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Old 04-23-2011, 06:01 PM #1
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Default PWMing verse DC output.

Hey all, my name is nate and i'm a electrical engineer working with johndeere.

Was looking at all your driver circuits and they all by majority look like DC supplies? . Now, I might be mistaken but I don't think dvd burners..ect operate at max power with a constant DC current. (aka most laser diodes are not built to run solid dc I think?) I would assume you guys are killing your diode life using those type of drivers.

Why don't you guys PWM your voltage regulators outputs?

For those who don't understand:
PWMing is basically like a square/sine wave voltage output. Many lights out there are constantly turning on and off faster than you can see it happen. 60 times a second in the usa ..ect. Via practical application this can conserve power and reduce heat. However, main reason we use ac current is transmission line based. (reduces power loss over great distances)


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Old 04-23-2011, 06:12 PM #2
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Default Re: PWMing verse DC output.

I see that you have not done much research or reading on the
Forum pertaining to Laser Diode requirements as to power
supply requirements... either CW or pulsed...

A Laser Diode is a DC current device... It requires a regulated current
source for proper operation...
To get a pulsed regulated current source is more difficult and many
LD PS circuits do exist for Laser Projector use...

It all depend what you want to do with your Laser....
1) do you want to burn things and pop ballons
2) do you want to use your Laser in a projector

Don't forget that a LD at 100mW CW will give you 100mW
of output power and a 100mW pulsed (@50% duty cycle @
60Hz) will onlt give you ~50% of that power...

Jerry
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Old 04-23-2011, 06:31 PM #3
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Default Re: PWMing verse DC output.

Quote:
Originally Posted by lasersbee View Post
I see that you have not done much research or reading on the
Forum pertaining to Laser Diode requirements as to power
supply requirements... either CW or pulsed...

A Laser Diode is a DC current device... It requires a regulated current
source for proper operation...
To get a pulsed regulated current source is more difficult and many
LD PS circuits do exist for Laser Projector use...

It all depend what you want to do with your Laser....
1) do you want to burn things and pop ballons
2) do you want to use your Laser in a projector

Don't forget that a LD at 100mW CW will give you 100mW
of output power and a 100mW pulsed (@50% duty cycle @
60Hz) will onlt give you ~50% of that power...

Jerry
Hey Jerry, thanks for your post.

But hmm, not sure if I agree with you. a pulsed current driver is by no means more difficult than a dc supply. All it requires is an oscillator and a bjt hooked up to the adj pin of a voltage regulator. Hardly a source for additional complexity.

However your right, that would reduce the power output. However, i'm curious if that translates into practical application if pwmed at a high frequency. Thermal dynamics are abit more complicated then that. I wonder if practically speaking, it would take double the time to burn something if PWMing at 50% duty cycle. I'll have to test that out at some point cause i'm rather curious.
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Old 04-23-2011, 06:44 PM #4
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Default Re: PWMing verse DC output.

Quote:
Originally Posted by random person View Post
Hey Jerry, thanks for your post.

But hmm, not sure if I agree with you. a pulsed current driver is by no means more difficult than a dc supply. All it requires is an oscillator and a bjt hooked up to the adj pin of a voltage regulator. Hardly a source for additional complexity.

However your right, that would reduce the power output. However, i'm curious if that translates into practical application if pwmed at a high frequency. Thermal dynamics are abit more complicated then that. I wonder if practically speaking, it would take double the time to burn something if PWMing at 50% duty cycle. I'll have to test that out at some point cause i'm rather curious.
That's what a lot of members do here... then they share their test
results with the community...

Don't forget the Data sheets of some of the LDs state a frequency
in the Mhz range for pulsed applications.... not 60Hz...

It would be interestiing to see your design of a current regulated
PWM Laser Diode Power supply.... I would test one....

Jerry
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Old 04-24-2011, 04:08 PM #5
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Default Re: PWMing verse DC output.

Quote:
Originally Posted by random person View Post
a pulsed current driver is by no means more difficult than a dc supply.
O rly? How about these means?:

Quote:
Originally Posted by random person View Post
an oscillator and a bjt hooked up to the adj pin
It takes some balls to come in here and tell everyone they're doing it wrong, I'll give you that much.
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Old 04-24-2011, 04:17 PM #6
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Default Re: PWMing verse DC output.

Quote:
Originally Posted by Cyparagon View Post
It takes some balls to come in here and tell everyone they're doing it wrong, I'll give you that much.
Don't worry ma'am, I'm from the internet.

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Old 04-27-2011, 12:07 AM #7
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Default Re: PWMing verse DC output.

Quote:
Originally Posted by Cyparagon View Post
O rly? How about these means?:



It takes some balls to come in here and tell everyone they're doing it wrong, I'll give you that much.
Now now.... don't attack people for asking a questions. =P I believe that is abit different then telling everyone on here that they are wrong. Now thanks to laserbee I see why you guys use DC, to increase your power output.. my badd.. as evidence by my gigantic post count I'm still abit new here.

Annyyhooww as for creating a simple PWM voltage regulator I was originally thinking this.

