Short Circuit Protection

Delta Prime

Joined Nov 15, 2019
1,150
Hello there :)

Once a MOSFET is switched on, it will not switch off again even if the voltage is removed, because the parasitic capacitance between gate and source (1nF to 5nF typical) will store charge and will maintain the gate-source voltage even when the signal is removed. To remove this charge quickly, a resistor is required in parallel with this capacitor.
 

Alec_t

Joined Sep 17, 2013
12,206
Are you sure that's how the 100mΩ is wired? It would make sense as a current-sense resistor if it were between source and ground.
How is the gate voltage being switched? D2 has no clear purpose unless it is in parallel with a gate drive resistor to speed up the gate discharge.
 

Thread Starter

mishra87

Joined Jan 17, 2016
908
Are you sure that's how the 100mΩ is wired? It would make sense as a current-sense resistor if it were between source and ground.
How is the gate voltage being switched? D2 has no clear purpose unless it is in parallel with a gate drive resistor to speed up the gate discharge.
Hi,

I know the circuit you are taking about.
Yes Resistor is connected correctly between drain and source.
 

Thread Starter

mishra87

Joined Jan 17, 2016
908
Hello there :)

Once a MOSFET is switched on, it will not switch off again even if the voltage is removed, because the parasitic capacitance between gate and source (1nF to 5nF typical) will store charge and will maintain the gate-source voltage even when the signal is removed. To remove this charge quickly, a resistor is required in parallel with this capacitor.
Hi,

I may not be agree with your reason !
 

OBW0549

Joined Mar 2, 2015
3,566
Instead of asking us here on AAC what the purpose of that 100 mΩ resistor is, why don't you do the smart thing and ask whoever designed that circuit? They put that resistor there; we didn't.

Where did you get this stupid circuit from, anyway? That resistor serves NO plausible purpose; its presence makes no sense whatsoever.
 

AnalogKid

Joined Aug 1, 2013
9,356
Once a MOSFET is switched on, it will not switch off again even if the voltage is removed, because the parasitic capacitance between gate and source (1nF to 5nF typical) will store charge and will maintain the gate-source voltage even when the signal is removed. To remove this charge quickly, a resistor is required in parallel with this capacitor.
That is a description of R8, not R10.

D2 usually is there to prevent excessive Vgs. It is a 4.7 V zener, and would reduce a 24 V input to under 20 V (a common max Vgs spec). However, if that is its purpose, that is not the placement for that function.

ak
 
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OBW0549

Joined Mar 2, 2015
3,566
Why do I suspect that someone saw a notation that a MOSFET with 100mOhm on resistance was required, and they put it in the schematic as a resistor?
Good point; although a person clueless enough to do that probably shouldn't be playing around with electricity...
 
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