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Gate and source of P-channel mosfet breakdown

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wealth210

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I'm using a type of P-channel mosfet in my project.I find this mosfet is easily damaged whose gate and source is short.I guess the cause of ESD.There is no High voltage and no big current occur in Mosfet,but the defect samples can be picked up by our testing fixture.I know Mosfet is sensitive static electronic part,but How much ESD will destroy the Mosfet,I don't know?Looking forward for your help.Thanks!

Best regards
 
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ericgibbs

Well-Known Member
Most Helpful Member
I'm using a type of P-channel mosfet in my project.I find this mosfet is easily damaged whose gate and source breakdown.I guess the cause of ESD.There is no High voltage and no big current occur in Mosfet,but the defect samples can be picked up by our design fixture.if so,what's the relationship between the Gate and ESD,I know Mosfet is sensitive static parts,but How much ESD will destroy the Mosfet,I don't know?Looking forward for your help.Thanks!

Best regards
hi,
Post the circuit of your PCBA.
 

crutschow

Well-Known Member
Most Helpful Member
The maximum gate-source voltage is listed on data sheet for the transistor. If you exceed that voltage you can damage the transistor. This level of voltage can often be achieved by just walking across a room and touching the transistor. You should work on a grounded table or ESD pad (through a large resistor) and use an ESD wrist strap connected to ground whenever you work with such devices.
 

wealth210

New Member
I agree with crutschow analysis.Mosfets are sensitive static electronic part.we have weared an ESD wrist strap under operation.In my opinion,the cause of defect samples come from two side,one is coming material quanlity,and another is ESD.Now,we are still finding the true root of making problem.I attach the driving PCBA schematic for your reference.
 

wealth210

New Member
I'm sorry that I can't find the 'add attachment' command per WilliB's method,so I don't know how to show my PCBA driving schematic here.
 

ericgibbs

Well-Known Member
Most Helpful Member
I'm sorry that I can't find the 'add attachment' command per WilliB's method,so I don't know how to show my PCBA driving schematic here.
hi wealth,
Click the REPLY box
scroll down the screen to find the Manage Attachments button click it

This should open a Manage Attachments window.
Use the BROWSE button to find the PCBA drawing on your computer.
On the Manage Attachments window is a list of acceptable file types.

Click the UPLOAD button and wait until the file is transferred.

Then click 'close window'

Submit Reply....:)
 
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wealth210

New Member
Thanks Eric.
I can manage to add attachment per your direction.Thank you very much.
I just issued a new post whose title is:Making a output voltage,Would you like to review it when are feel free.Thanks in advance.
 

Attachments

ericgibbs

Well-Known Member
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Thanks Eric.
I can manage to add attachment per your direction.Thank you very much.
I just issued a new post whose title is:Making a output voltage,Would you like to review it when are feel free.Thanks in advance.
hi,
What is the value of the Vbatt.?

Have those capacitors and small inductors around the motor been fitted to reduce EM radiation into the radio section.?

My first point would be that FET's are not being protected against 'electrical switching spikes' from the motor, thats why the FET's are failing short circuit.
 

OutToLunch

New Member
the body diodes of the complementary FETs will handle the time between the switch of the h-bridge. that is, of course, if they are rated for the current that is going through them. It would probably be prudent to put some schottky's across the FETs to reduce losses anyway.
 
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ericgibbs

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the body diodes of the complementary FETs will handle the time between the switch of the h-bridge. that is, of course, if they are rated for the current that is going through them. It would probably be prudent to put some schottky's across the FETs to reduce losses anyway.
hi OTL,
Spoke to this guy on the chat line yesterday, apparently the P MOS FETs keeping dying on the production pcb's while on test.

Failure mode he says, is blown Gate/Source short.
 

smanches

New Member
Good idea to put some 15V zeners on the gates to dump any high voltage spikes. Also need snubber caps across the drain-source pins as physically close to the FET as possible.

And as mentioned, freewheel diodes are needed to provide a path for the inductive kick from the motor. This is probably what is blowing the FETs, since it's either going to develop a high enough voltage to breakdown the gate-source junction, or it's blowing the reverse diodes due to too much current. Do NOT rely on the MOSFET built in diodes for the freewheel current. They are not meant for that purpose and have very slow recoveries, which can cause problems if you are switching faster than they can recover. Put the diodes close the motor.
 
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ericgibbs

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so can we assume that this went through some sort of pre-production qualification process?
hi OTL,
From what he said on the chatline, he has made a test rig to determine why they are failing after build during final testing.
But he now also concerned that his test rig maybe taking out the FETs'!!!!

He also suspects the 'quality' of the bought in FET's maybe low.

He is also not confident that ESD rules are not being applied during assembly.

I suggested he makes up simple pre-assembly test rig, get a good sample batch and follow them thru production.
 

millwood

Banned
I have used mosfets for over 20+ years and I don't remember one of them being damaged by esd.

most likely failures I have encountered include thermal stress, overdriven (Vgs > 10v), or reverse bias (Vgs < -10v).

other than that, they are pretty robust.
 

smanches

New Member
ESD is mostly affected by environment though, which makes it very hard to determine what might be causing it. Your environment may naturally have low static build up, where wealth's environment might be very prone to it.

I have never had problems with ESD either, but I actively try to keep any static buildup to a minimum. I agree that it's probably the lack of FET protection in the circuit that is blowing the fets.
 

Pommie

Well-Known Member
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Modern devices are much less prone to ESD. Twenty years ago things were vastly different.

Mike.
 
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