more mosfet questions

[ca178858]

I'm trying to figure out some of the finer points of mosfet switching, let me see if I've got this right:

Lets assume were talking about an logic level n-channel here...

To turn on, we apply a positive voltage to the gate (with respect to the source). Once the gate is charged (measured in nano-coulomb) then current is conducted across drain and source...

The data sheet lists different gate charges- 'total', 'gate-source' and 'gate-drain'. I'm guesing that the 'total charge' has to be met for the device to start conducting. Not sure what use the other charges are...

It would appear that if I was driving a logic level mosfet from a PIC then when I switched on a mosfet it would draw the maximum amount of current from the PIC until the total charge was met- this would only take a few tens of nano-seconds- but would probably be hard on the PIC.

I could add a resistor is series of say 200 ohms to ensure that no more than 25mA is drawn from the PIC, and this would cause the mosfet to turn on much slower (looks like about 120ns if the gate charge was 3nC). Is this nescasary? Am I right about the values?


This is all pretty new thinking for me, but I'm guessing this is why mosfet drivers are used, and why they are rated for several amps- because the more current you drive with the faster the mosfet switches...

 

[tavib]

Usually the gate charge is higher than 3nC for rich the ON point.
The gate-source charge is only up to start conducting a small amount of current, to achieve the complete ON state you must supply the gate-source charge and the gate-drain(Miller) charge. These two will make the switching time and loss(especially the gate-drain charge which corespond to drain-source voltage rise or fall time).
For more complete information see this

 

[ca178858]

Thats exactly what I was looking for!!

One more step closer to understanding mosfets...


Thanks