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For a project I need to make a custom laser driver for a dot green laser module.

Below and here 1 you can find my design, the laser I've using now is a PLT5 510 OSRAM, the VSYS is 3,7 V DC from a Li-Ion battery.

The optical power output measured with this set-up are around 1.5 mW, I need to increase the optical power output around 10 times more. I can adjust the feedback current changing the value of the resistors.

If I increase the optical power output, the temperature of the module increase a lot, 90 degrees instead 35 degrees in the normal use.

How can I make a good regulation of the current of the laser module, and get almost 10 mW of optical power output, with a less temperature in the module?

I've no idea if I can use PWM to drive the laser and how to make this regulation, because the laser has a Feeback PIN and I'm thinking about the PWM can affect to the normal and properly running of the laser module.

Please can anybody help me?

Thanks in advance!

Driver laser module

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  • \$\begingroup\$ What sort of heat sink do you have on your laser? \$\endgroup\$ – WhatRoughBeast Apr 18 '17 at 13:19
  • \$\begingroup\$ Only the casing where it is housed, is made of plastic, ASA \$\endgroup\$ – Tom Bromel Apr 19 '17 at 7:41
  • \$\begingroup\$ I'm thinking about to change ASA to CoolPoly, with a better conductive coefficient, 20 times more air, 40 times less Alluminium... \$\endgroup\$ – Tom Bromel Apr 19 '17 at 9:18
  • \$\begingroup\$ PWM would work IF you can use a low enough duty cycle AND can tolerate the on/off beam. If this is the case you should say so in your question. |Saying you cannot heatsink the LASER and then saying you can change the surrounding material seems very contradictory. Copper is good. A heat pipe is even better and may be able to take the heat to a pl;ace where it can be more easily dealt with. Blown air will help in most cases. Blown cold air more so. Liquid cooling and heat pipe on the outer surface may be very useful. We cannot tell what may work as the details provided are too low. \$\endgroup\$ – Russell McMahon Apr 19 '17 at 11:42
  • \$\begingroup\$ Well, I can use PWM, but where? In the EN of the Step-up? or where? \$\endgroup\$ – Tom Bromel Apr 19 '17 at 11:52
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You probably can't. And it has zero to do with how your circuit works.

First line from datasheet (seriously!!!):

• Optical output power (continuous wave): 10 mW (T case = 25 °C)

Acting as if you knew that and would be willing to stress your device beyond that:

The optical power output of this kind of lasers is, sufficiently above the lasing threshold, a linear function of the current flowing through the diode. See datasheet p.3.

So, to get 10x as much output power, you need to put in 10x as much current. Simple as that. Following said figure from said datasheet:

P/I curve and V/I curve

at ~40°C and ~1.5mW output, you're currently driving the device with 36 mA. With the right curve, you get a forward voltage of about 4.7 V, meaning your device is converting about 4.7 V· 36 mA = 0.17 W into heat.

15 mW is already out of the range of that curve, but let's extrapolate.

The slope of the power/current curve at 40 °C is roughly 0.3 mW/mA. You need around 14 mW of additional power, so that's about 46 mA more current, or (36+46) mA = 82 mA in total. Right chart tells us you'd have a forward voltage of ~ 5.2 V there. That means you're basically converting (nearly) 5.2 V * 82 mA = 0.43 W into heat here; that's 0.16 W more than you're currently doing.

The danger here is that you have a control loop that keeps the output constant. Now, when the temperature slowly rises, you'll need to push through more current to get the same output. That in turn will lead to more heat production, will lead to higher temperature, and will in turn lead to your control pushing through more power. That'll not end well for the diode.

conclusion

By far your best bet with this specific laser diode is to:

Actively cool the device, possibly with liquids, to keep its temperature as low as possible, so that you get the "better" P_out/I_in curve

Realistically, get a stronger laser diode. These might not be fun to hit your eye with.

