Power supply anomaly causing headache - Page 1
Forum section: electronics / projects

author: DeadPixel
author_rank: Newbie
timestamp: June 23, 2025, 09:59:40 am
content: Hello everybody,I'm having a rough time getting to the bottom of a flyback power supply issue for a product of a customer of mine. Customer is experiencing an increased defect rate with this product, a seemingly sensitive cellular modem module on the board that is powered by the flyback supply voltage breaks, i exhibited ICs failing open/short or even cracking. This to me indicates electrical overstress and not ESD, as these modules are handled within an enclosure normally.The flyback regulated voltage (+Vsystem) is +3.95V nominally, but when (dis)connecting 230V mains I can measure brief (<100ns) output transient on the secondary side, swinging the output voltage from ~0V to 10V. Normally this product is not hot-swapped, but this way I can reproduce the behavior.I believe these transients cause the electrical overstress on the module.I have linked relevant datasheets, the customers flyback transformer design, some snippets of the SCH and PCB and some measurements for context.I would be so thankful if anyone would like to take a look at this. I am open to suggestions regarding what to do/try/measure next.Kind regards,Hidde de Laat

author: wraper
author_rank: Unknown
timestamp: June 23, 2025, 10:07:06 am
content: Quote from: DeadPixel on June 23, 2025, 09:59:40 amMost likely a red herring and is a measurement issue, not a PSU issue because it's way too short. Measure with a probe directly connected to capacitor and use a short probe ground connection. Alligator clip wire easily picks up transient caused noise. Short ground spring contacts are included with probes for a good reason.

author: DeadPixel
author_rank: Newbie
timestamp: June 23, 2025, 10:11:28 am
content: Hi! To measure the supply line I've made sure to keep the loop small by soldering a wire to the GND plane to form a small loop.I believe this measurement to be correct, but if it is not somehow, how can it be that the primary current relates to this transient? (see attachment of measurement)Context for the measurement screenshotCH1: TAP1500 1.5Ghz Active probe DC coupled 3.95V offsetCH2: TMDP0200 200Mhz diff probe DC coupled (connections can be seen on scope screenshot)CH3: TCP0030A 120Mhz current probe DC coupled

author: Andy Watson
author_rank: Super Contributor
timestamp: June 23, 2025, 10:45:32 am
content: Quote from: DeadPixel on June 23, 2025, 10:11:28 amOne way to check is to keep the same set-up but probe the ground (where you have attached the wire). Obviously, this should return nothing. If you still see the signal then it's an artifact.

author: PGPG
author_rank: Frequent Contributor
timestamp: June 23, 2025, 11:12:13 am
content: Do your scope is battery powered or AC mains powered?If AC mains than you probably have ground loop with probe shield in that loop and I suppose that what you measure is something induced in that loop.2 years ago I had a battle with disturbances in AC powered device that when measured (350MHz scope) I got something like you.We have ordered USB scope (to be able to work from laptop battery), and waiting for it I found (20+ years old) 10 ferryte cable clips (240 ohm each if remember well). I clipped 6 of them at scope supply cable, 2 at probe cable and 2 at device supply cable. That reduced disturbance seen at screen dramatically. When we got this USB scope (250 MHz, I think) the results were like at this AC supplied scope with all my clips.We solved our problem, but I can't say that I have understood what really was going on.The 50V ceramic capacitor working at 24V and being after SMB 33V 600W transil liked to break. In one serie because of supply chain problems the capacitor from different manufacturer were used.The main problem with diagnose was that it was in devices that are whole filled with wepurene (I'm not sure of name) mass.In 90s we have bought Microchip super-universal PIC programmer that was braking each IC when you run only Blankcheck. The reason was step-up DCDC making VPP. When it got signal to change output voltage from 5V to 21V its way from 5 to 21 were going through over 40V.