However, you might get current spikes as voltage goes from low to high so probably not the best idea for a diode. In addition, theres no telling how fast the voltage regulator would respond to changes in the adj pin.

Would be better to do a simple high side driver configuration. Representation of a HSD in image below.


However, if your trying to do a battery operated build it might be abit more difficult because you will increase the size of the overall driver by adding the oscilator + bjt+additonal resistors.
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Old 04-27-2011, 12:33 AM #8
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Default Re: PWMing verse DC output.

Quote:
Originally Posted by random person View Post
Now now.... don't attack people for asking a questions. =P I believe that is abit different then telling everyone on here that they are wrong. Now thanks to laserbee I see why you guys use DC, to increase your power output.. my badd.. as evidence by my gigantic post count I'm still abit new here.

Annyyhooww as for creating a simple PWM voltage regulator I was originally thinking this.

However, you might get current spikes as voltage goes from low to high so probably not the best idea for a diode. In addition, theres no telling how fast the voltage regulator would respond to changes in the adj pin.

Would be better to do a simple high side driver configuration. Representation of a HSD in image below.


However, if your trying to do a battery operated build it might be abit more difficult because you will increase the size of the overall driver by adding the oscilator + bjt+additonal resistors.
No one is attacking you....
You stated how easy it is to build a PWM Laser Diode Driver
and we told you that it is not...

It is now up to you to show us the error of our ways...

From what you posted...... I don't see a PMW Current
regulated Laser Diode driver...

I see a Voltage regulated driver.... in both drawings..

Not quite the same animal......


Jerry
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Old 04-27-2011, 04:04 AM #9
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Default Re: PWMing verse DC output.

Quote:
Originally Posted by lasersbee View Post
No one is attacking you....
You stated how easy it is to build a PWM Laser Diode Driver
and we told you that it is not...

It is now up to you to show us the error of our ways...

From what you posted...... I don't see a PMW Current
regulated Laser Diode driver...

I see a Voltage regulated driver.... in both drawings..

Not quite the same animal......


Jerry
not sure there errors in either way. Hence, i'm just trying to learn why you guys do things the way you do. I'm glad you noticed that both are voltage sources. That would be my next question, why the huge emphasis on current drivers? unless i'm mistaken on this(which is entirely possible), as your increasing the current output of your drivers all your really doing is just upping the voltage(ohms law). So why not just build a voltage driver specific to the current you want? Just use ohms law... So if you have a voltage regulator hooked up to a diode and a resistor in series it would simply be (Vin - Vdrop across diode)/R=current across diode. To get Vdrop across the diode use a high end DMM on a diode setting to determine the voltage drop across the diode and pick the resistor and voltage accordingly to give you the current you want. Or a way with no math would be to hook up your DMM in series with the resistor and diode and just keep upping the voltage till you get the current you want.

Once you do that you can interchange voltage drivers and current drivers.. At that point, the simple PWM circuit I showed works perfectly. However, then again, i'm still new to all this so if laser diodes have some properties I'm not aware of that all kinda falls apart.
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Old 04-27-2011, 09:42 AM #10
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Default Re: PWMing verse DC output.

In essence, you are right, and I am going to try out constant voltage drivers in the near future.

Quote:
However, then again, i'm still new to all this so if laser diodes have some properties I'm not aware of that all kinda falls apart.
Laser diodes change their forward voltage as they warm up (they always do under operation),
in fact they lower their Vf, so you better be careful enough to set your constant
voltage driver to a voltage that still is safe with a warm LD.

Otherwise, you might blow the diode with a voltage that is fine at ambient, but not
so fine at operating temp.

Also, the slope that relates voltage to current is a really steep one, take a look at the
graphs inside this thread:
3x 445nm LD PIV Plot
Means you have to be extra precise and careful when using voltage instead of current
to regulate a laser diode.
It is possible, but it is less headache and generally safer for the non-expert to drive
them with regulated current instead of voltage.
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Old 04-27-2011, 10:32 AM #11
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Default Re: PWMing verse DC output.

If you want to PWM drive a LD, better to use a sink driver based on an op-amp and a power mosfet, and inject the PWM signal as modulation in the feedback loop ..... like this one, as example (but there are a lot of other possibilities, anyway) ..... this way, it still act as a current regulator, plus the modulation capability (both PWM/TTL and analog, if needed )




EDIT: also, ofcourse all our current drivers works changing the voltage (a current is a derivate, you can't regulate directly it ..... it's like to ask someone to measure directly a speed ) ..... the difference is that, in constant voltage drivers, the current is not monitored, and when the LD increase the temperature, the current can go higher, and this go in "avalanche (more current > more heat > more current > more heat ..... etc) very quickly, and you find yourself with a burned LD in seconds (very few seconds )

The current drivers, instead, check continuously the current through the load, and adapt the output voltage on-the-fly, for prevent this ..... for this reason, laser diodes drivers are or constant-current types, or APC with feedback types, but never only constant-voltage types.
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Old 04-27-2011, 06:16 PM #12
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Default Re: PWMing verse DC output.

