Creating a negative voltage rail for MOSFET driver

[DaDes]

TL;DR

I am trying to create a negative voltage rail using a zener diode for driving a MOSFET

I am trying to create a negative voltage rail using a zener diode for driving a MOSFET. I've read several placed online about using a zener diode for this purpose but I cannot seem to get it to work. I am simulating using LTspice but the negative rail just isn't there. I would be very grateful if someone can help me with this and also understand what I'm doing wrong. This seems like something that should be easy but I'm missing something.

I've attached the schematic and the LTspice asc file renamed as .txt.

 

[Baluncore]

Welcome to PF.

The Zener diode will clamp the voltage at -4.7 V, but first you need a charge pump, to pull that rail negative.

The MOSFET you are using does not require a negative rail, as a gate voltage below +2.5 V will turn it fully off.

The MOSFET is a power device and has a gate capacitance of about 2 nF. It will take a good driver to charge that capacitance. At the moment, you are driving it with 17 amps from an ideal voltage source. I would expect a logic input MOSFET gate driver, would source or sink between one and two amps.

How will you control the unspecified gate driver?

 

[DaveE]

It's hard for us to help unless you explain more about your specific problem. There are many MOSFET gate drive circuit solutions. A negative PS isn't often required, but there are some cases where it is beneficial.

The short answer is if you want a negative power supply, you have to make one (or be given one). As @Baluncore said, this is usually a charge pump of some sort, since MOSFETs don't need a lot of power.

There is also a lot of info on the web. Here's one, kind of randomly chosen, example:
https://www.ti.com/lit/ml/slua618a/slua618a.pdf?ts=1733237400369

Often you will do well to just buy an appropriate driver IC and do what they say to use it properly.

 

[Baluncore]

@DaDes
I see you are lucky to be getting circuit design help from the LTspice group.io; Normally, they will only discuss the operation of LTspice, with answers like:

The SPICE directive; VN VN 0 -5 ; will generate a negative 5V rail node called VN, without having to place a voltage source or wires on the schematic. It also conveniently provides the ground connection needed by SPICE to function.

 

[Baluncore]

Here is your circuit modified to pump the negative rail down to -4.7 volts.
The MOSFET gate now switches between -4.7 V and 15.3 V.
Note that it takes about 3 k * 10 uF = 30 ms, for the rail to stabilise.
file.asc is attached as .txt file.

 

[DaveE]

If you have extra gate 'on' voltage you can add a series capacitor to level shift your gate drive to create a negative 'off' bias. As shown below. Like the charge pump it takes several cycles to work. I've used this a few times to deal with turn off problems caused by the miller capacitor charge injection back into the gate. But, it's not a "cookbook" solution, you have to make some design choices.

Without more information we can't tell which sort of solution is best for your gate drive issues.