MAX771 behaving oddly [SOLVED] - Page 1
Forum section: electronics / projects

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 06:00:44 am
content: EDIT: For anyone having this issue (if anyone even uses this chip anymore) Add a 1nF cap between FB and HV out. Solved it! Thanks to blueskull for the suggestion, and everyone else for helping me figure this out! Now I can get my clock working.I am making a nixie clock. I am following MikesElectricStuff's design off of his website, for the most part.After stupidly not including a load resistor and not having any tubes on, which I realized when I powered it up (with huge current limiting to slow the increase in voltage) I watched it climb to 180 and then past it and continue on to 250 before I shut it down, I sorted that out, and now it tops out to 180. Great!Except, not really. It regulates solidly to 180 with a 56k resistor as load. But if I change the power supply at all (say from 11 up to 12 volts) the voltage goes up to 190. That's not really much of a regulator, is it? Or is it supposed to do that? I want to feed it with 12 volts, but if it's not really regulating with varying input, then will it regulate with varying output load?I hope I made a stupid mistake that I can fix without re spinning the board. ugh. Thanks for the help!

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 06:47:08 am
content: Further testing shows that its not regulating at all. It regulates up to a voltage, but doesn't try to keep it there. I added another 56k resistor across its output and now at 12v instead of 185v, it stops at 150. What am I doing wrong? I already inspected the connections under a microscope and there are no bridges, so it must be something wrong with my layout.. Im going to cease testing until someone replies, because I am deeply worried about blowing something up, I don't like it when high voltage stuff doesn't do what you're expecting.Also I hope that I didn't kill it when I ran it with no load. It never got above 250v as I had constant current on and it was slowly increasing voltage (20v / second) and I cut it when I noticed it was going higher and kept going.

author: matseng
author_rank: Frequent Contributor
timestamp: December 05, 2015, 07:18:52 am
content: Your schematics indicates a 220mH inductor, most nixie smps uses something in the neighbourhood of 100uH..

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 07:28:18 am
content: I was going off of mikeselectricstuff's notes.http://www.electricstuff.co.uk/nixpsu.htmlL1 is typically 220 microhenries, although the physical size, DC resistance and current rating is more important than the exact inductance, and values from 100 to 470uH will normally work OK, depending on the supply voltage and required output voltage and current. DC resistance should be less than 1 ohm.What voltage zener should I add? And I'll try to find a cap now.How would not having a cap there prevent it from regulating at all?

author: matseng
author_rank: Frequent Contributor
timestamp: December 05, 2015, 07:32:19 am
content: The schematics says mH not uH. mH=millihnries, uH=microhenries

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 07:38:46 am
content: Sorry.. I have the flu right now and I'm a bit out of it. It is actually running a 220uH inductor like it should be. I just labeled it wrong. I'm sure I couldn't get that tiny of a 220mH inductor while being able to flow an amp.I am running 6 nixie tubes and two dots. So.. assuming 3-4mA per device, 24-32mA.What if I raise the supply voltage to 18 volts?Or, if I lower the inductance, how much do I have to lower it by? I may have some lower values laying around.

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 07:58:41 am
content: 100uH while voltage is at 12v or raised up?

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 08:06:28 am
content: Hm. It appears the only other kind of inductor I have is a 15uH. Which is waaay lower then what you said.So if I have to order a new inductor, I will, but what will the effect be of raising the voltage up? Or is there a way to use the 15uH part by changing my power limiting resistors R5 R6 and R7?

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 08:28:19 am
content: Also, I just realized I inadvertently put 3 3.3 ohm resistors on R5 6 and 7 instead of 1 ohms. Would that also cause this? I'll probably swap them now because your calculations probably worked with them in mind.I'll try that, too. Thank you!

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 08:45:39 am
content: Makes sense. I'll go a little lower then that, then. I'll parallel the inductors too and if it works, I'll order the proper size.What is the minimum idle current draw that this regulator would want to be happy so I also know what resistor to put on as a load resistor?Thank you so much for your help, I'm glad that I got it mostly right. =)

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 08:55:33 am
content: Affirmative, added to my to-do list.If I may ask, what is the purpose of that cap? I'm curious. Is it for stabilization of the feedback loop?

author: dannyf
author_rank: Super Contributor
timestamp: December 05, 2015, 12:53:39 pm
content: QuoteIs L1 really 220mH?I would also check wiring and compare the design to datasheet. Copying off a circuit off the internet has its inherent risks.

author: diyaudio
author_rank: Frequent Contributor
timestamp: December 05, 2015, 01:39:51 pm
content: Quote from: blueskull on December 05, 2015, 08:03:41 amThe datasheet sample clearly shows a fsw=300kHz @ 22uH, a 100uH will influence the di/dt current slope,questionWhat type of inductor are you using ? datasheet? selected a random core material from Ebay?.. MAX1771 can output 2A so inductor core should be Ipeak=2.5A

