No need of pulldown there.
It's much simpler to use P-channel fets.
You can sample the voltage after the first mosfet, knowing there's a diode in between, and turn the fets on when a safe voltage is detected.
It's the gates which may need pull-up/down resistors.
The circuit doesn't make much sense on it's own, even with the brief description. What sort of signal is it switching?
A floating gate can charge from "thin air" to a point where the mosfet turns on. It needs to be actively driven or furnished with a pull up or a pull down.
What about using a single BJT?
A BJT can work in reverse, given certain limitations: the hFE is much lower, typically only about 5 to 10 and the base-emitter voltage rating limits it to 5V.
Here's an example of a voltage level shifter which uses a single BJT and works in the reverse bias region. The transistor doesn't block a significant reverse voltage and the hFE requirement is low, so it works perfectly.
Level shift bi-direc BJT 3.3V 5V.asc
Using N-channels on the high side means your gate voltage to turn on the mosfets will have to be Vth+ above the highest voltage that will be flowing through. So I agree that P-channel is the way to go here.
And for some reason I see this with the drains connected together. It doesn't matter if the drains float.
What about using a single BJT?
A BJT can work in reverse, given certain limitations: the hFE is much lower, typically only about 5 to 10 and the base-emitter voltage rating limits it to 5V.
Here's an example of a voltage level shifter which uses a single BJT and works in the reverse bias region. The transistor doesn't block a significant reverse voltage and the hFE requirement is low, so it works perfectly.
Level shift bi-direc BJT 3.3V 5V.asc
I sort of get that. Except I want no modification of the signal. No level shifting, no amplification. When closed I want the right side completely blocked up to 30V or so. And when open, I want the left side to pass through as unmodified as possible including peaks/waveshape etc.
So... Ok, hear me out a bit.
PNP-Drain to Drain-PNP, right? Ok the gate, I'll need a pullup to some voltage, and then to ground that out to a voltage lower than source. So, I can't just pull up to 5V and then ground. This is where the extra NPN would come in right? That I'd use that to pull a negative potential on the PNP gate right? So in this case, I need a gate driver. That's an extra part.
So... Ok, hear me out a bit.
PNP-Drain to Drain-PNP, right? Ok the gate, I'll need a pullup to some voltage, and then to ground that out to a voltage lower than source. So, I can't just pull up to 5V and then ground. This is where the extra NPN would come in right? That I'd use that to pull a negative potential on the PNP gate right? So in this case, I need a gate driver. That's an extra part.
I think I have misunderstood what signals you are transmitting through the mosfets - what the possible voltage range is. So I'm going to be quiet and see if this becomes more clear.
JTAG/SWD, so in this case 3V3.
So... Ok, hear me out a bit.
PNP-Drain to Drain-PNP, right? Ok the gate, I'll need a pullup to some voltage, and then to ground that out to a voltage lower than source. So, I can't just pull up to 5V and then ground. This is where the extra NPN would come in right? That I'd use that to pull a negative potential on the PNP gate right? So in this case, I need a gate driver. That's an extra part.
I think I have misunderstood what signals you are transmitting through the mosfets - what the possible voltage range is. So I'm going to be quiet and see if this becomes more clear.
JTAG/SWD, so in this case 3V3.
JTAG/SWD, so in this case 3V3.
But also zero volts. Right? So the JTAG lines are signal and clock lines which switch between high and low, and the data line is bidirectional. Back-to-back mosfets I think are normally used to switch power or ground lines. Wouldn't it be more appropriate to use an analog switch for signal lines? Maybe something like the SN74LVC2G66.
So... Ok, hear me out a bit.
PNP-Drain to Drain-PNP, right? Ok the gate, I'll need a pullup to some voltage, and then to ground that out to a voltage lower than source. So, I can't just pull up to 5V and then ground. This is where the extra NPN would come in right? That I'd use that to pull a negative potential on the PNP gate right? So in this case, I need a gate driver. That's an extra part.
I think I have misunderstood what signals you are transmitting through the mosfets - what the possible voltage range is. So I'm going to be quiet and see if this becomes more clear.
JTAG/SWD, so in this case 3V3.The gate capacitance in the NX2007 could be a problem. The data sheet specifies 23.6pF and you have two of them.
https://assets.nexperia.com/documents/data-sheet/NX7002BK.pdf
A smaller device such as the 2SK1829 might be more suitable.
https://docs.rs-online.com/3357/0900766b814b2fb8.pdf
You'll also need a zener diode to protect the gate if the voltage can exceed the VGS rating.
Why not use integrated solution? Something like https://www.analog.com/en/products/max4505.html