It does make sense. The MOSFET will not conduct when VGS is 0V, therefore the source voltage will not be higher than the gate voltage.
Obviously there is some critical concept here I'm having a hard time grasping.
Let's say I have a 5V power source, a resistor, and my 3V microcontroller pin. If I connect the 5V source to the resistor and then the resistor to my microcontroller, I will destroy the microcontroller.
Now I do the same with a mosfet in the resistor's place. What's changed?
We know the mosfet must be on and in a low resistance state, because otherwise the microcontroller could not pull the 5V side down when it's pin goes LOW. The mosfet's resistance and the pullup form a voltage divider, and the drain pin is the point between them we tap and connect to he PFET. For that point to be near 0V, the mosfet MUST be almost fully on, with a resistance of less than a few ohms.
And if the mosfet is almost fully on, then that means there's almost no resistance between that 5V pullup and the uC pin. Which is the same as my example with just the resistor. And the uC will be destroyed.
Except you say it won't. That the source can never go above the gate. That's fine, and I'd accept that, except it directly conflicts which what I just said about how the mosfet's resistance must be low, and therefore effectively a dead short between my uC pin and that 5V pullup... which would destroy the pin.
What am I missing here?
If that were true, I could not switch a 20V source using logic level mosfet because the gate would never see more than 3-5V.
The 20V switch is a common source circuit ("low side switch"). The level translator is a common drain circuit. Totally different. In fact, you can't easily use an N MOSFET as a 20V high side switch.
I don't understand the difference. If I took that drawing and put my uC pin where the ground is, and set it low, it would look very much the same, and would the lamp not still light when 3.3V is applied at the gate? That is, until the pin burnt out.
uC pin goes high, gate sees 5V, source is connected to 20V source powering lamp. Source sees more than 5V.
Now this does not make sense. In the 20V switch circuit, the source is grounded.
And it is grounded when I set my uC pin low as well, is it not?