TL431 range stability - Page 1
Forum section: electronics / projects

author: b_force
author_rank: Super Contributor
timestamp: June 08, 2018, 06:53:03 pm
content: The TL431 is used in almost every SMPS.I want to change the output voltage of one specific power supply and I was wondering if this is also gonna have effect on the stability?I have been reading through some very good documents of ONSemi and some other ones.https://www.onsemi.com/pub/Collateral/TND381-D.PDFhttps://www.plexim.com/files/plecs_tl431.pdfAs far as I understand the lower resistor doesn't effect the loop stability, although the total gain also has an effect of it.Can I just change that resistor or do I have to compensate something else as well?Would it be easier to change the upper or lower resistor?My control theory is getting a little rusty these days, so maybe one of you can just answer this question?

author: Zero999
author_rank: Super Contributor
timestamp: June 08, 2018, 07:50:58 pm
content: Yes it can affect stability, especially lowering the voltage, which would decrease the gain and could require an increase in any frequency compensation capacitor.This isn't something I've tried to do myself, but I've seen the question asked here before.

author: b_force
author_rank: Super Contributor
timestamp: June 08, 2018, 08:01:23 pm
content: Quote from: Hero999 on June 08, 2018, 07:50:58 pmOk, thanks. That was already what I thought.Yeah, I have been looking around for a while (in fact a few days), but I haven't found anything.So if someone actually knows these topics, please share them!I know that many power supplies of a certain range (but different output voltages) use exactly the same components and only change the feedback circuit.So if I understand you well, your response suggest that increasing the voltage would be far less problematic?

author: Zero999
author_rank: Super Contributor
timestamp: June 08, 2018, 08:19:43 pm
content: Quote from: b_force on June 08, 2018, 08:01:23 pmIncreasing the voltage poses its other set of problems: higher duty cycle, so more power dissipation in the driver transistors, increasing the flux in the transformer and greater stress on the components on the secondary side, especially the filter capacitors.Of course it's possible to tweak the voltage on a switched mode power supply, designed to work at a fixed voltage, but it's something one does at their own risk. There will be limits on much the voltage can be changed, before it behaves erratically or is destroyed completely.

author: b_force
author_rank: Super Contributor
timestamp: June 08, 2018, 08:29:08 pm
content: Quote from: Hero999 on June 08, 2018, 08:19:43 pmyes, I agree (and was already aware) that of course increasing the voltage will bring new issues.But as far as I know, most of these power supplies simply use the same components, even the transformer + caps.If that's true (which I really think it is, but correct me if I am wrong), that suggests that they engineer the power supply on these worst case specs and than simply turn everything down.Anyone having experience with this?

author: b_force
author_rank: Super Contributor
timestamp: June 08, 2018, 08:41:33 pm
content: I just bumped into this document;https://www.power.com/sites/default/files/product-docs/an57.pdfQuoteThat would suggest my first thoughts that the lower resistor doesn't really effect the stability of the circuit?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 08, 2018, 08:59:29 pm
content: BS. The compensation cap works into RF1 || RF2.Think of the TL431 as an op-amp with open-collector output, a single (inverting) input pin, and a massive yet suspiciously accurate (~2.50V) input offset voltage.If RF1 || RF2 were nearly zero, the input AC voltage, coupled in by the capacitor, would be nearly zero, and compensation would be impossible.There is a second aspect to compensation here, that the output can only rise and fall as fast as the TL431 responds, and with as much gain as it has; and that the output response depends on what's loading down the output. But TL431 is not a transconductance amplifier. This isn't a workable method.They also don't show a series resistor on the cap (in Fig. 5), which is almost always required for reasonable phase margin in real designs. Curiously, they do show it in Fig.1 (R7). Also, CF2-RF4 will hardly do anything; actually, they will do something, and that something isworse, not better. RF4 should be connected to the TL431 input side, not the output side!Regarding transconductance (Gm) amps:These are compensated by connecting the R+C network from output to ground. This works when the node's internal resistance is very high (i.e., a constant current -- hence, having gain in units of A/V, or conductance), and when the gain is constant.TL431's transconductance varies extremely widely over its operating range, so this isn't an applicable method.I have seen this method applied on the TL494 and related parts, implying their error amps have some Gm-amp characteristics. But it's not clear to me if that was something the designer just happened to get away with, or if that's actually characteristic of the amp. The datasheet is certainly not written to suggest it is anything but a normal voltage-mode amp.A good example, that's both intended, and actually works quite nicely, is a TPS54233 (and all the others in that family). The amp is explicitly defined as, and rated as, a Gm amp.http://www.ti.com/lit/ds/symlink/tps54233.pdfThis allows simple calculation of compensation, without having to worry about input and output impedances.Tim

author: b_force
author_rank: Super Contributor
timestamp: June 08, 2018, 09:39:40 pm
content: Well, that doesn't make the story not much clearerI dove into some more documents.For people who are lurking and are interested;www.ti.com/lit/an/slva685/slva685.pdfThis kept me thinking about the OnSemi document mentioned earlier.http://cbasso.pagesperso-orange.fr/Downloads/PPTs/Chris%20Basso%20APEC%20seminar%202015.pdfPage 16 clearly says;" Because of local feedback, the op amp builds a virtual ground"" The 0-Hz gain is indeed changed but not fc!"The next page very clearly shows that the phase isn't changing" The division ratio is changed to adjust Vout: fc is unchanged"This is in line with the other documents that you can actually change the lower resistor without to much trouble?

author: David Hess
author_rank: Super Contributor
timestamp: June 09, 2018, 12:35:27 am
content: Quote from: T3sl4co1l on June 08, 2018, 08:59:29 pmTI publishedinternal details of the TL494which make me think that the designerwas on crackjust got away with it. It sure looks to me like they left a internal Miller compensation capacitor out of the schematic. The compensation node is sure not a transconductance output.

author: b_force
author_rank: Super Contributor
timestamp: June 09, 2018, 10:38:56 am
content: Well, I found some more documents and they ALL suggest that the lower resistor can be changed without to much trouble?So I am confused now?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 09, 2018, 10:53:00 am
content: You're welcome to trust appnotes... at your own risk.If they don't give verifiable justification for their claims... it's just hearsay.Tim

author: b_force
author_rank: Super Contributor
timestamp: June 09, 2018, 11:56:14 am
content: Quote from: T3sl4co1l on June 09, 2018, 10:53:00 amLol, yes, I know what you mean, but I guess what I meant to say is that I see a lot of well meant critique on these articles.On the other hand, I still don't have a clear answer, let alone a practical solution?