MOSFET keeps failing - Page 1
Forum section: electronics / beginners

author: timdriedger
author_rank: Contributor
timestamp: October 27, 2017, 10:21:35 pm
content: Hi Everyone,Looking for some advise on the attached schematic.The MOSFET in the design keeps failing. I suspect some type of transient voltage. How could I better protect the MOSFET?D5 is connected to an Atmega. Supply is 12VDC. The motor draws about 2A max. PWM duty cycle about 50-70%.Thanks!

author: tatus1969
author_rank: Super Contributor
timestamp: October 27, 2017, 10:26:07 pm
content: most probably overvoltage at the gate, try adding a 4v7 zener.

author: wraper
author_rank: Unknown
timestamp: October 27, 2017, 10:34:12 pm
content: Quote from: tatus1969 on October 27, 2017, 10:26:07 pmAtmega cannot output more than 5V. I think that Atmega has too weak output to charge/discharge gate capacitance fast enough. So at such PWM frequency mosfet stays in linear range big portion of time, exceeds safe operation area and fails. Use proper MOSFET driver IC or make driver from discrete components.

author: danadak
author_rank: Super Contributor
timestamp: October 27, 2017, 10:35:08 pm
content: Use an ohmeter and measure gate to drain, gate to source,to see if you have a blown gate.Some protection methods -https://www.alliedelec.com/images/Products/mkt/pb/cornelldubilier/pdfs/snubbersdesign.pdfhttp://pdfserv.maximintegrated.com/en/an/AN848.pdfhttp://www.semtech.com/images/datasheet/transient_voltage_protection_of_mosfets.pdfRegards, Dana.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: October 27, 2017, 10:44:06 pm
content: You mentioned the PWM duty cycle, but not the PWM frequency. How fast are you trying to run this?

author: timdriedger
author_rank: Contributor
timestamp: October 27, 2017, 10:45:52 pm
content: 32KHz

author: wraper
author_rank: Unknown
timestamp: October 27, 2017, 10:49:23 pm
content: Quote from: timdriedger on October 27, 2017, 10:45:52 pmAtmega certainly is too weak to directly drive gate of this MOSFET at such frequency.

author: rx8pilot
author_rank: Super Contributor
timestamp: October 27, 2017, 10:49:47 pm
content: That 100k resistor is pretty slow....keeping the GET linear and hot.The 32khz is way fast for the available drive current.Sent from my SM-G892A using Tapatalk

author: Dave
author_rank: Super Contributor
timestamp: October 27, 2017, 10:51:24 pm
content: You're cooking them to death.You need a proper gate driver that can switch the transistor in a respectable amount of time. A higher drive voltage would also be nice, but not crucial.

author: wraper
author_rank: Unknown
timestamp: October 27, 2017, 10:53:29 pm
content: Quote from: rx8pilot on October 27, 2017, 10:49:47 pmthere is 100R , not 100k. And gate resistor is not the major part of the problem, although should be reduced IMO. But only with conjunction of using proper gate driver.

author: rx8pilot
author_rank: Super Contributor
timestamp: October 27, 2017, 11:24:46 pm
content: Quote from: wraper on October 27, 2017, 10:53:29 pmThe 100k is the only path to discharge the gate it seems. Not sure if the PWM is able to sink current.Sent from my SM-G892A using Tapatalk

author: timdriedger
author_rank: Contributor
timestamp: October 27, 2017, 11:28:54 pm
content: Here is the signal at the gate pin.

author: wraper
author_rank: Unknown
timestamp: October 27, 2017, 11:31:14 pm
content: Quote from: rx8pilot on October 27, 2017, 11:24:46 pmatmega certainly can pull the current. But resistor is good for cases when MCU output is in high impedance state. Although 100k is not enough if pull-up in MCU is enabled. But I don't remember if default state of atmegas is with or without pull-ups enabled.

author: timdriedger
author_rank: Contributor
timestamp: October 27, 2017, 11:32:22 pm
content: I don't have the atmega pullups enabled.

author: wraper
author_rank: Unknown
timestamp: October 27, 2017, 11:32:38 pm
content: Quote from: timdriedger on October 27, 2017, 11:28:54 pmThese are not fast enough transitions.

