>I would like to drive a SFP9520 FET (or alternate)
>(24VDC, 800mA) >I'm considering adding a small FET
>stage if required to drive this larger FET.

>I have omitted the filtering op-amps circuit to try
>to get this circuit functioning. The input to this FET
>circuit now is 5volts rail to rail PWM 20kHz from the
>opto-coupler (pic driven) . I will test with just static
>condition at first. I am requesting

I take it that the IRF9520 is an equivalent of the part number you quote.


First thing is that the FET is not going to saturate with a 5V gate drive.
On page 2 of the data sheet, check the Rds(on) value, it is given with 10V
gate drive. This is typical gate drive for most FET's unless they are
classed as logic level devices. Logic level devices tend to have other
limitations such as higher Rds(on) values.

To check if you can get a low enough Rds(on) check figure 6. This would
suggest you are going to have about 2V across the FET at 0.8A current with
5V gate voltage.

The other thing that I suspect is stopping your circuit is how you drive the
gate. This looks like a large capacitor to your circuit, so if you are
trying to switch it fast, you will need a heavier current drive than the
opto can give you.

You may need to give a reasonable sketch of your circuit for further help.

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