturer has different labels for pins than LD, gnd, and PD) so I'm looking into a feedback loop for the current portion using an op-amp and a transistor now instead of the lm317.

I found a circuit using a LMV2011 and a transistor though I'm going to have to look more into that.
 
You can use free LTspice and a collection of models. The model collection includes laser models. The models have parameters that allow you to simulate direct voltage, the dependence of light power on the laser current, as well as photocurrents. I warn you that lasers spoil when the power is turned on and the power rattles! Therefore, it is worth to simulate the work of the driver and the laser in these modes, so as not to get upset.
https://forum.allaboutcircuits.com/...nents-models-of-ltspice-free-download.133690/
 
You can use free LTspice and a collection of models. The model collection includes laser models. The models have parameters that allow you to simulate direct voltage, the dependence of light power on the laser current, as well as photocurrents. I warn you that lasers spoil when the power is turned on and the power rattles! Therefore, it is worth to simulate the work of the driver and the laser in these modes, so as not to get upset.
https://forum.allaboutcircuits.com/...nents-models-of-ltspice-free-download.133690/
Thanks, I'll try to use LTspice and simulate this out first.
 
Here's a different approach. Take a look at Analog Devices/Linear Tech LT3092 spec sheet.
It's a precision current regulator which is ideal for driving LASER Diodes.
It can deliver several mA to 400mA. See spec sheet for paralleling to get higher currents.
This should minimize the complexity as the LT3092 is a 3 pin device. 40 volt max input, and a couple resistors
to set the desired current out. Or if you want adjustable output current a pot instead of one of the resistors.
The set current is 10uA so watch out for leakage current paths on your PCB. Check the App Notes.
If you have questions get back to me at [email protected]
 
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