[scilab-Users] Scicos MOSFET gate voltage drive

[Simone Mannori]

Dear Shriram,

I would apologize for the late answer (we are in a constant overload
situation for the next
Scilab 5.0 release).

I would say a big "thanks" to Masoud for his indispensable and timely help
(Masoud is the
main contributor of  Modelica integration in Scicos).

Some notes about the diagram:

- before start a simulation is better use reasonable values (close to the
real one) for components
  parameter's.  Avoid this rule at your risk when are playing with POWER
circuits !
  I choose L=0.1Henry and R=0.1Ohm.  The relative  time constant is L/R =
1s.  Probably the real circuits
  is faster: you are more expert than me.

- I left the MOSFET parameter's  as default. It is not a very good choice
because I'm breaking the rule #1.
  The simulation woks quite good, but for useful results you need to tune
MOSFET parameter's in order to
  "clone" the performances of the real one. I would suggest a "Italian Made"
STB150NF55 by ST .
   Masoud still have MY book on MOSFET modeling, so ask him for more
information about MOSFET
   parameter's tuning ;).

- I added a small value resistor (100Ohm) in series with the gate of the
MOSFET. Drive a gate with an ideal
  voltage source is not a very good idea. The MOSFET equivalent input
circuits is a  capacitance and drive
  a capacitor with a ideal voltage source can create issues in the
calculation of the input gate current :
  i = C d vg / dt . If "vg" is a voltage step the input current is infinite
and you have a convergence problem.
  In the "real world" the input equivalent circuits is a non-linear, time
varying capacitance with a parallel non linear,
  time varying, conductance. This conductance  can  be NEGATIVE.  If you add
the inevitable inductance of the
  wiring, you can obtain a nice 10MHz oscillator. Normally these oscillation
destroy the real device.
  Usually the minimum gate resistor is in the order of 10 Ohms. An
alternative, safer, solution is drive the gate
  with a "voltage clamped" current source.

- Push the KP gain is not dangerous (in the simulation) because the system
is - basically - first order... BUT
  look at the Gate voltage: more than 30 volt ! You will destroy the real
device. I introduced a saturation to
  limit the gate voltage (look at the second attached file). In the real
circuits a Zener diode will clamp the voltage.

- Using linear drive you need to add a big heath sink to the MOSFET. Use a
PWM circuits is a better solution.
  Masoud has some prepared some ready made examples inside
"demos/Electrical" folder.

Please don't hesitate to contact us for further help for your application:
Laurent Vaylet is a mechanical engineer with
some first-hands experiences in the automotive world.

Best Regards

   Simone Mannori - Scilab/INRIA Rocquencourt


P.S. Thanks to Sylvestre for his "postman" job ;).
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