Flyback over- voltage shutdown! Please, help! - Page 1
Forum section: electronics / general

author: tinker65
author_rank: Contributor
timestamp: July 23, 2020, 04:38:45 pm
content: I am stuck trouble shooting a flyback converter. It works fine at no load, but once load is applied, Aux voltage seems to rise rapidly with the load and causes OV shutdown. It goes from 15V at no load to 26V @ 10% load, while output voltage stays constant. What could have caused that? I thought Vaux should stay constant and proportional to VoutController is NCP12510

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 23, 2020, 04:48:01 pm
content: Guessing transformer windup is shite.Schematic, layout, and transformer winding schedule if possible?Tim

author: tinker65
author_rank: Contributor
timestamp: July 23, 2020, 06:31:53 pm
content: I am using off the shelf transformer:https://www.we-online.de/katalog/datasheet/7508116312.pdfReduced the schematics down to a basic application example from ncp12510 datasheet, still same results.Managed to get ~35% of design power by doubling current sense resistor and reducing Ipk to ~0.55A. Something is off, that transformer is specified at 1.4A saturation, should work fine at 1.1A Ipk, right? reducing current sense resistor just a little, causes shutdown at even lower power levels (either OV on Vaux or OC on Isense)Seems like current is rising way faster than what I designed it for, causing a shorter than designed duty cycle.

author: trobbins
author_rank: Frequent Contributor
timestamp: July 24, 2020, 06:18:15 am
content: As indicated - please show your layout (eg. a few photos) including pcb details - it's very easy to get switching noise ingress in to control signals unless sufficient care is taken.

author: tinker65
author_rank: Contributor
timestamp: July 24, 2020, 08:31:31 pm
content: Here is a layout:

author: tinker65
author_rank: Contributor
timestamp: July 24, 2020, 09:41:37 pm
content: Here is the highest output power I managed to get:With Rsense =1.5R, at Vdc=170V I peak =0.53A after MOSFET was on for 2.5us. This matches up calculations for 800uH L primary. Vout is 12V and Iout 0.7A, while Vaux is 25.3VSomething is missing!!!Vaux should not increase with the load, right? at no load it is 14V as its supposed to be per turns ratio when Vout is 12VHere is the schematics. I tried with and without snubbers, with synchronous rectification and simple diode, even tried disconnecting Vstartup after start, nothing makes a difference. Vaux still rises with the load!

author: tinker65
author_rank: Contributor
timestamp: July 24, 2020, 10:42:21 pm
content: I guess that transformer was designed to provide way more current on Aux winding than needed for my controller. Adding 2k resistor across Vaux capacitor lowered Vaux to 18V at full load. Does that sound right? I've never heard of a solution like that.Is that a problem that I am using pins 4 and 5 of the AUX winding and leaving pin 6 to float?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2020, 11:08:58 pm
content: It may be that aux is better coupled to pri than sec, i.e., much of the leakage inductance spike seen on the primary is also seen on the aux winding.This spike is proportional to current at turn-off, and therefore to load. Yes, this is normal behavior, and it can generate dangerous voltages if the transformer is wound poorly (possibly the case here) or aux is lightly loaded.A peak clamp snubber on the primary would seem like a good idea.The phenomenon of some outputs increasing while others decrease (due to load) is called cross regulation.Tim

author: ocset
author_rank: Super Contributor
timestamp: July 24, 2020, 11:35:28 pm
content: Did you put a series resistor in series with the aux diode?...you need to do this because the overvoltage ringing of the leakage inductance in the aux winding is peak charging your lightly loaded auxiliary up to 20v or so...this is normal...you can reduce it by sandwiching the aux between the two primary halves...............Also, yes, if the aux is lightly loaded, then you may need to dummy load it slightly aswell to again reduce the peak charging.overall, did you sandwich the sec and the aux between two primary halves?..or actually, i am not sure if its even possible to do that with that transformer....it looks like you will have to reduce leakage inductance by sandwiching the pri and the aux between two paralleld secondaries.look at page 10, R39...https://led-driver.power.com/sites/default/files/PDFFiles/rdr347.pdfR39 with the aux cap helps to filter out the leakage spike which is causing your problems

author: trobbins
author_rank: Frequent Contributor
timestamp: July 25, 2020, 01:07:19 am
content: Wrt to the pcb layout:- why is there a ground loop for the current sense negative, including some common trace for pin 3 circuitry ?- why is there no de-glitch cap for the current sense input to pin 4?- what is your sense resistor - is it a large smt part ?- what were you trying to achieve with the gate drive external components?What layout do you have for the secondary diode and filter, and for the primary winding? What waveform do you measure on the primary, given you have no form of snubbering?Are you using the 65kHz or 100kHz IC ?What diodes are you using?Can you get away with using the 14V section of the aux - that may have a lower leakage than if using the full 20V winding ?

