LMR14206 caught fire! - Page 1
Forum section: electronics / beginners

author: stafil
author_rank: Regular Contributor
timestamp: August 01, 2025, 06:34:45 pm
content: I am trying to convert 24VAC to 3.3V DC to power my ESP32 using the circuit attached. However after I plugged it in and as soon as tried to measure the voltage across the diode, the LMR14206 caught fire. Do you see anything wrong with the schematic that could explain the flame?Thanks so much in advance!

author: The Soulman
author_rank: Super Contributor
timestamp: August 01, 2025, 06:45:51 pm
content: I do not have direct experience with this chip and came only for the pics..While I'm here I might as well try to help.Comparing your circuit to the one in the datasheet (TI) it looks like your start-up capacitor is 10 times to large and your output capacitor is 10 times to small, also your feedback divider may be to high impedance.Things worth checking.Also did you measure the input voltage to your ic? An unloaded transformer will produce a higher voltage than specified..

author: stafil
author_rank: Regular Contributor
timestamp: August 01, 2025, 07:00:36 pm
content: Don't have a microscope, so I tried my best for a photo with my old iPhone...Ugh, I think i screwed up with the output capacitor and read 4.7 instead of 47, you are right!But for the input capacitor the datasheet says: "A low ESR ceramic capacitor (CIN) is needed between the VIN pin and GND pin. This capacitor prevents largevoltage transients from appearing at the input. Use a 2.2 μF-10 μF value with X5R or X7R dielectric." So 10uF is good enough? But even if it wasn't, it shouldn't cause it to catch fire..?I calculated the freedback resistors based on this:"VOUT=0.765V(1+(R1/R2)) (2) Typically R2 will be given as 100Ω-10 kΩ for a starting value. To solve for R1 given R2 and VOUT, use: R1=R2((VOUT/0.765V)-1)."

author: voltsandjolts
author_rank: Unknown
timestamp: August 01, 2025, 07:05:04 pm
content: Quote from: stafil on August 01, 2025, 06:34:45 pmDid you solder it the right way around

author: stafil
author_rank: Regular Contributor
timestamp: August 01, 2025, 07:20:41 pm
content: > Did you solder it the right way aroundthat's a good question. all evidence are gone now, but I guess I could try again...

author: MK14
author_rank: Super Contributor
timestamp: August 01, 2025, 07:33:28 pm
content: Quote from: stafil on August 01, 2025, 06:34:45 pmWhat are you using, to get the 24 volts AC from?If it is a rather small, cheap, mains transformer (bigger transformers, tend to have better/smaller voltage variation with load). Their output can increase rather significantly, while the output load is small. E.g. Transformer load regulation could be 20% or more. The mains voltage can be higher (than nominal), at times, as well, which is another factor.So, if while lightly loaded, it was 30 VAC or more, along with the almost x1.5 (1.414 etc, root 2), when the bridge rectifier (diodes) convert it to unregulated DC. It could easily exceed the absolute maximum input voltage, which seems to be 45 volts.Which could easily blow that simple switcher.

author: wraper
author_rank: Unknown
timestamp: August 01, 2025, 07:39:38 pm
content: L1 as seen on PCB is a joke. Do you think you can take some tiny inductor and use it as power inductor in buck converter? Also it's 1.25 MHz switcher, layout is a joke as well to be used at such frequency. Also good luck trying to use a tiny electrolytic capacitor on the output.

author: stafil
author_rank: Regular Contributor
timestamp: August 01, 2025, 07:49:14 pm
content: Quote from: MK14 on August 01, 2025, 07:33:28 pmPhoto attached of the transformer. It has ReliaPro in its name. So it must be good, right?

author: stafil
author_rank: Regular Contributor
timestamp: August 01, 2025, 07:50:45 pm
content: Quote from: wraper on August 01, 2025, 07:39:38 pmI am here to learn, so don't just say it's a joke. Be specific what is wrong with the layout and suggest tips for fixing it. Your post comes across as very unfriendly and negative

author: MK14
author_rank: Super Contributor
timestamp: August 01, 2025, 07:54:40 pm
content: Quote from: stafil on August 01, 2025, 07:49:14 pmIt would be interesting to measure the DC voltage going into the input of the simple switcher. You probably need to remove the old (burnt/broken) one, as it may have gone short-circuit.Then you will have a better idea, of what happened.Alternatively, you could measure the AC output voltage, from the transformer, without a load. Then estimate the final DC voltage, going into the simple switcher.The 24 V AC, is when it has a load of 1 amp (as mentioned on the transformers case). But unloaded, with a very light load, (as mentioned in my previous post), it can go up by a surprising amount, such as +25%.

author: wraper
author_rank: Unknown
timestamp: August 01, 2025, 07:57:41 pm
content: Quote from: stafil on August 01, 2025, 07:50:45 pmL1 is a short circuit as much as IC is aware, it will saturate instantly and act as a dead short. Actually it most likely got fried and lost all inductance even before IC emitted magic smoke. As of electrolytic capacitor, you'd be lucky if there is 0.5uF left in it at such switching frequency. Not to say it has too high ESR and won't surive ripple current anyway. Lookhttps://www.ti.com/tool/LMR14206XMKDEMOfor inspiration.

author: stafil
author_rank: Regular Contributor
timestamp: August 01, 2025, 08:00:09 pm
content: Quote from: wraper on August 01, 2025, 07:57:41 pmThanks! That helps a lot, especially the LMR14206XMKDEMO!

author: stafil
author_rank: Regular Contributor
timestamp: August 01, 2025, 08:02:02 pm
content: Quote from: MK14 on August 01, 2025, 07:54:40 pmI measured the output of the Bridge Rectifier, after the LMR14206 got burned, and it was bang on 38V. But I think you have a point, that maybe I need and IC with larger margin, and not just 42V!

