Need an explanation of how this circuit works

Thread Starter


Joined Nov 19, 2022
I came across this bidirectional switch circuit.

Screenshot from 2023-08-04 13-33-37.png

I know how two MOSFETs configuration works but when I apply the pulse to turn it on and off the NMOS it turns on the back-to-back PMOS si I can see the capacitor voltage across the load resistor but it does not change its state as the falling edge of the pulse is present:

Screenshot from 2023-08-04 13-31-34.png
Screenshot from 2023-08-04 13-30-57.png

I know that I'm missing something here but I can't see it. How am I supposed to turn it on and off so it can also turn the back-to-back PMOS on and off?


Joined Apr 5, 2008

The gates of the upper mosgets are likely gone defective.
The Vgs was to high and has blown the gate. (Vgs max 8 Volts)
Also the Vds of 20 Volts is a problem.




Joined Mar 14, 2008
I see no reason for C16.
It can be removed.

Also for 30V, R54 and R59 should be both changed to about 1kΩ to meet both your switching speeds and keep Vgs below its maximum of 20V.
Last edited:


Joined Mar 31, 2012
Where is your choice of component values coming from? In particular, why is R59 only 5 Ω while R54 is 10 kΩ?

You need to look at the PFETs to see what is happening. Keep in mind that when you turn on the NFET, you are pulling the gate voltages on the PFETs low, which is going to give you full charge on the capacitor of about 30 V. Then, when it is disconnected by turning off the NFET, the only path for it to discharge if via the 10 kΩ resistor. So you've got a 1 ms time constant, so it will take considerably longer than that for the 30 V to decay to the point where the PFTs shut off, but your simulation is only for a fraction of that.