Opto feedback for Flyback SMPS with "Darlington opto" - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: July 08, 2023, 11:50:55 am
content: Hi,On page 7 of the below, the opto has a NPN connected to it, in like a "Darlington" configuration...https://www.power.com/sites/default/files/documents/der243.pdf..so, one presumes that the Feedback loop equation for the transfer function of the Error Amplifier is the same as if an opto alone had been used...but you just consider the "increased CTR" of the Darlington configuration?

author: tggzzz
author_rank: Super Contributor
timestamp: July 08, 2023, 12:01:57 pm
content: Quote from: Faringdon on July 08, 2023, 11:50:55 amAt least this faringdon question doesn't include "...do you agree that..."You need to work out the operation of the circuit on the phototransistor side, in terms of its output as a function of current into Q2 base. Then work out what the current into Q2 base is as a function of the voltage across C14.Since you are a very experienced PSUdesignerwith plenty of experience of analogue circuits and control loops, you should have no trouble doing that.EDIT: inserted/quoted the first post, so it won't be lost to posterity.

author: Faringdon
author_rank: Super Contributor
timestamp: July 08, 2023, 01:37:09 pm
content: Thanks,The V across C14 will be 24V whatever the load, since ayk, its regulated.Wouldnt you get it on max load..then measure the current through the opto diode......and also measure the current coming out of the NPN collector, then divide one by the other, and get the "new CTR"?

author: tggzzz
author_rank: Super Contributor
timestamp: July 08, 2023, 02:06:20 pm
content: Quote from: Faringdon on July 08, 2023, 01:37:09 pmAnd there we have the "would/wouldn't you" question.I wouldn't do anything like that. You might decide that's the right thing to do.What makes you think the voltage across C14 is 24V? Have you even looked at the schematic (and spec)?Your starting point appears to be that thefeedbackcircuit's operation isprimarilydependent on how heavilyloadedthe output is. That makes me wonder whether you have any cluewhatsoeverabout what the feedback is intended to achieve. If you don't understand that then there is no point in your continuing. Or ours, for that matter.

author: Faringdon
author_rank: Super Contributor
timestamp: July 08, 2023, 10:45:00 pm
content: thanks, the vout is regulated to 24v.

author: tggzzz
author_rank: Super Contributor
timestamp: July 09, 2023, 12:54:43 am
content: Quote from: Faringdon on July 08, 2023, 10:45:00 pm

author: floobydust
author_rank: Super Contributor
timestamp: July 09, 2023, 07:17:40 am
content: It's a 14V zener VR1 ZMM5244B, not 24V. It's not regulating voltage. TheIC datasheetexplains it more, it's muddied by the Control pin having many functions:Increased Output Overvoltage Shutdown Sensitivity• Transistor Q1 {Q2 in app note} and VR1 added to improve the output overvoltage shutdown sensitivity.During an open-loop condition the output and therefore bias winding voltage will rise. When this exceeds the voltage of VR1 plus a VBEvoltage drop Q1 turns on and current is fed into the VOLTAGE MONITOR pin. The addition of Q1 ensures that the current into the VOLTAGE MONITOR pin is sufficient to exceed the latching shutdown threshold even when the output is fully loaded while the supply is operating at low-line as under this condition the output voltage overshoot is relatively small"4.14 Output Feedback[...]Bias winding voltage tuned to ~9 V at no-load, high-line to reduce no-load input power {hence me thinking the 14V zener is normally off}To minimize the dissipation of the secondary side feedback circuit Q2 was added to form a Darlington connection with U3B. This reduced the feedback current on the secondary to ~1 mA. The increased loop gain (due to the hFEof the transistor) was compensated by increasing the value of R16 and the addition of R25. A standard 2.5 V TL431 voltage reference was replaced with the 1.24 V LMV431 to reduce the supply current requirement from 1 mA to 100 uA.Control loop compensation is provided by C16 and R19 with C19 and R22 forming a phase boost to improve the phase margin. Resistor R16 limits the DC gain of the feedback system to ensure power supply stability throughout the range of operation. Resistor R27 ensures the minimum bias to U2 and C22 provides a soft finish rise of the output. The relatively small value for C22 is due to the reduced feedback currents."

