Let's say I want to use an nMos FET in a low side switching circuit as follows:
Datasheet for IRF530 here.
Now, during my education in EE, I have learnt some operation regions for MOSFETs and formulas for drain current related to these regions. And one parameter Kn which was the key parameter shows up in everywhere.
But now, I am having hard time in my calculations, because, there is not a Kn value in the datasheets of such MOSFETs. If it were present, what I would do is:
(For Vgs = high case, SW1 is closed)
- Assume an operation region for the nMOS, I start with SAT generally
- Use Idsat formula for the SAT region.
Idsat = Kn/2*(Vgs-Vtn)^2
- Find the drain voltage using
Vd = Vdd - Rload * Idsat
- Check if
Vds >= Vgs - Vtnor not, if so, assumption for SAT is validated, if not, then MOSFET is in linear region, since it cannot be in cut-off
This was working great while doing it on paper, however, now in reality, I am stuck how to approach this kind of a design using the information present in datasheets.
I'm not able to make any comments on the operation region of the MOSFET because I cannot guess drain voltage without any information about drain current flowing through the circuit.
So, what I am asking is something like the followings:
- In which operation region a MOSFET should be operated for switching applications?
- For ON(mosfet conducts) part of the operation, what should I replace MOSFET with? Rds(on)? Or a voltage difference between source and drain? How should I decide whether I can directly replace the MOSFET with a resistance of Rds(on) or not. What criterias do I have to satisyf for it to act as an almost short circuit when it is ON.
And as an extra, if anyone knows, an answer to the following would be nice:
Why Kn is not present in these datasheets while it is heavily used in theoretical calculations, at least in my college education?