Electrical – why MOSFET Driver


  1. Why do we youse MOSFET Driver when we use MOSFET as a switch in a high side configuration and we do not use it in low side configuration?

  2. Can we drive a MOSFET ON/OFF with the OUTPUT of UC3842?

Best Answer

Firstly the UC3842. It has a push-pull BJT driver and is quite suitable for driving the high capacitance load of an N channel MOSFET's gate-source region: -

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And here's an example circuit: -

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Picture source. If you read the UC3842 data sheet you'll see that it is good for driving a couple of hundred mA so for small to moderately sized MOSFET's (generalism alert) it's a decent choice.

Why do we youse MOSFET Driver when we use MOSFET as a switch in a high side configuration

In a high side application, the preferred method is to use an N channel MOSFET and specialist bootstrapped driver. The more natural choice would be a P channel MOSFET but these are usually deemed more inefficient at fast and hard switching.

To drive an N channel MOSFET, the gate must be driven higher than the source and, if you want to form a low ohmic on state between drain and source you need to drive the gate about 10 to 15 volts above both drain and source. Given that the maximum voltage seen will be at the drain this creates a problem and therefore specialist drivers are preferred because they incorporate a bootstrapping circuit to deliver higher-than-drain voltages to the gate. Here is an example: -

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The high-side N channel MOSFET steals a bit of energy from the output switching waveform by using a capacitive voltage doubling circuit that feeds the boost pin.

This is also know as bootstrapping and isn't of course needed when driving the low-side MOSFET however, the LTC4444 device above can also drive a low side MOSFET and the advantage here is that it incorporates "adaptive shoot-through protection" that inserts a small dead-time between one MOSFET turning off and the other one turning on. This prevents both devices being partially on simultaneously and dramatically restricts the phenomenom of current shoot-through.

In the application note, they have treated a flyback power supply, so the MOSFET is in the low side configuration, but I want to use this IC for buck converter power supply where a MOSFET switch is in the High side configuration

You are out of luck in that case - the output of the UC3842 is only suitable for directly driving a low-side MOSFET or BJT but, if you are prepared to do a few experiments you could drive an NPN BJT that can interface with a top-side P channel MOSFET but then there are better chips around than this solution.