Specialized Power Supply

[rocketPack]

Hello!

I am trying to build a low cost power supply (driver). It needs to be current limited (adjustability is good). It's going to be used to operate an optical device so regulation based on feedback from a photodiode would be superb. I've looked around at a lot of circuits but none seem to fit the bill quite right. Perhaps I can modify one, but I want to see what ideas you guys might have. The specs are:
Voltage: 3v constant
Current: Up to 750mA max required

The input is still up in the air - but it would be highly preferred if it were somewhat 'flexible'. For example, I can use a 12v DC power supply but it's not going to be a rock solid 12v. I'm also considering 120VAC but using an already regulated and limited supply (such as a 12v one like I have) seems safer, easier, and more ideal.

Here comes the hard part... It would be really nice if the unit could produced a high speed pulsed output, somewhere in the 50kHz range.

Other features:
- Soft start (for equipment protection)
- Photodiode feedback control (like I mentioned earlier)

Constant output is probably tolerable, however, and it can be done without the feedback control as well, though less ideally.

I've looked at several dozen circuits and they all seem to have problems in my specific application. One thing I considered was this:
http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=776079&e_categoryid=100&e_pcodeid=65253

My only problem really is finding a way to accurately and reliably control current; I want to avoid a simple 'put a resistor in series' approach.

If anyone has links, suggestions, or reference designs I could work from I'd be very greatful. Thank you!

 

[Tony_Stoynov]

Hi,
you can use the SMPS, some like the LM2575, but with a current feadback. Another choicesh is t use linear regulator, LM317 etc., but in this case the power over the regulator will be higer, for example, if you use 2575, and for 500 mA he voltage over the photodiode is around 3 V, and your power supplay is a 12 V, then over the LM2575 you will have around 0.5 W, in too same case over the LM317 you will have a 4.5 W, this is a big different. If you have a uC you can use the integrated PWM module and ADC or compartor to realize the current source.

 

[rocketPack]

I googled the LM2575 and I found an amazing feature of national.com - I told it what I wanted from my power supply, and it picked all the components, made a schematic, and even ran an analysis for me! This is incredible! You definately gave me a 'push' in the right direction... thank you!

 

[ante1]

Hi rocket,

You might want to try this one:

View attachment 38622

 

[rocketPack]

Hey, I wish I'd seen that last night...  :-X anywho, I ordered a 'sample' of a LM5010, their website drew me up a schematic and listed components and stuff so I bought them all from mouser (only about $12 + $11 for the 'sample'). Let me know what you think of this circuit, it looks very well regulated and filtered so it should be a good starting circuit (money is tight so I imagine this is about as good as I will get for this $$$). Opinions? I can still make adjustments if necessary!

Link to FULL SIZE version (1086x445, 180kb):

Sorry for the size, I just got a new widescreen monitor so I've got 1680x1050 pixels to play with... but I forget that not everyone has ;D

 

[ante1]

Hi rocket,

You said “It needs to be current limited (adjustability is good).” can this be accomplished here?

 

[rocketPack]

So I'm told... I'll show you what I did:
I googled "LM2575", and found this result:
http://www.national.com/pf/LM/LM2575.html
I went there and filled out the " LM2575 WEBENCH® Custom Design/Analyze/Build It/Test It" box with my exact specs, and then it said I could not achieve those specs with the LM2575, but I could with the LM5010. So I went to the LM5010 page and did the exact same specs, and it told me exactly what to buy, a schemat., etc. My settings were:
Vin MIN: 8v
Vin MAX: 16v
Vout: 3v
Iout: 0.75A

The output is a 676kHz PWM signal @ 2.25W max.

Do you see an issue though? It looks to me like the Iout is maintained by the feedback and the parallel resistor/cap, but I'm no expert at this stuff. If you see an issue let me know before I hook expensive stuff up and 'let the smoke out'

Thanks!

-----------

Edit: Upon re-reading your question, it seems you might be asking more about the adjustability - on that matter, I don't know how easily it can be adjusted, honestly. Like I said, I think it has to do with the parallel resistor/cap and the feedback circuit; do they make super low resistance, high power pots at decent prices (<$5)? If someone could explain the circuit to me a bit better (I'm only slightly able to understand it) that would be good too. I don't mind doing the leg work, but I need to know at least a little bit before I can do anything useful.

I should also mention that EWB doesn't have an LM5010 or any other IC that has the same footprint (well, honestly, I didn't look very hard) so I took some other IC and renamed everything; that's why there is an extra pin...

 

[gogo2520]

Hi Ante
i Like you power supply Using LM 317's Lm350's I got some of both. If I were to use Lm350's and used resistors for ajusting current on the first stage what wat resistors should I use. I'm looking at maybe 2 to 3 amps
                                    Thanks gogo

 

[ante1]

Hi gogo,

Yes, the formula is right there on the drawing (Ohms Law) but I would use at least twice the calculated wattage eg. for 3A I pick a 0R4 - 3W resistor (1.2W calculated).

 

[gogo2520]

Thank You Ante

  Bigger is better, good thing because I have a bunch of 5 watt resistors I have been  saveing. I have been tearing apart old dot matrix printers and finding all kinds of good stuff. What a goldmine and people almost pay you to take em off there hands.
                            Thanks again
                                gogo

 

[rocketPack]

Well, I tried pricing out the ckt that Ante mentioned, but it's coming out at $38 before you count a proto board and a box + wiring (just the components themselves), which is out of my price range.

I was wondering though - all I really need is a stable, regulated current supply at 3v. Does that make things any easier if you take v. adjustment out of the picture and just focus on obtaining a stable current supply? Well, I imagine it does... I did some googling and I didn't come up with much. I tried to use EWB to simulate using a voltage regulator set for constant voltage feeding into another set for constant current. It did not work well... at all. I can't remember which one I was using (perhaps something similar to LM117?) but I followed the manuf. data sheet's "typical application" and everything. Any ideas? I really appreciate all your help!

 

[ante1]

Hi rocket,

Even if the voltage is as you put it “out of the picture” we must still know the voltage range in which you want a constant current. What kind of circuit is this going to power, does it have a constant internal resistance?

 

[rocketPack]

Same paramaters, 3v. I didn't mean "out of the picture" entirely, I just meant a fixed voltage, constant current. Anyway, amperage would also be the same... minimum adjustment of down to around 25 or 50mA (or less) preferred, and it has to go up to at least 750mA. Anything more is fine, I just wont be using it. The resistance will vary a little bit as it heats up, like you'd expect. The circuit is a diode laser, by the way - so it's obviously going to sink as much current as it can find without some kind of limit. Thanks for your input!

 

[ante1]

Hi rocket,

A guy named Ohm once decided that U=R*I and R=U/I and I=U/R.  ;D
And I agree!

 

[rocketPack]

right... that'd be the first thing I learned in electronics class, except that I don't have a 3v power supply, nor the interest in buying hundreds of resistors of various values. I was looking for something more or less.... usable.

 

[ante1]

Ok, for a laser you need a specified voltage in this case 3V. If you current limit the voltage source below what the laser draws then the voltage will drop too. So you can't have both!