Looking for failure mode troubleshooting advice - Failing N-Chan Power Mosfet - Page 1
Forum section: electronics / projects

author: rfdes
author_rank: Regular Contributor
timestamp: January 18, 2024, 05:13:11 pm
content: Hi -I am helping a co-worker resolve a 'failed' mosfet problem and could use some advice.This is a simple device that pulses ON a power mosfet in order to sink currentthrough a tungsten 'igniter' element, vaporizing the igniter. The schematic isreally quite basic (shown below). The power source is an 18V cordless power drillbattery. The power mosfet being used is a Vishay SQJQ160E device with a D-S breakdownof 60V and a continuous drain current of 600 Amps. The mosfet's pulsed drain currentspecification is 655 Amps.The issue that is being observed is that the mosfet is failing shorted when pulsed onuntil the tungsten element has vaporized. Unfortunately, I do not have access to anyof the hardware and am going on the reports that are being handed down to the failure.I have no idea what the current capacity of a Lithium celled 18V power tool battery has butI would have thought that it wouldn't be near 600 Amps.My question is, do these battery's have the capability of exceeding the Mosfet's currentspecification of 650 Amps? We are trying to avoid adding current limiting resistors dueto size constraints.Also, is there a possibility that the failure is due to something other than over current?Any ideas and pointers?Thanks

author: berke
author_rank: Frequent Contributor
timestamp: January 18, 2024, 05:24:00 pm
content: Did you check the safe operating area curves?

author: rfdes
author_rank: Regular Contributor
timestamp: January 18, 2024, 06:21:52 pm
content: Quote from: edavid on January 18, 2024, 05:34:12 pmGate voltage is being driven to 10V and the gate cap is a ' do not place' so it is not being used.The cold resistance of the igniter is approx 0.050 ohm.I've asked for this to be measured and am struggling to get this information. Why, I do not know. Laziness, I suppose.

author: rfdes
author_rank: Regular Contributor
timestamp: January 18, 2024, 06:47:44 pm
content: Quote from: berke on January 18, 2024, 05:24:00 pmYes, we've checked the conditions and they appear to be within the safe area but we need measurements to confirm.Thanks

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 18, 2024, 07:49:35 pm
content: The schematic is really quite basic (shown below).So basic in fact it is missing the gate drive circuit entirely!!

author: Andy Chee
author_rank: Super Contributor
timestamp: January 18, 2024, 08:08:03 pm
content: Quote from: rfdes on January 18, 2024, 05:13:11 pmLanguage is important here.Is the mosfet failing shorted? Or is the mosfet failing to turn off?There is a big difference between the two issues.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 18, 2024, 08:12:47 pm
content: Quote from: rfdes on January 18, 2024, 05:13:11 pmA quick google finds 18V battery packs rated at 20A or 30A peak.You would need multiple packs and multiple FETs.If your intention is to whack something until it fuses, you will need very careful and aggressive voltage clamping, as the load 'goes open' at peak current.

author: inse
author_rank: Super Contributor
timestamp: January 18, 2024, 08:41:33 pm
content: I assume the gate drive circuit is not supplied by the 18V battery, which surely will break down during the pulse.The wiring of the MOSFET is capable of conducting the 100‘s of amps and the gate driver is directly connected to the source of the MOSFET?So there is no ground shift that might limit the effective G/S voltage.The suppressor diode is in place and good shape?If possible, measure gate/source and drain/source voltage during the pulse.Current measurement might become tricky

author: berke
author_rank: Frequent Contributor
timestamp: January 18, 2024, 09:33:54 pm
content: Quote from: PCB.Wiz on January 18, 2024, 08:12:47 pmThat could be the problem, depending on how fast the tungsten igniter goes open.The resistance is given as .1 ohm which should be what you get from 1" of 0.005" diameter tungsten. The energy required to vaporize that is about 30 J. If the voltage source was ideal the power supplied would be 3240 W and it would take about 4.7 ms. At fusion radiative losses should be around 86 W. However if the battery pack is limited to 30 amps full vaporization could take 7 seconds.In any case, if the vaporizing rod still conducts at 10 S (0.1 ohm) and if it vaporizes instantaneously the dI/dt could be enormous. Depending on how things are wired one can expect for example 200 nH inductance in the wiring. Then a quick LTSpice simulation gives ridiculous dI/dt values of 3 TA/s. A simple 5 pF stray capacitance to the gate of the MOSFET could be enough to make things go poof, as in:

author: rfdes
author_rank: Regular Contributor
timestamp: January 22, 2024, 04:47:46 pm
content: Quote from: fourtytwo42 on January 18, 2024, 07:49:35 pmYes, that is intentional. Some of the circuitry is sensitive. I was simply trying to explain my issue.Thanks.