author: DeadPixel
author_rank: Newbie
timestamp: June 23, 2025, 11:18:08 am
content: Quote from: Andy Watson on June 23, 2025, 10:45:32 amThere is return on the GND, what does this mean? Am I not measuring 'neat' enough? or is this some sort of ground bounce due a bad GND reference/PCB design? I am a bit puzzled

author: tszaboo
author_rank: Super Contributor
timestamp: June 23, 2025, 11:55:13 am
content: Your probing setup looks quite OK. I don't think you can improve it much, if you use that active probe.You might want to try a power rail probe instead of an active one, if you have one. That has a ~100KOhm DC input resistance and 50 Ohm AC. With a soldered in coax pigtail on C300 you can probably eliminate the radiated noise as much as possible and only measure the conducted one.As how to resolve it? I'm guessing capacitor right before the ferrite bead, or RC snubber?It's hard to analyze circuits from afar, but I don't immediately see any effective bypassing in the ~10-100MHz region, so maybe that's what's causing it? Which I'm not saying is a design fault, often times you just don't bypass everywhere as you don't expect the device to se noise (or load) in certain frequencies. Did you send it through any EMC testing?

author: DeadPixel
author_rank: Newbie
timestamp: June 23, 2025, 11:56:33 am
content: I double checked a random product sample transformer, and got the following numbers:Primary (100k 0.5V test V)Inductance: 805uHLeakage: 50uHSecondary (100k 0.5V testV)Inductance: 1.6-1.7uHLeakage: 160-170nHWhat do you folks think about these coupled vs leakage inductance numbers? Can they be a contributing factor to this issue?

author: DeadPixel
author_rank: Newbie
timestamp: June 23, 2025, 12:04:04 pm
content: Quote from: tszaboo on June 23, 2025, 11:55:13 amThanks for the assuranceI am currently looking for a Power Probe, but have got no such unit at the momentHave you got any clue as to why there is measurable primary current upon attaching/connecting mains 230V? It seems to correlate with the secondary side output voltage transients.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 23, 2025, 12:09:46 pm
content: Quote from: DeadPixel on June 23, 2025, 09:59:40 amIMO it's highly unlikely that this is directly causing damage to anything. Even if it is "real" at the the place you're probing, it's unlikely to be seen by any IC or module (assuming there is at least a tiny bit of local decoupling capacitance).Which net is actually the supply for the module? +Vsystem or VCCM?Have you actually reproduced the failure or the module when disconnecting the mains?edit: also don't use a fancy active probe like the TAP1500 for troubleshooting power supplies!!

author: DeadPixel
author_rank: Newbie
timestamp: June 23, 2025, 12:15:14 pm
content: Quote from: mtwieg on June 23, 2025, 12:09:46 pmHi mtwieg, thank for replying. I'm chasing this phenomenon due to lack of other leads.+Vsystem is powering the module, i have included the SCH page of the module.I have reproduced the issue, having 5 units toggled on mains using a relay board switched by a sig-gen at 16Hz 50%dc. Three of the five units failed after 4 hours of switching.

author: tszaboo
author_rank: Super Contributor
timestamp: June 23, 2025, 12:18:53 pm
content: Quote from: DeadPixel on June 23, 2025, 12:04:04 pmI know it's painful to see the prices of those probes, that's why I made my own. Some people set up an entire electronics lab from the price of one of those probes.Why is there a current? Those switcher ICs can do some things during transients. Or inrush into capacitors. There could be many reasons, I'm not sure if getting rid of that is even possible. You should probably focus on the transients on the secondary side.

author: floobydust
author_rank: Super Contributor
timestamp: June 23, 2025, 05:41:23 pm
content: I think C241 is a bad idea and asking for problems with overshoot or worse.If (yellow) is MLCC you're hard pressed to realize 1/2 that capacitance from the voltage coefficient, and with ageing. Depends on the manufacturer too.TLV431 properties varies between manufacturers looking at stability vs load capacitance:TIFig. 5-18On-SemiFig. 18Diodes Inc.What I've found is plan for up to 2A TX current drain, and then see how the PSU is with light or no load. So try apply a pulsed load and see how the regulation is. The design hints at a problem with it or maybe it was not polished. Purchasing making changes in brand to some components can upset a design.R204 seems too low 100R 26mA for the opto LED is up there.I don't see a resistor/LED to give a minimum load for the SMPS and it might need several mA dummy load when the modem is sleeping.VSystem has no RF bypass cap. L202 won't do much for HF ripple, usually these are several uH not a ferrite bead.C500, C501 I would think at least one is required to keep the modem module rail low impedance.I would remove C241.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 23, 2025, 07:25:38 pm
content: Quote from: DeadPixel on June 23, 2025, 12:15:14 pmOk so in between +Vsystem and the module is Q500/Q501, C521/C520, and L500. I'm guessing there is more capacitance inside the module on the VBATT pins. I don't think there's any way that transient is actually making it inside the module. May want to probe there to make sure...Another thing to consider is if maybe the failure is due to improper sequencing of these different supply rails (+Vsystem, VCCM, V_USB, SIMVCC, etc).

