I am trying to understand mosfet drivers in general, no specific part. And I am encountering low side and high side drivers. My question is that if for example: I have a N channel mosfet as a high side switch and another identical N channel mosfet as a low side switch. Why do I need a different driver for each? Ultimately you want to charge gate capacitance quicklyand since they are both N channel what is the difference of how I need to charge it if its high side or low side?

  • \$\begingroup\$ Vgs must be a few volts above source in either case; when switching 12v low, let's say 5v (0+5), but when switching high it's 17v (12+5), and a lot of times the switched voltage is the max voltage used by the circuit, so the high side needs special handling to get to 17v when the max of all else is 12v. \$\endgroup\$ – dandavis May 16 '18 at 16:11

A low side FET driver's circuitry is referenced to ground. Therefore it's easy to interface to controllers or even logic signals.

A high side N-FET has its source flying up and down between ground and the drain voltage, maybe higher with parasitic ringing. So does the source reference connection on the driver.

So a high side driver generally has level shifting circuitry that allows you to control Vgs even though the source is at high voltage. It may also have features to help with bootstrapping the high voltage, or desat protection, etc.

For N-channel switch applications the high side driver may have a charge pump built in to allow it to drive the gate above the source.


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