Been contemplating switching over to the new-fangled Linear Technology 3080 three terminal regulator. This one has some nice features, specifically a very low internal noise floor and programmable output with just a single resistor. Several Spice simulations show that is has both plusses and minuses with respect to the LM317. In addition to lower noise floor, the other plus is about 3 dB better power supply ripple rejection than the LM317. Minuses are that it has about 8 dB worse load regulation than the LM317. So, for now, I do not see that the benefits outweigh the drawbacks in this application, so I’m sticking with the LM317 as I continue to plod through this thing.
Not all LM317s are created equal! The On Semiconductor LM317T oscillates madly at about 55 KHz when used in this application. It has done this consistently with three different samples, so I don’t believe it’s just a “bad” sample I have. So far, the ST Micro LM317T performs admirably—extremely quiet DC coming out and is able to hold its liquor…until you exact max current (350 mA) at somewhere around 30V output, which means the fets are blocking somewhere around 520V DC. Under those conditions, the STMicro can’t control the gates, and poof… Oddly there is no oscillation that I can see on the scope at the moment of failure.
There are two additional brands of LM317 I’m going to try: Texas Instruments and Fairchild. I also ordered a few STMicro LM217s (a bit better performance than the 317). Perhaps one of these will do the trick. I have high hopes for the TI brand. TI seems to make high quality stuff.
By the way, it did occur to me (several times) that maybe this design simply can’t do what I’m asking it to do, namely the hard task of supporting max current at min voltage output. I realize this is a hard requirement, but a full-proof design must be able to support it. It’s not realistic to ask the operator to “please make sure you don’t ever short the leads”—it’s going to happen once or twice I just know it. If I were making a “simple” relatively small adjust range that did not need the requirement of surviving a full-on short, I’d be done weeks ago. Indeed, this has been a most challenging project.
But I’m not to the point of throwing in the towel yet, however I admit I've been tempted a time or two (or three) over the last few weeks.
If these other brands of LM317 do not work out, I think I will have exhausted all of my available options, although I could try a smaller form factor fet (TO-220 outline instead of the TO-247 outline I’m currently using). That might give the LM317 an easier load to work with….dunno.
Not all LM317s are created equal! The On Semiconductor LM317T oscillates madly at about 55 KHz when used in this application. It has done this consistently with three different samples, so I don’t believe it’s just a “bad” sample I have. So far, the ST Micro LM317T performs admirably—extremely quiet DC coming out and is able to hold its liquor…until you exact max current (350 mA) at somewhere around 30V output, which means the fets are blocking somewhere around 520V DC. Under those conditions, the STMicro can’t control the gates, and poof… Oddly there is no oscillation that I can see on the scope at the moment of failure.
There are two additional brands of LM317 I’m going to try: Texas Instruments and Fairchild. I also ordered a few STMicro LM217s (a bit better performance than the 317). Perhaps one of these will do the trick. I have high hopes for the TI brand. TI seems to make high quality stuff.
By the way, it did occur to me (several times) that maybe this design simply can’t do what I’m asking it to do, namely the hard task of supporting max current at min voltage output. I realize this is a hard requirement, but a full-proof design must be able to support it. It’s not realistic to ask the operator to “please make sure you don’t ever short the leads”—it’s going to happen once or twice I just know it. If I were making a “simple” relatively small adjust range that did not need the requirement of surviving a full-on short, I’d be done weeks ago. Indeed, this has been a most challenging project.
But I’m not to the point of throwing in the towel yet, however I admit I've been tempted a time or two (or three) over the last few weeks.
If these other brands of LM317 do not work out, I think I will have exhausted all of my available options, although I could try a smaller form factor fet (TO-220 outline instead of the TO-247 outline I’m currently using). That might give the LM317 an easier load to work with….dunno.