author: wraper
author_rank: Unknown
timestamp: October 27, 2017, 11:38:26 pm
content: Quote from: timdriedger on October 27, 2017, 11:32:22 pmI was meaning default state after reset before pins are configured.

author: timdriedger
author_rank: Contributor
timestamp: October 27, 2017, 11:44:29 pm
content: I believe the default state is hi-Z.

author: Ian.M
author_rank: Super Contributor
timestamp: October 27, 2017, 11:45:49 pm
content: At 14V Vdd, its only got about 8nC of gate charge. Even if the Atmega + the 100R resistor limits the average gate current during transition to around 10mA, its unlikely to be spending over 1us in the linear region, and for sub-10us pulses its good for around 70A. It shouldn't be running hot either unless you've skimped on the copper area - worst case it should be dissipating an average of under a watt due to transition losses, and a couple of hundred mW in I2R losses during the PWM on period.However a 2A motor could easily have a stall current of around 30A (see:Getting a handle on brushed DC motor current @ EDN, and take several ms for the current to decrease to within the MOSFET's DC SOA rating. and the 1ms curve on the SOA diagram shows its only good for 15A at 12V Vds, rising to a peak of 30A as Vds drops to 4V. I suspect its either going to need a *MUCH* beefier MOSFET, active current limiting, or a soft start, ramping up the PWM from zero duty cycle to cure this one.Edit: while I was typing this the O.P.'s posted a scope trace of the gate waveform that confirms its sub-1us rise and fall. Stiffer gate drive would be nice, as there appears to be some oscillation on the rising edge, but I wouldn't call it essential.Insert 0.01R (or a 50A rated current probe) between source and ground, and do a one-shot capture of the current waveform for the first 10 PWM cycles from stopped, with the full mechanical load on the motor. I suspect you'll see a grossly excessive current peak.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: October 27, 2017, 11:59:43 pm
content: Quote from: wraper on October 27, 2017, 10:34:12 pmImproper Vgs clamping/driving is one of the most common causes of MOSFET destruction.It's not usually the drive voltage that is the culprit but the inductance in the drive circuit (including pcb traces) and failure to adequately snub switching transients (Vds).A clamping zener is a good start, like 12V or so, to clamp transients. A good low impedance drive is also required, as the energy stored in Cgs needs to be removed quickly when switching off.

author: timdriedger
author_rank: Contributor
timestamp: October 28, 2017, 12:05:15 am
content: Images attached to show more detail on the rising/falling edges of the gate signal.I am ramping up the PWM duty cycle when stating the motor over ~1 second.The motor runs for about 45 seconds every 2 -3 minutes.We've run cycle testing for weeks at a time with no thermal issues. We've also passed all the EMC immunity testing including static discharge testing.When a customer reports an issue (motor turning continuously with now software control) I replace the MOSFET and everything is back up and running.

author: timdriedger
author_rank: Contributor
timestamp: October 28, 2017, 12:25:21 am
content: Image of Vgs and motor current attached.

author: rx8pilot
author_rank: Super Contributor
timestamp: October 28, 2017, 01:07:15 am
content: Curious how you are measuring the motor current.Sent from my SM-G892A using Tapatalk

author: Ian.M
author_rank: Super Contributor
timestamp: October 28, 2017, 01:13:12 am
content: We need the scale for the current. How many mV per Amp?Also its the startup transient that's of interest, not the steady state current triangle wave.A further thought - if the customer has loose connections to the motor or a brush stuck in its holder, it could be causing a startup surge outside your 1 second PWM ramp-up period, that you just aren't getting with your (presumably) well maintained test jig.

