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Old 01-14-2016, 05:17 AM #1
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Default How to drive 2x40W Coherent Laser diode bars?

Hello HIMNL9 or any other Gurus out there,

I have 2x40w Coherent diode bars from Surplus. I want to run both similtaniousely (at the same time together) from one driver supply if that possible.

I am planning to have these two bars (simultaneously) but transversely pumping a small (28mmx9mmx2.5mm) Nd:YLF slab. I have a switch mode power supply that can deliver 5V at 40Amp (regulated a bit like the computer powersupply).

Can I have these two laser diode bars run on the same driver schematic as from HIMNL9 and ronald1711 (there was a thread in year 2010, but can't find it no more).

If so how do I connect the diodes (can I connect them in parallel or series together).

I will very much appriciate any experts' comments and insight or anything I overlooked.

Thanks.



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Old 01-14-2016, 02:39 PM #2
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Default Re: How to drive 2x40W Coherent Laser diode bars?

Best way to connect them would be in series if you want to use just the one driver, that said, your SMPS may not provide enough voltage for two diodes in series once you account for the dropout on the driver. In that case, you'd probably be best using one driver for each diode.
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Old 01-14-2016, 05:15 PM #3
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Default Re: How to drive 2x40W Coherent Laser diode bars?

Thanks diachi,

Ok, I can make 2 x sink drivers (driver schematic as from HIMNL9 and ronald1711 as per the thread in year 2010). Each will drive each diode bars seperately.

I can supply these sink drivers from the SMPS that can deliver 5V at 40Amps (regulated). What is the best way to connect these two sink drivers to the SMPS.

I was thinking parallel.

Any suggestions please.
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Old 01-14-2016, 05:16 PM #4
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Default Re: How to drive 2x40W Coherent Laser diode bars?

Quote:
Originally Posted by Vis View Post
Thanks diachi,

Ok, I can make 2 x sink drivers (driver schematic as from HIMNL9 and ronald1711 as per the thread in year 2010). Each will drive each diode bars seperately.

I can supply these sink drivers from the SMPS that can deliver 5V at 40Amps (regulated). What is the best way to connect these two sink drivers to the SMPS.

I was thinking parallel.

Any suggestions please.
Yes, they'd need to go in parallel for that to work.
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Old 01-14-2016, 05:21 PM #5
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Default Re: How to drive 2x40W Coherent Laser diode bars?

the diodes need around 40A each to run at full power , if you run the in parallel , the 5V 40A SMPS wont be able to run them both a full power
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Old 01-14-2016, 05:27 PM #6
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Default Re: How to drive 2x40W Coherent Laser diode bars?

Quote:
Originally Posted by ionlaser555 View Post
the diodes need around 40A each to run at full power , if you run the in parallel , the 5V 40A SMPS wont be able to run them both a full power

Hmm, good point. I was thinking they were both closer to 20A, but you're right, they'd be closer to 40A each.

You probably could run them both off of that SMPS at full power, but not with a linear driver, it'd need to be a switching buck driver.
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Old 01-14-2016, 07:58 PM #7
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Default Re: How to drive 2x40W Coherent Laser diode bars?

'You probably could run them both off of that SMPS at full power, but not with a linear driver, it'd need to be a switching buck driver'

Can you please elaberate on that. Linear sink driver will be the best choice for the diode bars, I thought. I don't understand how the design principle for the switching buck driver works.
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Old 01-14-2016, 08:02 PM #8
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Default Re: How to drive 2x40W Coherent Laser diode bars?

Quote:
Originally Posted by Vis View Post
'You probably could run them both off of that SMPS at full power, but not with a linear driver, it'd need to be a switching buck driver'

Can you please elaberate on that. Linear sink driver will be the best choice for the diode bars, I thought. I don't understand how the design principle for the switching buck driver works.
Exactly the problem, building a switching driver would be much more of a challenge. Much easier to go with a linear driver and a bigger switching supply than you currently have to provide power to your linear driver.

