Laser diode driver - Page 1
Forum section: electronics / projects

author: LukeW
author_rank: Frequent Contributor
timestamp: March 28, 2016, 09:27:08 am
content: The goal is to take a 12V supply rail and provide power to a laser diode (2W 445nm), with microcontroller PWM modulation.Normally, with no PWM signal present, Q1 should be off and Q2 on, bringing the noninverting input to 0V and therefore zero laser current.The laser should, ideally, be absolutely off in this condition without the slightest hint of output.With Q1 on and Q2 off the noninverting input will come up to the reference voltage, determined by R7 and the other components, and the laser will run, at whatever the constant-current setpoint is (say 1.5A).The components on the microcontroller board (Q1 and LED1 in the marked box) should be considered a fixed interface - i.e. no changes or edits on that board, and the laser board should be edited to suit.The laser board and MCU board share a common ground and +12V supply, along with the open-drain output signal.LED1 should reflect the laser state.Would appreciate your thoughts

author: Colin55
author_rank: Contributor
timestamp: March 28, 2016, 12:46:00 pm
content: With 1R5 you are going to get much more than 1.5 amps through the laser diode

author: chris_leyson
author_rank: Super Contributor
timestamp: March 28, 2016, 01:10:42 pm
content: With R10 = 1R5 the source current will be exactly 1.5A. If your laser forward voltage is say 3V then the voltage drop across the mosfet is going to be 12-3-1.5 = 7.5V. At 100% duty cycle the mosfet will be dissipating 7.5*1.5 = 11.25W !

author: chris_leyson
author_rank: Super Contributor
timestamp: March 28, 2016, 01:23:56 pm
content: You might need to add a small capacitor across the opamp, pins 1 and 2, say 47pF, to prevent the laser current overshooting.Depends on you PWM frequency but you might need to try a faster opamp. Test the circuit using a low value resistor that matches the laser charateristics just in case.

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 28, 2016, 02:16:58 pm
content: @lukeWAll the comments thus far are certainly valid. However, driving a laser with solely a constant current is seldom the right way to get good results. Depending on what you are doing with the laser, some feedback from an external element like a photodiode or the internal monitor diode is an effective means in achieving stability, both thermal and optical. Think of a laser like an opamp. Without some feedback, things can get a little unstable.Look at some APC (automatic power control ) circuits to get some idea how to implement something that provides some stability.Of course, if you drive the laser at a reasonable current level and keep the thermal limits within an acceptable range and tolerate some power and wavelength drift, then ignore my advice and just implement what you have. I would reconsider some of the components you already selected... LM358 and a Mosfet( an NPN transistor would work better). If not careful, your current ciruit will oscillate due to inadequate phase marginMoreover, you might consider some better protection against accidental reverse spikes( no, a 1N4001 is not fast enough to do any good), turn on transients, surges, etc to prevent the magic smoke scenario. Semiconductor lasers are not the most robust devices@grouchobyte

author: LukeW
author_rank: Frequent Contributor
timestamp: March 28, 2016, 02:45:55 pm
content: Most of the diodes in this sort of 1-2W range, such as the Nichia NDB7775 just as a representative example, don't have a feedback photodiode built in.Building external electrooptical feedback into the optical path seems like a pain.

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 28, 2016, 03:05:44 pm
content: Quote from: LukeW on March 28, 2016, 02:45:55 pmIn this case, just keep the laser diode in a stable thermal/ mechanical mode or mount and live with the delta P and delta lambda. Alternatively, you could use thermal feedback via a thermistor or temp sensor. This is less accurate and slower but you would have to know the tempco slopes which are negative for temp vs power and positive for temp vs wavelength(lambda) for the selected laser.I would still tweak your driver based on the other comments. Good luck to you and pay attention to laser safety at those dangerous power levels

author: Marco
author_rank: Super Contributor
timestamp: March 28, 2016, 05:43:53 pm
content: Quote from: grouchobyte on March 28, 2016, 02:16:58 pmWith 150 nF across it speed is not of the essence.PS. non-Schottky "fast" diodes can actually have higher overshoot during forward recovery, talking about fast is misleading.

author: chris_leyson
author_rank: Super Contributor
timestamp: March 28, 2016, 06:49:48 pm
content: I just noticed that Marco, you don't need the capacitor across the laser diode.