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  • \$\begingroup\$ He says he's getting 1.5 mW out. He says and your lh graph confirms that he should get about 10 mW max. What is he doing that prevents him getting the nominal maximum optical output that the device "should" produce? \$\endgroup\$ – Russell McMahon Apr 18 '17 at 10:47
  • \$\begingroup\$ @RussellMcMahon I didn't say he couldn't do it at all, I just said that playing with the PWM doesn't change the inherent power/current relations, and that the way to go is active cooling or larger diode. The "about 10mW" requirement he sets clashes with the "10x output power" requirement further up in his question – so I took the harder one. \$\endgroup\$ – Marcus Müller Apr 18 '17 at 10:49
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    \$\begingroup\$ I wasn't questioning your figures or method - rather noting that if his Tc is 90C and he is getting 1.5 mW out there is something very wrong. As Rth_jc is acceptable his Rth_ca must be terrible, so his Tj is high, so the efficiency drops way way way off and he is chasing his tail. The key, which I didn't completely see when I mad that comment-question, is that the falloff in efficiency with temperature is so marked with this device (unlike many other devices where the dropoff happens but is not so pronounced) that he MUST keep it cool to keep efficiency up to have any chance of success. \$\endgroup\$ – Russell McMahon Apr 18 '17 at 11:31
  • \$\begingroup\$ ... ie often enough a low Tc means you can use a lower spec heatsink to keep Tjmax just in spec whereas with this device you MUST keep Tj low for it to work anywhere near max output. \$\endgroup\$ – Russell McMahon Apr 18 '17 at 11:32
  • \$\begingroup\$ Thanks @RussellMcMahon and (@)Marcus Müller for your explanations. Due the design of the mechanical casing I cannot use any cooler way to decrease the temperature of the laser, only can change the conductivity of the material of the casing, because this need to be waterproof and closed. I asked about the PWM because the feedback of the laser and the PWM I had read that they do not get along. Can I used PWM to drive the EN of the Step-UP and decrease the power momentaneously (At the frequency enough so that the eye doesn't see it) and decrease the temperature this way?. \$\endgroup\$ – Tom Bromel Apr 18 '17 at 14:38
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There is a special "problem" with this device (and presumably with many LASERS) compared to many other electronic components. Whereas we usually cool a device

  • To limit Tj (junction temperature) to a value that allows reliability and long life. Typically Tjmax may be in the 120-150C range BUT Tj operating is often kept much lower than this. This is to some extent changing - eg modern LEDs are now often specified to operate at 85C and some at 105 C and Cree lighting LED spec sheets give data for 105C operation and do not give specs at low temperatures.

  • To limit Tc (case temperature) to a temperature that is not "offensively high" even though electrically it may not matter. eg Tc above 50 C means the heat sink will burn fingers and Tc > 100C means the heatsink will boil water and cook eggs.

Whereas, in this case the efficiency of the device drops very significantly for increasing Tj. At a constant 60 mA you may get about 10.25 mW optical power out at Tc = 20C (Tj = say 30-45C) but only about 60% of that at Tc = 60C. As Tj and Tc rise power out drops, you need more power in to compensate, temperature rises further and efficiency drop further and you end up with VERY high Tc, low optical output and unacceptable efficiency. (Marcus covers that partially with his comment on the control loop.)

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Using the Osram PLT5 510 data sheet that Marcus cites.

Page 2 gives condition for 10 mW output at 10 mW optical.

At an unrealistic Tcase = 25C the specifications are:

Vf = 5.5V typical, 7.0V max. Idc = 60 mA typical, 100 mA max.
Internal thermal resistance of the device Tjc = 34 K/W

Neglecting the small amount of "cooling" due to radiant LASER energy, the heat power to be dissipated = V x I = 5.5V x 60 mA = 330 mW typical and 7V x 100 mA = 700 mW max.

Internal IC temperature rise = Tjc x power = 34 x 0.33 to 0.7W = 11 C to 24C rise junction to case.

