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Creating a negative voltage rail for MOSFET driver

 

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 uploaded the file MOSFET Driver with Negative Rail.asc
 

On 04/12/2024 09:40, dakeller00 via groups.io wrote:
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 uploaded the file MOSFET Driver with Negative Rail.asc
You can't create a negative rail with this circuit - where's the current going to come from?

If you want to drive the NMOS gate with a source that goes negative, use AC coupling from your pulse source and an input clamp on the gate - see MOSFET Driver with Negative Rail_ATC.

--
Regards,
Tony
 

You can generate a negative rail using a simple cmos Charge pump. Your positive rail has an oscillator and a driver stage and level shifter, the output of the driver pushes down on the top of a capacitor.  The Level shifter moves the oscillator signal down to a driver running from the negative rail, the output of the driver grabe the bottom end of the capacitor and dumps it's charge into a storage capacitor from ground to the negative rail. or you can do the "bottom side" just with a pair of diodes diodes & the storage cap.  so in it's simplest form, a single cmos schmitt inverter generates a square wave (one R and one C) driving the top of the charge pump capacitor, it has two diodes on the other end, one to the negative rail and one to ground, plus the storage capacitor. 


On Wed, 4 Dec 2024 at 13:41, Tony Casey via groups.io <tony=ritecom.com@groups.io> wrote:
On 04/12/2024 09:40, dakeller00 via groups.io wrote:
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 uploaded the file MOSFET Driver with Negative Rail.asc
You can't create a negative rail with this circuit - where's the current going to come from?

If you want to drive the NMOS gate with a source that goes negative, use AC coupling from your pulse source and an input clamp on the gate - see MOSFET Driver with Negative Rail_ATC.

--
Regards,
Tony
 

On Wed, Dec 4, 2024 at 04:59 AM, <dakeller00@...> wrote:
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 uploaded a schematic "MOSFET Driver with Negative Rail-AI.asc" to the Temp folder.  It shows three more ways to drive the MOSFET's gate with a negative bias.
  • The first example is yours, but with a resistor and another voltage source added to make the Zener diode function.  That is how to use the Zener.
  • The second one adds a negative voltage source.  It has no zener diode.
  • The third one just changes the values of your PULSE source.  It's the simplest, but best for simulations only.
 
The first example might be better for building it in hardware.
 
I removed your resistor R1 because it did not do anything useful.
 
I also want to note the following.  The vast majority of MOSFETs are enhancement-mode.  This means the N-channel MOSFET does not conduct unless its gate voltage Vgs is greater than about Vto or around +2.2 V for this MOSFET.  Gate voltage Vgs more negative than about +1 V turns the MOSFET fully off.  By making it even more negative, it does not make the MOSFET be more OFF than it already is at Vgs = 0.  So I don't see the point of using a negative voltage rail or negative offset in this circuit.  Why do that?
 
Andy
 

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