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 04:11:00 pm
content: @dannyf Yeah, its actually 220uH. I was tired and sick lol@diyaudio the inductor is a SDR0805 220uHhttps://www.digikey.com/product-detail/en/SDR0805-221KL/SDR0805-221KLCT-ND/2127164I am going to replace it with:http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=185291394&uq=635849070180056196

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 04:36:18 pm
content: Good news, I located a 100uH inductor in an led boost driver. Going to slap that in as its very slightly beefier as well. I did all the other changes too, lets see what happens.

author: diyaudio
author_rank: Frequent Contributor
timestamp: December 05, 2015, 04:36:48 pm
content: Quote from: ITman496 on December 05, 2015, 04:11:00 pmSDR0805-221KL 220Tol=± 10Q=12Test Fsw (Mhz)=0.796, 796kHzSFR (MHz)=5RDC (ohms)=0.96IRms(A)=0.49ISat(A)=0.80Have you tried to slow increase load (using various dummy loads), keeping the power circuit isolated.. monitor your scope whilst probing the current waveform across (L), you should see almost see a Iramp + Core Reset waveform decay... with no waveform you have no clue or method to debug the problem....its all a guess game from here onwards.

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 04:42:32 pm
content: I would want to probe across R5/R6/R7, correct?

author: diyaudio
author_rank: Frequent Contributor
timestamp: December 05, 2015, 04:46:38 pm
content: Quote from: ITman496 on December 05, 2015, 04:42:32 pmno. across power inductor (L)..

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 04:50:43 pm
content: How do I avoid exposing my scope to 200+ V?Also, I just tried powering up, and it seems to want to run away on me again. I slowly ramped it up with a 56K resistor as load and I gave up when it hit 230V, still in current limit mode on the power supply. It shouldn't be trying to make more then 180ish. I can't really test it until I can at least get it to be stable.. Any idea why it would be running away on me now?Current status is: total resistance across R5 6 7 = 0.25 ohms. Inductor is 100uH, beefier then the 100uH version of the SDR0805, so probably okay.

author: diyaudio
author_rank: Frequent Contributor
timestamp: December 05, 2015, 05:06:09 pm
content: Quote from: ITman496 on December 05, 2015, 04:50:43 pm1) You are measuring L1 (I) between node 12V and L(out) that is differential between those nodes ground clip and probe... there is no reference to ground.2) You can start debugging the usual suspects...mosfet gate, duty-cycle, FET turn on, gate current requirements, L1(I) current.. all these can be be done with a cheap scope.Its not worth the effort copying and pasting from the interweb... next-time use a datasheet and application notes as a baseline for a design at least to have the groundwork mathematics and specification data at hand.

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 05:12:08 pm
content: I'll admit at this point I wish I had consulted you guys first before building it. I'm working on probing stuff now, I'll get a shot of it when I'm done. I'm going to probe the fet gate, logic supply, and feedback pin to the 771 first, and then I'll probe L1 itself after I pull all that apart. be back in a moment.Thank you for the help, and for putting up with me. I've designed a lot of things but I'm very new to dc/dc converters and have not done much with them.

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 05:38:38 pm
content: Interesting twist.. My feedback line has an unexpected thing on it.Yellow is gate drive on the fet, blue is logic voltage, purple is feedback. I dont know if my wiring hanging off of it (an inch of wire on the resistor divider and my scope probe on 10X) could be causing that, but .... im pretty sure its not supposed to look like this?EDIT: should note that output across the 56k resistor / caps was around 111V.

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 05:51:04 pm
content: https://www.digikey.com/product-detail/en/UUB2E4R7MNL1GS/493-9966-1-ND/3974001This is what I was using. I'll try to find another cap.

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 06:11:18 pm
content: I found a 0.47V 400V cap. It took me a while, I dont usually stock all this HV stuff.Does it look like it'll work, or will it's ESR be too high?SPRAGUE 730Phttp://www.datasheetarchive.com/dl/Datasheets-SW8/DSASW00153505.pdfIt looks like it might work.. it claims 37 milliohms at 20-100khz. That sounds a lot better then what the other caps are doing.

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 06:42:36 pm
content: Hm. It does not appear to have reduced the feedback wobbling or stopped the regulator from freaking out and shooting way up. I watched it nearly hit 300v before I pulled the plug. Maybe I need more capacitance? It just appears like the regulator is sitting at maximum drive and refuses to stop doing so and the voltage just keeps going up and up and up.

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 06:59:40 pm
content: It's hard to tell because its going up and down so much. It doesn't seem very stable. I'm going to reduce my dummy load.

author: ITman496
author_rank: Regular Contributor
timestamp: December 05, 2015, 07:25:05 pm
content: Blueskull you are a hero!!I realized I read the value on my cap wrong when I added the 1nF to the feedback loop. I added the proper one. SOLVED. BAM.Thank you man! It's regulating now to EXACTLY 180.0V and I can vary the voltage from 9-24v and its staying perfectly and the gate drive is fluttering around as I change the voltage. Everything is good!Thank you! I'm gonna try to take the gigantic ceramic cap off now to see if I really need it because now its a bit big for my design but I can leave it there if I must. Experimentation time!I can't believe how on the money you were. I wish I had done that first.