Low impedance Injector Driver

[Brian Dessent]

Dave Bannister wrote:

> I'm trying to modify my EEC computer on my mustang to drive low impedance
> injectors..

Just about any power mosfet should be able to do that with no problem. 
Using a bipolar transistor will result in more headaches (base drive and
vce(sat) issues) so I recommend staying clear of them.  Mosfets are
usually specified by maximum Vds, Ids, and Rds ("on" resistance.)  As
long as you have some headroom on all these, pretty much any fet will
do.  You most likely want N-channel, as they are intrinsically superior
to P-channel.  It really depends on the wiring though, n is for suited
for low side or pull-down switching where it is completing the path on
the ground side, p-channel is best for high side switching (pull-up)
where it's completing the path at the positive terminal.  

Your biggest problem will be designing the gate drive, because you must
ensure that 1) the fet turns on fully and 2) it turns on fast.  Get the
datasheet for whatever device you're considering (go to
www.questlink.com and enter the part number in the search box) and look
at the gate voltage requirements, usually labeled Vgs.  I don't know
what the stock computer uses as output drivers so I can't say what
you'll be dealing with here.  In general though, mosfets are pretty easy
to drive compared to bipolar (npn or pnp) because they require almost
zero gate current.  So as long as you can get a high enough gate voltage
(look into the "logic level" mosfets which generally require only 5V Vgs
if this is an issue) with a fairly fast risetime, you'll be fine.  You
probably won't even have to worry about heat sinking the fets.

(Note: this "gate drive is simple" argument really only applies if you
are doing low-side switching.  If you are doing high-side switching you
run into a slew of issues that usually make designers pull out their
hair.)

For example: IRLIZ24 : 55 Vds max, 14 amps Ids continuous, Rds(on) of
0.06 ohms, $1.44.  See digikey p. 383.

Brian

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