Re: 3.3v logic level mosfets

colin wrote:
"Anthony Fremont" <spam-not@xxxxxxxxxxx> wrote in message
news:12ue23pdlhmvd2c@xxxxxxxxxxxxxxxxxxxxx
colin wrote:
"Anthony Fremont" <spam-not@xxxxxxxxxxx> wrote in message

the graph shows at 1 amp its saturated at vgs=3v., I hadnt intended
to need more than 1amp,

If I understand what I'm reading, Vds will be 1V at 3Vgs and 1A,
that's a dissipation of 1W. It's gonna get real hot if you draw an
amp thru it.

its an inverter so the current is ramped from 0-1amp, average 50%
duty cycle, so 0.25A avg
I only need about 1.5W real power output from the device im driving,
although it seems I need a fair bit more due to reactive currents in
the load,
at 4vgs its over 4amps wich should be ample for my needs, typical
ofc.

Am I not reading this right? At Vgs=4V and Id = 4A, I come up with
a Vds of 5V. Isn't that 20W of dissipation?

yea those dissipations are quite high, its a current ramp though so
only the tail end power will be that high, the average will work out
a lot lower. the on resistance would of course severly limit
operation to less than 4amp as id rather use 5v for the inverter. At
12v i would need less current too ofc.

Theres a quad package too, maybe 2 in parallell will easily do, or I
can use the other 2 to save a discrete elsewhere.

As for devices with large input capacitance such as igbt, the
current from my PIC is somewhat limited, so switching speed will
become unaceptable so I would need a driver anyway wich then makes
the fet selection easy anyway. the output from the inverter is 1/2
sine wich is why its PWM is driven from the PIC. it should drive
the 2nc gate charge of the vn2222 quick enough.

I agree that the PIC should have no trouble charging the gate of that
device.

Well depends how fast you want to switch it, 100khz would be ideal.
the dspic33 is a bit weedy on the io pin current side.
rated 4ma but seems to easily sink a peak of 10ma.

Ok, most of the ones I tinker with will supply 20mA.

All I know is that I couldn't find any FET that I could afford/use at
400V, I had to use an IGBT and a gate driver:
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1335&dDocName=en010666

I had a dual SO8 500v mosfet wich i used as a class d stage, fed by an
inverter.
Im trying to simplify it drastically by doing away with the inverter
and the hi and low side driver.

Think I might just plop for a driver and be done with it. I just
tried a dual driver I had available but cldnt get it to work, then
reading the data sheet realised the shutdown input had a threshold
of 11v lol.

TC4423 by microchip $2 in singles at Digikey. Good to 18V on the
output and should work fine at 3.3V.
4423 dual inverting
4424 dual non-inverting
4425 dual one-of-each

dual low side drivers should be cheaper than that I hope,
I dont need 3amp gate drive, thats as much as drain current,
or discrete will do.

Those are for fairly healthy sized MOSFETs and IGBTs. Microchip's AN786 app
note might interest you. It has some good info on how to "correctly"
calculate the amount of drive current you will really need to switch in a
certain amount of time. Those microchip gate drivers above will switch in
30nS or better. I guess it all depends upon what you need. :-)
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en011878
It's about PWM'ng power supplies that have high and low side drive, but
might interest you anyway.


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