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| Laser Diode driver - kHz modulation & 1% duty cycles |
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| mikerj:
--- Quote from: smoothVTer on April 23, 2019, 10:34:58 pm --- --- Quote from: mikerj on April 23, 2019, 10:07:51 pm ---Just to make sure I understand your intentions, you want to provide a constant average power output independent of PWM duty cycle, so low duty cycles require much higher peak currents than high duty cycles? I don't see any current limiting in the circuit, does the 2.5v laser supply guarantee you can't blow it up if the drive transistor is saturated? --- End quote --- The idea is to be able to set the peak LD current to a constant, known level, independent of the LD's own Ipd characteristic. Once the LD is "peaked", so to speak, the idea is to modulate at several kHz and vary the duty cycle to scale the output power. The complication arises because the same part number laser diode ( red or IR, in this case ) can have an Ipd anywhere from 50uA to 250uA for an output power Po = 10mW The LD is most efficient at this "peaked" output level, so to achieve highest efficiency, you want to always reach that peak power during the PWM "on" time. I guess another way to word it is that I want to achieve a constant peak power, that I can then PWM/vary the duty cycle to scale the perceived average power. To achieve this constant peak power, I run this system as a fast linear regulator, using the DAC as my peak power control signal. --- End quote --- Sounds to me like you want a constant current current drive. Using a continuous feedback with an integrator to control laser current will result in the loop trying to maintain a constant average power, i.e. laser current will rapidly increase at low duty cycles. If you want closed loop power control only whilst the laser is operating you will need some kind of sample and hold on the feedback signal gated by the PWM. You may be able to do the entire power control loop by triggering an ADC conversion from the PWM output and using this to adjust the DAC which controls a current source. |
| smoothVTer:
--- Quote from: StillTrying on April 23, 2019, 09:11:48 pm ---"If you meant transimpedance amplifier, there is a single reason I cannot use one: This is a single supply system, and as such, I cannot use a TIA because using a TIA with an N-type laser diode would generate a negative output voltage in proportion." All you've got to do is leave out RPD, and provide a resistance path opamp output to -IN, the PD's current is already in the right phase for overall negative feedback. --- End quote --- OK, I tried this approach but I am not sure how to get it working correctly. As a starting point, here is a simulation of the steady state system in its current form ( ignoring any PWMing of the LD ) The laser diode is modeled more or less with D1/D2/D3 B1 is a simplistic model of the photodiode: below lasing threshold, the photocurrent is very low. As it approaches lasing threshold ( ~20mA ) the photocurrent increases drastically, and thereafter, approximates a linear curve for increasing laser diode forward current. C2/R4 are there for OPA stability. With this simulation, I can finely control the forward current through D1/2/3 using the Vref_drive ( which would come from my internal DAC ) When I apply your suggestion to use the PD feedback current directly in a TIA approach, I cannot get a usable current through D1/2/3 anymore, no matter what value R4 I choose. It seems to regulate, albeit to a very low current: Does the 2nd schematic/simulation I have shown here reflect the general idea of your suggestion? |
| smoothVTer:
--- Quote from: duak on April 24, 2019, 03:55:57 am ---I last worked with laser diodes in the early 90's. I don't think things have changed much though: https://www.newport.com/medias/sys_master/images/images/he9/hd7/8797049520158/AN05-Laser-Diode-Characteristics-Overview.pdf It explains some aspects but for what it's worth, here's my spin on it too: - it's normal to have a variation in monitor current. The photodiode sits at the other end of the laser diode chip and measures how much light leaks out of the cavity back facet or mirror. I can't remember the exact values for the leakage but you can see that a difference between 99.9% and 99.99% reflectivity is an order of magnitude variation in monitor current. - a laser emits non-coherent radiation as an LED below lasing threshold. The amount and wavelength is not usually specified. - the LD_supply voltage seems a little low. It should work but Q1 is running close to saturation ie., VBE > VCE - you've got to be really careful not to exceed either the forward current limit or the power limit spec or you will blow the facet off the die and it will no longer lase. This may have changed over the years, but back then the manufacturer wasn't kidding about exceeding limits. We joked that you could kill a $100 device in 100 ns for a burn rate of $1B per second. - back when I worked with these things, if the laser was modulated, the monitor was sampled and used to determine the drive current. This was then switched or steered so as to not cause any current spikes that could damage the laser diode. --- End quote --- Thanks for the resources and your inputs. The devices I am working on use very cheap laser diodes ( 635nm reds and 850nm IR's ) that cost about $3/ea. so even if I blow several it is not a big deal ( phew! ) The system itself is tiny and runs off a small, weak battery. Therefore the smallest PCB I can get away with steers me towards a fully integrated solution, if I can get away with it. Also having these weak batteries makes it critical to drive these LD's at peak efficiency to get the longest battery life. |
