Would it be a good idea to use the internal Rds of a MOSFET as a high side shunt. or would it be better to get a dedicated shunt resistor separately to measure current passing though the MOSFET?
Some DC/DC stabilizers on-resistance of the transistor is used to limit the current.
In this case, voltage control is performed in the open transistor.
The Rds ON of a MOSFET varies significantly from unit-to-unit and also has a positive coefficient with the MOSFET junction temperature.
So Rds would not be good for the purpose of measuring the MOSFET ON current unless your accuracy requirements are very low.
Some DC/DC stabilizers on-resistance of the transistor is used to limit the current.
In this case, voltage control is performed in the open transistor.