author: tggzzz
author_rank: Super Contributor
timestamp: July 09, 2023, 07:54:20 am
content: Quote from: floobydust on July 09, 2023, 07:17:40 amThat might be what faringdon is thinking, but I'm not sure. What he has written appears, to me, to be incomprehensible - i.e. I can't comprehend what he is thinking.Apart from that, his questions are not what I would expect from someone that has spent years designing SMPSs - albeit in, ahem, 40 companies. They are "not even wrong"https://en.wikipedia.org/wiki/Not_even_wrong

author: Faringdon
author_rank: Super Contributor
timestamp: July 09, 2023, 09:29:19 am
content: Thanks very much, indeed its regulated to 19V , not 24V...my bad.However, the question of the top post still stands.......ie, do you just consider it like the "normal" error amp feedback equation, but the CTR value changed to the new "Darlington CTR" value.For anyone's interest, the attached is the feedback loop equation for the "TOPswitch with TL431 Error Amplifier", but without the Q1 added.And so the question is, how does that equation vary, when the Q1 NPN is added? (its equation 8.35 in the attached page 467)

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 09, 2023, 02:46:12 pm
content: Basically yes, adding a bjt Q2 in that way should increase the overall gain of the feedback by the beta of Q2 (at whatever collector bias current it happens to be operating at). I see it as a separate gain stage in the feedback, but one may also choose to see it as a modifier to the CTR of the opto, I suppose.It's not clear why one would add such gain, I'd assume that topswitch designs their chips to work with common optocouplers alone. And since the beta of a bjt is not very well-controlled, Q2 adds additional uncertainty to the actual feedback gain. Only good reason I can see to do this is to get away with a much lower bias current on the secondary side circuitry while still having high DC loop gain.

author: tggzzz
author_rank: Super Contributor
timestamp: July 09, 2023, 04:39:44 pm
content: Quote from: Faringdon on July 09, 2023, 09:29:19 amSo does my question in reply #3, viz...QuoteBut at least you have - eventually and after several prompts - read a few key numbers in "your" application note.Now try the key letters and sentences and hence concepts and operation.

author: floobydust
author_rank: Super Contributor
timestamp: July 09, 2023, 07:51:30 pm
content: Quote from: Faringdon on July 09, 2023, 09:29:19 amTell us about your transformer, what is the bias winding like for voltage. Same as the app note? Or is this some "we have boxes of old ones"?Because a lovely equation will still be quite wrong if it's different. Changing the output voltage too, is it 24V now? also affects the feedback loop design. Of course you know this.

author: Faringdon
author_rank: Super Contributor
timestamp: July 09, 2023, 08:40:02 pm
content: QuoteThanks, yes , you are rightly speaking of the transfer function of the error amp being [control_current(s)/Vout(s)]...yes i see your good pointQuoteThanks, its 28V at full load, and 17V at no load.

author: floobydust
author_rank: Super Contributor
timestamp: July 09, 2023, 11:14:22 pm
content: It's very difficult to be spoon fed details. Some might say frustrating.What are you trying to do? Use the IC and app note to make a ____ with a ____ and a ____ to generate __ volts, watts etc. Can't reveal details or the IP will be stolen?The feedback equation is the last of your worries, it's trivial to tweak values after the PSU is up and running. Adding the one little transistor is not rocket science the "new" CTR did not go quite to Mars. Indeed, the extra gain was shed by a few resistors explained by Power Integrations.In production, will the feedback factor vary more that usual? YES the hFEwith batch, temperature etc will move around, as the opto's does as well.

author: tggzzz
author_rank: Super Contributor
timestamp: July 10, 2023, 12:05:53 am
content: Quote from: floobydust on July 09, 2023, 11:14:22 pmAsk your (useful) "XY question" another couple of times, and he might get around to responding to it.This is faringdon. He seems to tenaciously latch on to unimportant/irrelevant/incorrect notions such as you mention.Elsewhere in this topic he has the notion that having the output "on max load" is important to measuring the loop gain...Quote from: Faringdon on July 08, 2023, 01:37:09 pm

author: floobydust
author_rank: Super Contributor
timestamp: July 10, 2023, 01:05:59 am
content: Sometime knowledgeable, other times boneheadedThis inconsistency I have seen before by someone who was a fake- just trying to appear as a smart expert by copying/quoting other coworker's/engineer's smarts and work. When boxed into a corner, isolated from Internet, peers etc. they know nothing on their own.I worked with a guy like that, a charlatan EE. Noticed he could not connect an LED indicator and transistor correctlygave me hand waving and mumbling when I pointed out the mistake. I did not give him the answer. Eventually I could tell he was asking others for the answer, while I kept asking him for a corrected schematic. Never happened. Told the boss who finally did the homework if the diversity hire guy had real EE knowledge or what. He could not sketch connect the LED and transistor on a whiteboard, so he was sacked.Now, for all job applicants I give them a 10 question quiz, very basic on a few things to see if they are what they advertise.

