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
       overcome possible peak voltages while dealing with 320 volts dc
     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'm not completely sure about this,but it
       seems to me that if ,and most mosfets do, you have 20 v as maximum
       voltage drop allowed between source and gate,and you suddenly put 315 v
       on the gate before the mosfet is in full conduction,which means,before
       there are 300 volts on source,the mosfet should get fried.
       There must be some way to gradually raise voltage on gate,as it is raised
       on source,keeping 15 volt drop between G-S that will ensure
full conduction.

Thanks,
Walter.

>(given you don't want to use a chip designed for this purpose)
>How about an isolated power supply and a optical coupler.
>Mike

>>Hi all,
>>       I've been toying arround with power mosfets for some time
>>now,hanling some high voltage switching (300 volts,maybe 1 or two
>>amps).N-channel mosfets require at least gate voltage to be 10-12
>>volts above source to get in full conduction,which means that
>>if your high voltage rail is 300 volts,and you have two mosfets
>>in a half bridge configuration,the upper mosfet will require
>>300 + 12 = 312 volts for turning on.There are drivers from IR to
>>achieve this,but I'd like to do it without them.The approach is
>>usually making the source voltage float to the source level,using
>>a bootstrap diode and cappacitor to provide the overvoltage when
>>the mosfet is to be turned on.This has the inherent limitation of not
>>being able to keep the mosfet in conduction for a period longer than
>>the charge of the cappacitor allows it to.Then,the lower mosfet must
>>put the upper one's source to ground to allow the cappacitor to
>>recharge from a 15 volt power source through the bootstrap diode.
>>       Please excuse the roughness of the exposition,but this is
>> mainly the result of months of burning things on the protoboard
>>with very little theoretical background.
>>       Can any of you show me how you would build the circuit to
>>drive the upper side mosfet,with the bootstrap diode and cappacitor?

>>Thanks,
>>Walter