Mosfet Gate Voltage help

Thread Starter

dazzer7

Joined May 7, 2016
19
Its more of a project from Electronics school where we get a faulty laptop motherboard and have to try and find where the faults are, I am a little confused because the motherboard is not powering on , 19volts is on the motherboard from power supply . power button is getting 3volts . so I look for the first Mosfet ( B04 N03 EID 5106) that is closest to the 9 volts power from the board which is PQ4 , source 3pins are 19volts - train 4 pins 19 volts - gate is 25 volts . and the same for the PQ3 which is just to the side -so I was thinking and I could be completely wrong but is 25 volts not to high on the gate for this Mosfet ? I tried to get the data sheet up for the Mosfet and if it the same Mosfet and I think it is - gate source volt -limits 20v. ?
<a href="https://imgur.com/onATIJZ">
 

sagor

Joined Mar 10, 2019
422
Correct, based on a quick search for a datasheet for B04N03, maximum Vgs is +20V. If you are seeing 25V on the gate, then odds are the MOSFET is blown. You have to track down the source of the gate voltage, find what is causing the overvoltage. Or, it could be that it is surviving the overvoltage, and is just fully turned on. Or, it is shorted out, a common result of a MOSFET failure (short between D and S) A short can be detected by removing the gate voltage and seeing if it still passes current. However, too much current may eventually destroy the internal die in the MOSFET and cause it to open up...
 

Thread Starter

dazzer7

Joined May 7, 2016
19
Correct, based on a quick search for a datasheet for B04N03, maximum Vgs is +20V. If you are seeing 25V on the gate, then odds are the MOSFET is blown. You have to track down the source of the gate voltage, find what is causing the overvoltage. Or, it could be that it is surviving the overvoltage, and is just fully turned on. Or, it is shorted out, a common result of a MOSFET failure (short between D and S) A short can be detected by removing the gate voltage and seeing if it still passes current. However, too much current may eventually destroy the internal die in the MOSFET and cause it to open up...
yes they is something short them smd capacitors beep in a continuity test that are around the mosfet . but I think I have found another problem or it could be part of this problem.
them 3 smd capacitors above them 4 mosfets to the right side don't show a short in a continuity test, and they have 19 volts and are giving PQ803 and PQ801 19v on the 4 pins train , PQ803 train has 19volts on 4 pins 0 volts on source 3pins or gate, PQ804 has 0 volts on source and gate and train ? PQ801 has 19volts on 4pins train - 0volts on source and gate , PQ 805 has 0 volts on all. so I am wondering why 0 volts on all on the PQ804 - PQ805, and why know source and gate on PQ803 and PQ801 which are both get 19V on the train 4 pins.
<a href="https://imgur.com/N32bQlm">
 

BobTPH

Joined Jun 5, 2013
3,104
What is the reference point you are measuring to? The limit if 20V is netweem the gate and source, is that what you are measuring?

Bob
 

Thread Starter

dazzer7

Joined May 7, 2016
19
What is the reference point you are measuring to? The limit if 20V is netweem the gate and source, is that what you are measuring?

Bob
Yes the first picture I measured the gate which had 25volts , the source each pin 19volts . but we think 25 volts is to high , But I think I have noticed something else in the first picture PQ4 PQ3 PQ5 I can only see two mosfets , It looks like PQ 3 is not there ? I could be wrong PQ3 has 3 solder dots .
 

BobTPH

Joined Jun 5, 2013
3,104
Yes the first picture I measured the gate which had 25volts , the source each pin 19volts
If the gate is at 25V and the source is at 19V, the voltage between the source and gate is 6V which is perfectly okay.

There is no such thing as absolute voltage, a voltage needs to measured between two points. The MOSFET has no way of knowing what any voltage in your circuit is except the voltages on it's 3 pins. It does not know that the gate is 25V above ground, it could be hundreds of volts above ground, and as long as it is less than 20V above or below the source pin, it is okay.

Bob
 

jpanhalt

Joined Jan 18, 2008
11,088
Do you have a way to measure frequency? It is conceivable that the gate is pulsed, which will give a pulse at the source too.
 
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