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Mosfet with open ground and source.

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Arya Kumar

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Hello,

Generally i place mosfet with Source connected to ground and do negetive switching. But i am coming across a problem when there will be circuitry after ground.(So i cannot connect my Mosfet Source to ground)
I see people put a diode. Is it the solution, Please verify
This is the problem
Capture.PNG
Can this be its solution
Capture2.PNG.

Also comment if there can be better design for this.
 

I do not get why there is D2. Q1 gonna be off when switch is off
 

Do you really connect a push-button at MOSFET source? What is "phase" voltage?

Consider that the MOSFET will be destroyed if source voltage against ground exceeds max Vgs of e.g. 20V.
 

ok i have just modeled Switch with some external circuits which can be in high impedance state.
Just because the source is in high impedance state and i need ground to create Vgs , to ON the mosfet.
My problem is i dont get ground due to high impedence circuit.
I see in google people use diode in similar circuit.

- - - Updated - - -

i have just modeled Switch with some external circuits which can be in high impedance state.

- - - Updated - - -

Yes sure, I am considering that.
Please dont look into resistor values, i have not labled that correctly as of now
 

Source node will be pulled up by Id until Id is turned off. The diode isn't servicing a useful purpose.

To avoid misunderstandings, it would be better if you show a complete schematic including the circuit connected to drain.
 

No this diode, connected this way, is not a solution. Your Mosfet would always be off.

What do you mean by:
"But i am coming across a problem when there will be circuitry after ground.(So i cannot connect my Mosfet Source to ground)"

If there is circuitry after ground, then that "ground" is not really a ground.

If you really want the forum to help you, you require to provide absolutely all the details.
 

The only thing that protects a power MOSFET gate
from high drain voltage, is the depletion region of
the D-B (B=S) junction. If there is no stiff source
potential then the gate oxide will see full D-G voltage
and pop.

A reverse biased diode helps none. The source relative
to gate potential must hold position. If you want the
pushbutton to interrupt current, put in the drain leg
(if switch has tolerable isolation for safety etc. - if
not, use pushbutton to control a NO relay perhaps).
 

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