I keep killing mosfets - what I do wrong? - Page 1
Forum section: electronics / beginners

author: PMA
author_rank: Contributor
timestamp: September 28, 2019, 06:15:41 am
content: I have medium sized PCB that have three identical driver circuits:When I assembled the board, one of the circuits worked, one was constantly on and was constantly off. Then I changed Q3 for both and one of channels started to work. However, I cannot get third channel to work, it is constantly on. Channels seems to work reliable once they work first time.Checked so far:-I haven't goofed with resistors, they are correct and there are no short circuits-D32 works as expected-Control signal from micro is ok, I can wiggle it low/high. By default it is low.-BTS555 works as expected - if I remove Q3 it stays off-When I install new Q3 and measure the circuit, it gives identical reading than two other circuits, but when I switch power on, Q3 pulls low (enough).Do I just have bad luck with ESD/components or is there something fundamentally wrong with the circuit?Edit:datasheets for major componentsQ3:https://assets.nexperia.com/documents/data-sheet/2N7002P.pdfU11:https://media.digikey.com/pdf/Data%20Sheets/Infineon%20PDFs/BTS555.pdfD32:http://www.vishay.com/docs/87787/v20dl45bp.pdfSolvedGround of the soldering iron was in different potential than ground of the PCB.

author: magic
author_rank: Super Contributor
timestamp: September 28, 2019, 06:25:14 am
content: You drive Q3 with 5V divided by 6 due to R83/R89. That's less than 1V, not really a lot.The first guess is that most of 2n7002s have insufficient gate threshold voltage to turn on in such circuit. Remove R89. Perhaps reduce R83 value if Q3 takes too long to switch after a command from the MCU.If BTS555 doesn't have an internal pullup resistor on IN, an external resistor should probably be connected. Presumably to VBB, but I don't know what this chip is so check the datasheet.

author: PMA
author_rank: Contributor
timestamp: September 28, 2019, 06:29:46 am
content: R83 is only 100 ohms, so there is practically 5V on the gate. R83 is there to protect output of the micro.Pin 2 of the BTS555 is pulled to VBB internally.

author: oPossum
author_rank: Super Contributor
timestamp: September 28, 2019, 06:31:48 am
content: Try testing the 2N7000 before assembling the board to determine if you have bad parts or they are being damaged later.

author: PMA
author_rank: Contributor
timestamp: September 28, 2019, 06:40:53 am
content: I kind of know how to test them, but just to avoid mistakes by being overly self-confident: What is the best/correct way to test mosfets (I don't have speciality test gear available, only scope and good quality multimeter)?

author: magic
author_rank: Super Contributor
timestamp: September 28, 2019, 06:45:45 am
content: Quote from: PMA on September 28, 2019, 06:29:46 amSorry, I somehow saw 100k

author: JustMeHere
author_rank: Frequent Contributor
timestamp: September 28, 2019, 06:55:15 am
content: Quote from: PMA on September 28, 2019, 06:40:53 amThe make component testers like this:https://www.ebay.com/i/263099848341?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=263099848341&targetid=474173447189&device=c&mktype=pla&googleloc=9010735&poi=&campaignid=6469750210&mkgroupid=79220309442&rlsatarget=pla-474173447189&abcId=1141186&merchantid=118875589&gclid=EAIaIQobChMInLfwgvby5AIVRtbACh2STgBHEAQYAyABEgJlOPD_BwEI'm thinking it's something to do with your soldering. The circuit looks good. There's an example of it in your BT555 datasheet.Make sure voltages are good coming out of our micro.Do you have the actual board file? It would be good to check it for mistakes.

author: PMA
author_rank: Contributor
timestamp: September 28, 2019, 07:51:04 am
content: Quote from: JustMeHere on September 28, 2019, 06:55:15 amCorrect, but not the way you expected.My soldering iron was in different power outlet than power supply and there is quite large potential difference on ground planes between the outlets. I left PSU connected to the board when soldering to avoid ESD problems. In reality there was large potential difference between GND of the PCB and tip of the soldering ironLuckily I didn't break anything else from the board.Edit: it is chooching now. Thanks for the help.

