> Thanks,Mike
>        I see two problems with isolated power supply and
>        optocoupler:
>      1.I can't find an optocoupler rated for switching 800 volts (at
>      least to

Most of the QT electronics opto's listed in Mouser have 5.3 KV
isolation.  Should be plenty.

>      2.When the mosfet is off,source is grounded,which means that at
>      the very
>        beginning of the switching you would need 12 volts on the
>        gate,and
> the        voltage should have to raise as the mosfet goes into
> conduction,being
>        always 12 volts above source


I struggled with this idea myself.  The trick is, the power supply is
NOT connected to ground, it is connected to the Source of the Mosfet,
and it is completely isolated from Ground.  Using an isolated tap
from a small transformer is sufficient, as long as it can block the
several hundred volts you are dealing with.

I've seen some designs that just tap off a diode-resistor-zener
power supply from the Drain to Source, to get the 12 volts.  This has
always, IMHO, been one of the problems with Mosfets.

-- Lawrence Lile

     "The ideal design has zero parts"  -
            (attributed to Harold Hallikainen)

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