AVR-Chat

On Feb 19, 2005, at 10:06 AM, Robert Adsett wrote:

> That should be driving the power section off of more than 5 or 6 Volts 
> is
> asking for trouble.  You want the gate drive voltage to be 10-15V above
> it's reference.

Datasheet reference please for your 10-15V recommendation?

Rule of thumb which has worked well for me is to drive the gate with at 
least 2 volts more than the source. And remember the "source" is the 
output of the MOSFET while drain is the input.

Yes, drive the gate harder and "on" resistance will be lower. But the 
most bang-for-bucks is just over 2 volts.

One of my favorite parts is the IRF7307:
http://www.irf.com/product-info/datasheets/data/irf7307.pdf

Notice in figure 3 how they have about 9 amps flowing thru a 5A part, 
pulsing, with 2 volts GS. Figure 6 shows gate charge and you can see 
how the gate takes quite a charge before it rises above 2 volts.

I don't recommend the IRF7307 for this application as its SOIC-8 and 
therefore hard to prototype with. Also its only 20V.

Digikey labels the IRF7307 "logic level" but not the IRF7317:
http://www.irf.com/product-info/datasheets/data/irf7317.pdf

In Figure 5 of the IRF7317 datasheet we see a knee in the plot of "on" 
resistance vs. gate voltage at 3 volts.

One other point is that maximum Vgs is +-12 volts on these parts while 
the maximum Vds is 20. If one were to drive the gate +15 then it would 
be outside of its ratings.

The real reason for speaking up is to suggest the combination of P- and 
N-channel MOSFETs for the purpose of switching a higher voltage with a 
lower voltage.

--
David Kelly N4HHE, dkelly@HiWAAY.net
========================================================================
Whom computers would destroy, they must first drive mad.