What are the reasons why a negative gate-source voltage (that is, a bipolar supply) would be preferred?
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3 Answers
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In the context of a n-type MOSFET, a negative Gate-Source Voltage will result in a secure switching off of the MOSFET when needed. The reasons:
- A negative voltage ensures that no noise can allow for an undesired switch on of the MOSFET, which in case of a unipolar supply, may be possible.
- A Bipolar supply (i.e., introduction of a negative \$V_{GS}\$ ensures that due the MOSFET is not switched on due to influence from a rising \$V_{DS}\$, caused by the Miller-Effect. Since \$C_{GD}\$ and \$C_{GS}\$, or rather their loading processes are tied to one another because of Miller-Effect, it may happen that while switching off the MOSFET, when \$V_{DS}\$ rises, \$V_{GS}\$ also rises leading to a switching on of the MOSFET when the intention was to switch it off.
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Applying a negative gate source voltage will ensure that the N channel MOSFET will switch off/deactivate a bit more quickly. It's as simple as that. Having a negative voltage will discharge gate-source capacitance more effectively. The downside is that it will delay a few ns when activating the device.
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To answer your question, no a negative voltage is not need to turn off an N-MOSFET and is seldom done in practice.
A Vgs of 0V will fully turn it off.
