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PWM drivers for lasers. Why/why not?

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Canuke

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The discussion in this thread on low power usage for 445nm diodes, which lase at a minimum 200mA or so, leads me to this question for our driver experts.

To me, the best option for operating this diode at low powers, given the minimum lasing current and very low efficiency at those currents, would be to control intensity via pulse-width modulation (PWM) duty cycle adjustment.

It seems to me, however, that unlike in the LED domain, PWM doesn't seem to be used in any of our custom drivers, though it seems to be used for this purpose by some pointer manufacturers here and there.

Is this simply because the demand at LPF has always been for max power?

Or, are there technical hurdles? In the aforementioned thread, safety was an objection raised to PWM, since peak power would still be high. I countered that a sufficiently high PWM frequency mitigates that danger. Another poster then replied that common LED drivers work in the Hz range (too low).

So the next question is: Are higher (>1kHz) PWM frequencies difficult/expensive/complex to achieve in Flexdrive or Microboost type drivers pushing 1-3A? Perhaps it's because of capacitance/overshoot etc.?
 
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Benm

Benm

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I dont know about the specific drivers you mention, but in general, PWM can easily be applied on laser diodes. The benefit is often very limited - we operated these diodes well beyond manufcaturer ratings as it is.

There could be advantages in pwm with some very specific laser diodes, but research on that is pending and will be made available once completed,
 
C

Canuke

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Ah, that makes sense.... PWM is mainly a way to throttle back a diode, not push it (not that there won't be people doing that :evil:), and now that we've finally found a diode that's just TOO powerful for some applications, there's some interest there.

Along these lines: I don't know whether the Flexdrive v5 is supposed to be PWM, but right now my 445 build using a Flex set at 400 is showing a distinct PWM at around >1500Hz. So, either Flexdrives use PWM, or I've done something bad to mine and it's malfunctioning...
 
Bionic-Badger

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Well, it's not really PWM. There's no modulation going on. Those types of drivers are "buck" types of DC-DC converters which use an oscillating (switching) voltage in conjunction with an inductor and diode, to produce a different voltage/current even above the input voltage level. There are a number of pre-made driver chips for this purpose that just need an inductor and some other supporting parts to function. What you're seeing are the oscillations produced by the voltage conversion. They're far more apparent when the battery is nearly depleted, meaning the driver can't operate correctly.

See if a fresh battery helps, or see if the battery you're using is sufficient to power your laser. Regular alkaline batteries usually can't handle the current requirements, so you should use a Li-Poly battery instead.
 
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Benm

Benm

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You should see constant current to the laser from a switching regulator. Depending on design and operating conditions there will be a ripple on the output in most practical drivers, but that is a flaw, not a feature.

PWM can be very useful for turning power -down- reliably. Lets say you have a 200 mW red, but want a safe 5 mW setting for that. Reducing current is not reliable as threshold changes with temperature, and since you would be operating just a tiny bit above treshold, output power would not be steady.

With PWM you can run it at the current for 200 mW, but just at a 2.5% duty cycle, which gives you 5 mW on average. You have to do this rather quickly to make it a safety addon though - several kHz at least. Also, you'll notice a dotted line when you wave the laser around quickly.
 
Xplorer877

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I've made diodes operate successfully with a 555 timer PWM driver. Just have a transistor control the current flow after the driver. With a potentiometer you can have an adjustable Po without dialing with efficiency and stability problems of near threshold currents. I'm working on a RGB laser with a 3 channel PWM controller that will cycle through all the hues.

-Tony
 
Xer0

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Any micro boost-driver here which could be TTL modulated in kHz Range?
 
Xplorer877

Xplorer877

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http://hacylon.case.edu/ebay/laser_diode/FlexMod01.php

^ That driver is ready made for TTL Mod. If the TTL input is 5v the laser is on, if it's 0v the laser is off. I'm sure that driver can handle KHz frequencies.

Or you can build this

Untitled-7.png


And have the pulsed output trigger a transistor that controlled power to the diode.

-Tony
 
Benm

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There are circuits enough to make pwm modulation.