If you want an explanation of the difference between them in terms of how they work then let me know. That said, Ionlaser can probably offer a better explanation than I can ...
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Old 01-14-2016, 08:37 PM #9
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Default Re: How to drive 2x40W Coherent Laser diode bars?

If you can explain the diffrences that would be great.

What is the spec for the SMPS should be? Should 5v 60A SMPS do? (It is being used on another ptoject at present, I can compenscate it for this project for a moment.

But, what spec SMPS be better?

Thanks.
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Old 01-14-2016, 09:18 PM #10
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Default Re: How to drive 2x40W Coherent Laser diode bars?

Also had another thought. I've just found another SMPS for sale from the same vendor. that is exact same spec as the one I already have. Wondering if I should just use one 5v 40A SMPS via liner sink driver for driving one 40W diode bar. If so, will each provide enough 'juice' for the LD bar each. I will have to live with the double bulky PS setup.

I guess I can live with that.
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Old 01-14-2016, 09:24 PM #11
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Default Re: How to drive 2x40W Coherent Laser diode bars?

Quote:
Originally Posted by Vis View Post
If you can explain the diffrences that would be great.

What is the spec for the SMPS should be? Should 5v 60A SMPS do? (It is being used on another ptoject at present, I can compenscate it for this project for a moment.

But, what spec SMPS be better?

Thanks.
I'd go for at least 80A@5V, 100A would be better - that gives you some overhead, seeing as each diode will need nearly 40A, that puts you at nearly 80A total. Two 40A @ 5V supplies would work too, although you might be cutting it a little close if you want to run the diodes at full power.

Linear supplies dissipate the excess energy as heat, much like a resistor in series with a load would. So if you have a load that needs 1A with a resistor in series, 1A will still flow through the resistor (Components in series see the same current!). Say you have a 10 ohm resistor and you have 1A going to your load through the resistor, that's a voltage drop 10V across the resistor, which means you are dissipating 10W as heat. I didn't add the calculations but the formulas used are P=IV and V=IR.

Switching supplies generally act more like a transformer. Many are basically transformers with an intermediate stage that rectifies the high voltage AC from the wall into high voltage DC then converts it into high frequency high voltage AC which allows the use of a much smaller/lighter transformer to drop the voltage to what is required, then rectifies the output again back to DC at the desired output voltage. Instead of dropping the excess voltage as heat they convert the voltage directly to what you need. So say your load needs 100W of power at 10VAC (That's 10A current [P=IV]). You provide 100VAC to a transformer which converts the voltage to 10VAC. Power must remain equal on both sides of the transformer, so if your load is consuming 100W your input to the transformer must be 100W. But instead of being at 10V it's at 100V.

Using P=IV we can see that the Current (I) is equal to 100W/100V, which is 1A@100V on the input vs 10A@10V on the output. In theory there is no energy wasted here, you put in 100W and you get out 100W at the desired voltage. Unlike the above linear example where you'd need to put in 110W to get 100W out, thus using 1.1A input instead of 1A input.

Switching supplies can also use PWM (pulse width modulation) with an inductor and capacitor instead of AC and a transformer with a rectifier to achieve the same result. See the faucet analogy below for a basic explanation of the PWM system.

An analogy:

Quote:
You can think of a linear supply as a sink with a faucet and a drain in which faucet flows at a constant rate, and the drain drains at a constant rate, but the flow of the faucet is greater than the flow of the drain. In order for this arrangement to work, you have to dump excess water somewhere; in the case of a linear power supply, the excess is turned into heat.


You can think of a switching power supply as a sink with a faucet and a drain in which the faucet can be rapidly turned on and off. The drain still doesn't flow as fast as the faucet does, but by toggling the faucet on and off you can make sure you don't overflow the sink.
I hope that wasn't too much for you ... you do need a basic understanding of electrical theory to understand it really, and I'm not so good with my words, so that doesn't help.
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Old 01-14-2016, 09:44 PM #12
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Default Re: How to drive 2x40W Coherent Laser diode bars?