Quote:
Originally Posted by random person View Post
Now now.... don't attack people for asking a questions. =P I believe that is abit different then telling everyone on here that they are wrong. Now thanks to laserbee I see why you guys use DC, to increase your power output.. my badd.. as evidence by my gigantic post count I'm still abit new here.

Annyyhooww as for creating a simple PWM voltage regulator I was originally thinking this.

However, you might get current spikes as voltage goes from low to high so probably not the best idea for a diode. In addition, theres no telling how fast the voltage regulator would respond to changes in the adj pin.

Would be better to do a simple high side driver configuration. Representation of a HSD in image below.


However, if your trying to do a battery operated build it might be abit more difficult because you will increase the size of the overall driver by adding the oscilator + bjt+additonal resistors.

Let's see the simple oscillators. top pic is the app example of remote shutdown. where's the drive.

The popular homebrew driver here is a constant current source from a monolithic Ereg. It will limit current to a range in a preset level. Using the Ereg as a Ereg only limits the current to the capability of the device with respect to the supply.. That would result in a poped and smoking LD, even with AAA's as a source. The key with LD's appears to be achieving limited (and hopefully constant) current with small component count. Don't even suggest just a simple resistor, they will shoot you as a KipKay spy (but that's another tale).

You leave yourself open to question when you say no big deal building pulsed supply while only presenting a (from the data book) application suggestion with remote shutdown. That's not even half of the circuit requirement.......
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Old 04-27-2011, 11:40 PM #13
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Default Re: PWMing verse DC output.



HIMNL9, that was basically the idea... kinda looks abit messy though. I'm not quite sure you need alll that.. Danger of using a mosfet as a driver is you gotta be sure your in the correct region of operation or else you will get the high heat. However, even pumping acouple amps through those shouldn't heat them up tooo much so not sure if your in the correct region of operation there. You want to make sure your in the saturation region of operation rather than the triode. Whats up with the 100ohm going to the gate of the mosfet?

anselm:
Quote:
You leave yourself open to question when you say no big deal building pulsed supply while only presenting a (from the data book) application suggestion with remote shutdown. That's not even half of the circuit requirement.......
Let me google that for you

Must I show everything? Oscillators are a dime a dozen. (theres numerous of them and you would pick one to suite your application) HIMNL9 pointed out a version of the simple ones called a op amp relaxation oscillator which happens to be my personal fav. (kinda hard to see it in the wiring mess). However, there are BJT oscilators, 555 timers... ect...ect.

Updating my schematic... (the +5V at the top being the voltage regulator circuit that you choose to pick)

this is one of thousands of ways you could do this... This would be best suited to be powered by a 2 cell build (so you can create your virtual ground and thus acheive a + and - to power the op amp) or power it with your own custom variable supply.... If you want I could show you also how to build a simple current controller into this while were at it..
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Old 04-27-2011, 11:56 PM #14
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Default Re: PWMing verse DC output.

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Originally Posted by anselm View Post
In essence, you are right, and I am going to try out constant voltage drivers in the near future.
Constant voltage drivers may be decent (enough) when you have 405 and 445 PIV curves that look like THIS.

But they start to get a little scary when you have voltage curves that are flatter like THIS
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Old 04-28-2011, 12:13 AM #15
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Default Re: PWMing verse DC output.

This link always helped me to understand why a constant current is important. I'm guessing most of the electrical engineers in this thread will understand but some others may benefit from watching it.

YouTube - Constant Current vs. Constant Voltage in use with Laser Diodes

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Old 04-28-2011, 12:49 AM #16
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Default Re: PWMing verse DC output.

Quote:
Originally Posted by random person View Post

HIMNL9, that was basically the idea... kinda looks abit messy though. I'm not quite sure you need alll that.. Danger of using a mosfet as a driver is you gotta be sure your in the correct region of operation or else you will get the high heat. However, even pumping acouple amps through those shouldn't heat them up tooo much so not sure if your in the correct region of operation there. You want to make sure your in the saturation region of operation rather than the triode. Whats up with the 100ohm going to the gate of the mosfet?

anselm:


Let me google that for you

Must I show everything? Oscillators are a dime a dozen. (theres numerous of them and you would pick one to suite your application) HIMNL9 pointed out a version of the simple ones called a op amp relaxation oscillator which happens to be my personal fav. (kinda hard to see it in the wiring mess). However, there are BJT oscilators, 555 timers... ect...ect.

blah blah other stuff blah

While I like the fact that you are introducing new ideas (actually revisiting some old ideas, but they haven't been posted recently) laser diodes are a well-understood thing, and your tone of conversation isn't exactly friendly.. it's more like you're talking to a 9 year old.. I'm not 9 and neither is anyone else in this thread, so my suggestion to you is to rethink how you come at people.

You can definitely PWM them and get a bit of extra life, or you can run them CW within datasheet specs and have many thousands of hours of useful lifetime per the datasheet MTBF figure.

There are times when you might want CW and there are times when you might not. It's application dependent and dependent on personal taste.

@HIMNL9: I like what you're doing there.. I think I remember seeing you post that circuit before. How confident are you in the cleanliness of the modulation?
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