author: Someone
author_rank: Super Contributor
timestamp: July 10, 2023, 01:51:03 am
content: Quote from: floobydust on July 10, 2023, 01:05:59 amThere are several users on this forum doing just that, be careful with your contributions if you don't want the population of charlatans to increase.

author: floobydust
author_rank: Super Contributor
timestamp: July 10, 2023, 04:42:08 am
content: Yes, I do notice some people use the forums for fame and glory, a boost above their level.OP seems to ask questions across many EE forums for a project, collect answers here and there and then leaving threads that have no closure, come to no conclusion.https://www.edaboard.com/threads/what-is-open-loop-gain-of-tl431.407628/"Helped: 0"https://www.edaboard.com/threads/fsgm0565-has-no-mains-input-uvlo.407624/using the FSGM0565 although the thread here is about replacing it due to obsolescence. On EDA board "... does anybody know when TOPswitch will be obseleted?"So it's power trolling between forums (pun intended). #6 top topic starter here.

author: tggzzz
author_rank: Super Contributor
timestamp: July 10, 2023, 08:18:43 am
content: Quote from: floobydust on July 10, 2023, 01:05:59 amI've never done the 10 simple questions thingper se.I've found it adequate to get them to explain what they did on a past project. Then I ask questions about what they did, why, and what they would do better next time. I might then go on to ask questions that are relevant to the job but which don't have specific "correct" answers, and which can illuminate how my company works and whether they have the right mindset. Seeing what questions they ask is also a good indicator of their background too. If they "reverse interview" me, so much the betterWorks less well with people from a classified job, and with new graduates that haven't done much on their own. But it can indicate which new grads have/haven't done extra projects on their own, which is extremely useful information.

author: floobydust
author_rank: Super Contributor
timestamp: July 10, 2023, 04:03:45 pm
content: Quiz with just the basics for op-amp configuration, need for termination resistors, LED resistor calc. does weed out the nincompoops.In interviews I want a sense of their mindset, their approach to design and dealing with obstacles, and learning. Ask a tough hardware question, answer: "oh I would use google"There is a generation not of solving the puzzle but look for the answer to the puzzle instead.Another owner I know will ask techs in an interview to troubleshoot a mock controller. He gives out a multimeter and schematic and says "fix it". He pays a wage for their time on it.Surprisingly, many of them run away. They are fakes in the sense they just keep replacing things until it's fixed, no troubleshooting skills.Fake engineers take a miniscule problem addressed, and add all the marketing hype possible. One interviewee put in a LM317 on a product to save the day on an obsolete IC. Is this worthy of a paragraph on a resume and blah blah blah me good? I'll bet he posted on Internet forums lol.I see posts all over the Internet for this 20W SMPS project and there's something amiss in the common sense department. As he says, these are commodity designs nowadays.

author: tggzzz
author_rank: Super Contributor
timestamp: July 10, 2023, 05:13:49 pm
content: OK, I'll play devil's advocate with some points...Quote from: floobydust on July 10, 2023, 04:03:45 pmValid if they also say they've never worked in that area, but would consider X and Y and anything else that gurgle might raise as being possibly relevant.QuoteTypical field service engineerQuoteIt would be reasonable if mentioned in conjunction with it being a high pressure "fault-find and fix" on-site task where the customer was prone to throwing chairs around.Internet forums, not so much. Unless on Hackaday or similar!QuoteNo argument there, even from a devil's advocate One interviewer asked me how, if I was an foreman when building the pyramids, I would know when it was time to change shift. I took this as a "Q: how would you use a barometer to measure the height of a building? A: give it to the janitor after the told me the height" question.Hence I spent quite a few minutes dreaming up "Holywood Script" solutions; eventually he became tired and asked me if I knew the answer he was looking for. Yes, it was a sand eggtimer; boring.

author: Faringdon
author_rank: Super Contributor
timestamp: July 10, 2023, 07:53:07 pm
content: QuoteThanks, you are correct, TOPswitch-HX would have 9mA coming from the opto transistor when in light load...(if the NPN wasnt also used with it in Darlington).I guess this is why TOPwitch-HX is no longer supported in their PI design software. They likely dont want people to use TOPswitch-HXany more.The emitter degen is to tame the NPN, but the 20R they use isnt likely enough...but you cant use more or else too much voltage drop when in light load and your bias coil has dropped right down in voltage.