author: tinker65
author_rank: Contributor
timestamp: July 25, 2020, 01:39:33 am
content: Quote from: treez on July 24, 2020, 11:35:28 pmI will try that and see if it makes a difference. Damn reference design did not have it. This makes sense as AUX is very lightly loaded. I am using standard off the shelf transformer, no idea how it was wound and the datasheet doesn't even give a spec for what current AUX winding was designed for. I bet a lot more than that tiny controller. Assuming resistance is proportional to rated current, its a whooping 300mA on the AUX!Quote from: trobbins on July 25, 2020, 01:07:19 amIdea is to keep gate drive currents flowing through a thicker trace, while current is sensed over the thin one (4 terminal sensing)Quote from: trobbins on July 25, 2020, 01:07:19 amprobably a good idea and wont hurt to add one, reference design/eval board did not have it, so I skipped it tooQuote from: trobbins on July 25, 2020, 01:07:19 am2x 2521 SMT resistors in parallelQuote from: trobbins on July 25, 2020, 01:07:19 amdifferent turn off and turn on times and rise/fall rates. I have snubbers in the design, just omitted for clarity. Also, adding/removing snubbers has not affected the issue of overvoltage on the AUX winding, just the overshoot and ringing, as it was designed to.Quote from: trobbins on July 25, 2020, 01:07:19 am65kHz, using 14V section only, pin 6 left floatingI am using synchronous rectification, but tried with diodes too, same issue on the AUX overvoltage, happens even at a lower power with diodes due to larger voltage drop.

author: trobbins
author_rank: Frequent Contributor
timestamp: July 25, 2020, 02:07:30 am
content: Imho the IC doesn't support 4 terminal sensing - you only have one 0V IC terminal to work with for gate drive and sense resistor sensing. As such, I would recommend just using a single thick trace from the most appropriate take-off location for the sense resistor, over to pin 1, and as you are doing, make pin 1 the star node for all 0V related control circuitry (although the pin 3 link to 0V looks a bit suspect).Do you have a scope waveform of the aux winding voltage at no output load, and with some load, and what happens at turn-on?Would the feedback path be influenced by any switching frequency ripple (a 431 has quite a lot of gain at 65kHz)?

author: ocset
author_rank: Super Contributor
timestamp: July 25, 2020, 09:50:22 am
content: Bottom of page 9 of the following...https://www.ti.com/seclit/ml/slup261/slup261.pdf...explains the problem you described in your top post(i think its actually the tenth page, but is page "9")

author: tinker65
author_rank: Contributor
timestamp: July 26, 2020, 01:05:24 am
content: YES, that was the reason. Over-voltage ringing was peak charging the cap and controller did not draw enough power to compensate for it. 33 Ohm resistor in series with AUX winding solved the issue and dropped Vcc to ~18V at full load. And why in a world reference design from ON semi did not have it???Now my problem is that duty cycle stays virtually constant, and it enters skip cycle mode instead of reducing duty cycle at nearly 100% load which is causing a really annoying audible noise. Per data sheet, it should not skip cycles until Vfb drops to below 0.8V, so I checked Fvb wit the scope and it goes to 0 for a short time and then goes back up to 3.8V. This repeats at roughly 1kHz at full load and causing controller to enter skip frequency mode. Compensation/control loop issue?Here is reference design I've been using, looks like adding R2 =R6 solves the problem. But my transformer etc are different and ON semi FB loop calculator does not support R2. Anyone has a good link on how to calculate FB loop for the configuration with the R2 installed?P.S. Start up behavior is also not that great, regardless of how large the C10 is, it takes 3 attempts for it to spin up at no load.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 26, 2020, 02:58:18 am
content: Great, the forum's broken add-inline-attachment bug strikes again. I no longer know what C10 was.Yes, increasing phase margin by using an R+C in the compensation network helps. How much depends on the converter gain, which depends on several things. It's not always possible to calculate, and you may find it's easier anyway to find by adjustment.For that, set up a pulsing load, say 10ms on, 10ms off. Set an idle load as well, so it goes between say 10 and 20% of full load, or 50 and 100% (preferably, try several ranges and make sure it's working in all of them). Monitor the output voltage and adjust the compensation components to give the best transient response and ripple.Tim

author: ocset
author_rank: Super Contributor
timestamp: July 26, 2020, 01:20:29 pm
content: QuoteYes its on page 448 to 450 of Basso's book "Designing control loops for linear and switching power supplies"The expression for the TL431 based errror amp trxfer function is on page 450....you should put "(R1 + R2)" instead of "R1"...that then gives it with your extra added resistor (your R2)(These are the book designators, not yours)Basso is the master of the TL431 based feedback loop...and he has a whole chapter on it.But anyway, if you post the Error amp trxfer function that you have in your app note, then i will adjust it to include your R2.QuoteYes, its gone unstable and as T3sl4co1l said, you need to adjust your compensation Rs and Cs to get rid of it.QuoteIs that a problem?Its likely because your output caps are large and excite the inbuilt overload protection in the controller.......this is fine....the overload protection feature is a good one to have, so worth tolerating the hiccups at startup........IMHO , this overload timout feature should IMHO be essential for all offline SMPS, to prevent too much power dissipation in the mains system due to gone-bad PSUs----------------------By the way i would fit R3 ~1K in your schem so the TL431 can have its bias current without pulling it through the opto diode, which as you know, affects feedback...or can potentially do so.