author: timdriedger
author_rank: Contributor
timestamp: October 28, 2017, 03:05:19 am
content: Quote from: Ian.M on October 28, 2017, 01:13:12 amHantek current clamp. Scale is 1mA/1mV (1:1)

author: KL27x
author_rank: Super Contributor
timestamp: October 28, 2017, 05:03:10 am
content: What is D4? It's drawn in there like a zener. Google search returns a shottky.Ahh, neermind. I see it's just snubber. Shottky. And I now realize I have never used the proper symbol for a shottky nor put a motor snubber in my schematic.Did you check the snubber to see if it is still alive when you changed the FET?I'll be checking back to see what went wrong. Sounds like it could be pretty interesting to know... the mysterious things that happen so fast they can't be scoped. Unless Ian is right. Then it's just boring, lol.I don't get how transient from the motor or VDS would get all the way to the gate, though. Why we have a few folks suggesting a zener or crowbar on the gate. The ucontroller pin has an ESD diode between its pin and Vdd, already... Is there a way for the transient to occur on the GATE side of the 100R resistor? I have never blown a FET with too much gate voltage... And I have never blown a gate without also killing the micro.

author: xavier60
author_rank: Super Contributor
timestamp: October 28, 2017, 11:31:04 am
content: I would also like to see the Drain voltage waveform.

author: danadak
author_rank: Super Contributor
timestamp: October 28, 2017, 12:13:45 pm
content: Transients on Drain get coupled back to gate via Cgd.http://www.ti.com/lit/ml/slua618/slua618.pdfRegards, Dana.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: October 28, 2017, 07:19:02 pm
content: I'm just wondering if there is a problem with the peak current draw at startup, or perhaps if it's a commutator motor, with momentary shorting of the segments drawing a huge pulse per revolution.Something of that kind could certainly explain why they work for a good while, then pop.

author: Bootalito
author_rank: Unknown
timestamp: October 29, 2017, 11:48:37 am
content: As a minor problem, 100R resistor is too low. Atmega2560 can output 40ma. At 5v this is 120R.And as other people have said you cant charge and discharge the gate capacitor at those speeds. I made a spreadsheet a while back to help me quickly calculate the max frequency given a particular mosfets specs. I'll see if I can dig it upSent from my VS990 using Tapatalk

author: Dave
author_rank: Super Contributor
timestamp: October 29, 2017, 12:20:41 pm
content: The output resistance of the pins is roughly 40 ohms, so it's fine.

author: Bootalito
author_rank: Unknown
timestamp: October 29, 2017, 12:54:16 pm
content: Quote from: Dave on October 29, 2017, 12:20:41 pmOh okay I didn't know that was a thing. But yeah I looked at some data sheets and there it is!Sent from my VS990 using Tapatalk

author: tatus1969
author_rank: Super Contributor
timestamp: October 30, 2017, 09:32:20 am
content: Quote from: wraper on October 27, 2017, 10:34:12 pmThere are basically only two failure modes of a MOSFET, thermal overstress and Vgs overvoltage. Modern MOSFETs can handle Vds overvoltage well, they simply avalanche and act as a zener diode. However, this can again lead to thermal overstress.Your oscilloscope shots seem to show the motor running in steady state, right? Parasitic effects will exaggerate at higher currents, I would go to the extreme there and drive a blocked motor for a short time, and check the waveforms (and the MOSFET temperature rise) then.EDIT: how is your 12V supply bypassed right on the freewheeling diode? You need low ESR capacitors with substantial capacitance there, the freewheeling current must have a good (AC) return path to ground. I suggest to monitor the 12V on a scope with that blocked motor as well.

author: Zero999
author_rank: Super Contributor
timestamp: October 31, 2017, 09:21:59 am
content: Quote from: IanMacdonald on October 28, 2017, 07:19:02 pmThat's possible. Check for swarf shorting the commutator contacts, although I would have thought the high current would vaporise it. Another possibility is a shorted turn, in which case the motor should be replaced, if it can't be economically rewound.

author: bson
author_rank: Unknown
timestamp: October 31, 2017, 10:30:32 pm
content: What's the breakdown of D4? If it's anywhere around 12V or less you could be shorting to ground when fully open.Also, the body zener's purpose seems to be mainly to keep static Vds buildup under control, not to dump several amps of reverse current - so I'd give it a proper external protective diode.

author: timdriedger
author_rank: Contributor
timestamp: October 31, 2017, 10:56:00 pm
content: Quote from: KL27x on October 28, 2017, 05:03:10 amI have two failed boards here on the bench. After removing the bad MOSFETs, D4 snubber tests okay with the DMM.