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PMOS Turn on problem

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NewBuilder

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I am trying to use a PMOS as a high side switch. The source voltage is ~14V. I am trying to create a load switch control circuit using an NMOS and resistor so that a small driver can be used for the NMOS to pull the gate of the high side PMOS to ground, turing on the PMOS when necessary. I am having a problem with getting the PMOS to turn on all the way though. I don't really have experience with PMOS devices and cant figure out how to turn it on. I'll attach a schematic. I've tried 1k, 2k, 3k-10k, 100k resistors for the "bias resistor" or whatever it would be called with no luck. I am getting mA compared to 3-5 A.

PS: I know there are drivers available to easily switch a PMOS at 14V, but I plan to be at hundreds of volts in the future.

Thanks in advance
Schematic.PNG
 

Hi,

For the benefit of some-one who will be able to answer, what's the NMOS gate voltage you're using? Is that a Buck or a Boost converter or something?

What do the graphs in the datasheet say you should be seeing through and out of that specific PMOS with those voltages used?

I got - assuming the PMOS drops 1V of the 14 going through it - 165mA out with that load, but I admit I've ignored the inductor and any effects it may have, sorry.

Aside - any point in putting a pulldown resistor from NMOS gate to ground?
 
Your "bias resistor" turns off the Pmos and the Nmos turns it on if the Vgs for the logic level Pmos is 4.5V or more and the Vgs for the Nmos is high enough (usually 10V but it could be a 4.5V logic level Nmos).
 

Audioguru and d123,

While the NMOS is enabled, Vg of the PMOS is connected to ground so Vg=0. I thought that would turn off the PMOS? I am having to actually connect a positive source to the gate, no less than 12V in this case, for it to turn off. Is there another way for me to switch the PMOS using a low voltage? Eventually, I'm going to have ~500V on the input. It is not possible for me to apply a 500V signal to switch the FET.


d123,

Thanks for pointing out the resistor value. It makes sense why it was in the mA regime. So I guess the real problem is its just not turning off when the gate is connected to ground. I was originally using a generic LTspice PMOS but picked one from the bucket of models just to see if it worked using one. it didnt.

This is a buck converter. And I was using a 5V source to switch the NMOS because it's just a generic LTspice model and works for it.
 

Hi,

While the NMOS is enabled, Vg of the PMOS is connected to ground so Vg=0.

Read datasheets.
Vg is not important.
V-GS is what counts. It is the voltage between gate and source.

This voltage controls whether the Mosfet is ON or OFF.

Klaus
 
While the NMOS is enabled, Vg of the PMOS is connected to ground so Vg=0. I thought that would turn off the PMOS?
Nope, it turns it on if the gate is 4.5V more negative than the source pin.

I am having to actually connect a positive source to the gate, no less than 12V in this case, for it to turn off.
Of course, a low voltage or no voltage from gate to source turns it off.

Is there another way for me to switch the PMOS using a low voltage? Eventually, I'm going to have ~500V on the input. It is not possible for me to apply a 500V signal to switch the FET.
The Nmos is switching the Pmos on and off with a low voltage but the gate-source voltage of the Pmos must be limited with a voltage divider and a zener diode.
 

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