A 555 controlling blanking will work fine for experiments.
 
ossumguywill

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I was gonna make a new thread, but I saw this.... Why don't we have any "smart" drivers? It would be really simple to make a driver with modes and memory (low/med/high/etc.) with PWM for lasers. With a µC, you can tailor the PWM frequency to whatever you want and get 8 bit resolution easily (255 brightness levels+off). I would personally like adjustable power output like what wicked is doing in a torch. Perhaps a standard flashlight-like operation where power cycling controls level selection. A fast strobe feature would be equally cool.
 
Xer0

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NE555 can do pulse lenght adjustment too? thought only TL494 can.

Besides, why Wicked has only an 6Hz PWM?
 
L

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What you have to do is define the difference between PWM and unfiltered switching drivers. Some very cheap diode drivers and many led drivers use a unfiltered switching power supply as the energy source. Since a inductor is used to store the energy in a switcher, the energy comes out, in some cases, with a huge spike. A led will handle the spike just fine, but laser diodes have a much smaller, more fragile die structure, and thus the switching spike will kill them.

So yes, you can PWM a proper linear driver and control energy in the beam. Its done all the time for laser projectors to several megahertz. What you have to watch out for is a poorly designed switching driver with no output low pass filters. Non low passed switchers without clamping or post regulation kill laser diodes. This is why using a 1 amp led driver with a 1 amp 445 laser diode can be fatal.

That is where the confusion starts, as switching power supplies are based on using PWM to control the charge stored in a inductor. Dimming is also based on PWM. That makes it a hardware question, not a PWM question. I don't know how to make the distinction more clear.

So what you hear about PWM killing diodes is "actually" poorly engineered switching drivers killing diodes.

So if your laser diode current source is purely a switcher with no filter or linear regulator, you have a problem.

If you have a switching step up supply followed by a linear regulator, such as DR Lava's products, your fine, because the designer took steps to clamp the inductive pulses. If in doubt, use a dummy load and watch the output on a fast oscilloscope, for spikes.

Steve
 
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ossumguywill

ossumguywill

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LSRFAQ said:
What you have to do is define the difference between PWM and unfiltered switching drivers. Some very cheap diode drivers and many led drivers use a unfiltered switching power supply as the energy source. Since a inductor is used to store the energy in a switcher, the energy comes out, in some cases, with a huge spike. A led will handle the spike just fine, but laser diodes have a much smaller, more fragile die structure, and thus the switching spike will kill them.

So yes, you can PWM a proper linear driver and control energy in the beam. Its done all the time for laser projectors to several megahertz. What you have to watch out for is a poorly designed switching driver with no output low pass filters. Non low passed switchers without clamping or post regulation kill laser diodes. This is why using a 1 amp led driver with a 1 amp 445 laser diode can be fatal.

That is where the confusion starts, as switching power supplies are based on using PWM to control the charge stored in a inductor. Dimming is also based on PWM. That makes it a hardware question, not a PWM question. I don't know how to make the distinction more clear.

So what you hear about PWM killing diodes is "actually" poorly engineered switching drivers killing diodes.

So if your laser diode current source is purely a switcher with no filter or linear regulator, you have a problem.

If you have a switching step up supply followed by a linear regulator, such as DR Lava's products, your fine, because the designer took steps to clamp the inductive pulses. If in doubt, use a dummy load and watch the output on a fast oscilloscope, for spikes.

Steve
Click to expand...
yep-you can't use PWM off a standard laser driver, ESPECIALLY if it has a capacitor to smooth out the voltage parallel to the diode. That will give you a big boost of current and you're no dimmer than when you started.
 
Benm

Benm

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Xplorer877 said:
I'm working on a RGB laser with a 3 channel PWM controller that will cycle through all the hues.
Click to expand...

What are you going to use that rgb laser for anyway?

If its for a scanner, PWM isn't likely to be desireable, as it will give a dotted-line-appearance in the projection, unless your pwm is extremely fast (several MHz range).
 
ossumguywill

ossumguywill

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Also Xplorer877, what power source are you using that won't freak out when you use fast PWM like that? I think both constant current and switching drivers HATE it when you use PWM.

Edit:I was just looking at the datasheet... even the DIP version of the attiny2/4/85 series should be under 8mm wide. I'll try sticking one in a pen host a little bit later. Maybe I can make my own "smart" PWM driver...
 
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drlava

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The MicroBoost does have PWM input control capability.. accepts modulation to 10kHz I believe. PWM input requires a small modification to a trace on the driver.
 
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