Root Cause of FDC6303N FET drain and gate short circuit?
in [Design]
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From: Winfield Hill on 30 Jun 2010 21:11 Myauk wrote... > > Winfield Hill wrote: >> Myauk wrote... >> >>> In a typical PCBA design which consists of relay driver circuit, >>> we found the FET drain and gate short circuit problem causing >>> damage to MCU DIOs due to excessive sink current in production >>> on 5 or 6 failed out of 100 PCBA units. >>> 2. Or is there any possible fault condition which can cause a >>> good working FET to have gate and drain shorted? >> >> Drain-to-gate shorts is a typical failure mode for power mosfets. >> [ snip ] >> >> Lot's of handwaving there, but the standard remedies apply, a diode >> across the coil, a gate resistor to slow down switching speed, etc. > > Thanks Win, > > But I think it is not really a design issue. We have already added > freewheeling diode verifying the back emf is already eliminated by > checking the waveforms. And 10k Gate resistor is there. So there > is no way possibly to damage the FET. Whoa! When I said, "a gate resistor to slow down switching speed" I could not have meant a resistor to ground, how would that slow the mosfet's switching speed? No, you may need a series resistor. A 10k resistor to ground might make you feel comfortable, in case the logic supply isn't present (could that happen?), but here I was talking about an everyday series gate resistor. Maybe as high as 470 to 1.8k or 3.3k or so, to slow your low-capacitance mosfet's switching speed, and help it survive high-inductance PCB wiring. Recall the damaging equations: high dI/dt ==> high V = L dI/dt. The FDC6303N datasheet says Crss = 9pF. Assume a modest 50mA gate drive from your processor, and we get dV/dt = I/C = 5.5V/ns, which says your 5V 60mA relay current could shutoff in 1ns. Going back to our equations, that would imply Vs = 60V for say 1uH of PCB wiring inductance. OK, less for slower switching. Say 1/3 as fast, that's still 20V on the source with the gate pinned, and with an 8V max Vgs rating, 20V could certainly zap your gates. And yes, that's a design problem. I seriously doubt your mosfets would fail incoming inspection. BTW, what are you doing with the FDC6303N's second mosfet? -- Thanks, - Win
From: Myauk on 30 Jun 2010 21:40 On Jul 1, 9:11 am, Winfield Hill <Winfield_mem...(a)newsguy.com> wrote: > Myauk wrote... > > > Winfield Hill wrote: > >> Myauk wrote... > > >>> In a typical PCBA design which consists of relay driver circuit, > >>> we found the FET drain and gate short circuit problem causing > >>> damage to MCU DIOs due to excessive sink current in production > >>> on 5 or 6 failed out of 100 PCBA units. > >>> 2. Or is there any possible fault condition which can cause a > >>> good working FET to have gate and drain shorted? > > >> Drain-to-gate shorts is a typical failure mode for power mosfets. > >> [ snip ] > > >> Lot's of handwaving there, but the standard remedies apply, a diode > >> across the coil, a gate resistor to slow down switching speed, etc. > > > Thanks Win, > > > But I think it is not really a design issue. We have already added > > freewheeling diode verifying the back emf is already eliminated by > > checking the waveforms. And 10k Gate resistor is there. So there > > is no way possibly to damage the FET. > > Whoa! When I said, "a gate resistor to slow down switching speed" > I could not have meant a resistor to ground, how would that slow > the mosfet's switching speed? No, you may need a series resistor. > A 10k resistor to ground might make you feel comfortable, in case > the logic supply isn't present (could that happen?), but here I > was talking about an everyday series gate resistor. Maybe as high > as 470 to 1.8k or 3.3k or so, to slow your low-capacitance mosfet's > switching speed, and help it survive high-inductance PCB wiring. > > Recall the damaging equations: high dI/dt ==> high V = L dI/dt. > > The FDC6303N datasheet says Crss = 9pF. Assume a modest 50mA gate > drive from your processor, and we get dV/dt = I/C = 5.5V/ns, which > says your 5V 60mA relay current could shutoff in 1ns. Going back > to our equations, that would imply Vs = 60V for say 1uH of PCB > wiring inductance. OK, less for slower switching. Say 1/3 as > fast, that's still 20V on the source with the gate pinned, and > with an 8V max Vgs rating, 20V could certainly zap your gates. > And yes, that's a design problem. > > I seriously doubt your mosfets would fail incoming inspection. > > BTW, what are you doing with the FDC6303N's second mosfet? > > -- > Thanks, > - Win- Hide quoted text - > > - Show quoted text - Thanks Win, sorry that I understand it wrongly. I am using both MOSFETs for driving latching relay. One for ON coil, one for OFF coil. Regards