| smoothVTer:
--- Quote from: mikerj on April 24, 2019, 11:00:33 am --- --- Quote from: smoothVTer on April 23, 2019, 10:34:58 pm --- --- Quote from: mikerj on April 23, 2019, 10:07:51 pm ---Just to make sure I understand your intentions, you want to provide a constant average power output independent of PWM duty cycle, so low duty cycles require much higher peak currents than high duty cycles? I don't see any current limiting in the circuit, does the 2.5v laser supply guarantee you can't blow it up if the drive transistor is saturated? --- End quote --- The idea is to be able to set the peak LD current to a constant, known level, independent of the LD's own Ipd characteristic. Once the LD is "peaked", so to speak, the idea is to modulate at several kHz and vary the duty cycle to scale the output power. The complication arises because the same part number laser diode ( red or IR, in this case ) can have an Ipd anywhere from 50uA to 250uA for an output power Po = 10mW The LD is most efficient at this "peaked" output level, so to achieve highest efficiency, you want to always reach that peak power during the PWM "on" time. I guess another way to word it is that I want to achieve a constant peak power, that I can then PWM/vary the duty cycle to scale the perceived average power. To achieve this constant peak power, I run this system as a fast linear regulator, using the DAC as my peak power control signal. --- End quote --- Sounds to me like you want a constant current current drive. Using a continuous feedback with an integrator to control laser current will result in the loop trying to maintain a constant average power, i.e. laser current will rapidly increase at low duty cycles. If you want closed loop power control only whilst the laser is operating you will need some kind of sample and hold on the feedback signal gated by the PWM. You may be able to do the entire power control loop by triggering an ADC conversion from the PWM output and using this to adjust the DAC which controls a current source. --- End quote --- Thanks mikerj, I found this app note from Maxim about doing pretty much what you describe in the first half of your reply, I think. Yet I do not understand how this circuit could possibly work ( regulating average power ) because then there is no limit to how much the current can increase at low duty cycles ... this circuit below would push the current very high and blow the laser diode at low duty cycles. Taken from https://www.maximintegrated.com/en/app-notes/index.mvp/id/1811 |
| pwlps:
--- Quote from: smoothVTer on April 23, 2019, 06:30:15 pm --- The theory of operation: When a current flows through the LD, a reverse current proportional to the light output is generated through the PD. The LD and PD are in the same 3-pin package ( this being an N-type LD ) Rpd converts this reverse current into a feedback voltage. The OPA's inverting input is connected to this feedback point. The OPA non-inverting input is connected to a 10-bit DAC which I can digitally control through the MCU. Therefore, this DAC sets the peak LD current/optical power. The OPA drives enough current through Q1's base in order to achieve the requested peak voltage at the Rpd point. The LD I am using are all either IR or red LD's. Their monitor photodiode current can varying wildly: I've seen as low as 50uA all the way to 250uA for the same optical output power using LD's from the same box. Now in order to achieve varying optical powers, I utilize PWM and vary the duty cycle of a special feature of this OPA. This particular OPA can switch its output pin within 50ns between (a) actively driven and (b) high-Z When driving, the OPA servos current to Q1's base and ramps to/holds the peak laser power. When in high-Z, Rbe removes base charge and quickly puts Q1 into cutoff. --- End quote --- I don't understand the principle of operation as you explained, but I admit I didn't make much effort to analyze it in details. I don't get: why should you need any digital stages if your pwm duty cycle is only controlled by an analog input ? You might be interested to compare it with my approach here: https://www.eevblog.com/forum/projects/simple-lifi-transmitting-audio-signal-with-a-led/msg2282771/#msg2282771 A slight modification would be needed if you want to have the feedback go through a measuring photodiode rather than to be taken directly from the LED output voltage, otherwise this sigma-delta converter circuit can give a high-fidelity PWM without any digital components. |
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