Blown Mosfet Porn - Page 1
Forum section: electronics / general

author: jastervoid
author_rank: Newbie
timestamp: February 08, 2016, 02:12:29 am
content: This showed up in the shop today and I had to share. Don't think I've seen this type of failure on a motherboard before. It was a Dell Inspiron desktop computer.

author: Brumby
author_rank: Unknown
timestamp: February 08, 2016, 02:17:02 am
content: Yes, yes. I think I see it.

author: retrolefty
author_rank: Super Contributor
timestamp: February 08, 2016, 02:24:47 am
content: Don't lick that wound, could be hazardous.

author: Halcyon
author_rank: Unknown
timestamp: February 08, 2016, 04:35:59 am
content: Suspiciously close to the USB ports. Do you think they might have plugged one of those "USB killers" in?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 08, 2016, 05:21:40 am
content: Nah, looks like the high side switch for the CPU power.Could be a toast CPU in there (likely as a secondary effect). As for what cause that, who knows...Here's one of mine,Thermal failure, note the high fidelity impression of the tin plating against the sil-pad finish.Hmm, sides are kind of cracked. They went with quite a pop. They seem to pry apart...Ah, such a lovely mess.These are IGBTs, by the way. The shiny rectangle is the co-pack diode. It was perfectly fine (probably), just fractured in the disassembly process. The transistor die clearly didn't fare as well.The sequence of events was:1. Transistors (namely, the IGBT die) got way too hot. This depresses Vgs(th) (so eventually, an "off" voltage won't actually be "off" anymore), stretches turn-off time (IGBTs use minority carriers, so recovery time is relevant, which is worse at high temperatures), and increases leakage current (so "off" is less fully off as temperature rises).2. When it reaches some critical temperature, all these effects combine, to generate a hot spot where power dissipation is growing exponentially. This melts, becoming a three-way short.3. About ten microseconds later, the opposite side transistor turns on, into the short. This draws massive current from the supply bypass, killing that transistor as well.4. With the supply now fully shorted, the supply bypass (here, something like 1mF at 160VDC) discharges through the shorts, vaporizing them in about 100us.5. The expanding arc flash splits the case apart, emitting a loud report, and occasionally, sending shrapnel into the air. The capacitors finish discharging within 100us-2ms, depending on type and value.6. After one or two line cycles (~20ms), the short circuit draws excessive line current, blowing the fuse.Not very graphic for TO-247 sized parts, but the whole series of events can be rather hazardous when it occurs with industrial size modules and 480V mains.Tim

author: all_repair
author_rank: Frequent Contributor
timestamp: February 08, 2016, 05:30:41 am
content: Power MOSFET are not cheap parts. IF you are going to fix them, you have to heck through all the surrounding components. Use a current limiter, and use old MOFET (for test). Why? Very painful if your new MOFET pops because of an oversight. For some stubborn fault, it could be a few rounds of pop before the real cause is found.

author: tszaboo
author_rank: Super Contributor
timestamp: February 08, 2016, 11:07:11 am
content: Wait till you see a SOT227 fet blown. There was a 3 cm hole on that PCB, and damaged heatsink. Yes, it damaged a block of aluminium.

author: digsys
author_rank: Unknown
timestamp: February 08, 2016, 12:07:08 pm
content: .. amateurs !! :-) ... try Tantalum caps that go wild !! I've had a few awesome fireballs in my time ...

author: DerekG
author_rank: Frequent Contributor
timestamp: February 08, 2016, 12:41:45 pm
content: Quote from: jastervoid on February 08, 2016, 02:12:29 amYes, I have seen this board failure before. When I asked the customer what he was doing on his computer at the time, he replied "watching the latest very hot porn movie".See, watching porn can do more than just making you blind.

author: nanofrog
author_rank: Super Contributor
timestamp: February 08, 2016, 01:11:22 pm
content: Quote from: DerekG on February 08, 2016, 12:41:45 pmI was always told it made you grow hair on your palms as well.

author: jastervoid
author_rank: Newbie
timestamp: February 08, 2016, 04:03:30 pm
content: Quote from: Halcyon on February 08, 2016, 04:35:59 amDoubtful. The mosfet in question is part of the multi-phase dc-dc converter for the CPU power supply and should have no connection to the +5V for the USB. I think the charring vented in that direction due to the way the chip removed itself from the pcb. I suspect the mosfet itself failed short. I won't be repairing this particular board. I don't have another LGA 1150 motherboard to test the CPU in but my gut says its probably OK.

author: station240
author_rank: Unknown
timestamp: February 08, 2016, 05:07:41 pm
content: I wish I had photos of the mains side of an ATX PSU that blew after a 415V switchboard with a bad neutral fried a few things.

author: AF6LJ
author_rank: Unknown
timestamp: February 08, 2016, 05:21:17 pm
content: Quote from: jastervoid on February 08, 2016, 04:03:30 pmI don't think I would be repairing it ether. looks like there is internal damage to the PCB....Not Good...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 08, 2016, 08:03:04 pm
content: Quote from: NANDBlog on February 08, 2016, 11:07:11 amI suppose you didn't have semiconductor fuses anywhere in the circuit...Now I'm sad! I've exploded one or two before, and the PCB was just fine! Well, aside from the scorch marks, and eroded/burned/melted pads and traces in the general vicinity...(FYI: where transistors are involved, semiconductor fuses only serve to reduce the shrapnel output -- terminating the expanding arc flash earlyish. Under fault conditions, they clear in about a millisecond. That's a darn sight better than the 10-100ms of a traditional beefy mains fuse, especially for industrial 480V. But certainly not enough to avoid any exploding, let alone protect the devices themselves, which again is a 10us kind of thing. Semiconductor fuses are just fast enough to protect diodes and SCRs, though!)Tim

author: tszaboo
author_rank: Super Contributor
timestamp: February 08, 2016, 08:51:02 pm
content: Quote from: T3sl4co1l on February 08, 2016, 08:03:04 pmBunch of FET in parallel*, only one fuse. Not my design. Those fuses are expensive anyways.Found the picture for you guys.