Constant current laser driver with modulation using switching DC supplies - Page 1
Forum section: electronics / projects

author: moonie223
author_rank: Contributor
timestamp: January 30, 2021, 05:02:34 am
content: Hello!I am trying to figure a way to make a set of RGB laser drivers for a laser scanner. I have a few working examples of using opamps and transistors as a modulating supply, but I would like to come up with a more efficient scheme, and switching regulators and mosfets fully switching seems like the only way. The three laser diodes I would like to use have pretty drastically different Vf, although they are all low current. The lower powered diodes focus better, I've got a pretty fast set of galvos and would nice, clean output for neat graphics. In the end if I had something that wasn't 90% heatsink I will be pleased!I did buy a RGB laser module off ebay which was supposed to have switching analog modulation drivers, but they leave a lot to be desired. The modulation voltage range is 0.3-1.5V instead of 0-5V. Scaling a 5V DAC signal to fit in the smaller voltage range does not increase the dimming fidelity, it's still ~5 distinct steps in output current. I can fix things somewhat with software, but trying to match colors is pretty much impossible because of the modulation steps. I believe it's using a basic LED switching regulator (MT7201 LED driver) with PWM dimming, the datasheet for the regulator isn't the greatest.A mix of all the colors I can manage, better than then 7 with TTL I guess!I do know of some switching constant current supplies, but none with modulation of the output current. I've spent the last few days trying to figure something out in simulations and have got close, but the bandwidth I can manage is abysmal (~1khz from a 1.4MHz regulator). I tried using a current sense amp for feedback which was summed with a trimmed offset voltage to match the regulator's feedback which is then subtracted from a modulation voltage and used as the feedback voltage, after a low pass filter.Anything I have tried with a comparator has been a pretty big failure, I am not sure how to sync the regulator and the comparator...I guess my question is, is what I'm trying to do even possible (switching DC regulator with ~100KHz 0-5V modulation) and if so, do you have any examples or pointers that might set me in the right direction?Thank you!

author: ajb
author_rank: Super Contributor
timestamp: January 30, 2021, 07:23:49 am
content: Your modulation bandwidth is going to be limited to a fraction of your converter switching frequency, because the the converter cannot respond to a change in command or feedback any faster than that. In practice, of course the response is going to be even slower than that for loop stability. You also have four op amps in your feedback path, which is going to make things harder on you.If you could get your switching frequency up to a few tens of MHz or something (GaN?), and tighten up your feedback loop, then maybe you could get something that would give you sufficient modulation bandwidth for light show use. I don't think there's anyone doing direct switching drivers for light show use for this reason. A much more realistic (and common) approach would be to use a switching converter to preregulate your supply down to something a bit above your diode Vf and then use a linear constant current stage to drive the diode. Low-side current sensing/driving will make this all a lot easier as well. A linear stage that only needs 2V of headroom is pretty straightforward, but you can do better with some care and careful transistor selection.

author: moonie223
author_rank: Contributor
timestamp: January 30, 2021, 08:32:42 am
content: Thank you for the reply.I guess it makes sense that you can't modulate faster than the switching frequency, I just thought 1+MHz switching regulator might be able to manage a tenth of that instead of a thousandth.I didn't bother looking for optimal opamps, or much of anything else. To be honest, kinda glad I got something set up that even works, even if it's just in a sim and it's not fast enough!I don't know of any high speed DC regulators, and trying to build one from exotic components ain't my cup of tea. Unless someone knows of any ways to link the actual switching of the regulator to the hysterics of the comparator I'm not currently using I doubt I'll be able to improve the bandwidth without lethal amounts of oscillation. I tried a few things, but nothing consistent. I guess two or three switching regulators probably could be made smaller than all these opamps, and that's easy enough I might not even mess it up...http://www.die4laser.com/dvd-rec/Die4Drive.htmI can use something like this instead, although I forget if each individual diode's isolation prevents that. Might try and mess with some component selection and maybe try and learn to import spice models into ltspice and see what I can come up with. At least the sims run faster with the linear stuff...Thanks again!

author: ajb
author_rank: Super Contributor
timestamp: January 30, 2021, 09:07:27 am
content: Happy to help! Although I probably should have safety brigaded you about the lasers, but in lieu of that of course you should be careful to avoid direct or reflected eye exposure, including diffuse reflections with higher powers, generally never aim them near people or animals, avoid aiming them at combustible materials, and never let them enter airspace. Since you have a us flag you should also know that laser systems like this are subject to federal regulation and require a variance from the FDA to enter into commerce. Your state or local jurisdiction may also have an opinion.Anyway most diodes in light show use are isolated cans, at least in the higher power multimodes. Still possible to see single mode parts that aren't, but not sure how common they are these days.

author: moonie223
author_rank: Contributor
timestamp: January 30, 2021, 09:40:22 am
content: I've heard the rundown on laser safety before, but won't discourage posting it again! At least I'm not looking to put multiple watts in a flashlight, although that would be easier...If I were to sell anything it'd be piece parts, used to machine mirror mounts, diode housings, and all that neat stuff. It has been a while, the casio projectors and cheap blue was where I left off. Green and high power 638nm popping up in the meantime and is just a super bonus.I went back and looked at the diodes I bought a while back and they are all isolated. Looks like that might be the best way forward. The die4drive doesn't appear to have any issue modulating at 1MHz, although the 1ohm sense resistor is rather toasty and it needs about 220pf of capacitance on the modulation input/soft start to keep from spiking current on a very fast square wave, so I guess that's a slight phase offset at high frequency. FDMC8462 looks like it might be a good choice for a mosfet besides the price, very low rds and gate charge and a very high thermal dissipation capability, and it's in ltspice already.I might look into a switching regulator with very good transient response and go from there!