Why is a 22.1kΩ resistor getting cooked in this cicruit? - Page 1
Forum section: electronics / beginners

author: FlakeyCrust
author_rank: Newbie
timestamp: July 10, 2021, 12:15:14 am
content: I'm trying to make a very compact set of four switching voltage regulators, based onTPS54428chips. I only have a basic understanding of electronics so I'm just following the guide from the data-sheet. I got it sort-of working. If I power the entire set with an old 9V battery, it works. If I power one regulator with a server power supply, it works. If I power two with the power supply, it works. But when I tried all four, I heard a noise. I immediately unplugged it and then checked it with the 9V batter again. The regulator closest to the power input was now outputting the wrong voltage. Instead of 5V, it was outputting slightly over 5.3V. Replacing the 22.1k resistor got it back to 5V.The second closest regulator is also outputting the wrong voltage (but it's slightly less than what it's supposed to be) so I believe another component was damaged, but I haven't tried to fix it yet. The last two regulators seem to be fine.I have attached the example schematic from the data-sheet as attachment 1. The values of R1, L1 and C4 vary depending on the intended output voltage. R2 is the resistor that got fried.I suspect the problem was that I completely ignored the layout guideline from the data-sheet, mainly the part that says "make a single point connection from the signal ground to power ground", and instead crammed everything as close as possible. The data-sheet has an example layout (see attachment 2). I also deviated from the example by using one 100μF capacitor instead of two 10μF capacitors per regulator (the third 0.1μF capacitor is optional), but I don't think that's the problem.I have included photos of my circuit as attachments 3 and 4 (I actually ordered a custom PCB for this from China, but there was a mix-up with the order and they had to re-send it, and I got tired of waiting, resulting in the mess you see here).So, can someone tell me why the resistor got cooked and if having a separate ground line would fix it. I would try it and see, but I have already fried a few components from mistakes, including at least one TPS54428 chip, and soldering this is as much of a pain as it looks.

author: cortex_m0
author_rank: Regular Contributor
timestamp: July 10, 2021, 02:17:03 am
content: The main way resistors fail is by overheating, which would normally mean too much power dissipation, or a very hot ambient environment. And in my experience, it is rare for a resistor to shift in value slightly. Normally a failure is either a catastrophic open or short circuit (depending on resistor type).There is, however, no reason to believe your circuit should be generating much power across that resistor. My math suggests it should be about 1mW with a 5V output, which would be fine for even the tiniest SMT resistors.One thing that can happen with switching regulators is oscillation and ringing, which can produce high voltages. The fact that your layout is crude would make that somewhat more likely. Still, it should take quite a remarkable malfunction to damage the resistor.If you have an oscilloscope, measuring the voltage across the resistor while trying to make it fail again would be my suggestion. Or wait for the PCBs, where it will surely be easier to probe the circuit.

author: WattsThat
author_rank: Frequent Contributor
timestamp: July 10, 2021, 02:30:42 am
content: I would expect that anything that is out of tolerance is due to having been cooked by your soldering iron. I don’t see 4 pcs of C6 and really cannot figure out what connects to what, it’s just too much of a jumble.The 9v battery is severely current limited due to its internal resistance so the circuit will not behave the same way with more available current as would be available from a server supply.There is no doubt a bunch of bad solder joints in there. Learning how to properly solder would be the best first step before attempting such things. While you could dead bug build something like you’ve done on just a piece of FR4 copper clad, your soldering must improve before things will work. Search for ”Manhattan dead bug construction”.Are you making things more difficult by using lead free solder?

author: bobbydazzler
author_rank: Regular Contributor
timestamp: July 10, 2021, 02:57:15 am
content: I think I can see an smd resistor in there so if that's the case you should be able to put 40v+ across it, I don't see how it could die from normal operation in your circuit. Maybe extreme ringing on the ceramic capacitors could kill it? Or maybe it's just not wired up properly. Can't you make your circuit on some perfboard instead while you're waiting for your pcb?

author: FlakeyCrust
author_rank: Newbie
timestamp: July 10, 2021, 04:26:00 am
content: I am absolutely making things more difficult by using lead free solder. I don't think my soldering is that bad. It's not great, but the reason it looks so lumpy is it's lead free and I used a flash when taking a picture (to minimize exposure time and allow an in-focus close-up using a hand-held phone. To put it into perspective, the coarse-looking fabric in the background is actually a smooth bed sheet). Also, the thicker parts have some particularly ugly spots because the soldering iron was only hot enough to properly melt the solder in the immediate vicinity of the iron, so some solder flowed off of the iron to the sides and didn't fuse properly, but there is more than enough solder that is properly fused. I can easily lift the entire thing by the tiniest components using fine tweezers without anything coming loose.I have a really cheap hand-held oscilloscope. I didn't find any voltage spikes across the 22.1k resistor when hooking up the 9V battery. I will try soldering a piece of wire to the circuit so I can use alligator grips on both sides of that resistor before doing any tests with the power supply.As for using a perfboard, I only ordered enough parts to make two circuits and be left with two spare TPS54428 chips, and I have already remade the 5V regulator part twice, so I'm going to wait for the PCB before trying to build another circuit.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: July 10, 2021, 04:31:10 am
content: Well everything's probably connected properly if it works fine from the 9v battery so the culprit is most likely a large current surge when you power it from the server power supply. A 0.5ohm or less resistor in series with input might fix that.