Solution depends on your budget and electrical experience. If you have little money to spend, if you require no modulation, and are set on using the existing 5V supply, you can power them in series (provided their heat sinks are electrically isolated) with a big resistor to limit the current and drop the extra voltage. Something on the order of 25mΩ at 50W.

If you've got electrical knowledge, you can modify the feedback circuit in your supply to operate in constant current instead of constant voltage. Sounds like that's out of the question though.

If you've got $200 to spare, I can sell you a 0-7.5V 0-125A adjustable power supply that specializes in this sort of thing.
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Old 01-15-2016, 04:30 AM #13
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Default Re: How to drive 2x40W Coherent Laser diode bars?

I have a Coherent FAP800 with a 40 watt bar in it and I have been experimenting with a 60 amp CC power supply by driving it to full output at about 2 VDC without the current limiting turned down to keep the current below 50 amps and it ran just fine without heating up so much the current would begin to creep up, but I do have a very good heat sink on it too.

I then drove the bar directly with a Cyclon 2 VDC round lead acid battery cell which can supply hundreds of amps and had no problems with it. I suppose if the unit isn't heat sinked well enough the current would start to increase above the maximum recommended, but I never had a problem hooking it directly up to a battery without a CC driver using my 5 pound heat sink with fins on it. Would I recommend this? No, I'm just telling you what I experienced.

Edit: I'm back to add more info....

There is a LPF member using a Texas Instruments buck on a 40 watt Coherent bar and although it isn't a constant current regulator, it's working well for him. He's trying to wire it up using the sense leads and a resistor to keep it from changing current, not sure what twists he is using to do that. However, you might look at this as a possible driver, you can input up to 14 VDC and it will produce up to 50 amps out from .7 to 3.6 VDC, but you will have to use two of them for your bars, the output voltage isn't high enough to run them in series. I don't think they are very expensive and they run cool due to their high efficiency.

Constant voltage (not constant current) buck: http://www.ti.com/product/pth08t250w

I have a power supply much like Cyparagon offered you and it works well for me, you can run two bars in series with it and set the current too. His price is good.
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Last edited by Alaskan; 01-15-2016 at 05:32 AM.
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Old 01-15-2016, 05:23 AM #14
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Default Re: How to drive 2x40W Coherent Laser diode bars?

Thanks Diachi for explaning it using simple Ohms law.

I understood it thanks.

Also all the others for offers and your experiences with this type of LD. I thank you all.

Just, wondering. I realise that I don't have much head room if I use a 40A SMPS, but, will the LD run at lower currents. I take it the closest it going to recieve from the power supply will be around 38A or there about.

I am limited by funds as to howmuch I can spend on this project.

Having these run with 2x40A SMPS (one each) with their own linear sink drivers, giving them a dedicated current source that will improve practicality, and also thinking that, if these diodes runs just below their max threshold, then their life time will increase. Will this be so.

Though for my purpose (pumping), I would prefer them running at full power to get good gain on the YLF crystal.
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Old 01-15-2016, 05:41 AM #15
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Default Re: How to drive 2x40W Coherent Laser diode bars?

It will run just fine at lower currents, all the way down to about 12 amps, but only puts out a couple of watts at that low of a current, but 38 amps, you will get plenty of power out of it, probably 30+ watts.
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Last edited by Alaskan; 01-15-2016 at 06:14 AM. Reason: I said 35 watts, but probably closer to 30 watts out at 38 amps, edited it.
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Old 01-15-2016, 08:53 AM #16
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Default Re: How to drive 2x40W Coherent Laser diode bars?

Thanks Alaskan,
30+ watts of laser power and 60+ for both LD will be enough pumping for squezing a nice cool 'juice' of 1053nm wavelengths @ hefti ~26+ watts YLF output I think.

Which also means I don't have to drive my LDs hard at their max either.

Any sugestiins guys. Thanks.
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