author: LaserSteve
author_rank: Super Contributor
timestamp: March 28, 2016, 08:01:39 pm
content: He should have a commercial laser diode protection device such as a Lasorb by Pangolin. Lasorbs need to be within 10-20 cm of the device, and should be right at the diode if possible. The Lasorb is more then fast enough. A small cap across the diode won't hurt, either.My concern is unknown startup conditions in the LM part.Usually we use a soft start circuit or a floating current limiter circuit on the laser diode, giving the opamp time to become stable.You may hear that those blue diodes are robust, but they still suffer from spikes just like any other laser diode.Light feedback is quite easy, a piece of beamspiltter made microscope slide has ~ 3.96% reflection at shallow angles, from each side. That goes into a scattering material such as opal glass or sandblasted glass or drafting mylar..Then you have your photodiode.Steve

author: grouchobyte
author_rank: Regular Contributor
timestamp: March 28, 2016, 09:10:51 pm
content: SteveI attempted to quell the enthusiasm for this diode driver with a few cautionary points.,,,,you can see that, right?It never ceases to amaze me the amount of enthusiam and determination people on this forum have in driving lasers like an LED. At least @lukeW asks for help.I suppose after a few accidents, some blown lasers and bruised egos the exuberance will wear off and the sentiment that playing with fire is exactly that@grouchobyte

author: LukeW
author_rank: Frequent Contributor
timestamp: March 29, 2016, 06:04:34 am
content: A buck regulator would be better, for efficiency if nothing else. But that will add extra lead time, drawing it up, spinning a board and ordering components, and a fast turnaround basically working solution was needed now.But I needed to throw together something quickly for a test using components already on hand, which motivates some of the perhaps crude design choices shown.The linear regulator is quite toasty, obviously, even after I've changed the input spec from 12V down to 8V to cut out some of the power. But a bit of inefficiency is OK in this case.The 1R5 sense resistor is quite high, much higher than it needs to be, therefore with more power dissipation. But it was the only power resistor on hand with about the right value.

author: LaserSteve
author_rank: Super Contributor
timestamp: March 29, 2016, 07:31:05 pm
content: A Linear will get you there quicker then building a Buck regulator. Designing a Buck with good enough transient suppression to avoid sub-microsecond spikes blowing the laser diode is non-trivial. Many good LED oriented Buck controllers will quickly fry laser diodes.Don't omit the Lasorb.Steve

author: LukeW
author_rank: Frequent Contributor
timestamp: March 30, 2016, 12:09:37 pm
content: The Lasorbs look like very nice devices, but they're not exactly DigiKey stock, and finding a distributor that will cost-effectively sell them in small volumes seems like a pain.It may not be as good, but what is the best protection choice across the laser diode other than a Lasorb, using jellybean components?A fast schottky across the laser only protects in one direction, but it's better than nothing.

author: LaserSteve
author_rank: Super Contributor
timestamp: March 30, 2016, 05:18:50 pm
content: The reason for the invention of the Lasorb was that jellybean components typically failed to protect the diode. They simply are not fast enough, nor do they latch off until certain faults are cleared.Lasorb was first developed to protect arrays of up to 120 high power diodes in touring laser show projectors, where even well engineered "Jellybean" parts constantly failed to protect the diode. At 100-500$ a diode, having parts of arrays fail got expensive fast. The last thing you want to do is tour with Roger Waters or have an install at Disney and have a projector quit. The part has since spread into fiber optic telecommunications and other applications, meaning it is in production and is going to be around a long while.The laser show industry always looks for lowest cost, we would not be installing these if there were viable alternatives in low cost drivers with modulation.It only takes a few nanosecond spike... Pangolin sells by phone or web and doesn't mind if you order onesies.Classical protection is REALLY done with power supply sequencing, ESD shielding, current limiting, active shunts during startup, and soft-start circuits.Steve

author: chris_leyson
author_rank: Super Contributor
timestamp: March 30, 2016, 06:29:51 pm
content: Never used Lasorb, never needed it. For the stuff I work on the laser is only a few inches from the controller, has optical feedback and it's all contained in a sealed enclosure so don't really need a Lasorb.QuoteAre we talking lasers with or without a monitor photodiode, I guess it would be without the monitor photodiode as people seem to want to drive high power class 4 lasers that generally don't have photodiode monitoring.All you need is a well designed control loop with one opamp, two at the most, and a few additional components is case of supply brownout. I've had to fix control loops with too much phase lag or overcomplicated loops with 5 or 6 opamps, in both cases I've seen 50% overshoot in laser current, designed by numptys and never tested obviously !My concern here is not about using Lasorb protection devices but laser safety, where exactly are the keyswitches and warning lamps for the class 4 laser you are using ? Due diligence, I don't see it, well you won't be seeing it if a class 4 laser burn your retiners, and maybe you get a spell inside if you burn someone elses retiners.