Allowable Tj operating = 120 C (page 1) BUT allowable Tcase_operating = 60C. These two specs plus the expected temperature rise internally "do not make sense", but we'll see what can be managed.

Looking at the optical power output curve cited by Marcus we can see that at constant 60 mA, output at Tc (Tcase) =
Tc mW-out
0 11.5
20 10.25
40 8.5
60 6.5

ie power drop per degree C increases with increasing temperature and at 90C case that you report, at 60 mA you'd expect Pout of perhaps 3+ mW - about twice what you report at your unstated V_LASER and I_LASER.
Essentially, you are baking it to death.

As Marcus says, it MUST be cooler.
To get it to TCase = 20C and worst case input of 24 C rise Tjc at 0.7 W you'd need a heat sink between ambient and case of thermal resistance
Rac = (Tc-Ta) / Power
(as Trise = power x resistance so resistance = delta_T/Power.)

Unless you are working in an unheated lab in higher lattitudes in winter, to get a Ta that works in practice you need perhaps a Peltier cooler or some other source of coolth in the say 0-10C range.
eg at Ta = 10C, Rac = (20-10)/0.7W ~= 14 C/W heatsink, and at Ta = 0C (eg ice + water) you need a 28 C/W heatsink.
So eg maintaining a bath of meltwater around a block of water ice as your cold source allows a relatively modest heatsinnk and passive cooling or an even more modest one and a small fan.

IF your Ta / air temperature is not over say 20C you could use vigorous air blowing and as useful a heatsink as you can manage and probably maintain Tc in the 25-30C range. This will decrease your power output by about 10 - 15% compared to 20C at constant current - but if you can test select diodes by hand the spec sheet suggests that a better than 2 times improvement in output may be possible.

SO:

  • Select for a good output diode.

  • Heatsink as well as possible

  • Air blow heatsink.

  • Use as cool a source as possible - 0-10C ideal. 20C maybe bearable.

This is, not too too surprisingly, similar to what Marcus concludes, but may give additional insight into why it works. Or not :-).

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You have badly misunderstood your requirements and your options. Applying PWM will only improve the temperature of your driver, not your laser. And the laser is already the big heat source. Well, actually the driver IC in your schematic already uses PWM, which is why it is efficient.

"The laser is in a closed plastic casing and I can change the material, but I cannot add pipes or copper in the casing to cool it". If you want to increase the power of the laser, you need to pull heat away from it. this will have two limits - material and geometry. The obvious first step is to go to a better material than ASA. I'd recommend aluminum. Coolpoly seems an unlikely choice unless you have the ability to make your own injection molds.

But even with a better material (and aluminum is about a thousand times better than ASA), you may still run into problems with your housing geometry. If the material conducts heat away from the laser, it still has to go somewhere, so the housing will get warm. At diode levels of 10 mW I would not expect that to be a problem, but you need to keep it in mind.

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  • \$\begingroup\$ Yes, I need to make the inyection moulds, ASA and CoolPoly have the same mechanical properties, but different thermal coefficient, maybe it's better? or you think that it's the same? I cannot make in alluminium because the price and the complexity of the casing \$\endgroup\$ – Tom Bromel Apr 19 '17 at 12:31
  • \$\begingroup\$ @TomBromel - Unless you are making a commercial product, an injection mold will cost far more than machining a few housings out of aluminum. If you are making a commercial product, what you are proposing is almost certainly illegal. So which is it? \$\endgroup\$ – WhatRoughBeast Apr 19 '17 at 12:37
  • \$\begingroup\$ I have a manual plastic Injection in my workshop and a CNC router to make the moulds (3 axis), it's easy for me, because if i make the machining of the pieces I need at least 5 axis, and i cannot use my router. \$\endgroup\$ – Tom Bromel Apr 19 '17 at 13:09
  • \$\begingroup\$ Sorry, but what is illegal? \$\endgroup\$ – Tom Bromel Apr 19 '17 at 13:10

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