Linear Voltage Regulator current limits... - Page 1
Forum section: electronics / projects

author: westfw
author_rank: Super Contributor
timestamp: February 12, 2017, 03:08:59 am
content: Most of us have learned by now that if you have a linear voltage regulator in a typical "Amateur" project (say, an LM7805-like regulator on an Arduino-like board), the current you can draw is more likely to be limited by power dissipation issues (and heating) than it is to be limited by the "maximum current" specification of the regulator. And yet, in a given package style, you are likely to find regulators with several different "maximum current ratings. (for a TO220, you might seen 500mA, 1A, 1.5A, 3A...) And you'll get the same chip inside different packages whose hope of dissipating the needed power is ... questionable, at best.So exactly WHAT do those current limits mean? I guess some of the regulators have built-in current limits as well as built-in "thermal protection" - does that mean that I'd actually be better off picking a regulator with a LOWER current rating that more closely matches the expected dissipation/draw? I mean, I'd expect the over-current protection to be both faster and less "harmful" than thermal protection; right? Is there any advantage at all to picking a regulator that is over-spec on the max current, if the only way I'd hit that current would also send me into "everything gets very hot" territory?

author: StillTrying
author_rank: Super Contributor
timestamp: February 12, 2017, 03:26:50 am
content: Many of them also detect a high input - output voltage difference, and won't supply their max. currrent because of that, even before theyget too hot.

author: Brumby
author_rank: Unknown
timestamp: February 12, 2017, 03:45:37 am
content: Don't forget - issues from thermal effects take time to manifest. Issues from peak current demands do not.Your design decisions will be based on both.

author: David Hess
author_rank: Super Contributor
timestamp: February 12, 2017, 11:06:40 am
content: Picking a regulator with a lower current limit is a good idea to minimize the short circuit current limit.The dies are *not* the same even in the same package. The higher current regulators have a larger die simply because the output transistor has to be larger to support the higher current limit and they do not make the transistor any larger than neccessary. One reason to use a higher current regulator in a low current application is for the lower junction-to-case thermal resistance provided by the larger die if a large external heat sink is used.

author: 2N3055
author_rank: Super Contributor
timestamp: February 12, 2017, 11:17:29 am
content: Current limits are peak values. Thermal limits are slow, average process. This is one place where RMS abstraction is helpful.Let's say you have a circuit that has 10 msec 1A pulses every second, and is drawing 1mA in between.. That is on average 11mA , and that is what you'l have heating your VR.Of course, if it has enough thermal mass not to overheat in 10 msec..And of course, voltage dropout over said VR.. There are tiny SMD regulators rated for several A, but you can use them only to drop 3.3V to 2.5V for instance, making it less than 1W..The relevant term here is SOA (safe operating area), both static and dynamic... And not only device, but of the device combined with your cooling solution.Thermal dissipation will be input /output voltage difference multiplied with average current, making sure that thermal mass is sufficiently smoothing out the power peaks....It gets very complicated quickly, if you need very optimized solution.In small batches, prototypes, and hobby use just massively oversize... On thermal side.. You want to limit short circuit current though..LM317 is popular because it has overcurrent, thermal and SOA protection built in... But all that is there only to give time to fuse or some other protection to react. You can't rely on it to keep indefinite short circuit.. (although I've seen it survive all kinds of abuse, but that's not guaranteed.)So yes, no universal answer.

author: 2N3055
author_rank: Super Contributor
timestamp: February 12, 2017, 11:24:46 am
content: Good read by ON semi..Linear & Switching Voltage Regulator Handbook ( HB206-D.pdf )

author: drussell
author_rank: Super Contributor
timestamp: February 12, 2017, 02:47:25 pm
content: Quote from: 2N3055 on February 12, 2017, 11:17:29 amMost linear regulatorsarespecified to be able to operate safely into a continuous short circuit indefinitely.

