Hi Everyone.
What would be the best approach for a Pwm power supply?
If I do this (below) the GND will not be Common , since we always want the GND's to be common.
Is it possible to only use a Mosfet with filtering on the output?
Is a Buck cct a better way?
What makes you think that?
Possible yes, if you don't care about a stable voltage. Otherwise you need a voltage reference that can control the FET.
A buck converter is way better.
You can try to find a "high-side driver" schematic. If you need to control more than 5V, It will have at-least one more transistor (probably a regular bipolar transistor rather than a MODFET).
The problem is... To turn-off the MOSFET the gate-source voltage needs to be (about) 0V. If you have a 12V power supply and you use a P-Channel MOSFET, you'll need 12V on the gate to turn it off, so you can't control it directly with 5V (or 3.3V) logic.
You can find Solid State Relays that can do it. Just make sure to get a DC SSR. AC & DC SSRs are not (usually) interchangeable.
https://forum.arduino.cc/t/common-ground-and-why-you-need-one/626215
There will be a Voltage difference between the Batt negative and the Mosfet negative output.
I can just feed the output back to A0 and adjust the PWM.
Look at this cct what GND must be used for the voltage divider GND1 or GND2?
You're right, I wasn't thinking of that.
On the other hand, a buck converter doesn't have a common Vcc either, so the outcome effectively is the same.
They are? Why put a chip with Vin and Vout, if Vcc is the same across those pins?
Sorry I mixed them VSS Gnd is common on a Buck converter.
No worry, we both did the same type of mistake 
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