author: wraper
author_rank: Unknown
timestamp: August 01, 2025, 09:52:12 pm
content: It's certainly not enough margin to account for possible mains voltage variation. However it's certainly not what caused the failure. Circuit is broken in several ways but the reason it burned is absurdly insufficient inductor.

author: stafil
author_rank: Regular Contributor
timestamp: August 01, 2025, 10:10:04 pm
content: Quote from: wraper on August 01, 2025, 09:52:12 pmOther than the inductor, in what other ways is the circuit broken? How can I fix it? Assuming I fix the inductor.

author: wraper
author_rank: Unknown
timestamp: August 01, 2025, 10:35:28 pm
content: As I said, wrong output capacitor, poor layout (very important for high frequency converters). Not sure about the diode as no part number was given, it might be too slow but long leads are no good as such. My suggestion is to basically replicate LMR14206XMKDEMO PCB layout and use the same or very similar inductor, capacitors and diode.

author: stafil
author_rank: Regular Contributor
timestamp: August 01, 2025, 11:50:19 pm
content: Quote from: wraper on August 01, 2025, 10:35:28 pmSorry, thought it was a different user

author: lakis70
author_rank: Contributor
timestamp: August 02, 2025, 01:45:03 am
content: why you choose 24V for 3.3V?Take a 5V supply and a low drop linear regulator for 3.3V or some LM.

author: amyk
author_rank: Super Contributor
timestamp: August 02, 2025, 04:07:57 am
content: Never heard of this IC before and looking at the PCB layout, thought it was a linear regulator at first!That's how far off your layout is.Just search online for "buck regulator PCB layout", you will find plenty of information.

author: MariuszD
author_rank: Regular Contributor
timestamp: August 02, 2025, 10:15:48 am
content: Quote from: stafil on August 01, 2025, 06:34:45 pmDid you connect the multimeter to D2? Only the oscilloscope probe can be connected there.

author: stafil
author_rank: Regular Contributor
timestamp: August 02, 2025, 08:53:49 pm
content: Thanks a ton everyone for the feedback. I have updated the buck layout and was wondering if anyone has any feedback. It's a 2 layer board. Bottom layer all ground, on the top layer I will expand the ground zone to occupy the rest of the PCB. The VIN zone will go to the output of bridge rectifier.

author: BennoG
author_rank: Frequent Contributor
timestamp: August 03, 2025, 03:38:03 pm
content: C2 will start ringing the Vin when you connect the 34V DC. This will immediately kill your controller chip (ringing will be over 100V)You can partially dampen the ringing by putting a 10uF electrolytic capacitor in parallel with C2.Benno

author: wraper
author_rank: Unknown
timestamp: August 03, 2025, 04:24:16 pm
content: Quote from: BennoG on August 03, 2025, 03:38:03 pmIt's only a problem if you're hot plugging it. Op seems to have a rectifier and a smoothing cap on the PCB. Also putting something as low as 10uF makes little sense. Use hundreds uF, if you're adding an electrolytic cap there.

author: BennoG
author_rank: Frequent Contributor
timestamp: August 03, 2025, 04:45:14 pm
content: Quote from: wraper on August 03, 2025, 04:24:16 pmI don't see a rectifier on the new PCB.Also the internal ESR on high uF electrolytic is to high to prevent ringing.

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 03, 2025, 05:50:27 pm
content: The new layout apparently uses another controller IC instead of the LMR14206, for some reason. The layout look plausible, but can't tell without the updated schematic too...

author: stafil
author_rank: Regular Contributor
timestamp: August 03, 2025, 08:35:37 pm
content: Hey, sorry for not updating the schematic. I updated the LMR14206 to LM5164 to have higher margin in case of spikes. Please see attached for the schematic and BOM.Once again thank you so much @mtwieg @BennoG @wraper*C1 is there with 0 value because not sure if i need a high value electrolytic smoothing capacitor there or not.

author: stafil
author_rank: Regular Contributor
timestamp: August 03, 2025, 08:40:45 pm
content: Quote from: BennoG on August 03, 2025, 03:38:03 pmI have removed all the electrolytic capacitors and replaced them with ceramic capacitors. Please see previous reply for updated schematic and BOM. C2 is an X7R capacitor now:https://product.tdk.com/system/files/dam/doc/product/capacitor/ceramic/mlcc/catalog/mlcc_commercial_midvoltage_en.pdf

author: wraper
author_rank: Unknown
timestamp: August 03, 2025, 08:49:42 pm
content: Layout is reasonable. I suggest removing ground pour / via stitching from under the inductor. EDIT: Traces to C5, R2, C3 do not need to be thick, it will only cause soldering problems with no benefit.

author: Eraldo
author_rank: Regular Contributor
timestamp: August 04, 2025, 12:54:51 pm
content: I would recommend not putting every component so close together, assuming you are hand soldering. Mostly around the inductor since its the biggest componentAn easy trick would be to print it at 1:1 with a printer and see if you can fit your soldering iron between the components. It also gives you a perspective of the size.

author: wraper
author_rank: Unknown
timestamp: August 04, 2025, 01:07:56 pm
content: Quote from: Eraldo on August 04, 2025, 12:54:51 pmIf hand soldering, the only problem could be C3 if you assemble parts in sensible order.

author: stafil
author_rank: Regular Contributor
timestamp: August 04, 2025, 08:45:37 pm
content: Quote from: Eraldo on August 04, 2025, 12:54:51 pmI'll just put it in the oven at 250 C and hope for the best