author: 2N3055
author_rank: Super Contributor
timestamp: February 12, 2017, 06:54:17 pm
content: Quote from: drussell on February 12, 2017, 02:47:25 pmWell you are right. Let's take LM317 (and its variations) as example, datasheet mentions that.SOA protection in LM317 will allow in excess of 0.5 A at I/O differential of 35V. That's more than 17W.So in that case, only thing that protects LM317 is thermal protection. And in LM317 thermal protection kicks in when crystal is at more than 170 °C or so..It shuts off the device,and waits for it to cool down with some hysteresis. So what you got is thermal feedback relaxation oscillator, keeping your device at the very end of it's thermal envelope. Indefinitely.And device will fail doing that. Not overnight, but few hundreds of hours or more it will decrease it's reliability immensely. And some time after that all of them will come back to you.. Broken. It is not something you should rely on, it's there to help device survive occasional events, not permanent overload. If you carefully design test circuit, you can make it survive permanent short circuit. But in controlled environment, at specific working criteria.. Not across whole operating area.Proper foldback current limiting with thermal design that keeps temperatures within normal operating envelope during worst case scenario, can be engineered to withstand continuous current limit. Or combined thermal and current limit designed to keep temperature much lower than 170°C.But my favorite simple solution is that if you have short circuit or overheat condition for more than few seconds, you should shutdown device and wait for repair.........And combining LM317(for quick reaction) and thermal fuse (for slow reaction) works really well. It will survive transients and blow when something shorts permanently in your circuit. You can put polyfuse too. That works well too.. It will keep resetting, but will keep thermal overload in PTC not regulator.You must remember this is not laboratory supply we're talking about. A power bus in a device should never encounter short circuit. If it does, device is malfunctioning with God knows what consequences. Shutdown ASAP and call maintenance..

author: SeanB
author_rank: Super Contributor
timestamp: February 12, 2017, 07:03:22 pm
content: Quote from: drussell on February 12, 2017, 02:47:25 pmNever trust that, it really is there for in case there is a short on the output, running into a short indefinitely will result in a shorted IO regulator very fast, or a regulator that is shorted to ground and blows the case off in the case of the supply being up to dumping the energy in the bond wires. Running into a short might activate foldback limiting, but that is not a parameter tested in production, and it often does not really work on some devices with certain combinations of input voltage and die temperature.Linear regulators are best operated with a differential close to dropout voltage and with die temperature kept under 70C at worst. That way they last almost for ever, as evidenced by the dozens of S100 bus systems in control applications still in use, with a 7805 or three per board to provide power from the 8V rail.

author: 2N3055
author_rank: Super Contributor
timestamp: February 12, 2017, 07:18:15 pm
content: Quote from: SeanB on February 12, 2017, 07:03:22 pmI agree with all 100%!!

author: David Hess
author_rank: Super Contributor
timestamp: February 12, 2017, 09:30:35 pm
content: The safe operating area protection in integrated regulators also includes protecting the output transistor from secondary breakdown when a high input to output voltage difference exists. In an extreme cases where the input voltage is high, a regulator may not even start into a negative resistance load. Some recent regulator designs have very low secondary breakdown ratings compared to older ones.Thermal protection is deliberately designed with hysteresis so the regulator can hard start into a difficult load.

author: 2N3055
author_rank: Super Contributor
timestamp: February 12, 2017, 09:52:44 pm
content: Quote from: David Hess on February 12, 2017, 09:30:35 pmDavid is right on the money, as always..LM317 can have trouble starting with inductive load, for instance...Protection is there to protect VR as much as it can, but it cannot interfere with regulator stability, dynamic performance etc..In cases where protection would interfere with regulator performance, protection lost... :-)And as David said, all of them are different. For instance 78xx has more aggressive SOA protection than LM317.. But protection for LM317 works even if you disconnect sense pin...Three pin regulators are easier than "roll your own PSU from discrete parts", but they are sometimes not that easy.. Simple performance and requirements can be done simply.. Sophisticated features and excellent performance require a bit more work, even with 3-terminal regulators.

author: David Hess
author_rank: Super Contributor
timestamp: February 13, 2017, 02:07:43 am
content: Quote from: 2N3055 on February 12, 2017, 09:52:44 pmI think Robert Pease mentioned this in one of his articles. A new designer at National did a new regulator but left out the thermal hysteresis resulting in the regulator getting "stuck" when trying to restart some loads after a thermal overload. The output voltage and thereby current would rise just high enough for the regulator to maintain its junction temperature at the thermal limit. If the regulator had been able to achieve its full output voltage, then the decrease in input to output voltage would have lowered its power dissipation enough for it to continue supplying the load.Foldback current limiting can suffer the same problem into negative resistance loads like switching regulators.QuoteI had forgotten about that; the regulator's quiescent current runs its control circuits. So the 7805 quiescent current runs out of its common pin but the 317 quiescent current runs out its output pin so the later has its protection circuits still active if its adjustment pin gets disconnected.I like using 3-terminal regulators as protected power pass transistors in place of theLM395which is expensive and only available as an NPN device. With a proper current sharing and thermal design, an integrated regulator can protect the high power transistor(s) that it is driving.

author: westfw
author_rank: Super Contributor
timestamp: February 13, 2017, 04:18:05 am
content: QuoteI'm not talking about higher-rated regulators in different packages; I'm talking about the SAME regulator with the same rating, offered in a wide variety of packages. Like the National LM1117 witch is available in 4 packages with thermal resistance ratings ranging from 3 (TO220) to 15 (TO-223) C/W...