Why does this PMOS turn on?

Thread Starter

bmbouter

Joined May 29, 2021
56
See the attached asc, screenshot of the rendered asc, and graphs probing the source and drain side of the PMOS.

I'm pretty new at analog electronics, but with source and gate being the same net there shouldn't be any difference in voltage between them (unless maybe capacitance loading differences?) So I expect the PMOS to remain off.

Why does it turn on?
 

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Papabravo

Joined Feb 24, 2006
19,570
You are not able to see the parasitic capacitors from source to drain and gate to drain. The parasitic capacitor from gate to source is shorted. What is happening at the drain is characteristic of a series capacitor whose voltage increases as V1 turns on and then discharges to ground through R1 because the MOSFET is off, and there is little to no drain current. At steady state, no drain current means no voltage drop across R1.

Note: To turn the MOSFET on, so it can draw appreciable drain current, it must be negative with respect to the source.
Note: The exponential decay of the drain voltage should allow to estimate the magnitude of the parasitic capacitance discharging through the 100KΩ resistor
 

crutschow

Joined Mar 14, 2008
31,112
If you reduce the value of R1 or add a small capacitor (e.g. 1-10nF) from the MOSFET drain to ground, you will see a corresponding drop in the voltage spike.
 

eetech00

Joined Jun 8, 2013
3,418
See the attached asc, screenshot of the rendered asc, and graphs probing the source and drain side of the PMOS.

I'm pretty new at analog electronics, but with source and gate being the same net there shouldn't be any difference in voltage between them (unless maybe capacitance loading differences?) So I expect the PMOS to remain off.

Why does it turn on?
If the voltage source is supposed to be a power supply, then:

The voltage source needs a small amount of internal resistance.
A real power supply won't reach nominal level in 20uS, more like 200mS.
There would probably be smoothing cap (maybe 470u) at the power supply.
 
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