L200 Dual Voltage Design Help

[FenderBender]

I don't really have a "good" power supply...I have some cheap wallwarts that I can rig up with an LM317 and get the job done, but nothing dedicated, which becomes frustrating.

So I'm embarking on a mission...I'd like to build an L200 dual voltage power supply. I believe +/- ~15V will be sufficient. I am trying to keep cost at a minimum and quality high. I don't need tons of features, but I do want it to last a while. Blah blah blah

Okay, so I looked at the L200 designers guide and picked out the Fig. 21 design which is a positive/negative supply. http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/APPLICATION_NOTE/CD00003773.pdf

I have a few questions because I can't really see the drawing too well. There is a dotted line between the positive section and the negative section of the power supply. It's labeled "tracking". Is that saying that the wipers of the voltage adjustment potentiometers are connected to eachother? I don't really know what the term "tracking" means...

Also, can I simultaneously control current by putting a potentiometer between pins 5 and 2? Or will the output voltage be affected? ( I don't think it should).

The reason I ask is if you look at the regular L200 datasheet, http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000053.pdf, if you look at figure 23, they use a 741 op amp with the L200 to get variable current and voltage. Is this not possible to do without an external opamp? So if I wanted to do +/- 15V with variable current and variable voltage, I would need to use op amps? If I wanted a fixed current limit, say 1A, set by a single resistor, would I need the opamps? This is all going back to the original figure 21 dual supply.

Thanks a lot.

[FenderBender]

Alright feel like an idiot. The reason I guess pots are not frequently used with the current limit is because the value is like 0.5ohm...I've yet to find a 0.5ohm pot. It would also need to be like 3-5W. Bleh, I'm dumb. However, I could simply throw in a switch and have say a 500mA limit and a 1A limit, or something along those lines.

[Bored@Work]

I am trying to keep cost at a minimum

Then you are going the wrong route. You will not be able to beat the cheap Chinese "lab" power supplies you find everywhere. Already the basic parts (enclosure, heatsink, transformer, panel meters, etc.) will cost you more than a complete Chinese power supply with similar specs. These Chinese power supplies typically happen to be build around ICs like your L200, the LM723 or the LM317.

This is of course usually the moment where someone come up and tells us that he managed to build his first-class power supply for less then $5 - because he happened to have all the parts in his part's drawer. Fat change that you have them. And I tell you, not everything someone happens to have in his part's drawer happened to end up there free of charge. Or someone comes up telling us is so great to use an old PC fire starterpower supply ghetto style.

Regarding your L200 in a bench power supply, it has the problem most of the linear regulators of its generation have, the inflexible current sensing (if any) on the high side. The old-school solution to that is to add separate current sensing in the form of a rail-to-rail OpAmp. Either configured as a differential amplifier (with the usual problem of matching resistors), or configured as a current mirror (with an additional NPN transistor). That way you get your current sensing reference with respect to ground, i.e. a  normal pot can be used. Figure 36 in your app note shows the second version (current mirroring). Another common way is to use a current limiting circuit with a second transistor.  Figure 35 in your application note shows such an example.

[FenderBender]

Thanks. I'm not sure if I'd go the op-amp way or do something simple with the integrated current limit on the L200.

For the dual voltage supply in the "Designers Guide", are you just making two positive supplies but flipping the output on one so that it is a negative voltage?

[Bored@Work]

For the dual voltage supply in the "Designers Guide", are you just making two positive supplies but flipping the output on one so that it is a negative voltage?

Figure 21? One is not really flipped. That would kill the IC in the worst case. It is two times the same supply, just that the + output of the lower one is connected to the - output of the upper one, and that connection called ground then.

Such a dual voltage supply is kind of a running gag in those old data sheets and application notes. Completely superfluous, because it is trivial. Companies not having a matching negative regulator for their positive regulator like to show them, so they can claim that one can be build with their regulator, too. I find that habit annoying.

A key requirement for such a supply is to have a transformer with two separate secondary windings. A transformer with a center tapped winding will not work. If you have a transformer with a center tap only you need a negative regulator. Figure 19 and 20 show some versions with an OpAmp on the negative rail. But these versions are crutches. It looks as if they desperately tried to stick with ST amps, but didn't have anything good enough.

[FenderBender]

Oh...I see...I need two completely separate windings. Well I mean I guess I could just use two transformers, but who wants to do that?

I'd do a switching supply but I don't know of any simplistic switchers with matching positive and negative parts. Actually I don't know of too many negative switchers in the first place...

Well I'm planning on doing some work with op-amps and some lower power stuff, so I don't need a crapton of current output. I might just go with a simple LM317/LM337 dual supply. Perhaps I could throw in two more of those in current regulator mode...ideas?

Thanks.

[Bored@Work]

Well I'm planning on doing some work with op-amps and some lower power stuff, so I don't need a crapton of current output. I might just go with a simple LM317/LM337 dual supply. Perhaps I could throw in two more of those in current regulator mode...ideas?

The LM317 has a similar inflexible way of current limiting than the L200. Just that they don't have an extra pin, and that the reference voltage for voltage regulation is also on the high side, allowing it to be (ab)used for current limiting, too. Adding an opamp for flexible current limiting is even more of a pain. I think the LM317 datasheet has an example.

[FenderBender]

I meant do a way with current limiting...

[FenderBender]

Well here's the new plan. I want to use an LT3080 in adjustable current limit mode, and then follow the LT with an LM317/350/338. And then repeat that for the negative side. I can pretty easily set up two separate secondaries. Not really an issue, for me.

Key question, in regards to the dual supply thing where the output of one is switched to achieve "negative voltage"...can this be done on a switching regulator as well, perhaps an LM2576? Or do smps designs not make this possible? Maybe I wouldn't get such abysmal efficiency, though I'm not sure how badly the LT3080 would screw around with efficiency in current regulator mode.

Thanks.