author: tooki
author_rank: Super Contributor
timestamp: July 10, 2021, 12:08:42 pm
content: Quote from: FlakeyCrust on July 10, 2021, 04:26:00 amNot to be rude in any way, but no, it kinda is “that bad”. The fact that there are spikeseverywhereis indicative of a consistently bad process. A good joint looks good even under magnification and unflattering lighting.The chances of a fault in this construction are quite high. Could easily be an intermittent fault caused by movement, or even something like a path that the low current of the 9V battery can’t sufficiently energize, but the server PSU can. Chip capacitors and resistors are not designed to handle mechanical loads, and could easily be damaged.Also, I’m trying to figure out the input and output wiring… it looks like you’ve got black as outputs and red as ground (), but where is the positive input?Finally, the server PSU is a switching power supply. If your circuits are on the brink of instability, the higher ripple of the server PSU could be enough to push yours into oscillation. (The battery, in contrast, is a very clean power source.)You should try TI’s Webench Power Designer. You can choose that chip, enter your parameters, and a) see what component values it spits out, and b) alter those values to what you actually used, and then run the simulation to see what effect that has. It also gives you a PCB layout.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 10, 2021, 12:12:23 pm
content: The biggest problem in that construction is the MLCCs. They are very likely to crack, either during soldering, or even more likely, when you apply even a tiny bit of force in that construction. I have done that and learnt my lesson. Only use MLCCs on a PCB and make sure the board doesn't flex. Be extra careful with anything larger than 0805.When the MLCCs crack, they transform into short-circuits, or worse, some lowish-value resistors. I have seen one glowing bright yellow and burning everything around it when connected to a battery.

author: StillTrying
author_rank: Super Contributor
timestamp: July 10, 2021, 01:38:40 pm
content: Quote from: FlakeyCrust on July 10, 2021, 12:15:14 amFor a switching IC and especially a power switching IC you do have to get the decoupling caps as near as possible to each IC's power pins, a 100uF a few cm away doesn't count because of the amount of inductance in the wires and capacitor.It looks like you could do with some flux, for a rough test circuit any will do.

author: tooki
author_rank: Super Contributor
timestamp: July 10, 2021, 01:43:30 pm
content: Quote from: StillTrying on July 10, 2021, 01:38:40 pmThose aren’t decoupling caps, they’re the bulk input caps. They definitely need to be close by of course.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: July 10, 2021, 04:45:14 pm
content: If both surfaces you're soldering together are already tinned then it doesn't matter to much if you use fluxless solder to then join them, sure the joint will look nasty/lumpy but the electrical connection should be fine at least for a prototype.

author: FlakeyCrust
author_rank: Newbie
timestamp: July 10, 2021, 09:52:22 pm
content: After fiddling with the circuit some more and giving it some thought, I think the problem probablyisthe soldering. I added a wire to be able to properly probe the aforementioned resistor, and not only was there no voltage spike, the entire 5V regulator was working, but then I measured the third regulator down and it was outputting only slightly above zero, this is in addition to the second regulator being a bit low. I'm going to try to shore up the joints and be more generous with the flux. If I can't get it to work, I'll wait for the PCB. Thanks for all the help.Also, I just want to point out a few things: I don't think the bad soldering is due to not knowing how to do it properly. The parts are just so small (the smallest being 0402 resistors) that it's a challenge for me to just hold the part and the soldering iron steady. With the PCB, it should be a lot easier because I can just press the part against the board. And yes, I was tinning everything before trying to join them. The red wire was indeed ground. I simply had not attached a second wire for power input. The wires weren't color coded. I had some wires salvaged from a couple of junk power supplies. The black wires were all cut from the same 24AWG wire and the red was cut from a 20AWG wire.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 11, 2021, 08:16:39 am
content: It's OK, many of us resort to crusty soldering when prototyping, it's not always easy and the results are ugly. Saving time in soldering something up quickly wastes time debugging said soldering but it is likely still much faster than designing and ordering a prototype PCB for example.Just take my advice about the MLCCs seriously, cracking is surprisingly common issue in such constructions, I'd say it's almost guaranteed to fail at some point, BTDT. A much better-looking MLCC contraptions of mine have catastrophically failed. Get some leaded MLCCs for prototyping, or use solderable breadboard to mount the SMD MLCCs, the rest of the components can hang in air.