Saturday 4 February 2017

Difference between n-channel MOSFET and p-channel MOSFET in a buck-boost converter



From TI's Understanding Buck-Boost Power Stages in Switch Mode Power Supplies:


buck-boost converter


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My question: why is a floating drive needed for an n-channel MOSFET but not for a p-channel MOSFET?



Answer



With a P-MOSFET the source is connected to the supply. This means that VG(off) is always less than the supply and therefore the transistor can be turned off easily.


With a N-MOSFET the source is connected to the output. This means that VG(on) is not only greater than the output (by definition), it may in fact be greater than the supply. This means that a special drive circuit may be required in order to turn the transistor on.


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