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High Current Laser Diode PSU

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EEYORE

Sun Mar 15 2009, 01:29AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello
I used the circuit in the app notes using a potentiometer on the trim pin. With a load resistor across the output, it will trim down past 2 volts actually.

It is very odd indeed. Seems it just doesnt regulate at all when drawing current (in the amp range). I havent scoped it yet for this, but scoped another of the same type while running a laser and saw a clean DC line of 5 volts.

I need to try hooking up another flatpac to rule out this one being faulty. Assume it is ringing, might some capacitance fix this?

Matt

Electroholic

Sun Mar 15 2009, 07:06PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

if your output current is dependent on input voltage, first thing i would check is Vgs on the fets. You will probably see it going down instead of up as you down trim the input voltage.

EEYORE

Mon Mar 16 2009, 04:52AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello,
The Vgs on the Fets is deffinately one reason for the problems. I took a closer look at the data sheet and they need at least 4volts to get past 30A. That solves the issue of not being able to get enough current, but it doesnt explain why the voltage drops from 5 volts to 4.6 volts when I pull 25A. I beleive I can pull max current as I cranked it up past 25A (meter only goes to 25A) and the flatpac shutoff (for overcurrent).

There is still the problem of hissing from the flatpac. It doesnt hiss below 5amps, but hisses on its way up to 15A alot and backs off some around 25A.

Weird!

So now comes the choice: Find FETs that can work at 2.7volts or get a stronger fan to cool off the heatsink. Without the ability to trim down some (Which I cant now), the power is going to be 200W - 80W (laser diode) = 120W!!!!!Yikes!

EDIT: I hooked up the old flatpac and got the same results. Lots of hissing as well from this one. Voltage drops as current goes up. I do have alot of long wires, maybe that has something to do with this? Maybe the output needs some capacitance? The output is also VERY erratic! Current goes up and down and the output voltage "quivers". It looks as if the thing is struggling to regulate..

Matt

hotcrazyfruit

Tue Mar 17 2009, 02:58PM

banned on 5/26/2009
Registered Member #1877 Joined: Mon Dec 22 2008, 02:03AM
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Posts: 147

for the mosfet's i think u mentioned you needed better ones, there are sample able ones on

that are rated for 40 amps at 1200 volts i think. u can sample 3 per month, and they are worth 30 bucks each!
hope i helped, Nick
edit : i found the link :

st.com/stonline/stappl/productcatalog/app?path=/ pages/stcom/ PcStComGenerateTableViewHTML.onClickOfRPN&primaryh eader=Transistors&secondaryheader=Power% 20MOSFETs&subclassheader=N-Channel%20(>650V) &subclassid=1168&rpncode=77616&producttype=product &open=&applilevel=

when posted as a link,m it doesn't work so copy and paste :)

EEYORE

Tue Mar 17 2009, 04:21PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

hotcrazyfruit wrote ...

for the mosfet's i think u mentioned you needed better ones, there are sample able ones on

That still just links to their homepage...Anyways, the transistor you are refering to likely is NOT a good choice seeing as its rated for 1200volts. Its likely got a high Rth and on-resistance. No idea of its minimum Vgs, but I suspect its high too.

My mosfets are pretty good. Pretty darn hard to beat their specs for this application. Remember, this is a low voltage high current application.
Matt

Proud Mary

Tue Mar 17 2009, 04:25PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

What about the popular and inexpensive LM338T adjustable regulator, which needs but a couple of discrete components to make an adjustable current or voltage regulator offering 0 - 5A programmable constant current, and/or 1.2 - 37V adjustable voltage. Current regulation is, I believe, better than 1%.

I'm not a laser person, and so don't know all the ins and outs of their PSU needs, but you'd be hard pressed to beat the simplicity of LM338T if you want a stable good quality variable current/variable voltage regulator.

I'd expect to pay about Â£1, a dollar and a bit for them in single units.

I use them a lot, and swear by 'em, and Eirik/"Uzzors" is now using one to control the filament current in his X-ray tube with good results.

You can download a data sheet for LM338T here:

5A is the minimum guaranteed output, with 7A cited as a typical maximum, so don't forget a really good greased heatsink if you choose this technology!

EEYORE

Tue Mar 17 2009, 07:00PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Harry wrote ...

What about the popular and inexpensive LM338T adjustable regulator, which needs but a couple of discrete components to make an adjustable current or voltage regulator offering 0 - 5A programmable constant current, and/or 1.2 - 37V adjustable voltage. Current regulation is, I believe, better than 1%.

I'm not a laser person, and so don't know all the ins and outs of their PSU needs, but you'd be hard pressed to beat the simplicity of LM338T if you want a stable good quality variable current/variable voltage regulator.

I'd expect to pay about Â£1, a dollar and a bit for them in single units.

I use them a lot, and swear by 'em, and Eirik/"Uzzors" is now using one to control the filament current in his X-ray tube with good results.

You can download a data sheet for LM338T here:

5A is the minimum guaranteed output, with 7A cited as a typical maximum, so don't forget a really good greased heatsink if you choose this technology!

Hello,
thanks for the tip, but I need to control up to 50A max.

Plus, the regualtors also waste alot of power as well as need large beefy rheostats for the constant current setup.
My setup is pretty good. I shortened the wires some and things are better. Voltage doesnt drop anymore when cranking up the current. There is still some hiss, but much less. (seems to go away once im up past 25A) I probably cut out 50% of the wiring lenght and also went to 12awg wire instead of two 16awg wires in parallel. Deffinately makes things harder to do with short wires

Heating will drop some with a new high power fan rather than the pitiful one I have now. (Heatsink gets hot with no load, but just gets warm with a laser connected). No idea of the flow rate of the fan, but its pretty quiet and doesnt seem to push much air.
Matt

Proud Mary

Tue Mar 17 2009, 07:50PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

mattrg2 wrote ...

5A is the minimum guaranteed output, with 7A cited as a typical maximum, so don't forget a really good greased heatsink if you choose this technology!

Hello,
thanks for the tip, but I need to control up to 50A max.

There's no need at all for giant rheostats, and one can use LM338 to feed the bases of some cheap, fat NPN pass transistors up to any amperage you care to mention.

But I agree that this kind of technology helps to keep down your central heating bills, but suspect it imay be too simple to design and implement to appeal to many experimenters today! (I know my suggestion has a very 1970s technology flavour to it!) But there's better ways of being Green than worrying over the dissipation of a voltage regulator, and Heaven knows, I might have suggested using a massive mercury-argon thyratron on one of my thermionic nostalgia days!

I wish you well with your very interesting project, and look forward to hearing of your eventual engineering solution to your PSU problem.

Electroholic

Tue Mar 17 2009, 10:58PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

Matt
to down trim Input voltage, you can feed the opamps with say a 12V source with the same ground, and keep the drain hooked up to the down trimmed flatpack.
something like this should work.

Sulaiman

Tue Mar 17 2009, 11:32PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I've got some SUM110N05-06, may be the kind of thing?
rated 110A 55V low gate voltage and charge, 6mOhm
(but you'll not get such low Rds(on) in practice due to temperature and gate voltage)
and smt about 10mm x 16mm including leads etc.
There are probably newer better variants now, try the Vishay Semiconductors site.
I think I have nine unused - for postage cost I could pop 2 or 3 in the mail.
Can't remember if I bought them or sampled them from Vishay, may be worth you looking at the Vishay site for current samplings?
Also, can't remember if Vishay do samples!

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