a
I understand the power dissipation during transition from on to off and off to on. Is my thinking that the the knee (circled in white on drawing) in the voltage across Vsd is do the change of voltage, Vsd, and acts as if gate discharge is decreasing slower than it actual is, assuming that is constant, Or does the gate discharge rate actually slow down? Or is it something else entirely that causes that knew?
Actually, the plot that you circled is the absolute value drain current. The V(12v)- V(out) plot is the voltage across the MosFet.
Think about it this way. Which terminal of the device controls Isd? To change the state of that switch, a charge, Q, must be pumped into and/or out of the gate to change the gate potential to change the conduction state. This essentially boils down to a RC circuit with the gate charging via R1 and discharging via R2. The rate of change of the gate potential is directly tied the rate of change in charge at the gate, which is the major factor impacting Ton and Toff.
Look at the datasheet below closely with particular attention to Fig. 5 and Fig. 13A.
https://www.electro-tech-online.com/custompdfs/2010/11/irf4905pbf.pdf
I have included two more plots with the addition of the gate potential V(gate) of those I posted yesterday. Look at the step in as the MosFet turns on and off and how that compares with the graph in Fig. 13A. Also, in the sim change R1 to 1ohm and R2 to 10ohms, and observe the changes in Ton and Toff.
OK, here is the symbol (.asy) file and I’ll briefly describe how and where to put it. I don’t believe you have this since you have used the include statement in the sim you posted.
1. Create a new folder in LTC\lib\sym\misc and name it MOSFET;
2. Copy the file below and paste it to a pure text editor…I use Notepad all the time. Using Notepad, under File select Save As then click on the box Save as Type and select All files. Then type in the file name as IRF4905.asy and save it in the folder created above;
NOTE: Be sure you include the .asy extension;
3. Then create a new folder in LTC\lib\sub and name it MOSFET also;
4. Rename the file IRF4905.sub if it is not already because the spicemodel line in the symbol file is pointing at that file name as MOSFET\IRF4905.sub
5. Move the MosFet subcircuit file to the new subfolder created then start LTC and test. If LTC was running shut it down and reboot so it can see the new part.
This method will work for MosFets, among others, that are spice subcircuit models only and the appropriate files should be placed in your two newly created folders with the same path information in the symbol file. If you come across a MosFet with a simpler .model file, that can be place in the cmp\M Standard file using the format convention used there. Here is the symbol file:
Version 4
SymbolType CELL
LINE Normal 48 48 48 96
LINE Normal 16 80 48 80
LINE Normal 16 48 24 48
LINE Normal 48 48 24 44
LINE Normal 48 48 24 52
LINE Normal 24 44 24 52
LINE Normal 16 8 16 24
LINE Normal 16 40 16 56
LINE Normal 16 72 16 88
LINE Normal 0 80 8 80
LINE Normal 8 16 8 80
LINE Normal 48 16 16 16
LINE Normal 48 0 48 16
WINDOW 0 56 32 Left 0
WINDOW 3 56 72 Left 0
SYMATTR Value IRF4905
SYMATTR Prefix X
SYMATTR SpiceModel MOSFET\IRF4905.sub
SYMATTR Value2 IRF4905
SYMATTR Description HEXFET® Power MOSFET Transistor
PIN 48 0 NONE 0
PINATTR PinName D
PINATTR SpiceOrder 1
PIN 0 80 NONE 0
PINATTR PinName G
PINATTR SpiceOrder 2
PIN 48 96 NONE 0
PINATTR PinName S
PINATTR SpiceOrder 3