From: Myauk on 1 Jul 2010 03:55 On Jul 1, 9:11 am, Winfield Hill <Winfield_mem...(a)newsguy.com> wrote: > Myauk wrote... > > > Winfield Hill wrote: > >> Myauk wrote... > > >>> In a typical PCBA design which consists of relay driver circuit, > >>> we found the FET drain and gate short circuit problem causing > >>> damage to MCU DIOs due to excessive sink current in production > >>> on 5 or 6 failed out of 100 PCBA units. > >>> 2. Or is there any possible fault condition which can cause a > >>> good working FET to have gate and drain shorted? > > >> Drain-to-gate shorts is a typical failure mode for power mosfets. > >> [ snip ] > > >> Lot's of handwaving there, but the standard remedies apply, a diode > >> across the coil, a gate resistor to slow down switching speed, etc. > > > Thanks Win, > > > But I think it is not really a design issue. We have already added > > freewheeling diode verifying the back emf is already eliminated by > > checking the waveforms. And 10k Gate resistor is there. So there > > is no way possibly to damage the FET. > > Whoa! When I said, "a gate resistor to slow down switching speed" > I could not have meant a resistor to ground, how would that slow > the mosfet's switching speed? No, you may need a series resistor. > A 10k resistor to ground might make you feel comfortable, in case > the logic supply isn't present (could that happen?), but here I > was talking about an everyday series gate resistor. Maybe as high > as 470 to 1.8k or 3.3k or so, to slow your low-capacitance mosfet's > switching speed, and help it survive high-inductance PCB wiring. > > Recall the damaging equations: high dI/dt ==> high V = L dI/dt. > > The FDC6303N datasheet says Crss = 9pF. Assume a modest 50mA gate > drive from your processor, and we get dV/dt = I/C = 5.5V/ns, which > says your 5V 60mA relay current could shutoff in 1ns. Going back > to our equations, that would imply Vs = 60V for say 1uH of PCB > wiring inductance. OK, less for slower switching. Say 1/3 as > fast, that's still 20V on the source with the gate pinned, and > with an 8V max Vgs rating, 20V could certainly zap your gates. > And yes, that's a design problem. > > I seriously doubt your mosfets would fail incoming inspection. > > BTW, what are you doing with the FDC6303N's second mosfet? > > -- > Thanks, > - Win- Hide quoted text - > > - Show quoted text - By the way, I have checked the rise time of the pulse to drive the relay iwith scope. It is 37.6ns and the VGS rise is up to 3.82V only at 3.3V dring singal to the FET. So it is safe to say that I won't kill the FET as it is less than half of VGS rating 8V. And checking turning on and turning off of the relay the VDS never exceeds 5.8V. This does not mean that I won't add the series resistor as I understand that it will also prevent from excessive drain of current into DIO of MCU in addition to slowing down the MOSFET switching speed. Special Thanks to you all on your suggestions and comments on this topic. Regards
From: Myauk on 1 Jul 2010 08:31 > Sorry, that's not the way it works. Yes of course your gate- > drive voltage is under 8V. And your risetime may not look fast, > but that includes the slow gnd-to-plateau, and plateau-to-Vcc > portions. It's during the short plateau when all the action > happens and drain voltage switches. I did see a plateau indeed. Is there any recommended source of information where I can learn all the details? Furthermore, what matters > is the true gate voltage, inside the die, hidden from you for > a short time on the far side of the gate spreading resistance. > > And it's the source voltage that has the damaging spike, You mean the spike goes negative causing VGS to be higher? > Is only one of the two mosfets failing? Yes, only one of two MOSFETs have obvious failure. Gate to Drain low resistance or short. Regards
From: Winfield Hill on 1 Jul 2010 09:50
Myauk wrote... > >> Sorry, that's not the way it works. Yes of course your gate- >> drive voltage is under 8V. And your risetime may not look fast, >> but that includes the slow gnd-to-plateau, and plateau-to-Vcc >> portions. It's during the short plateau when all the action >> happens and drain voltage switches. > > I did see a plateau indeed. Is there any recommended source > of information where I can learn all the details? There are a multitude of sources for information about FET gate-charge waveforms and drain switching analysis. But not so many about the effects of wiring inductance, etc. >> Furthermore, what matters is the true gate voltage, inside >> the die, hidden from you for a short time on the far side >> of the gate spreading resistance. >> >> And it's the source voltage that has the damaging spike, > > You mean the spike goes negative causing VGS to be higher? Actually, it's a resonant ringing, and is most likely to happen at shutoff, at the end of your coil-current pulse. It's possible it could fail on the very first relay pulse. As for whether it's a positive or negative voltage that does the job, can't say. But I'll wager that PCB wiring inductance is involved. >> Is only one of the two mosfets failing? > > Yes, only one of two MOSFETs have obvious failure. > Gate to Drain low resistance or short. That rules out ESD damage, I'd say. I see you're giving up on the relay. But I hope you add some resistance and do enough testing to see if it helps. If you replace a bad mosfet, and run it for a while, is there a very high chance of failure again? High enough to be able to get an idea if it's working. -- Thanks, - Win |