sensasi: [LTspice] Re: mosfet power dissipation discrepancy

Minggu, 10 Februari 2008

[LTspice] Re: mosfet power dissipation discrepancy - help!!!

--- In LTspice@yahoogroups.com, "Hendrik Jan Zwerver" <hjzwerver@...>
wrote:
>
> --- In LTspice@yahoogroups.com, "yeahboyyeah" <steve.harris@> wrote:
> >
> > Hi Group!
> > I have a circuit where I'm using an LT3782 and am doing efficiency
> > calculations using mosfet models provided by Fairchild and Vishay and
> > comparing the results with using the mosfet model creation program on
> > this website. I get very different mosfet power dissipation results.
> > Which results should I trust; the model from the manufacturer or the
> > program-created models from the datasheet? What default number should
> > I use if the datasheet doesn't have transconductance value or Gfs
value?
> >
> > I've posted the files in the Files/Temp section:
> > LT3782 mosfet efficiency new.asc
> > 40CTQ045.mod - schottky diode model used in the schematic
> > SUM75N06-09L.LIB - created by model program
> > SUM75N06-09L.SUB - provided by Vishay
> > FDB10AN06A0.LIB - created by model program
> > FDB10AN06A0.MOD - created by Fairchild
> >
> > The results are in the spreadsheet file,
> > MOSFET LTSPICE EFFICIENCY 2.XLS
> >
> > Thanks,
> > Steve
> >
> Hi Steve,
>
> I found one mistake in the VDmos version of the SUM75N06 model
> The CGDmax value is way off. The data sheet says at vds=0 it's about
> 1200pf so CGDMAX must be about 2000pf. Now the gate charge curve fits
> the data sheet. You will have more losses in the fet (twice) so this
> should fit the results you measured with the manufacturers model. The
> gate charges from the manufacturer model are ok.
>
> I've put the test schematic for the 4 gate charges in the temp folder.
> (4gatecharges.zip)
>
>
> Regards,
>
> Hendrik Jan
>
Hi Hendrik,
What value am I supposed to use for Cgdmax? I used 1200pF in the
program. The bottom of the graph in the program says to "Determine
Crss_min and Crss_max at Vds=0V." At Vds=0V, you only see the maximum
value (1200pF).

One other question: what if the datasheet doesn't specify a gate
resistance? What is a good default value to use?

Thanks,
Steve