Root Cause of FDC6303N FET drain and gate short circuit?
in [Design]
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From: Myauk on 30 Jun 2010 07:53 Hi all, 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. Circuit Configuration is as follows: The relay driver circuit consists of MCU DIO driving directly to the gate of FDC6303N. The Drain is connected to relay coil of internal resistance 86 ohms. The realy is safely freewhelled with S1MB Diode. The source is connected to ground. The supply for the relay is 5V, the MCU DIO is operating at 3.3V, CMOS totempole output has maximum sinking current of 15mA. The problem is obviously due to the shorted gate and drain of FDC6303N however my questions here are, 1. Is it possible that there are defective FETs with alreadyshorted gate and drain from the new lot of FETs catered for Production? 2. Or is there any possible fault condition which can cause a good working FET to have gate and drain shorted? Regards
From: Winfield Hill on 30 Jun 2010 09:38 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. Amusingly, the mosfet is still probably operating properly, that is it's operating the same as any other mosfet with its drain and gate externally connected together. But I digress. You want to know why the part failed. SFAIK, any of the various overstress conditions can cause this type of failure. E.g., overheating, in any of the ways Rds(on), switching losses, avalanche heating, or overvoltage. Including gate overvoltage, which can happen if a high current is switched too fast, causing high dI/dt, and a substantial source-wiring inductance, causing high V = L dI/dt, which can be a short damaging gate-voltage spike. 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
From: Myauk on 30 Jun 2010 09:49 On Jun 30, 9:38 pm, Winfield Hill <Winfield_mem...(a)newsguy.com> 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. > > Amusingly, the mosfet is still probably operating properly, that > is it's operating the same as any other mosfet with its drain and > gate externally connected together. But I digress. > > You want to know why the part failed. SFAIK, any of the various > overstress conditions can cause this type of failure. E.g., > overheating, in any of the ways Rds(on), switching losses, > avalanche heating, or overvoltage. Including gate overvoltage, > which can happen if a high current is switched too fast, causing > high dI/dt, and a substantial source-wiring inductance, causing > high V = L dI/dt, which can be a short damaging gate-voltage spike. > > 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 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. Does anyone have similar experience? Thanks and Best Regards
From: tm on 30 Jun 2010 09:58 "Myauk" <aungkokothet(a)gmail.com> wrote in message news:cc5dfca7-d7fd-4f9d-809a-9d5948063ca1(a)y32g2000prc.googlegroups.com... On Jun 30, 9:38 pm, Winfield Hill <Winfield_mem...(a)newsguy.com> 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? > ..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. .. ..Does anyone have similar experience? .. ..Thanks and Best Regards If you have a 10k gate resistor, how is it killing the DIO?
From: John Larkin on 30 Jun 2010 10:03
On Wed, 30 Jun 2010 04:53:20 -0700 (PDT), Myauk <aungkokothet(a)gmail.com> wrote: >Hi all, > >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. > >Circuit Configuration is as follows: > >The relay driver circuit consists of MCU DIO driving directly to the >gate of FDC6303N. >The Drain is connected to relay coil of internal resistance 86 ohms. >The realy is safely freewhelled with S1MB Diode. >The source is connected to ground. >The supply for the relay is 5V, >the MCU DIO is operating at 3.3V, >CMOS totempole output has maximum sinking current of 15mA. > >The problem is obviously due to the shorted gate and drain of FDC6303N >however my questions here are, > >1. Is it possible that there are defective FETs with alreadyshorted >gate and drain from the new lot of FETs catered for Production? > >2. Or is there any possible fault condition which can cause a good >working FET to have gate and drain shorted? > >Regards > > Wow. The fet is rated for 0.68 amps, 25 volts, and it's all zener gate/ESD protected. You are running it at 5 volts and 60 mA. There's no way it should fail. Are they dead at turnon? Are you buying the parts from an official distributor? John |