author: rfdes
author_rank: Regular Contributor
timestamp: January 22, 2024, 04:48:54 pm
content: Quote from: Andy Chee on January 18, 2024, 08:08:03 pmTotally agree. Poor choice of words. The Mosfet IS failing shorted. In fact the short exists between all three pins.Thanks.

author: rfdes
author_rank: Regular Contributor
timestamp: January 22, 2024, 04:53:23 pm
content: Quote from: PCB.Wiz on January 18, 2024, 08:12:47 pmNo, it is a single mosfet and a single fusible link. The mosfet is rated at >600 Amps peak and I was sure the battery couldn't support that. Problem is lack of measured data from my colleague and I was just trying to get some ideas as to where to start troubleshooting the issue. All I can tell you at this point is that the drive pulse is 250mS long, however, I have no knowledge how long it takes for the fusible link to open. Hopefully, I can come back with some useful data to present.Thanks.

author: rfdes
author_rank: Regular Contributor
timestamp: January 22, 2024, 04:55:42 pm
content: Quote from: inse on January 18, 2024, 08:41:33 pmThe pulse is derived from a 12V regulated circuit created from the 18V supply.Thanks.

author: rfdes
author_rank: Regular Contributor
timestamp: January 22, 2024, 04:57:22 pm
content: Quote from: berke on January 18, 2024, 09:33:54 pmThis is good information, I will forward this to my colleague. I appreciate your opinion.Thanks.

author: Slh
author_rank: Regular Contributor
timestamp: January 22, 2024, 05:24:53 pm
content: Something else to consider might be the order of the failure. Does the MOSFET definitely fail after the wire or does the MOSFET failing result in the wire vapourising?Also, what's the resistance of an arc in the medium that the ignitor is in (philosophical question - I don't expect a quantitive answer)? When arcs form, the area of plasma can increase rapidly so the resistance will drop like a stone and the current will increase correspondingly. This could be too much current for the MOSFET or just drop the 18V enough to also cause the 12V to drop and exceed the SOA ratings.

author: Wolfram
author_rank: Frequent Contributor
timestamp: January 22, 2024, 06:00:30 pm
content: What's your gate resistance, and rise time of the gate drive circuit? The MOSFET has a whole 200+ nC of gate charge, so it needs to be driven hard and fast to survive even a single switching event given the currents and voltages involved here. Consider the fact that the device will be dissipating significantly more while transitioning from off to on, compared to while fully turned on, and if this takes too long then the transistor simply overheats.Measure the voltages and currents in the final application, and the gate rise time. This allows you to calculate the dissipation during switching, and the peak junction temperature.

author: thm_w
author_rank: Super Contributor
timestamp: January 23, 2024, 12:07:43 am
content: Quote from: rfdes on January 22, 2024, 04:47:46 pmLeaving out relevant detail wastes your time and others. There is a good chance the gate drive is just an off the shelf part and not a sensitive design.Even if it were, I could buy the product, take it apart, and see what circuitry is there.

author: berke
author_rank: Frequent Contributor
timestamp: January 23, 2024, 08:18:32 am
content: In hindsight, a "tungsten igniter that vaporizes" is kinda sus."Hey guys, so I, uh, ran into this plutonium ball with a bunch of HE lenses around it and I'm trying to figure out the optimum delays, could you give me a hand?"

author: Slh
author_rank: Regular Contributor
timestamp: January 24, 2024, 07:15:38 am
content: I was assuming rocket ignitor but I guess that's also an option. Fortunately he claims to be in America so is just exercising his 2nd amendment rights...

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 24, 2024, 08:46:26 am
content: Quote from: rfdes on January 18, 2024, 05:13:11 pmOf course.You could look at this thread.https://www.eevblog.com/forum/projects/i-wanted-a-silk-purse-but-all-i-could-afford-was-this-sows-ear-ya-spot-welder/has some great pictures of the plumbing/wiring around TWELVE FETs they used to control ~900A

author: berke
author_rank: Frequent Contributor
timestamp: January 24, 2024, 08:50:43 am
content: Quote from: Slh on January 24, 2024, 07:15:38 amObviously no militia is well-regulated without a fission device.

author: moffy
author_rank: Super Contributor
timestamp: January 24, 2024, 10:02:10 am
content: Use an SCR instead of a MOSFET, they have much better surge ratings for a given current device. Also you only need to trigger it once and it will stay on until the igniter is vapourised.