author: Ian.M
author_rank: Super Contributor
timestamp: September 28, 2019, 09:04:30 am
content: Yes. Odd potential differences at the soldering iron bit are notorious for killing 'bare' MOSFETs and other sensitive components. Seehttps://www.eevblog.com/forum/beginners/grounded-soldering-iron-or-ungrounded-(eu-plug)/with link to case of blown 2N7000 MOSFETs in my reply #9

author: magic
author_rank: Super Contributor
timestamp: September 28, 2019, 09:51:08 am
content: Quote from: PMA on September 28, 2019, 07:51:04 amThere shouldn't be more than 20V between Earth pins in different outlets. In fact, there shouldn't even be 2V because no current flows through earthing cables except under fault conditions or with bad ground loops.OTOH, if the PSU isn't earthed at all (uses a 2 prong mains plug) then it will indeed inject low energy but high voltage charges into the powered circuit's ground. If something is powered by such a PSU and connects to any other circuit, its ground must be shorted to the other circuit's ground before making any other connection.The other day I have blown a GPS receiver module that way...editA better way is to ground the board to Earth through a 1MΩ resistor. This discharges the PCB before its touched by the iron but doesn't do so rapidly and parts have more chance to survive.

author: PMA
author_rank: Contributor
timestamp: September 28, 2019, 09:58:09 am
content: My iron was actually in ungrounded power outlet (only extension is grounded and I didn't noticed it / thought through), and there was computer etc. in the same extension. PSU was properly grounded.Luckily this is beginners area so I don't feel that bad

author: magic
author_rank: Super Contributor
timestamp: September 28, 2019, 10:01:13 am
content: Myself, I use a grounded iron so that it doesn't accumulate charge but I don't bother grounding the circuit. Instead, I touch some grounded metal and the board's ground simultaneously to discharge it and then just solder away. And that's only when dealing with MOSFETs.So far so good, but I can hear the ESD zealots screaming in horror

author: PMA
author_rank: Contributor
timestamp: September 28, 2019, 10:34:38 am
content: I agree with your method. I understand that ESD measures are taken seriously in production environment but there is no need to go overboard when prototyping with single or couple boards.Even single MOSFETs seems to be quite tough guys as I was able to assemble two with my original setup without killing themDifference between soldering iron and GND was probably 120volts (1/2 of supply).I found the problem when I was connecting serial cable from the board to PC. My hand touched PC and outer shell of the USB cable at the same time. I sweared twice - firstly because it hurt and secondly because I realized what I have done

author: Ian.M
author_rank: Super Contributor
timestamp: September 28, 2019, 10:55:09 am
content: As I commented in the thread I linked, a 1Meg resistor that is appropriate for ESD grounding wont help with the possible leakage current from a hot ceramic insulated mains powered soldering iron, as it only needs 10uA to develop 10V across the grounding resistor. It then becomes a matter of whether the peak voltage, low-pass filtered by the RC network of the grounding resistance with the MOSFET gate capacitance exceeds the gate oxide breakdown voltage.Its even worse if you have an un-grounded SMPSU feeding a low voltage iron. The max Y capacitor leakage current set by various safety standards is 0.75mA, and even if the leakage is only a tenth of that, it will develop far more than any normal MOSFET's gate oxide breakdown voltage across a 1meg resistor.Therefore, mains powered, and low voltage irons with a SMPSU that isn't specially constructed to avoid Y capacitor leakage current problems, need their bit hard-grounded to the same ground as the device you are working on (if its grounded).Yes it increases the risk of ESD damage if you aren't working on an ESD dissipative surface, but you should be on an ESD mat anyway. If you are dealing with notably ESD sensitive devices, avoid working on a silicone heat resistant mat unless its from a reputable manufacturer and specifically was sold as ESD safe.

author: Jwillis
author_rank: Super Contributor
timestamp: September 29, 2019, 01:47:10 am
content: According to the data sheet the maximum current to the in pin(2) is -250mA. In the diagram on page 7 it shows a resistor between the switch and pin 2. Maximum drain current on the 2n7002 is typically 115mA. Did you try putting a resistor between the drain and pin 2?Just a thought. If its shunting to ground then there must be current there.

author: PMA
author_rank: Contributor
timestamp: September 29, 2019, 05:38:47 am
content: I think that the resistor is needed for reverse battery protection (page 9). So it would be good to have, but not mandatory.According to page 13 (right side) and page 6 (bottom), typical current is only 1mA or so.

