[scilab-Users] Scicos MOSFET gate voltage drive

[Shriram Sampat]

Hello Simone,

Thanks a bunch for your vey detailed reply and thanks a lot to Masoud 
for his suggestion to use the controlled voltage source.

We have a similar working setup with heatsink and ventillators(as you 
guessed ;-) ), bcos we need to be driving around 150A over a 
considerable period of time. This setup and control for it, is totally 
with electronics. Now we want to increase the current driving 
capability(new MOSFET) and a higher control frequency (around 50KHZ PID 
loop), and high configurability via software. The initial scicos 
electrical model was just a simple experiment, whether I could do a 
simulation of this, to aid me write the control part in software and 
thanks to you guys i have the full set of tools to describe the system now.

One problem i have as of now, is defining the parameters for the MOSFET 
(attached datasheet). It is a high current (200A) MOSFET. I have asked 
the manufacturer for the spice parameters for his device and so far have 
got no updates. Can any of you guys help me parameterise this for Scicos ?

I am not much of a power electronics guy, as my expertise lies more in 
software and small electronics. I will go through your suggestions today 
and complete my simulation. I will also suggest it to my circuit design 
colleagues while we do the new design.

Thanks a lot guys for your effort and time. I really appreciate all of 
your help.

Ram

Simone Mannori wrote:
> 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 ;).
>
>

-- 
------------------------------------------------------------
Shriram R Sampat
Software Engineer - Embedded Systems

  EK Tronic Ing. R. Nitz        
  Tel.: + 49/(0)7154/8311-12
  FAX : + 49/(0)7154/70449
  email: s.sampat at ektronic.de
  www.ektronic.de
------------------------------------------------------------
  EK Tronic  Ing. R.Nitz       
  Enzstr. 9 , 70806 Kornwestheim
  Ust ID:  DE 146160631
  Geschäftsführung: Ing. Richard Nitz

-------------- next part --------------
A non-text attachment was scrubbed...
Name: semikron111AR.pdf
Type: application/pdf
Size: 513267 bytes
Desc: not available
URL: <https://lists.scilab.org/pipermail/users/attachments/20080125/4f87d77e/attachment.pdf>