author: liteyear
author_rank: Regular Contributor
timestamp: January 24, 2024, 11:24:30 am
content: Mysterious MOSFET failures that have plagued my career have generally turned out to be unexpected gate voltages, rather than SOA excursions per se.When the MOSFET turns off, the source-gate and drain-gate capacitance can be significant. For example, it's surprisingly easy to exceed thenegativeVgs limit due to a source voltage spike relative to the gate driver's ground reference. Or exceed the positive max due a drain voltage spike as @berke vividly describes.If you're managing an arc, then suppressing the ignition and extinguish events are probably of high importance.

author: Randy222
author_rank: Frequent Contributor
timestamp: January 24, 2024, 07:43:06 pm
content: Here's what I am figuring.When the load is about the same as wire resistance, things start to look like DS short analysis as the load-S voltage gets high enough to make huge amps. You can characterize the ckt shown as a short with huge ~300 amps, possibly more.So, now the ckt diagram as shown is not realistic at all. The wire from batt to load is R1, the load is R2, the wire from load to fet D is R3, the Rds(on) is R4, the wire from fet S to batt gnd is R5.With huge amps the GS voltage is not G-gnd. So, running gate at 10vdc != to 10v GS.Mostly likely the fet is "on" in the well defined non-linear region. The graph for the fet Rds(on) is clear. My good guess, GS voltage is likely around 6-7v with a 10v gate drive.How to test it? Each test is a melted fet. The only way I can think of a good/easy test is to do 1-shot trigger captures on a scope to capture the voltage pattern over a short period of time (1ms, 0.5ms, maybe 2ms). Capture the GS voltage when G voltage is applied. Then capture directly across fet G to S, then perhaps directly from fet S to batt gnd terminal.The fet failure is likely internal fet melt, high pulse current that is too long for the fet to survive.How you might fix the ckt to keep fet from burning up, add a heavy wire current-sense resistor between fet D and load, choose ohms wisely. Probably also wise to increase the gate voltage to almost near GS(max).Quote from: moffy on January 24, 2024, 10:02:10 amIdeally you want to quench the silicon switch some time during the plasma phase period. You don't technically need the silicon current when the igniter has already reached the plasma phase. This can perhaps be achieved by testing as I mentioned, this will get some timings on paper, then create a single pulse on the gate. That pulse length, plus the time it takes the gate charge to fall out to off state, should be how long the silicon conducts. In reality, the gate drive ckt is not as simple as a toggle switch to a 18v cordless drill battery.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 24, 2024, 07:49:46 pm
content: Quote from: moffy on January 24, 2024, 10:02:10 amInteresting idea. It would need to be a model without bonding wires.

author: Randy222
author_rank: Frequent Contributor
timestamp: January 25, 2024, 12:28:46 am
content: Quote from: PCB.Wiz on January 24, 2024, 07:49:46 pmDid you happen to check pricing for big amps SCR ?Another use with SCR is to use it to crowbar, creates a short path from fet D to S protecting fet from the long period of over current. Triggering the SCR as crowbar is where the handy math skills come into play.

author: moffy
author_rank: Super Contributor
timestamp: January 25, 2024, 12:40:44 am
content: Quote from: Randy222 on January 25, 2024, 12:28:46 amI would consider something like the VS-50RIA, stud mounted into a reasonable heatsink to help absorb the pulse heat:https://www.vishay.com/docs/93711/vs-50ria.pdfabout $23 AUD at Digikey.

author: Randy222
author_rank: Frequent Contributor
timestamp: January 25, 2024, 12:46:11 am
content: That's a 50A scr. How much current is actually needed? The original ckt seemed to indicate like 360A or so (given no limit from the 18v batt).Does the "igniter" actually vaporize in design, or is that just something that is part of the failure?If the goal was to vaporize it fast, then most likely the event is not long enough for joules to heat the SCR to any significant temp.So, although heatsink is ok, you mentioned "pulse", so assume to mean a very short event.Analogy is like this: if you touch a 1000C hot stovetop for 0.00000000001sec, not enough joule transfer to burn you.

author: moffy
author_rank: Super Contributor
timestamp: January 25, 2024, 12:58:47 am
content: Quote from: Randy222 on January 25, 2024, 12:46:11 am50A@94C, 80A RMS and 1400A non repetitive surge which is the most significant value, the heatsink will help, how much? that is dependent on the pulse shape and duration which is unknown and probably more dependent on the wires and battery internal resistance than anything else.P.S. I hope all of your power wires are twisted or held together to reduce inductance.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 25, 2024, 01:15:26 am
content: There is also BT158W-1200 SCRMouser have USD $5.89, lcsc have similar US$4.0139The OP could start with that, and work downwards, if that one does not fry ! (or go up, if it does fry !)Data says : Vo = 0.984 V; Rs = 0.0033 ΩITSM looks decent, the dI/dT could be interesting, says 150 A/μs MAX.

