Consistently destroying micro controller boards! Help! - Page 1
Forum section: electronics / projects

author: waymond91
author_rank: Regular Contributor
timestamp: May 04, 2016, 09:02:51 am
content: Hey there!For our senior project we have based our entire control system on the new STM32 F303K8 Nucleo Board.However, during our assembly process we have burned through 3 of these things! It is unclear what happened the first two times, but I think I have pretty good idea what happened this last time. I just wanted to share my thoughts here so that I could be 100% positive we have fixed this problem in the future.Our application uses a 7V bus to power some motors; this line is fed into the Vin pin of the Nucleo. This was all working fine, and when we went to attach some heatsinks to the motor drivers, we noticed that when we powered it up agian, there seemed to be a short. At first we assumed that our heat sinks had touched a couple pads, but then we noticed that the bus wires carryting +7v and +3.3V we making contact. After fixing this, there still seemed to be a short. When we remove the Nucleo board from the system, there is no short.Plugging the Nucleo into the computer still brings up the MBED usb drive along with a file titled "Fail.txt" and the chip gets REALLY hot and the board will sink over 600mA from power supply depending on what pin you power it through...My suspicion is that when we connected the two buses together we either: 1) blew out one of the voltage regulators when it was reversed biased or 2) Blew out some component of the microcontroller chip itself.When I plug in the nucleo to USB I get ~2.7V on the 5v rail (nothing on the micro should be hooked up to 5v if I'm not mistaken) and ~2.5 on the 3.3v rail.In the user manual for the chip I found the schematic showing the voltage regulation circuit. Looking up the part numbers it seems that they have no reverse-bias protection, the board itself DOES seem to include diodes to protect it in exactly this kind of situation.I was not there when this happened twice before. It seems possible that while testing the circuit a group mate may have briefly shorted these buses together. What do you guys think? Do the symptoms of the problem match my diagnosis? Obviously we will make sure the buses are well isolated in the future, but is this enough preventive action? We really need this to work and do not have a lot of extra time and money left. What checks should I do to thoroughly understand the problem and to ensure it is not repeated in the future?Here is the user manual for the nucleo, we are using a F303K8:http://www2.st.com/content/ccc/resource/technical/document/user_manual/e3/0e/88/05/e8/74/43/a0/DM00231744.pdf/files/DM00231744.pdf/jcr:content/translations/en.DM00231744.pdfHere are the datasheets for the two voltage regulators:http://www.mouser.com/ds/2/389/CD00227527-470539.pdfhttp://www.mouser.com/ds/2/389/CD00000544-310772.pdf

author: danadak
author_rank: Super Contributor
timestamp: May 04, 2016, 10:20:47 am
content: Anytime I hear motor involved large V transients come to mind,If any pin taken out of this range Vdd +.3 >= Vpin >= Vss - .3 youcould trigger the parasitic SCR that exists on pins in CMOS whichshorts the Vdd and Vss rails internal to the part. usually blows outthe bond wire(s) internal to chip for power supply rails. But sometimescauses a dead short due to melted aluminum pads, then chip getssuper hot. Term is latchup. Ap note attached.Power supply sequencing a major issue, and protecting inputs fromoutside rail excursions, normally done by combo of series R and eitherdiodes or zeners. Does the UP support power up defined states on I/Opins, if not you have to design in requisite protection and sequencing.Google this "cmos input protection images"Regards, Dana.

author: Ian.M
author_rank: Super Contributor
timestamp: May 04, 2016, 10:38:39 am
content: Applying 7V to the 5V or 3.3V buses will destroy the board.Don't do that!As the Vin pin is adjacent to ground, on the Nucleo's header it should be quite impossible to do that accidentally, unless you've done something stupid*Another issue could be spikes on the 7V rail exceeding the 15V Abs Max input voltage rating of the LM1117 regulator. I'd add a 10V unidirectional TVS diode between this rail and ground to provide overvoltage and reverse polarity protection..A photo of your project's boards and wiring would go a long way to help us understand how it's been possible to make a fatal mistake with the connections three times, or if your layout is such you are likely to have dangerous transients on your ground or I/O wiring that could be destroying the Nucleo due to latchup. However, if it lets the smoke out immediately on powerup, before any motors start, its almost certainly a short or wiring errr rather than transient induced latchup.In future, after any wiring changes check for rail to rail and rail to I/O shorts with a DMM on continuity at the Nucleo's Nano headersBEFOREapplying power.* I am ASS*U*MEing you aren't dumb enough to have used individual wires with separate connectors directly connected to the Nucleo's Nano compatible header pins.

author: Kalvin
author_rank: Super Contributor
timestamp: May 04, 2016, 11:30:25 am
content: +1 for the unidirectional TVS. I once worked with a 5V logic board hooked to a good lab power supply supplying the +5 voltage to the board. But for some reason my board got damaged from time to time when switching the power on-off-on. Checking the power supply output of the lab power supply with an oscilloscope revealed that there war a very short +30V voltage transient at the moment when the lab power supply was turned off: The lab power supply lost the control of its output voltage regulation during power off cycle and dumped the remaining 30V from its filtering capacitor to its output. The cure was to use a TVS until I built a decent power supply. So, if you are using a new lab power supply, it is always a good thing (TM) to check its output behavior when cycling on-off-on.

author: waymond91
author_rank: Regular Contributor
timestamp: May 04, 2016, 06:42:36 pm
content: Thank you for the replies!I think this must be the source of the problem for the previous nucleo boards as well.I think the culprits are the wires in the 3rd attached picture. These are the 3.3V and 7V bus voltage wires going into the motor control board. I think we can fix this via proper soldering.Seeing as how we don't have time to order new components from digikey (those clamping diodes seem like just the ticket, and will be included in the next revisions) I was going to try to bodge on an over-voltage protection circuit.I was thinking if I used a 3.6V zener diode to create a voltage reference in a comparator against the 3.3V bus; I could use any positive outputs to drive some sort of latching circuit that would essentially short the 3.3V bus to ground whenever it went too high. I was hoping that this in combination with proper soldering and attention to the problem would make our application more robust, assuming I have diagnosed the problem correctly.What do you think?Best regards**edit: looks like my photos were too large to upload. here are the links:http://s32.postimg.org/89fgzysmc/0504161127_HDR.jpghttp://s32.postimg.org/p0iezqzc4/0504161128_HDR.jpghttp://s32.postimg.org/etrwh62ck/0504161128b_HDR.jpg

author: helius
author_rank: Super Contributor
timestamp: May 04, 2016, 07:30:44 pm
content: If you are going to solder copper wires directly into your board, I highly recommend that you use short lengths of heatshrink tubing from now on. Cut the heatshrink to ~1 cm, push it over the wire, and then after soldering the wire to the board slide the heatshrink down to the board and shrink it in place. This will prevent shorts to nearby conductors and slightly reinforce the wire against flexural damage.You can use an ordinary hair drier to shrink if you don't have a heat gun.

author: danadak
author_rank: Super Contributor
timestamp: May 04, 2016, 08:22:54 pm
content: Shorting a buss to ground I would be hesitant, even if it supposedly has Isc protection.Rather, if you can, use a Zener on the buss, that way zener releases buss when V drops belowits setpoint. However Iz must be rated for full buss current. Another way is to limit buss currentto total of its loads. That can be done with a transistor -http://electronics.stackexchange.com/questions/85719/improving-a-simple-bipolar-current-limiterRegards, Dana.