Seems you are driving the source of the NMOS with two different supplies and I don't think it's recommended to use this way.
What you did is basically "translation" or "elevation": you have an independent 5V voltage source, and its reference (GND/return or whatever) is connected to the MOSFET source pin (Practically, the 1R gate-stopper resistor is still in series with the gate so I assumed the stopper resistor is connected between the gate and the positive of the 5V source.).
In practice, if the 5V source is isolated by default then it's totally fine to drive the MOSFET as shown in your circuit because you are free to connect this isolated source's reference to anywhere you want. But if the 5V source is normally GND-referenced (i.e. shares the same GND with 28V source) by default then connecting the 5V source's reference the MOSFET's source will short the load (10R resistor in your circuit).
If you want to drive an NMOS as a high-side switch, several methods exist:
- Bootstrapping
- Using gate-drive transformers
- Using dedicated gate driver ICs
- Using boosters (e.g. charge-pump).
The first two are useful when there's non-zero-frequency switching (e.g. PWM). If you want to drive the load indefinitely (e.g. no periodic on-offs or no pre-set on-time for the load) then these methods may not work or may not be practical.
The third one is usually a bit more advanced application of the first one.
The last one is basically generation of a higher voltage from a lower input voltage but this requires constant independent switching i.e. you need a separate oscillator to drive the pump and you can use the output of the pump which is going to be a DC and can share the same ground as 28V source (There are also ICs available for this job). Example: You can get 1.5 times the input voltage with a charge pump (~40V), and you can switch that voltage to the MOSFET's gate directly.
Buck converters usually use NMOS as the main switch which works as a high-side switch (the difference is the load is a more complex circuit there). To drive it, bootstrapping is one of the most common methods today. Bootstrapping is useful because it's basically a switching application.
