| Electronics > Projects, Designs, and Technical Stuff |
| Laser Diode driver - kHz modulation & 1% duty cycles |
| << < (6/7) > >> |
| David Hess:
--- Quote from: smoothVTer on April 23, 2019, 06:30:15 pm ---(1) Is there anything in this circuit that strikes you as faulty or terribly incorrect? Are any of my assumptions flawed or incorrect? --- End quote --- When the output is off, the external capacitor used for frequency compensation is charging up. This is called integrator windup and it results in the slow recovery. --- Quote ---(2) Is there a better way to go about compensating this OPA for a better transient response and reduce the ringing on the rising edge? Can I do this empirically as before? I do not have access to a VNA to do fancy stuff like the loop gain/phase measurement ... --- End quote --- The configuration of Q1 is a major problem; it is adding uncontrolled voltage gain within the feedback loop. Reduce the value of the series base resistor so that it only suppresses parasitic oscillation and add a low value resistor in series with the emitter to stabilize Q1's transconductance. With Q1 fixed, the external feedback capacitor should not even be required removing the slow ramp up. --- Quote ---(3) Why does the OPA seem to quickly slew to 500mV, but then exhibit that slow ramp-up to threshold? --- End quote --- I suspect this comes from the operational amplifier recovering from saturation of its internal circuits. --- Quote ---(4) Can this integrated OPA do what I am asking of it, given its GBWP and slew rate? --- End quote --- The topology is the problem and not the speed of the operational amplifier. A faster amplifier will not recover from integrator windup any more quickly. --- Quote ---(5) Any other ways to improve this circuit by adding or deleted external elements? --- End quote --- I might use a different circuit configuration. Replacing the operation amplifier with an operational transconductance amplifier would be one option but they are not common. (1) This provides a direct current output so Q1 is no longer needed and frequency compensation is considerably simplified. There is a trick I would at least evaluate though. Ground the operational amplifier's output through a low value resistor to a low impedance source, and then use the positive supply pin to drive the laser diode. This has the effect of replacing Q1 with the high side transistor of the operational amplifier's output stage and the output resistance now controls the transconductance. U111 in the first example shown below gives some idea of what this looks like. The power supply pins become current outputs, the normal inputs stay high impedance inputs, and the output becomes a low impedance input. The second example below shows the same idea but the output of the operational amplifier is being used for local feedback. Essentially this circuit configuration is an operational amplifier with a current feedback amplifier added to the output. --- Quote ---(6) Would an NMOS work better here, if I were somehow able to get the circuit to not oscillate? --- End quote --- A bipolar junction transistor is actually a better choice here because of low and predictable threshold voltage. --- Quote ---(7) Eventually I would like to get this circuit to work up to 10kHz modulation rates with 1% min duty cycles. Would that be possible given this OPA? --- End quote --- Absolutely, the operational amplifier is more than fast enough. (1) You could use a 723 voltage regulator; it has one. |
| smoothVTer:
--- Quote from: awallin on April 25, 2019, 06:30:07 pm ---not sure if it's been said/linked already but the usual constant-current circuit used for sciency-stuff is: Libbrecth-Hall http://www.submm.caltech.edu/kids_html/DesignLog/DesignLog179/MillerMUSICReadoutDocs/HEMT%20Power%20Supply/Libbrecht%20and%20Hall,%20A%20Low%20Noise%20High%20Speed%20Diode%20Laser%20Current%20Controller.pdf Erickson https://arxiv.org/abs/0805.0015 Seck https://arxiv.org/abs/1604.00374 --- End quote --- I have not seen this paper despite googleresearching for a couple of months now. A lot to digest in this paper! Thank you, I hope to gleam some insight from this. |
| smoothVTer:
--- Quote from: David Hess on April 26, 2019, 01:01:49 am --- --- Quote from: smoothVTer on April 23, 2019, 06:30:15 pm ---(1) Is there anything in this circuit that strikes you as faulty or terribly incorrect? Are any of my assumptions flawed or incorrect? --- End quote --- When the output is off, the external capacitor used for frequency compensation is charging up. This is called integrator windup and it results in the slow recovery. --- Quote ---(2) Is there a better way to go about compensating this OPA for a better transient response and reduce the ringing on the rising edge? Can I do this empirically as before? I do not have access to a VNA to do fancy stuff like the loop gain/phase measurement ... --- End quote --- The configuration of Q1 is a major problem; it is adding uncontrolled voltage gain within the feedback loop. Reduce the value of the series base resistor so that it only suppresses parasitic oscillation and add a low value resistor in series with the emitter to stabilize Q1's transconductance. With Q1 fixed, the external feedback capacitor should not even be required removing the slow ramp up. --- Quote ---(3) Why does the OPA seem to quickly slew to 500mV, but then exhibit that slow ramp-up to threshold? --- End quote --- I suspect this comes from the operational amplifier recovering from saturation of its internal circuits. --- Quote ---(4) Can this integrated OPA do what I am asking of it, given its GBWP and slew rate? --- End quote --- The topology is the problem and not the speed of the operational amplifier. A faster amplifier will not recover from integrator windup any more quickly. --- Quote ---(5) Any other ways to improve this circuit by adding or deleted external elements? --- End quote --- I might use a different circuit configuration. Replacing the operation amplifier with an operational transconductance amplifier would be one option but they are not common. (1) This provides a direct current output so Q1 is no longer needed and frequency compensation is considerably simplified. There