Electronic – Why would someone tend to use a high-side device instead of low-side device

drivermosfetmosfet-driver

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?

Best Answer

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