Root Cause of FDC6303N FET drain and gate short circuit?
in [Design]
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From: Myauk on 30 Jun 2010 10:08 On Jun 30, 10:03 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 30 Jun 2010 04:53:20 -0700 (PDT), Myauk > > > > > > <aungkokot...(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- Hide quoted text - > > - Show quoted text - They are already dead at turn on. We found it while we carried out the relay circuit operation test, MCU reset issue while controlling relay on/off. Regards
From: Myauk on 30 Jun 2010 10:10 On Jun 30, 9:58 pm, "tm" <nob...(a)msnbc.org> wrote: > "Myauk" <aungkokot...(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? Current from 5 V supply through relay is approximately 60mA sinking into the LOW Dio pin rated at 15mA. (4 times the rated value).
From: tm on 30 Jun 2010 10:20 "Myauk" <aungkokothet(a)gmail.com> wrote in message news:6c14ccac-9a18-43a5-9f05-b08e6d0c9fce(a)w15g2000pro.googlegroups.com... On Jun 30, 9:58 pm, "tm" <nob...(a)msnbc.org> wrote: > "Myauk" <aungkokot...(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? ..Current from 5 V supply through relay is approximately 60mA sinking ..into the LOW Dio pin rated at 15mA. (4 times the rated value). Then there is something you are not telling us. From your two comments, you say the MCU DIO pin connects to the fet through a 10k resistor. If the fet shorts to 5 volts (g to d), then the max current can be 0.5 mA. I=E/R= 5/10000=0.0005 Maybe you need to draw us a picture. T
From: SoothSayer on 30 Jun 2010 10:23 On 30 Jun 2010 06:38:25 -0700, Winfield Hill <Winfield_member(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. I always noticed on year, when doing a huge amplifier design, which contained a row of huge IGBTs and FETs on a 20 inch long sink, that we had some odd failures early on in the design that had no reason for being... Early failure modes were tied to coplanarity issues on the sink. It seems our assembler did not understand the concept of coplanarity until I showed him the proper assembly sequence. Without it, it was as if any part that did get attached right was not attached at all. Anyway, we experienced plenty of what appeared to be "immediate" failures due to the lack of a part being sinked properly. It was really a matter of a few seconds. and went away once assembly efficacy was achieved. It seems that those thermal spikes were fast enough to take the part to critical failure temperatures. Good heat sinking is REQUIRED, and that right where it belongs... on the back of that tab!
From: SoothSayer on 30 Jun 2010 10:30
On Wed, 30 Jun 2010 06:49:37 -0700 (PDT), Myauk <aungkokothet(a)gmail.com> wrote: >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 Do you have a thermal imager? If not, you could rent one. Examine the device as you utilize it. Watch the failure mode as it occurs and examine the waveforms during that period to determine if it is circuit design related of heat management related. Make sure that your heat sink attachment has a high integrity thermal pathway. Do not simply use a dry, "air" attachment. Make sure that it is well sinked. Once you rule out that, then get back to probing the circuit. Have you ever used 'transzorbs', or put ferrite beads on the gate lead during assembly? As close to the part as possible. |