LM317 pre-regulator for wide output voltage range - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: May 15, 2017, 05:33:15 am
content: Here's a little circuit I threw together back in about 2003 for a simple linear bench supply that could output 1.2 volts all the way up to 130V. It could go much higher if necessary. Has been working just fine all this time. There are some improvements that could be made. e.g it is not short circuit proof. The 15V zener might actually be a 10 volts. Couldn't quite see it. If anyone finds it useful then great.

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2017, 08:00:48 am
content: It should work. The zener might be better off 10V to ensure the MOSFET's gate voltage doesn't exceed the maximum rating.It's strange you should post this. I came up with a similar solution but using a Darlington pair. The advantage with a MOSFET over a Darlington pair, is it doesn't require any current to remain on, so less current runs through the zener, bypassing the regulator. The disadvantage of a MOSFET is it requires a higher voltage to turn on, so the drop-out voltage will be higher, than with a Darlington pair.The value of R4 might need to be reduced, since the LM317 requires a minimum 10mA of output current, otherwise the output voltage may rise, under light loads.https://www.eevblog.com/forum/projects/5-ampere-regulator-using-lm-317-or-lm723/msg1207823/#msg1207823How is it not short circuit protected? As long as the MOSFET can dissipate the short circuit current for long enough for the LM317 to shut down, I don't see a problem.

author: David Hess
author_rank: Super Contributor
timestamp: May 15, 2017, 04:15:52 pm
content: I have used the same type of circuit. For fixed output operation, I would include foldback current limiting to protect against continuous output shorts.

author: aandrew
author_rank: Frequent Contributor
timestamp: May 15, 2017, 05:31:28 pm
content: Interesting little circuit, thanks for sharing!Looks like a nice heart for a linear bench supply; add a programmable current limit for CC and some metering... nice!

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2017, 05:46:05 pm
content: Quote from: David Hess on May 15, 2017, 04:15:52 pmThe LM317 and LM78xx series do that anyway. Once short circuited, the current drops to a fraction of the original level.

author: David Hess
author_rank: Super Contributor
timestamp: May 15, 2017, 06:41:34 pm
content: Quote from: Hero999 on May 15, 2017, 05:46:05 pmThe LM317, LM78xx, and similar regulators have safe operating area protection which is not quite the same thing (1) but this does not operate when an input cascode is used to support higher voltage operation and the cascode itself is not protected.This is a real problem with floating regulators. The LM317 can regulate high voltages in a floating configuration but if there is an output short and the input voltage does not collapse, then the input to output rating of the LM317 will be exceeded destroying it. The cascode prevents this from happening up until the time that *its* input to output voltage rating is exceeded or it overheats.So to protect the cascode which protects the regulator, external foldback current limiting or SOA protection should be added.(1) Safe operating area protection limits the output current based on the input to output voltage difference. Foldback current limiting is simpler and just lowers the current limit as the output voltage falls. The results are similar but they operate in different ways. SOA protection in integrated regulators is actually preferred at low input to output voltage applications because it allows hard starting into a difficult load where foldback current limiting would fail.

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2017, 09:51:39 pm
content: Quote from: David Hess on May 15, 2017, 06:41:34 pmPerhaps it was the safe operating area protection kicking in when I short circuited it? I didn't measure the voltage across it or power being dissipated so can't be sure.Anyway, I don't see why the safe operating area protection on the LM78xx or LM713 can't be used to protect the cascode. All that needs to be done is the voltage across the regulator is high enough to trigger the safe operating area protection (but still within the maximum rating) when short circuited. The regulator will then cut its output current which will also limit the current through the cascode.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 15, 2017, 10:17:23 pm
content: Use an tl783 1.25 to 125 vdc regulator with a current foldback circuit, found schematics on the web and built a nice regulator, can't find my schematics

author: Seekonk
author_rank: Super Contributor
timestamp: May 15, 2017, 11:15:59 pm
content: Thanks, I've been meaning to build a low power supply like this. Wouldn't use it to invest a lot of time in it. This just might get me around to building one

author: Circlotron
author_rank: Super Contributor
timestamp: May 15, 2017, 11:24:01 pm
content: Hero999, I didn't realise your circuit was so similar to mine when I saw it the other day. I was thinking D1 was for some kind of reverse polarity protection for the LM317. But yeah, they are pretty much the same thing in principle. Great minds think alike.

