Can a N-channel MOSFET used as a highside switch
Is it possible to use an NMOS as a highside switch ?


Yes it is possible, see this circuit:



The issue is that you need to apply a gate voltage to the High side NMOS that is higher than the supply voltage. If you just applied the supply voltage (\$V_{IN}\$
in the picture) then the NMOS would not be fully switched on, the NMOS would drop a significant voltage between drain and source. That voltage is somewhat larger than the threshold voltage (\$V_t\$) of that NMOS.

The reason for this is that the gate-source voltage \$V_{GS}\$ of the NMOS must be at least \$V_t\$ for the NMOS to even conduct at all.

For the lowest resistance between drain and source (and that's what we want as we want to use the NMOS as a switch) we must apply a large \$V_{GS}\$. As we want \$V_{IN}\$ = \$V_{OUT}\$ when the NMOS is on that means that at the gate of the NMOS we need a voltage that is higher than the supply voltage.

One way to make this voltage is to use a bootstrap circuit. The "Built-in diode" and the "Bootstrap capacitor" form the bootstrap circuit. When the low-side NMOS conducts, then the High-side NMOS must be off (Do you see why?) then the capacitor charges to \$V_{IN}\$.

When state changes and the low-side NMOS stops conducting and the high side NMOS is switched on voltage at the pin called "BOOT" raises above the supply voltage. That is then used to fully turn on the high side NMOS.

The disadvantage of this circuit is that the bootstrap capacitor needs to be charged before it is discharged (due to leakage currents etc.). So for an application where \$V_{OUT}\$ is switched on/off regularly, this will work. For an application where the high side NMOS is switched on and must remain switched on for a long time this circuit is not suitable. Then a charge pump or a boost converter could be used to make a voltage that is higher than the supply voltage.