is a trick I would at least evaluate though. Ground the operational amplifier's output through a low value resistor to a low impedance source, and then use the positive supply pin to drive the laser diode. This has the effect of replacing Q1 with the high side transistor of the operational amplifier's output stage and the output resistance now controls the transconductance. U111 in the first example shown below gives some idea of what this looks like. The power supply pins become current outputs, the normal inputs stay high impedance inputs, and the output becomes a low impedance input. The second example below shows the same idea but the output of the operational amplifier is being used for local feedback. Essentially this circuit configuration is an operational amplifier with a current feedback amplifier added to the output. --- Quote ---(6) Would an NMOS work better here, if I were somehow able to get the circuit to not oscillate? --- End quote --- A bipolar junction transistor is actually a better choice here because of low and predictable threshold voltage. --- Quote ---(7) Eventually I would like to get this circuit to work up to 10kHz modulation rates with 1% min duty cycles. Would that be possible given this OPA? --- End quote --- Absolutely, the operational amplifier is more than fast enough. (1) You could use a 723 voltage regulator; it has one. --- End quote --- I will implement some of your suggestions here, David. Already have had some success with the base resistor swap out ... this and some other changes from replies above have been helpful. Once I figure this out, I will report back here with an updated status. I've been keeping scope shots along the entire process for posterity and will post back some of my findings. Thanks everyone! Couldn't have gotten this far without all ya'll. |
| StillTrying:
--- Quote from: smoothVTer on April 23, 2019, 06:30:15 pm ---(7) Eventually I would like to get this circuit to work up to 10kHz modulation rates with 1% min duty cycles. Would that be possible given this OPA? --- End quote --- With a 12pF photo diode and a 3.5MHz op amp 1us wide pulses will just be a little rounded hump. I use just a PD and 1 or 2 fast transistors to view square-ish 1us pulses. |
| LaserSteve:
I received your PM, fast drive is not my cup of tea, unfortunately. Having had my share of blown diodes due to poorly designed drivers in the laser show industry, I can suggested the following general additions. You haven't lived until 16 single mode red laser diodes die in a brief flash and go LED. This happened due to defects in a brand new commercial driver board, and at 35$ a diode... Some of my diodes cost 200$ each, so I tend to carefully obtain commercial drivers. When I'm not in the lab, my laser diodes are used in arrays for combination into one beam, so I tend to drive 4 diodes in series at a time. I only need 30 Khz at up to 1.5 amps, and no light feedback. I buy the drivers from a vendor known to not have issues. So I suggest the following, as my concerns are more practical, and not about the driver bandwidth: Add a Lasorb, make sure you have a fast, low Vf, reverse protection diode in series with the device, An independent upper limit current clamp circuit is a must, and insure you are ramping the power to the output stage after you know the op-amps in your circuit have stabilized. Thus some form of soft start. A warning about working with LDs on the bench, the little 10 to 100 uf capacitors across the current limited output stages of bench power supplies tend to store charge and blow up diodes, as does the surge that many PSUs have during startup. In other words, commercial constant current bench supplies often have only the steady state current well defined. :-\ If I could, I'd have a normally on FET (Depletion?) of some kind across the diode, until the circuit is stable. Normally closed mechanical shunt relays have traditionally been used for this, shorting the diode till the laser head cable is connected, or the PSU is stable, and they just are not fast enough. The Lasorb is a static discharge protection device that has a structure like a Mosfet/SCR hybrid and is triggered by a fast DV/DT. I know the inventor personally, they do work, and I strongly suggest having one within 6 cm of any valuable LD. www.lasorb.com I would have also sent you to Libbrecht and Hall as a start. There is a paper that was written in response to L&H that is worth a read: An Ultrahigh Stability, Low-noise Laser Current Driver with digital control Christopher J. Erickson, Marshall Van Zijll, Greg Doermann, and Dallin S. Durfee Department of Physics and Astronomy, Brigham Young University. REVIEW OF SCIENTIFIC INSTRUMENTS 79, 2008 Good, it is on line: https://www.physics.byu.edu/faculty/durfee/Publications/Erickson08au.pdf Which was a follow on to this: https://tf.nist.gov/general/pdf/739.pdf If you don't have academic access, emailing the professor will generally result in your receiving a preprint, if they are allowed. If I had to design a very low current diode driver, I'd drive the diode off a high side current source and shunt it with a FET or Bipolar for the modulation, being careful to ensure there is a always a below threshold "leak" of current through the LD to protect it from surges. Where you really should just go is here: I'd suggest taking a look at APC style LD driver chips with PD feedback by ic_Haus: https://www.ichaus.de/keyword/Laser%20Diode%20and%20LED%20Drivers Take a look here for addressing some additional concerns, his writeup is pretty good.. http://hololaser.kwaoo.me/laser/red_diodelasers.html#LDdrv http://hololaser.kwaoo.me/electronics/myLDdriver.html One last note, the traditional current source used for testing LDs by the laser show industry and laser hobbyists is a LM317 configured as a current source with just a non-inductive current sensing resistor. If the leads are kept short and the input power is filtered, this combination tends to lead to long diode life when used by amateurs. Evidently National's LM317 has the right startup characteristics, but others work as well. Hence my note on shunting the diode for regulation. Steve |
| Navigation |
| Message Index |
| Next page |
| Previous page |