I keep reading wrong description of MOSFET in tutorials...

[alexan_e]

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

 

[KJ6EAD]

Maybe it would be more correct to say that no current flows THROUGH the gate; that it just goes in to form an electrostatic field and is recovered when the field is collapsed, minus losses. Why do you keep reading MOSFET tutorials? Shouldn't one or two be enough, or are you keeping on until you find one that's right?:wink:

 

[alexan_e]

Maybe it would be more correct to say that no current flows THROUGH the gate; that it just goes in to form an electrostatic field and is recovered when the field is collapsed, minus losses.

It's what they actually mean by that, in the next page of that article it says:

"Because of the extremely high input or Gate resistance that the MOSFET has, its very fast switching speeds and the ease at which they can be driven makes them ideal to interface with op-amps or standard logic gates. However, care must be taken to ensure that the gate-source input voltage is correctly chosen because when using the MOSFET as a switch the device must obtain a low RDS(on) channel resistance in proportion to this input gate voltage. For example, do not apply a 12v signal if a 5v signal voltage is required. Power MOSFET´s can be used to control the movement of DC motors or brushless stepper motors directly from computer logic or Pulse-width Modulation (PWM) type controllers. As a DC motor offers high starting torque and which is also proportional to the armature current, MOSFET switches along with a PWM can be used as a very good speed controller that would provide smooth and quiet motor operation."

When anyone reeds this he will think that you can drive a MOSFET connected to stepper motor using a PWM from a logic controller that gives a few mA.
This is why some people (i did too) design bad motor drivers using MOSFETs, you think it is so easy to drive them until you understand that you are wrong.

Why do you keep reading MOSFET tutorials? Shouldn't one or two be enough, or are you keeping on until you find one that's right?:wink:

It is probably a side-effect of Johnnie Walker (keep on walking...):smile:

Alex

 

[dick_freebird]

It would be true in a discussion of DC characteristics.