I am working with a PMOS-based switching circuit where the PMOS turns on for 20 ms with a total period of 40 ms (i.e., a 50% duty cycle). In the schematic, there is a capacitor C1 and resistor R2 placed between the gate and source terminals of the PMOS.



I'm trying to understand the following:

1. What is the purpose of C1 and R2 between the gate and source of the PMOS?
2. Will the circuit still function correctly without these components?
3. If they are necessary, how should I calculate appropriate values for C1 and R2 based on the 20 ms ON time and 40 ms period?

Any insight or calculation approach would be greatly appreciated.

Thankyou

 

C1 is there to make it more likely to fail.
R2 is there to make sure it is OFF if your input gets disconnected.
Suggested values:
C1=0pF
R2 <1MΩ
[Edit] Mistyped R1 instead of R2 at first attempt

 

C1 does nothing in your circuit since V1 and V2 are ideal voltage sources with zero impedance.

 

In the schematic, there is a capacitor C1 and resistor R2 placed between the gate and source terminals of the PMOS.

Where did you get that circuit?

 

C1 is there to make it more likely to fail.

that's right but it can also slow down the turn-on.

 

that's right but it can also slow down the turn-on.

But why would you want to do that? That's what makes it fail!

 

it depends how long does it take, sometimes you need to start a load slowly. If you are inside the SOA it is ok.

 

it depends how long does it take, sometimes you need to start a load slowly. If you are inside the SOA it is ok.

Assuming that you have the infinite heatsink that the SOA graphs is based on. . .

 

I'm trying to understand the following:

1. What is the purpose of C1 and R2 between the gate and source of the PMOS?

that is your simulation problem. you really need to understand ideal sources...
once you do, you will note that values of C1 and R2 are irrelevant.

so first you need to add some values to those components.
then you need to add something to separate gate from V1... for example a switch, controlled by V1.
and THEN you can see effects of C1 and R2 values.

my guess is that someone tried (and failed) to make a delay circuit to create certain duty cycle.

1. Will the circuit still function correctly without these components?

Sure as long as you can find an ideal voltage source. which does not exist...

1. If they are necessary, how should I calculate appropriate values for C1 and R2 based on the 20 ms ON time and 40 ms period?

Simulator is one way to do that - as long as you understand what ideal components are. then you can model a real circuit.

for example, note that adding another transistor as a switch makes simulation realistic and effects of capacitor become visible.