author: Circlotron
author_rank: Super Contributor
timestamp: May 15, 2017, 11:27:52 pm
content: Quote from: coromonadalix on May 15, 2017, 10:17:23 pmI've also done one with a TL783. They are nice. Having a mosfet upstream though means you can choose one as big as you like.

author: VEGETA
author_rank: Super Contributor
timestamp: May 16, 2017, 10:04:55 am
content: I remember that LM317 has a voltage limit of something like 37v. How do you operate it at such high voltage? I saw people putting a power transistor in parallel to increase current but here it is in series.

author: JPortici
author_rank: Super Contributor
timestamp: May 16, 2017, 10:20:06 am
content: that's actually the difference between VI and VO. I don't see a max value on VI

author: VEGETA
author_rank: Super Contributor
timestamp: May 16, 2017, 11:04:51 am
content: Quote from: JPortici on May 16, 2017, 10:20:06 amYes you are right, it is differential voltage. However, I guess the darlington or mosfet is there to make sure this difference is maintained hence it is pre-regulator right?why not using a switching one like those on ebay then put lm317? much more efficient and better.

author: bktemp
author_rank: Super Contributor
timestamp: May 16, 2017, 11:36:41 am
content: The circuit does work, but shorting the output when set to a high voltage will certainly damage the whole circuit:When shorting the output, C4 gets discharged through the LM317 most likely frying the regulator -> add a diode from ADJ to OUT to provide a discharge pathThe same applies to C3: The input-output differential voltage will easily exceed 50V when the output gets shorted. This will not only fry the LM317 but also the mosfet: Output at 0V, therefore the gate at +10V, but source is still at >50V, generating a gate-source voltage of about -40V.The SOA protection of LM317 does not help in this application: If you look at the datasheet, it specifies 1.5A (min.), 2.2A (typ.) for 15V differential voltage. So in this circuit with a differential voltage of <10V, the SOA protection will never get activated and therefore the LM317 will limit the current to 2.2A (typ.), dissipating >300W in the mosfet. This will exeed the FBSOA quickly, resulting in a shorted mosfet and the full input voltage applied to the LM317.So how to solve these problems? Removing C4 or at least making it much smaller is probably a good start. Then I would add a current limiting circuit maybe combined with a foldback circuit for limiting the maximum power dissipation of the mosfet. Adding the current limiting to the mosfet is probably the easiest way to do it: A single transistor + 2-3 resistors should do the job.

author: Circlotron
author_rank: Super Contributor
timestamp: May 16, 2017, 12:18:34 pm
content: ^^ bktemp is dead right. The circuit is good enough for what I wanted it for but is by no means foolproof. There are a number of things that can be done to improve it. As it stands it is a demonstration of principle rather than a finished design.

author: David Hess
author_rank: Super Contributor
timestamp: May 16, 2017, 05:32:30 pm
content: Quote from: Hero999 on May 15, 2017, 09:51:39 pmExactly, at a high input to output voltage, the SOA protection operates like foldback current limiting. At low input to output voltage, it does not.QuoteThat would work. Another way and probably what I would try is to scale the input to output voltage across the regulator so that it tracks the input to output voltage across the cascode transistor. This is similar to how in the National application note on current boosting, the sharing of current between the regulator and current boosting transistor is carefully controlled so that the regulator's SOA protection including temperature and current limiting protect the transistor.But what I did in the past is simpler; I just implemented foldback current limiting which operates at any input to output voltage and effectively shuts off the regulator when an output short is present.