Re: gEDA-user: mosfet design help

[Dan McMahill <dan@xxxxxxxxxxxx>]

carzrgr8@xxxxxxxxxxxxx wrote:
> Here's a picture of what I'm trying to accomplish :
> http://www.geocities.com/fazool1_2000/geda/sample.pdf
>
> Switching frequency is around 500KHz - edges may be fast, but I'm not 
> yet sure how fast.  I've been simulating with 1ns rise/fall time, but 
> that number isn't based on any hard data.
>
> In simulation, this arrangement works - but I am now certain the lid 
> will blow off the mosfet :-)  Next step is to try to use the voltage 
> divider at the gate.
>
> Dan, are you saying that the Cgd will allow the fast transient input to 
> couple to the output?  Interesting - I didn't think about that.  That 
> sort of thing should show up in simulation, I expect.  There's some 
> small signal mosfet's from ON semi - just ran across them this morning. 
>  The Vgs max is +/-6V, and the gate has built-in transient supression ( 
> http://www.onsemi.com/pub/Collateral/NTA4153N-D.PDF ).  Maybe good?  I 
> need to look closer.

yes, Cgd can allow a fast transient to couple to the output.  It is more 
of a concern if you have a much larger input than output (like you do) 
and a following circuit which is not tolerant of that.

Ok, dumb question to follow here....  Why even bother with the level 
converter?  Is there a way to size your filter components to allow you 
to drive 50 V directly?    Obviously you have to watch out for 
transients.  If nothing else your particular circuit looks like a 
voltage divider would work quite well, just replace R1 with a series R 
to the 50V and a shunt R to ground.  Size them to give 4.12k looking 
back into it and to give the required attenuation.

-Dan



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