Re: Gatedrive:charge vs. capacitance vs. potential

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Jan Wagner <jwagner-at-cc.hut.fi> 

Hi,

On Mon, 22 Sep 2003, Tesla list wrote:
 > Original poster: Finn Hammer <f-h-at-c.dk>
 >
 > All,
 >
 > Can someone explain, how to arrive at the equivalent gate capacitance,
 > knowing the Qg(on) and Vgs.

If with Qg(on) you mean "max total gate charge at 10V Vgs, Vd=x V, Id=y A"
as often stated in the mosfet specs then you can just do Qg(on)/10V!

That's also a good simulation estimate for gate voltages beyond 10V,
because the mosfet is already fully enhanced at 10V Vgs i.e. the
drain-source voltage is minimal. The gate-drain miller capacitance won't
eat up any considerable further charge then. So even for 18V Vgs you can
use the Qg(on)/10V estimate.

There's more info on the mosfet capacitances and a description of
the mosfet turn on/off processes in:

   http://www-s.ti-dot-com/sc/techlit/slup169.pdf

It also shows how to calculate the total gate capacitance, switching power
losses, etc.

 > Is it correct that this gate charge varies with voltage across FET as
 > well as current trough it?

When the mosfet is used as a switch then it varies more according to the
drain-source voltage, e.g. 200VDC rails vs 400VDC rails makes a big
difference. It varies with current only as far as the drain current and
channel resistance raise the drain-source voltage (minimal effect, I'd
guess)

cheers,
  - jfw

--
**************************************************
  high voltage at http://www.hut.fi/~jwagner/tesla
  jwagner-at-cc.hut.fi - Jan.Wagner-at-cern.ch
  Jan OH2GHR