author: Randy222
author_rank: Frequent Contributor
timestamp: January 25, 2024, 01:18:13 am
content: It's the peak current needed, is what I am asking.A $6 SCR is not gonna support 300A.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 25, 2024, 01:22:01 am
content: Quote from: Randy222 on January 25, 2024, 01:18:13 amSee fig 4 above, for how many amps for how long graph...

author: Randy222
author_rank: Frequent Contributor
timestamp: January 25, 2024, 04:30:19 pm
content: I read all the specs in PDF. Some of the charts are more like transient specs, but that's ok for the problem ckt.1st, it's a no-no to run the device right at ABS-MAX specs.2nd, using the SCR alone means there needs to be some control of the large DC current period in order to protect the SCR. This is not easy with just an SCR alone. With DC it's gonna clamp on and stay on, until you turn off the large current flow.3rd, the event (whatever that is) needs to be less than the single half-wave AC sine 50/60Hz period. Maybe 2-3x that if the actual amps is lower. We have no idea how long the event takes, or when the silicon can be quenched during the event. Testing needed to know that.Would need to build models in spice, but perhaps crowbar a fet, or put SCR inline after fet and tie their gates together and drive with a proper gate drive pulse signal?Or, just make bank of 40v 1F caps, charge them to 35v, then dump them directly to the grounded load using "large" mechanical switch. The caps will do what they can in the "short" and then stop conducting when the load(plasma) halts.If there needs to be "control", then silicon is the way to go. If just needing a way to create "fast" event without control, then just use large source like cap bank or 12v car batt.

author: jonpaul
author_rank: Super Contributor
timestamp: January 26, 2024, 10:51:28 am
content: arc RFI , inductive load, use appropriate transient surpression Zener and /or snubber across the devicesj

author: Randy222
author_rank: Frequent Contributor
timestamp: January 26, 2024, 04:26:46 pm
content: Quote from: jonpaul on January 26, 2024, 10:51:28 amA zener to take ~1/2 the amps?There's a discrete zener in the posted ckt.

author: MarkT
author_rank: Frequent Contributor
timestamp: January 28, 2024, 08:45:11 pm
content: Quote from: rfdes on January 18, 2024, 05:13:11 pmThe power PAK package cannot take anything like 600A continuous, the bond wires will vaporize - that's the die-only rating I suspect (a common trick used by some FET manufacturers). If the pulse rating at 300us is 655A, the continuous rating is more likely to 60A or so. Note they also claim the 8mm x 8mm package can dissipate 600W at 25C - think about that...This is definitely an application for a chunky thyristor, not a FET. I've a hockey-puck thyristor that can handle 9000A pulse for real, it weighs about a pound!

author: johansen
author_rank: Super Contributor
timestamp: January 29, 2024, 07:40:09 am
content: So what happens to the gate driver circuit when the 18v battery is pulled down to about 6 volts by the 50mohm load?

author: Randy222
author_rank: Frequent Contributor
timestamp: January 29, 2024, 02:08:32 pm
content: Quote from: johansen on January 29, 2024, 07:40:09 amThe fet cooks in non-linear region.But, it all depends on how fast the event happens. Maybe adding 2-3 1F caps very close to fet D & S, you can somewhat mitigate the v drop.The source (batt) is just another point to capture on a DSO, or some form of DAQ.

author: jonpaul
author_rank: Super Contributor
timestamp: January 29, 2024, 02:53:29 pm
content: check for UHF parasitic oscillations in linear region.j

author: berke
author_rank: Frequent Contributor
timestamp: January 29, 2024, 05:10:59 pm
content: Simpler scenario. Couldn't find a SPICE for the MOSFET so I took something similar from Infineon. Even a small 50n inductance on the ground return is enough to cause a large spike on the gate, no need for capacitive coupling.

author: MarkT
author_rank: Frequent Contributor
timestamp: January 29, 2024, 05:43:25 pm
content: Thinking about this and thyristors - usually they have an I-squared-t rating for pulse rating, so if you know the max current and max time to fuse the igniter, you can determine I-squared-t and compare to datasheets.For instance this stud-mounted thyristor has a rating of around 10000 A^2s, which could handle 300A for 0.1s for instance:https://www.vishay.com/docs/93711/vs-50ria.pdfAnd being stud-mount there are no bond-wires to melt either, the die is sandwiched between flat electrodes.