author: Jwillis
author_rank: Super Contributor
timestamp: September 29, 2019, 07:58:37 am
content: Quote from: PMA on September 29, 2019, 05:38:47 amOk but that mosfet is pulling current from some where. Could always check to see how much current is being drawn with a ammeter. If it more than a milliamp then that chip might be faulty.

author: PMA
author_rank: Contributor
timestamp: September 29, 2019, 08:05:20 am
content: That is a good advice.However, the problem is already solved. My soldering iron was in ungrounded outlet which caused large potential difference between grounded PCB and tip of the iron.

author: Jwillis
author_rank: Super Contributor
timestamp: September 29, 2019, 05:26:26 pm
content: Thanks Ian.Something we all should take into consideration how lucky many of us have been with the old wood burners we've been using.The BTS555 is a class 3 Military spec (4,000 volts and above)ESD device .When you consider that some of the commonly use op amps like the TL082 are a mere class 1( 0 volt to 1,999 volts) or the 555 timer of a mere 500V .

author: Yansi
author_rank: Super Contributor
timestamp: September 29, 2019, 06:13:26 pm
content: and btw, when will people learn, thatZENER diodes are UNUSABLE as analog input protection?

author: PMA
author_rank: Contributor
timestamp: September 29, 2019, 06:17:03 pm
content: I don't disagree, but could you explain or point to articles or other sources?

author: Yansi
author_rank: Super Contributor
timestamp: September 29, 2019, 06:19:42 pm
content: I think that a VI characteristic of that zener is telling enough. The zener does not magically turn on exactly at the specified voltage. It begins to conduct well blow the rated voltage, skewing the measured voltage due to the voltage drop on the series protection resistor.This phenomenon is especially notable with low voltage zeners.

author: Per Hansson
author_rank: Unknown
timestamp: September 29, 2019, 07:32:46 pm
content: Thanks for this useful thread and the links to further discussions!My Weller WSD81 provides a 0-ohm connection to ground.However my old and venerable Weller WECP-20 had been modified by someone with an IEC plug.I thus assumed it was grounded but found out a few weeks ago it was not, no ground connected inside, yikes!Originally it must have come with a two-lead prong, I attached the ground now to the internal screw for the banana ground jack.Before there was ca 24VAC on the tip!That soldering iron is interesting though, there is a jumper inside and if you cut it you get a R/C filter 120 KOhm/22nF for the ground.I assume this might be beneficial if the device to be soldered actually does hold some charge above ground.I left the jumper intact for now though, what are your thoughts on it?http://remotesmart.wdfiles.com/local--files/weller-wecp-20/WECP_manual.pdf

author: PMA
author_rank: Contributor
timestamp: September 29, 2019, 07:45:07 pm
content: This has been taken into account (kind of).I have assumed that 5V1 zener starts to conduct in about 4.5V on small currents. However, I have never measured it. Because of the scrutiny and the fact that I had test setup practically ready made, I decided to take some measurements.There is four identical circuits side by side on the same board. To get different voltages to ADC circuit I used 2k4 potentimeter and 5V supplied by the board.Circuit on PCB:Measurements:Max/min are taken from the channel that gave max/min reading to that specific voltage.The data shows that 5V1 zener starts to effect measurements about one volt below nominal voltage, but this of course depends on many things and my sample size wasn't large enough to any definite conclusions.

author: Yansi
author_rank: Super Contributor
timestamp: September 29, 2019, 08:12:35 pm
content: Better than getting affected by a zener, is to use a Si diode clamp (BAV99, BAV199, double 4148, etc). These will work even for very high impedance measurement circuits. Leakage is very low, and when they are operated with a floating clamp voltage, the leakage is almost nonexistent.But be aware that neither of these solutions is very rugged against ESD events, additional protection may be required.

author: iMo
author_rank: Super Contributor
timestamp: September 29, 2019, 08:45:05 pm
content: Quote from: PMA on September 29, 2019, 08:05:20 amLong time back the mosfets (no esd diodes at that time on the chip) had the legs shorted by a piece of copper wire (a small spring actually). In harsh environment simply wind a thin wire around the legs and solder the transistor in. After soldering remove the wire.

author: Yansi
author_rank: Super Contributor
timestamp: September 29, 2019, 10:09:50 pm
content: That is unfortunately not that easy when soldering today's surface mount devices.