alexan_e
Administrator
I keep reading MOSFET tutorials that describe the MOSFET as a voltage controlled device that has extremely high input resistance (OK so far) but then state "NO current flows into the gate" (for example The MOSFET or Metal Oxide Semiconductor FET Tutorial).
Why do these tutorials ignore the fact that a MOSFET has a gate capacitance that you need to charge so that the MOSFET can turn on?
The "NO current flows into the gate" statement is true only when MOSFET capacitance is charged and you keep the MOSFET in that state, when you want to shut down the MOSFET then you need to discharge the gate capacitance and that means draw current from the gate.
When a MOSFET is used as a switch that turns on a load and then stays on (not fast switching) then driving the gate with a low current device (use of high gate resistor or low current driver ) is not a problem because the MOSFET will just take longer to charge and switch on.
On the other hand when you drive it with a frequency of 20Khz or 50KHz the driver current is a very important factor, you need to charge the gate very fast (and discharge it very fast to switch off) to minimize the power loss that will overheat the MOSFET and have it conducting long before you shut it down, for 50KHz switching the state changes every 10ns (50% duty cycle) and can be much lower for different duty cycles.
The MOSFET datasheets have a very important factor called Total Gate Charge and is given in nC.
It is actually the time * current at which the MOSFET is fully on, for example when you read 100nC it means that if you give 1A (limit the gate current to 1A is what i mean) the gate will be charged after 100ns (100nC /1A = 100ns ) or if you give 10mA the gate will be charged after 10000ns (100nC /0.01A = 10000ns ) etc
So when you choose a MOSFET and driver you should consider the parameters for both of them (MOSFET with lower nC are faster)
For mode details you can read
https://www.fairchildsemi.com/an/AN/AN-9010.pdf
see page 17 for the different gate capacitance charge stages
Alex
Why do these tutorials ignore the fact that a MOSFET has a gate capacitance that you need to charge so that the MOSFET can turn on?
The "NO current flows into the gate" statement is true only when MOSFET capacitance is charged and you keep the MOSFET in that state, when you want to shut down the MOSFET then you need to discharge the gate capacitance and that means draw current from the gate.
When a MOSFET is used as a switch that turns on a load and then stays on (not fast switching) then driving the gate with a low current device (use of high gate resistor or low current driver ) is not a problem because the MOSFET will just take longer to charge and switch on.
On the other hand when you drive it with a frequency of 20Khz or 50KHz the driver current is a very important factor, you need to charge the gate very fast (and discharge it very fast to switch off) to minimize the power loss that will overheat the MOSFET and have it conducting long before you shut it down, for 50KHz switching the state changes every 10ns (50% duty cycle) and can be much lower for different duty cycles.
The MOSFET datasheets have a very important factor called Total Gate Charge and is given in nC.
It is actually the time * current at which the MOSFET is fully on, for example when you read 100nC it means that if you give 1A (limit the gate current to 1A is what i mean) the gate will be charged after 100ns (100nC /1A = 100ns ) or if you give 10mA the gate will be charged after 10000ns (100nC /0.01A = 10000ns ) etc
So when you choose a MOSFET and driver you should consider the parameters for both of them (MOSFET with lower nC are faster)
For mode details you can read
https://www.fairchildsemi.com/an/AN/AN-9010.pdf
see page 17 for the different gate capacitance charge stages
Alex
