Simulation doubt with Zener diode & MOSFET

[FreshmanNewbie]

Hi,


I will try and do that later.


One important point to note here is that all voltages in the circuit are DC. Please bear this in mind as we proceed.

Now let's assume that you just supply power to the circuit at time t=0 and that all capacitors in the circuit (I'm referring to gate capacitances here) are discharged. From time t=0 to some other point in time (call it t_steady), there will be some transient in the MOSFET gate capacitor voltages and the 1nF capacitors externally connected to the MOSFET gates as charges flow into them and charges them into their respective steady-state voltages. Please ignore other MOSFET capacitances for now. These capacitors will be charged to a certain level depending on the voltage driving the charges. Then no further charges will be driven into the gates. This time is t_steady and from this time onwards we are in steadystate. Charges cannot leave the gates because no one condition for the charges to leave (i.e. either actively extracting the charges like MOSFET drivers do, or providing a path to a lower potential as in the case of pulling down the gate) has been provided. Since the driving voltages are still present, and there is potential difference in the circuit, then current keeps flowing along paths (or branches) that have potential difference.

In this circuit, at time t_steady, the Low-side MOSFET is already charged to 2.195V. The entire 1.62mA steady-state current that I calculated flows from the drain through the zener, through the gate drive resistor, to the -14V.

Thank you very much for the detailed explanation which was easy for me to understand.

 

[Akanimo]

...
I don't know where 6.4mA is coming from.
...

My assumption here is that the the 6.4mA was measured at the junction where the source of the High-side MOSFET, the drain of the Low-side MOSFET, the 100k resistor, the zener diode and the two 1nF capacitors all meet.

However, if the 6.4mA was the High-side MOSFET source current, then the Rds of the High-side MOSFET will be lower than calculated, the current through the Low-side MOSFET drain will be higher than calculated, the Rds of the Low-side MOSFET will be lower than calculated, the current through the 100k resistor will be unchanged, the current through the zener will be unchanged and, of course, the direction of current flow will remain unchanged.

--- Updated Jul 10, 2020 ---

I made a blunder in post#17 to say that the high-side MOSFET is operating in the triode region. It's actually operating in the saturation region like Klaus had said.

Every other analysis on post#17 is upheld.

 

[BradtheRad]

From the replies I see Falstad's newer version has more editable mosfet parameters. An improvement.

Nevertheless with a bjt it's easy to observe (by motion of yellow dots) how current flow is allowed or inhibited, depending on whether a current path is provided by neighboring components. Certain volt levels need to be present at transistor terminals.

Since mosfets are voltage controlled devices, there are no yellow dots to verify that the mosfet has a proper bias relationship among its terminals, making it less easy to detect the cause of bottlenecks.