I have a power P-channel MOSFET that can produce a max of -34 A and whose datasheet does not include parameters W or L. How can I model this PMOS without W or L if I only have \$R_{DS_{on}} \$ and \$Q_g \$?
Electronic – Simulating Power PMOS using LTSpice
ltspicepmospower electronics
Related Solutions
In The LTSpice help file you can find this table, which I'm too lazy to figure out how to reproduce completely:
The table is under LTspice IV -> LTspice -> Circuit Elements -> M. MOSFET in the help file contents tab.
I'm assuming you would relate this to the most basic forms of CMOS logic, the inverter:
For the PMOS it is still common to also use \$Vgs > Vt\$ for the ON condition. But note that then the PMOS Vt would then be negative !
Condition 2 \$Id = 0\$ can also be achieved with both transistors OFF. So condition 2 does not always imply condition 1. But for the inverter powered with a sufficient Vdd (If Vdd = 0, that would also imply Id = 0) then either the NMOS or the PMOS transistor is ON (in a conductiong state). That is assuming Vin is either equal to 0 V or Vdd. (If Vin = roughly \$Vdd/2\$ then both transistors are conducting and Id would not be 0).
Id = 0 is not related to the state of the circuit. It is similar to the situation that a light switch can be ON or OFF, it does not mean a current has to flow. Removing the lightbulb (so no current can flow) does not prevent the switch from being in the ON or OFF position).
Likewise in an inverter depending on it's state either the NMOS or the PMOS is in a conducting state, that does not mean a current Id has to flow. This is the great benefit of CMOS logic, (almost) no current flows when the logic is in a static position.
Best Answer
To model the P-MOS transistor in LTspice you do not need to know the \$W\$ and \$L\$.
The simples model used the \$K\$ factor and \$V_{TH}\$.
The drain currency is equal to:
$$I_D = \frac{K}{2}(V_{GS} - V_{TH})^2$$
And using the datascheetplot, we can also find \$V_{TH}\$ using this equation:
$$V_{TH} = \frac {V_{GS1} \sqrt{I_{D2}} -V_{GS2} \sqrt{I_{D1}}}{\sqrt{I_{D2}} - \sqrt{I_{D1}}} $$
And \$K\$ factor:
$$K=\left ( \frac{\sqrt{2I_{D1}}-\sqrt{2I_{D2}}}{V_{GS1} - V_{GS2}} \right )^2$$
For example, if we used the datasheet http://www.irf.com/product-info/datasheets/data/irf9z24n.pdf
We can use this plot:
And find that:
\$I_{D1} = 2A\$ and \$ V_{GS1} = 4.8V\$
\$I_{D2} = 4A\$ and \$ V_{GS2} = 5.5V\$
We have \$V_TH = -3.11V\$ and \$K = 1.4 \$
And finally, in LTSpice we can use this statement:
But most of the time we can find a spice model of a MOSFET on google.