author: thm_w
author_rank: Super Contributor
timestamp: June 23, 2025, 10:33:58 pm
content: If you can't trigger on the actual overvoltage failure, a quick hack might be to throw on a TVS and see if that saves the module during your tests.Quote from: floobydust on June 23, 2025, 05:41:23 pmIt seems they got the idea from this schematic or a similar one:https://oskolchip.ru/shop/293/desc/tny276pn-shim-kontroller-19-w-132khz-dip-8-tny276-dip-7But the capacitor is smaller (2.2uF), series resistor is smaller (47R) and other parts of the circuit don't match up either, like the cap across the TL431. Curious if OP made these tweaks for a specific reason or if they came from an existing schematic somewhere?

author: VectorBear
author_rank: Newbie
timestamp: June 24, 2025, 12:27:45 am
content: Two points.1) I used to cut the ground wire on 'scope AC power cables and put in a resistor (10k or 100k), the resistor sets a 'scope ground if the probes do not.2) That C111 does not ring right with me, although I may be the problem. It seems to add derivative feedback from the +3V95 rail. How about moving the cap to across R231 (or R208) with the same 430uS time constant. Even longer if you can accept the slower rise time.Otherwise all looks good, although I am a controls designer, not power supplies (hate magnetics).

author: jonpaul
author_rank: Super Contributor
timestamp: June 24, 2025, 07:05:06 am
content: Hello all: just saw thisSMPS: See many app notes and seminars on SMPS measurements,https://www.ti.com/lit/an/slva085/slva085.pdfProbing: Tektronix Circuits Concepts: Oscilloscope Probe Circuits and measurements.https://w140.com/tekwiki/wiki/Concepts_SeriesTransient fails: need experince and proper equipment to track these very tricky fails.At first sight, MOV protection is marginal. (red disc )Try a carefully specified TAZ at Vcc bus and at inputs of CPU.Bon chanceJon

author: temperance
author_rank: Super Contributor
timestamp: June 24, 2025, 07:27:41 pm
content: The control loop compensation looks very strange. Especially the 10 µF across the TL431. Who came up with this construction?The LDO can handle only 6 V at the input. The flyback is probably overshooting quite a bit and probably exceeding the LDO maximum input voltage specification when the modem starts to load the power supply in an intermittent way when transmitting.To verify this, you can connect a switched load (Function generator, MOSFET and a resistor) onto the flyback output. You don't need expensive wide bandwidth or fancy rail probes at all for this.Edit: The modems seems to be CAT M1 modem. They can hog up 500 mA from the supply when transmitting a frame.

author: DeadPixel
author_rank: Newbie
timestamp: June 26, 2025, 02:28:59 pm
content: Thanks everybody for your responses! I want to clear up the following: I have not participated in the design of this PCBA in any way, therefore i am unable to answer some of your questions :/I have been doing some measurements in the mean timeQuote from: temperance on June 24, 2025, 07:27:41 pmI did load step testing on the flyback, using a switched NFET with a 1A step, short wires and a fast input pulse. The results are linked as attachment, I measured the high- and low going current step with the effects on the output voltage. I did that with both an original PCBA, and one where I removed C241 (TLV output cap) It made no measurable differences, at least not with the load step measurementI am wondering, i do measure overshoot on during the low/current decrease of the load step. This overshoot is only tens of ns, is this transient potentially destructive for the module on the board? Or is this energy in this pulse considered too low to do any damage?