I am trying to understand why someone would want to use a high-side MOSFET to switch on and off a load if they could simply use a low-side MOSFET. Using a low-side MOSFET requires a much lower VG compared to a high-side device.

In the context of a half-bridge driver, I can see how you would need both low- and high-side devices with a charge pump integrated to the circuitry to charge the gate of the high-side MOSFET.

But my question is: How and why would you use a high-side device for various circuits other than half-bridge?

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    \$\begingroup\$ There are lots of times when a load has to be physically connected to ground e.g a starter motor. Then a high side switch needs to be used. \$\endgroup\$ – JIm Dearden Dec 14 '16 at 21:43
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    \$\begingroup\$ Sometimes there are good reasons not to disconnect the ground connection between that device and other circuitry. \$\endgroup\$ – Brian Drummond Dec 14 '16 at 21:43
  • \$\begingroup\$ same reason breakers are used on line instead of Neutral \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 14 '16 at 21:57

If your load is referenced to a system ground, putting a switch in series with the low side can result in a ground potential difference proportional to load current. (Load * RDSon)

This can erode noise margin on digital signals and inject noise and error into analog circuits.

To get around this, high side switching is often used. You can use a P-Channel FET for this, or develop a supply that's high enough above the rail you are switching to fully enhance an N-Channel FET.

For the same RDSon, the N-Channel FET will be cheaper than the P.

You can use a charge pump, develop an auxiliary supply to drive the FET gate. Some integrated load switch drivers have a charge pump built in and provide other useful features.

There are also integrated high side load switches that can be useful for certain load ranges. Here's an example from Micrel:

Micrel Driver

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    \$\begingroup\$ Switching the low side can also be dangerous in high-energy systems because the live conductor is always in circuit even when the unit is switched off. Switching in the high side ensures power is removed when the unit is switched off. \$\endgroup\$ – vofa Dec 14 '16 at 22:14

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