mount one, with the markings "223", the OnSemi logo ("On" inside a
circle), "D30" and "N02". My guess it's that some kind of power
MOSFET, but a trip to www.onsemi.com returned nothing useful.

TO 252 ?

I don't know where you're at, but Motorola came up with an intesting way of
marking mosfets, and they DO export them. They had MTP, MTH, and a few others,
but one number was the current rating and one was the voltage.

Now let me give you a REAL tip ! Determine the type and polarity of the unit.
If + voltage is put on the drain (case) it is an N channel device. Most likely
a regulator, you should see a coil nearby. After the coil look for a big cap,
see it's voltage rating, in fact you need a FET with the higher than the
highest cap voltage rating. ( electrolytics only )

On current, use the highest current you can get in that package. Get the
highspeed one too, can't hurt but might make a difference. We are talking about
the difference between $4 and $7 here, why skimp ? Just be sure it will work.
Since 1976 most of the world adopted a policy that to be "safe" safety cannot
require the FAILURE of a semiconductor. Almost universally this means you can
upgrade a semiconductor.

There is only one other thing. I assume there is no schematic. There are two or
three different types of FETs, there should be a Zener diode between the gate
and source, if it's like 5 volts that's probably a GEMFET or some equivalent,
5V is not enough to run a regular MOSFET at high currents. However if it calls
for a regular MOSFET, it'll blow a GEMFET type device. These things turn on
fully at 5V, not 20V, and in a PC I wouldn't be suprised if it is that class of
FETs. Perhaps, with all the shorted components removed you could put it in a PC
and measure the voltages at the pads. Might be revealing, in more ways than
one.

You could also probably order the fastest, highest rated regular MOSFET with
the highest gain and it should be alright, but transconductance (the measure of
gain of an FET or tube/valve) costs money. You can buy a transistor that will
work in damnear any circuit of the right polarity, but it won't be cheap. On
production there are different concerns.

Must be one heck of a video card, thought I'd give ya some ideas.

Good luck

JURB